HK1175579B - Methods for driving bistable electro-optic displays - Google Patents

Description

Method of driving bistable electro-optic display

This application is a divisional application of chinese patent application 200810176162.5 entitled "method of driving a bistable electro-optic display" filed by the applicant of yingke corporation on 31/03/2004.

This application is related to International application No. PCT/US02/37241, publication No. WO03/044765, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a method for driving an electro-optic display, in particular a bistable electro-optic display; the application also relates to an apparatus for use in such a method. More particularly, the present invention relates to driving methods and apparatus (controllers) for achieving more precise control of the gray state of the pixels of an electro-optic display. The invention also relates to a method of achieving long-term Direct Current (DC) balancing of drive impulses applied to an electrophoretic display. The invention is particularly, but not exclusively, intended for use with particle-based electrophoretic displays in which one or more types of electrically charged particles are suspended in a fluid and moved through the fluid under the influence of an electric field so as to change the appearance of the display.

Background

The term "electro-optic" as applied to a material or display is used herein in its conventional meaning in the imaging arts to refer to a material having first and second display states differing in at least one optical property, which changes from its first display state to its second display state upon application of an electric field to the material. While the optical property is typically a color perceptible to the naked eye, it may also be another optical property, such as light transmission, reflection, luminescence, or a false color in the sense of a change in reflection of electromagnetic wavelengths outside the visible range in the case of a display intended for machine reading.

The term "grey-scale state" is used herein in its conventional meaning in the imaging art to denote a state intermediate two extreme optical states of a pixel, and not necessarily to denote a black-and-white transition between the two extreme states. For example, several patents and published applications cited below describe electrophoretic displays in which the extreme states are white and deep blue, so that the intermediate "grey state" is effectively pale blue. Indeed, as mentioned above, the transition between the two extreme states may not be a color change at all.

The terms "bistable" and "bistability" are used herein in their conventional meaning in the display arts to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven by an addressing pulse of finite duration, that state will last at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element after the addressing pulse has ended, in order to assume its first or second display state. It is shown in published U.S. patent application No. 2002/0180687 that certain particle-based electrophoretic displays having gray scale capability are stable not only in their extreme black and white states, but also in their intermediate gray states, as is the case with certain other types of electro-optic displays. This type of display is correctly referred to as "multi-stable" rather than bi-stable, but for convenience the term "bi-stable" may be used herein to cover both bi-stable as well as multi-stable displays.

The term "gamma voltage" is used herein to refer to an external voltage reference used by the driver to determine the voltage to be applied to the pixels of the display. It will be appreciated that bistable electro-optic media do not display a one-to-one correlation between applied voltage and the optical state characteristics of the liquid crystal, and that the use of the term "gamma voltage" is not entirely equivalent herein to a conventional liquid crystal display, in which the gamma voltage determines a knee in the voltage level/output voltage curve.

The term "impulse" is used herein in its conventional meaning of the integral of voltage over time. However, some bistable electro-optic media are used as charge transducers, and for such media another definition of impulse, namely the integral of current over time (which is equal to the total charge applied), can be used. Depending on whether the medium is used as a voltage-time impulse transducer or as a charge impulse transducer, a proper definition of impulse should be taken.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type, such as described in U.S. patent nos. 5808783, 5777782, 5760761, 6054071, 6055091, 6097531, 6128124, 6137467 and 6147791 (although this type of display is often referred to as a "rotating bichromal sphere" display, more precisely the term "rotating bichromal member" is more appropriate because in some of the patents described above the rotating member is not spherical). Such displays employ a large number of small objects (usually spherical or cylindrical) having two or more portions with different optical properties and an internal dipole. The objects are suspended in fluid-filled vacuoles within the matrix, which vacuoles are filled with fluid so that the objects are free to rotate. The appearance of the display changes by applying an electric field thereto, thereby rotating the object to different positions and changing which parts of the object are visible through the viewing surface. This type of electro-optic medium is generally bistable.

Another type of electro-optic display employs an electrochromic medium, for example in the form of a nano-chromic film comprising an electrode at least partially composed of a semiconducting metal oxide and a plurality of dyed molecules having reversible color-changing capabilities attached to the electrode; see, for example, O' Regan, b, et al, "nature" (1991, 353, 737) and Wood, d, "information display" (18(3), 24 (3 months 2002)). See also Bach, u. et al, "adv.mater." (2002, 14(11), 845). Nanochromic films of this type are also described, for example, in U.S. Pat. No. 6301038, International application publication No. WO01/27690, and in U.S. Pat. No. 2003/0214695. This type of media is also generally bistable.

Another type of electro-optic display, which has been the subject of major development for many years, is a particle-based electrophoretic display in which a plurality of charged particles move through a suspending fluid under the influence of an electric field. Electrophoretic displays may have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, problems with the long-term image quality of these displays have prevented their widespread use. For example, particles that make up electrophoretic displays tend to settle, making these displays less useful.

A number of patents and applications have recently been published, assigned to or in the name of massachusetts institute of technology (mit) and eink corporation, which describe encapsulated electrophoretic media. Such encapsulation media comprise a plurality of small capsules, each of which itself comprises an internal phase containing electrophoretically mobile particles suspended in a fluid suspension medium, and a capsule wall surrounding the internal phase. The capsules themselves are typically held in a polymeric binder to form a tie layer between the two electrodes. For example, in U.S. Pat. nos. 5930026, 5961804, 6017584, 6067185, 6118426, 6120588, 6120839, 6124851, 6130773, 6130774, 6172798, 6177921, 6232950, 6249721, 6252564, 6262706, 6262833, 6300932, 6312304, 6312971, 6323989, 6327072, 6376828, 6377387, 6392785, 6392786, 6413790, 2003/0096113, 2003/0102858, 2003/0132908, 2003/0137521, 2003/0137717, 2003/0151702, 2003/0189749, 2003/0214695, 2003/0214697, 2003/0222315, 2004/0008398, 2004/0012839, 2004/0014265 and 2004/0027327, and international application publication nos. WO99/67678, WO00/05704, WO00/38000, WO00/38001, WO00/36560, WO00/67110, WO00/67327, WO01/07961, WO01/08241, WO03/092077 and WO 03/107315.

Many of the patents and applications identified above recognize that: the walls surrounding the discrete capsules in the encapsulated electrophoretic medium may be replaced by a continuous phase, resulting in a so-called "polymer dispersed electrophoretic display", in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material; and the discrete droplets of electrophoretic fluid in such polymer dispersed electrophoretic displays can be viewed as capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, e.g., 2002/0131147, supra. Thus, for purposes of this application, such polymer-dispersed electrophoretic media are considered to be subspecies of encapsulated electrophoretic media.

Encapsulated electrophoretic displays generally do not encounter the clustering and settling failure modes of conventional electrophoretic devices and offer other advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (the use of the word "printing" is intended to encompass all forms of printing and coating including, but not limited to, premetered coating such as sheet die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coating such as knife over roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, and other similar techniques.) thus, the resulting display may be flexible. Furthermore, since the display medium can be printed (in a variety of ways), the display itself can be made at a lower cost.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and suspending fluid are not encapsulated, but rather are held in a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, international application publication No. WO02/01281 and U.S. patent application publication No. 2002/0075556, both assigned to SipixImaging, inc.

Although electrophoretic media tend to be opaque (because, for example, in many electrophoretic media, the particles substantially block visible light from passing through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode," in which one display state is substantially opaque and one is light-transmissive. See, for example, the above-mentioned U.S. patent nos. 6130774 and 6172798, and U.S. patent nos. 5872552, 6144361, 6271823, 6225971, and 6184856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode; see U.S. patent No. 4418346.

The bistable or multistable performance of particle-based electrophoretic displays, as well as other electro-optic displays exhibiting similar performance, is in sharp contrast to conventional liquid crystal ("LC") displays. The twisted nematic liquid crystal action is not bistable or multistable but acts as a voltage transducer such that the application of a given electric field to a pixel of such a display produces a particular grey level at that pixel, irrespective of the grey level previously present at that pixel. Furthermore, LC displays are driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from a lighter state to a darker state is achieved by reducing or eliminating the electric field. Finally, the grey scale of the pixels of an LC display is not sensitive to the polarity of the electric field but only to its magnitude, and in fact, commercial LC displays often reverse the polarity of the driving electric field frequently for technical reasons.

In contrast, bistable electro-optic displays act quite approximately as impulse transducers, so that the final state of a pixel depends not only on the applied electric field and the time at which this field is applied, but also on the state of the pixel before the electric field is applied. Furthermore, it has now been found that, at least in the case of many particle-based electro-optic displays, the impulse required to change a given pixel by an equal change in grey scale (as judged by the eye or by standard optical instrumentation) is not necessarily constant, nor is they necessarily switchable. For example, consider a display in which each pixel can display a gray level of 0 (white), 1, 2 or 3 (black), preferably spaced apart. For example, the spacing may be linear in L (where L has the usual CIE definition:

L*＝116(R/R₀)^1/3-16，

wherein R is the reflectance, and R₀Standard reflectance values) or may be selected to provide a particular gamma; a gamma of 2.2 is often employed for monitors and the use of similar gammas may be desirable where these displays are used in place of monitors. ) It has been found that the impulse required to change a pixel from level 0 to level 1 (hereinafter referred to for convenience as a "0-1 transition") tends to be different than that required for a 1-2 or 2-3 transition. Furthermore, the impulse required for a 1-0 transition is not necessarily the same as the inversion of a 0-1 transition. In addition, some systems appear to exhibit a "memory" effect, such that the impulse required for, for example, a 0-1 transition varies somewhat with whether a particular pixel undergoes a 0-0-1, 1-0-1, or 3-0-1 transition. (where the notation "x-y-z" where x, y and z are all optical states 0, 1, 2 or 3 denotes a sequence of sequentially in time optical states that are listed from first to last.) although these problems can be reduced or solved by driving all pixels of the display to one of the extreme states for a sufficient period of time before driving the desired pixel to the other state, the resulting "flicker" of a solid color is often unacceptable; for example, a reader of an electronic book may wish to scroll the text of the book down the screen, but if the display needs to flash a solid black or white frequently, it may be distracting or not find the last read position. Furthermore, such flickering of the display increases its power consumption and may reduce the operational lifetime of the display. Finally, it has been found that in at least some cases the impulse required for a particular transition is influenced by the temperature and overall operating time of the display, and by the time a particular pixel remains in a particular optical state before a given transition, and compensation for these factors is desirable to ensure accurate grey scale reproduction.

Furthermore, as will be apparent from the foregoing discussion, the driving requirements of a bistable electro-optic medium make unmodified drivers designed for driving Active Matrix Liquid Crystal Displays (AMLCDs) unsuitable for use with bistable electro-optic medium-based displays. However, such AMLCD drivers are commercially available off-the-shelf with large voltage tolerance ranges and high pin count packages, and are inexpensive, making such AMLCD drivers attractive for driving bistable electro-optic displays, while similar drivers custom designed for bistable electro-optic medium-based displays are substantially more expensive and involve a large amount of design and production time. Accordingly, there are cost and development time advantages in modifying AMLCD drivers for use with bistable electro-optic displays, and the present invention seeks to provide a method and modified drivers that enable this.

Further, as mentioned above, the present invention relates to a method for driving an electrophoretic display, which enables a long-term DC-balancing of the drive impulse applied to the display. It has been found that encapsulated and other electrophoretic displays need to be driven with precisely DC balanced waveforms (i.e. the integral of the current over time of any particular pixel of the display should remain zero for extended periods of display operation) in order to maintain image stability, maintain symmetric switching characteristics, and provide the maximum useful operating life of the display. Conventional methods for maintaining accurate DC balance require accurate regulated power supplies, accurate voltage modulation drivers for gray scale levels, and crystal oscillators for timing, and the provision of these and similar components greatly increases the cost of the display.

(strictly speaking, DC balance should be measured "internally" taking into account the voltage to which the electro-optic medium itself is subjected, but in practice, it is not feasible to make such internal measurements in an operating display that may contain thousands of pixels, and in practice, DC balance is measured using "external" measurements, i.e., voltages applied to electrodes disposed on opposite sides of the electro-optic medium

The term "superframe" is used hereinafter to denote the sequence of consecutive display scan frames required to effect all necessary gray scale changes from the initial image to the final image. Display updates are typically only initiated at the beginning of a superframe.

The above-mentioned WO03/044765 describes a method of driving a bistable electro-optic display having a plurality of pixels each capable of displaying at least three grey levels (as is customary in the display art, the extreme black and white states are treated as two grey levels in order to calculate the number of grey levels). The method comprises the following steps:

storing a look-up table containing data representing an impulse required to convert the initial gray level to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

an output signal is generated representing the impulse required to convert the initial state of the one pixel to its desired final state, as determined from a look-up table.

For convenience, this method may be referred to as the "basic look-up table method" below.

Disclosure of Invention[0030]The lookup table used in the lookup table method may become very large depending on the number of previous states stored. To take an extreme example, consider an algorithm for 256 (2) that takes into account initial, final, and two previous states⁸) A look-up table method for a gray scale display. The necessary four-dimensional look-up table has 2³²An entry. If each entry requires, for example, 64 bits (8 bytes), the total size of the lookup table would be approximately 32 gigabytes. While storing this amount of data does not pose a problem for desktop computers, problems may arise in portable devices. In another aspect, the invention provides a method for driving a bistable electro-optic displayThe method, which achieves similar results to the look-up table method, does not require the storage of very large look-up tables.

One aspect of the invention relates to methods and apparatus for driving a bistable electro-optic display in a manner that allows a portion of the display to operate at a different bit depth (i.e., a different number of gray scale levels) than the remainder of the display. From the description of the sawtooth driving method described in fig. 11A and 11B of WO03/044765 above, it will be clear to a person skilled in the art that transitions between successive images in a general image stream of a bi-stable electro-optical display having a plurality of grey scale levels may be significantly longer than when the same display is driven in monochrome mode. The grey scale transitions may typically be up to four times the length of the corresponding monochrome transitions. A slower gray scale transition may not be objectionable when the display is used to present a series of images, such as a series of photographs or successive pages of an electronic book. However, it is sometimes useful to enable fast updates of limited areas of such displays. For example, consider the following case: users employ such displays to review series of photographs stored in the database in order to enter keywords or other indexing items for each photograph to assist in later retrieval of images from the database. In this case, a slower transition between successive photographs may be tolerable; for example, if a user spends one to two minutes studying each photo and deciding on index items, a one to two second transition between successive photos does not greatly affect the user's productivity. However, anyone who has tried to run a word processing program on a computer with insufficient processing power knows well that a one to two second delay is extremely frustrating and can lead to a large number of typing errors when updating a dialog box in which user-entered index items are displayed. In this and similar cases, it would therefore be advantageous to be able to run the dialog box in monochrome mode to allow for rapid transitions, while continuing to run the remainder of the display in grayscale mode to enable accurate rendering of the image, and the present invention provides a method and apparatus that enables this.

Another aspect of the invention relates to a method of achieving fine-tuning control of a gray scale of an impulse driven imaging medium without voltage fine-tuning control. Although electrophoretic and other electro-optic displays have been shown to exhibit bistability, this bistability is not unlimited and the image on the display decays slowly over time, so that if the image is to remain for an extended period, the image may have to be refreshed periodically in order to restore the image to its optical state at the time it was originally written.

However, such refreshing of the image may cause its own problems. As described in the above-mentioned us patent nos. 6531997 and 6504524, problems may be encountered and the operating life of the display may be reduced if the method for driving the display does not produce a net time-averaged applied electric field of zero or near zero across the electro-optic medium. The driving method that produces a zero net time averaged applied electric field across the electro-optic medium is suitably referred to as "direct current balancing" or "DC balancing". If an image is to be held for an extended period by applying refresh pulses, these pulses must have the same polarity as the address pulses used initially to drive the relevant pixels of the display to the optical state being held, which produces a DC-unbalanced drive scheme.

One challenge in achieving accurate gray scale levels in impulse drive media is applying an appropriate voltage impulse in order to achieve the desired gray tone. Satisfactory transitions between optical states can be achieved by fine-tuning control of the voltage of all or part of the drive waveform. The need for accuracy can be understood from the following examples. Consider the following: the current image consists of a screen of half black and half white, and the next image is expected to be a uniform gray scale between black and white. In order to achieve a uniform gray level, the impulses for going from black to gray and from white to gray must be finely adjusted so that the gray level resulting from black matches the gray level from white. Further fine-tuning is required if the resulting final gray scale is a function of the history of previous gray scales displayed. For example, as described above, the optical state achieved when going from black to gray may be a function not only of the applied waveform, but also of what state was reached before the current black state. It is then desirable for the display module to track certain aspects of the display history, such as previous image states, and allow fine-tuning of the waveform in order to compensate for this previous state history (see below for a more detailed discussion regarding this aspect).

The fine tuning of the impulse can be achieved by adjusting the width of the applied pulse with high precision, using only three voltage levels (0, + V, -V). However, this is undesirable for active matrix displays, since the frame rate must be increased in order to obtain high pulse width resolution. High frame rates increase the power consumption of the display and place higher demands on the control and driving electronics. It is therefore not desirable to operate an active matrix display at frame frequencies well above 60-75 Hz.

The fine tuning of the impulse can also be achieved when multiple finely spaced voltages are available. In active matrix driving, this requires a source driver that can output one of many sets of voltages available on at least a subset of the available voltages. For example, for a driver outputting between-10 and +10V, it may be advantageous to have available 0V, and two ranges of voltages between-10 and-7V and between 7 and 10V, with 16 different voltage levels between-10 and-7V and 16 different voltage levels between 7 and 10V, to bring the total number of required voltage levels up to 33 (see table 1). Fine control of the optical final state can then be achieved, for example, by varying the voltage between +7 and +10V or-10 and-7V for the last one or more scan frames of the addressing period. This approach is one example of a voltage modulation technique for achieving acceptable display performance.

Table 1: example of voltage required for voltage modulation drive

-10.0V	-7.8V	8.0V
			-9.8V	-7.6V	8.2V
-9.6V	-7.4V	8.4V
			-9.4V	-7.2V	8.6V
-9.2V	-7.0V	8.8V
			-9.0V	0.0V	9.0V
-8.8V	7.0V	9.2V
			-8.6V	7.2V	9.4V
-8.4V	7.4V	9.6V
			-8.2V	7.6V	9.8V
-8.0V	7.8V	10.0V

A disadvantage of using voltage modulation techniques is that the driver must have a range of voltage trim controls. The cost of the display module can be reduced by using drivers that provide only two or three voltages.

In another aspect, the present invention seeks to provide a method for implementing fine-tuning control of grey levels using drivers having only a small set of available voltages, especially if the control of the impulse is too coarse to achieve the fine-tuning required for acceptable display performance. Accordingly, this aspect of the present invention seeks to provide a method of achieving fine-tuning control of the grey-scale level of impulse-driven imaging media without the need for voltage fine-tuning control. This aspect of the invention is applicable, for example, to active matrix displays having source drivers capable of outputting only two or three voltages.

In another aspect, the invention relates to a method of driving an electro-optic display using a drive scheme that includes at least some Direct Current (DC) balanced transitions. For reasons detailed in the above-mentioned co-pending application, it is desirable to employ a DC balanced, i.e. having the following properties, driving scheme when driving an electro-optic display: for any sequence of optical states, the integral of the applied voltage is zero whenever the final optical state matches the initial optical state. This ensures that the net DC imbalance encountered by the electro-optic layer is limited by a known value. For example, a 15V, 300ms pulse may be used to drive the electro-optic layer from white to black. After this transition, the imaging layer encounters a DC imbalance impulse of 4.5V-s. To redrive the film to white, if a-15V, 300ms pulse is used, the imaging layer is DC balanced on the transition series from white to black and back to white.

It has also been found desirable to employ a drive scheme in which at least part of the transition itself is DC balanced; such transitions are hereinafter referred to as "DC balance transitions". The DC-balanced transition has no net voltage impulse. A drive scheme waveform that employs only DC-balanced transitions allows the electro-optic layer to remain DC-balanced after each transition. For example, a 15V, 300ms pulse followed by a 15V, 300ms pulse may be used to drive the electro-optic layer from white to black. The net voltage impulse on the electro-optic layer at this transition is zero. The electro-optic layer can then be redriven to white using a 15V, 300ms pulse followed by a-15V, 300ms pulse. The net voltage impulse is again zero on this transition.

The driving scheme consisting of all DC-balanced transition cells must be a DC-balanced waveform. It is also possible to formulate a DC balanced drive scheme that includes DC balanced transitions and DC unbalanced transitions, as discussed in detail below.

In one aspect, the invention provides a method of driving a bistable electro-optic display having a plurality of pixels, wherein each pixel is capable of displaying at least three gray levels, the method comprising:

storing a look-up table containing data representing an impulse required to convert the initial gray level to a final gray level;

storing data representing at least an initial state of each pixel of the display;

storing compensation voltage data representing compensation voltages for respective pixels of the display, the compensation voltage for any pixel being calculated from at least one impulse previously applied to that pixel;

receiving an input signal representing an expected final state of at least one pixel of the display; and

an output signal is generated representing a pixel voltage to be applied to the one pixel, the pixel voltage being the sum of the drive voltage determined from the initial and final states of the pixel and the look-up table and the compensation voltage determined from the compensation voltage data for the pixel.

For convenience, this method is hereinafter referred to as the "offset voltage" method of the present invention.

In this compensation voltage method, the compensation voltage for each pixel may be calculated according to at least one of a temporal previous state of the pixel and a gray scale previous state of the pixel. In addition, the compensation voltage for each pixel may be applied to that pixel both in a period in which a driving voltage is applied to the pixel and in a holding period in which no driving voltage is applied to the pixel.

For reasons explained in detail below, the compensation voltage used in the compensation voltage method of the present invention needs to be updated periodically. The compensation voltage for each pixel can be updated during each superframe (the period required for full addressing of the display). The compensation voltage for each pixel may be updated by: (1) modifying the previous value of the compensation voltage using a fixed algorithm independent of the pulses applied during the relevant superframe; and (2) increasing the value from step (1) by an amount determined by the pulses applied during the relevant superframe. In a preferred variant of this updating process, the compensation voltage for each pixel is updated by: (1) dividing a previous value of the compensation voltage by a fixed constant; and (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage/time curve applied to the electro-optic medium during the relevant superframe.

In the compensation voltage method of the present invention, the compensation voltage may be applied in the form of an exponentially decaying voltage applied at the end of at least one drive pulse.

The invention also provides a device controller for use in such a method of compensating voltage. The controller includes:

a storage section arranged to store a look-up table containing data representing pulses required to convert an initial grey level to a final grey level, data representing at least an initial state of each pixel of the display, and compensation voltage data for each pixel of the display;

an input component for receiving an input signal representing a desired final state of at least one pixel of the display;

a calculation section for determining a drive voltage required to change the initial state of the one pixel to a desired final state from the input signal, the stored data representing the initial state of the pixel, and the look-up table, the calculation section further determining a compensation voltage for the pixel from the compensation voltage data for the pixel and adding the drive voltage to the compensation voltage to determine a pixel voltage; and

an output component for generating an output signal representing the pixel voltage.

In such a controller the calculation means may be arranged to determine the compensation voltage in dependence on at least one of a temporal previous state of the pixel and a grey scale previous state of the pixel. In addition, the output section may be further configured to apply the compensation voltage to the pixel in both a period in which the driving voltage is applied to the pixel and a holding period in which no driving voltage is applied to the pixel.

Furthermore, in such a controller, the calculation means may be arranged to update the compensation voltage for each pixel during each super-frame required for full addressing of the display. For such an update, the calculation means may be arranged to update the compensation voltage for each pixel by: (1) modifying the previous value of the compensation voltage using a fixed algorithm independent of the pulses applied during the relevant superframe; and (2) increasing the value from step (1) by an amount determined by the pulses applied during the relevant superframe. In a preferred variant of this process, the calculation means are arranged to update the compensation voltage for each pixel by: (1) dividing a previous value of the compensation voltage by a fixed constant; and (2) increasing the value from step (1) by an amount substantially proportional to the total area under the voltage/time curve applied to the electro-optic medium during the relevant superframe.

The output means of the controller may be arranged to apply the compensation voltage in the form of an exponentially decaying voltage applied at the end of at least one drive pulse.

In another aspect, the invention provides a method for updating a bistable electro-optic display comprising: a plurality of pixels arranged in a plurality of rows and columns such that each pixel is uniquely defined by an intersection of a designated row and a designated column; and a drive means for independently applying an electric field to each pixel to change the display state of the pixel, each pixel having at least three different display states, the method comprising:

storing region data representing a defined region that includes a portion, but not all, of the display;

determining for each pixel whether the pixel is inside or outside the defined area;

a first drive scheme is applied to pixels within the defined area and a second drive scheme, different from the first drive scheme, is applied to pixels outside the defined area.

For convenience, this method is hereinafter referred to as the "define region" method of the present invention.

In this defined area approach, the bit depths of the first and second drive schemes may be different; in particular, one of the first and second drive schemes may be monochrome and the other may be a grey scale having at least four different grey levels. The defined area may include a text box for entering text on the display.

In another aspect, the invention provides a method of driving a bistable electro-optic display having a plurality of pixels, wherein each pixel is capable of displaying at least three gray levels, the method comprising:

storing a look-up table containing data representing an impulse required to convert the initial gray level to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

generating an output signal representing the impulse required to convert the initial state of said one pixel to its desired final state, determining from a look-up table,

wherein for at least one transition from an initial state to a final state, the output signal contains a DC imbalance trim sequence that:

(a) a net impulse with non-zero;

(b) is discontinuous;

(c) resulting in a change in the grey level of the pixel which is substantially different (typically by more than 50%) from the change in the optical state of its DC reference pulse, which is the voltage V₀Pulse of (2), wherein V₀For the maximum voltage applied during the trim sequence, but having the same sign as the net impulse G of the trim sequence, and the reference pulse having a duration of G/V₀(ii) a And

(d) resulting in a change of the grey level of the pixel with a magnitude smaller than the change of the grey level caused by its time reference pulse (typically less than half of it), wherein the time reference pulse is defined as a unipolar voltage pulse of the same duration as the trimming sequence, but wherein the sign of the reference pulse is the sign providing the larger change of the grey level.

For convenience, this method (and similar methods defined below) may be referred to hereinafter as the "discontinuous addressing" method of the present invention; when it is necessary to distinguish between the two methods, they may be referred to as a "DC unbalanced discontinuous addressing" method and a "DC balanced discontinuous addressing" method, respectively.

In a preferred form of this discontinuous addressing method the trimming sequence causes a change in the grey level of the pixel which is less than half the change in the grey level caused by its temporal reference pulse.

The invention also provides a method of driving a bistable electro-optic display having a plurality of pixels, each pixel being capable of displaying at least three gray levels, the method comprising:

storing a look-up table containing data representing an impulse required to convert the initial gray level to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

generating an output signal representing the impulse required to convert the initial state of said one pixel to its desired final state, determining from a look-up table,

wherein, for at least one transition from the initial state to the final state, the output signal comprises a DC-balancing fine tuning sequence that:

(a) has a substantially zero net impulse; and

(b) the grey level of a pixel is not made to differ from its grey level at the beginning of the trimming sequence by more than about one third of the difference in grey level between the two extreme optical states of the pixel at any point of the trimming sequence.

In both variants of the discontinuous addressing method of the invention, the output signal typically comprises at least one unipolar drive pulse in addition to the trimming sequence. The discontinuous output signal may be non-periodic. For most transitions in the look-up table, the output signal may have a non-zero net impulse and may be discontinuous. In at least one transition using a discontinuous output signal, the output signal may consist of only pulses having voltage levels + V, 0 and-V, preferably only pulses having voltage levels 0 and one of + V and-V. In a preferred variant of this method, for at least one transition using a discontinuous output signal, and preferably for most of the transitions in the look-up table where the initial and final states of the pixel differ, the output signal is formed by a pulse having a voltage level 0 followed by at least two pulses having the same voltage level of + V and-V. The transition table is preferably DC balanced. Additionally, for at least one transition employing a discontinuous output signal, the output signal may be comprised of a series of pulses that are integer multiples of a single interval.

The discontinuous addressing method of the invention may further comprise storing data representative of at least one temporally previous state of said one pixel and/or at least one grey scale previous state of said one pixel and generating an output signal in dependence on said at least one temporally previous state and/or at least one grey scale previous state of said one pixel.

The invention also provides a method of driving a bistable electro-optic display having a plurality of pixels each capable of displaying at least three grey levels, the method comprising applying to each pixel of the display an output signal effective to change the pixel from an initial state to a final state, wherein, for at least one transition in the pixel where the initial state and the final state are different, the output signal consists of a pulse having a voltage level of 0 followed by at least two pulses having the same voltage level of + V and-V.

In another aspect, the invention provides a method of driving a bistable electro-optic display having a plurality of pixels, each pixel being capable of displaying at least three grey levels, the method comprising applying to each pixel of the display an output signal effective to change the pixel from an initial state to a final state, wherein for at least one transition the output signal is non-zero but DC balanced.

For convenience, this method is hereinafter referred to as the "DC balanced addressing" method of the present invention.

In this DC balanced addressing method, the output signal may comprise, for at least one transition, a first pair of pulses comprising a voltage pulse and a preceding pulse of equal length but opposite sign. Alternatively, the output signal may also comprise a zero voltage period between two pulses, at least one of the pulses being interruptible by the zero voltage period. The output signal may also include a second pair of pulses of equal length but opposite sign; the second pair of pulses may have a different length than the first pair of pulses. A first one of the second pair of pulses has an opposite polarity to a first one of the first pair of pulses. The first pair of pulses may occur between the first and second of the second pair of pulses.

Additionally, in such a DC balanced addressing method, the output signal may comprise, for the above-mentioned transition, at least one pulse element effective to drive the pixel substantially into one optical rail (rail).

As discussed in more detail below, the DC balanced addressing method of the present invention may utilize a combination of DC balanced and DC unbalanced transitions. For example, the output signal may be non-zero but DC balanced for each transition where the initial and final states of the pixel are the same, and the output signal may not be DC balanced for each transition where the initial and final states of the pixel are not the same. In this addressing method, the output signal may have the form-x/Δ IP/x for each transition where the initial and final states of the pixel are not the same, where Δ IP is the difference in impulse potential between the initial and final states of the pixel, and-x and x are a pair of pulses of equal length but opposite sign.

The DC balanced addressing method of the invention may further comprise:

storing a lookup table containing data representing an impulse required to convert an initial gray level of a pixel to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

an output signal is generated representing the impulse required to convert the initial state of the one pixel to its desired final state, determined from a look-up table.

The invention also provides a method of driving a bistable electro-optic display having at least one pixel comprising applying a waveform v (t) to the pixel such that:

(where T is the length of the waveform, integration is over the duration of the waveform, V (T) is the waveform voltage as a function of time T, and M (T) is a memory function that characterizes a reduction in efficacy of the remnant voltage that causes the dwell time dependence arising from a short pulse at time zero) is less than about 1 volt-second. For convenience, this method is hereinafter referred to as the "DTD integration reduction" method of the present invention. Desirably, J is less than about 0.5 volt-seconds, and most desirably less than about 0.1 volt-seconds. In practice, J should be set as small as possible, ideally zero.

In a preferred form of this method, J is calculated from the formula:

where τ is the decay (relaxation) time, it preferably has a value of from about 0.7 to about 1.3 seconds.

Drawings

Fig. 1A-1E show five waveforms that can be used in the discontinuous addressing method of the present invention.

Figure 2 illustrates the problem in addressing an electro-optic display with various frame numbers of unipolar voltages.

Figure 3 illustrates one way of solving the problem shown in figure 2 using the discontinuous addressing method of the present invention.

Fig. 4 illustrates a second way of solving the problem shown in fig. 13 by using the discontinuous addressing method of the present invention.

Fig. 5 illustrates waveforms that can be used for the discontinuous addressing method of the present invention.

Fig. 6 illustrates basic waveforms that may be modified in accordance with the present invention to produce the waveforms shown in fig. 5.

FIG. 7 illustrates a problem in addressing electro-optic displays with various frame numbers of unipolar voltages while maintaining DC balance.

Figure 8 illustrates one way of solving the problem shown in figure 7 using the discontinuous addressing method of the present invention.

Fig. 9 illustrates a second way of solving the problem shown in fig. 7 by using the discontinuous addressing method of the present invention.

Figure 10 illustrates the gray scale levels obtained in a nominally four gray scale electro-optic display without employing the discontinuous addressing method of the present invention.

Fig. 11 illustrates the grey levels obtained from the same display as fig. 10 using various non-consecutive addressing sequences.

Fig. 12 illustrates the grey levels obtained from the same display as fig. 10 using a modified drive scheme in accordance with the discontinuous addressing method of the present invention.

Figure 13 illustrates a simple DC balance waveform that may be used to drive an electro-optic display.

Fig. 14 and 15 illustrate two modifications to the waveforms shown in fig. 13 to incorporate the zero voltage period.

Fig. 16 schematically illustrates how the waveforms shown in fig. 13 are modified to include another pair of drive pulses.

Fig. 17 illustrates a waveform generated by modifying the waveform of fig. 13 in the manner shown in fig. 16.

Fig. 18 illustrates a second waveform generated by modifying the waveforms of fig. 13 in the manner shown in fig. 16.

Fig. 19 schematically illustrates how the waveforms shown in fig. 18 are further modified to include a third pair of drive pulses.

Fig. 20 illustrates a waveform generated by modifying the waveform of fig. 18 in the manner shown in fig. 19.

Fig. 21 illustrates a preferred DC imbalance waveform that can be used in conjunction with a DC balance waveform to provide a complete look-up table for use in the method of the present invention.

FIG. 22 is a graph illustrating the reduced dwell time dependence that can be achieved by the compensation voltage method of the present invention.

FIG. 23 is a graph illustrating the effect of dwell time dependence in an electro-optic display.

Detailed Description

It will be apparent from the foregoing that the present invention provides a number of different improvements in the method of driving an electro-optic display and in the controller of a device or other apparatus which performs such a driving method. In the following description, the various improvements provided by the present invention are generally described independently, but those skilled in the imaging arts will appreciate that a single display may in fact utilize more than one of these major aspects; for example, displays employing the discontinuous addressing method of the present invention may also utilize the defined area method.

It initially appeared that the ideal method for addressing impulse driven electro-optic displays was the so-called "general gray scale image stream", in which the controller arranged each write of an image such that each pixel transitioned directly from its initial gray scale level to its final gray scale level. However, there is inevitably some error in writing the image to the impulse driven display. As mentioned in part, some of these errors that are actually encountered include:

(a) a previous state dependency; the impulse required to switch the pixel to a new optical state depends not only on the initial and desired optical states but also on the previous optical state of the pixel.

(b) Residence time dependence; the impulse required to switch the pixel to a new optical state depends on the time the pixel has spent in its respective optical state. The precise nature of this correlation is not fully understood, but in general, the longer a pixel dwells in its current optical state, the more impulse is required.

(c) A temperature dependence; the impulse required to switch the pixel to a new optical state depends strongly on the temperature.

(d) A humidity dependence; for at least some types of electro-optic media, the impulse required to switch a pixel to a new optical state depends on the ambient humidity.

(e) Mechanical uniformity; the impulse required to switch a pixel to a new optical state may be affected by mechanical changes to the display, such as changes in the thickness of the electro-optic medium or associated lamination adhesive. Other types of mechanical non-uniformities may result from unavoidable variations between different manufacturing batches of media, manufacturing tolerances and material variations.

(f) A voltage error; the actual impulse applied to the pixel will necessarily be slightly different from the theoretically applied impulse due to inevitable slight errors in the voltage delivered by the driver.

Typical grayscale image streams suffer from an "error accumulation" phenomenon. For example, consider that the temperature dependence produces a 0.2L error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10L. Perhaps more practically assuming that the average error per transition expressed as the difference between the theoretical and actual reflectivity of the display is ± 0.2L. After 100 consecutive transitions, the pixels will show an average deviation from their expected state 2L; such deviations are readily apparent to the average viewer of certain types of images.

This error accumulation phenomenon is applicable not only to errors due to temperature but also to other types of errors. Compensation for such errors is possible, but only to a limited degree of accuracy. For example, temperature errors may be compensated for by employing a temperature sensor and a look-up table, but the temperature sensor has limited resolution and may read a slightly different temperature than the electro-optic medium. Similarly, previous state dependencies can be compensated by storing previous states and employing a multi-dimensional transition matrix, but the controller memory limits the number of states that can be recorded and the size of the transition matrix that can be stored, thereby imposing a limit on the accuracy of this type of compensation, as described above.

Thus, general grayscale image streams require extremely precise control of the applied impulse to provide good results, and it has been empirically found that general grayscale image streams are generally not feasible in commercial displays at the current level of electro-optic display technology.

Almost all electro-optic media have a built-in reset (error-limiting) mechanism, i.e. their extreme (usually black and white) optical states, which act as "optical rails". After a particular impulse has been applied to a pixel of an electro-optic display, that pixel cannot become whiter (or blacker). For example, in an encapsulated electrophoretic display, after a particular impulse has been applied, all electrophoretic particles squeeze each other or onto the capsule wall and can no longer move, thereby creating a finite optical state or optical rail. Due to the electrophoretic particle size and charge distribution present in such media, some particles reach the rail before others, creating a "soft rail" phenomenon, so that the required impulse accuracy is reduced as the final optical state of the transition approaches the extreme black and white states, while the required optical accuracy increases sharply in transitions ending near the middle of the optical range of the pixel. It is clear that a typical pure gray scale image stream driving scheme cannot rely on the use of optical rails to prevent errors in gray scale levels, because in such a driving scheme any given pixel may experience an infinite number of changes in gray scale levels without touching any of the optical rails.

As described in the above-mentioned us patent nos. 6504524 and 6531997, in many electro-optic media, and in particular in particle-based electrophoretic media, it is desirable that the drive scheme for driving such media be Direct Current (DC) balanced in the sense that the algebraic sum of the currents through a particular pixel over an extended period should be zero or as close to zero as possible, and the drive scheme of the present invention will be designed with this criterion in mind. More specifically, the look-up table should be designed such that any sequence of transitions starting and ending in one extreme optical state (black or white) of the pixel should be DC balanced. As may be seen initially from the above, such DC balancing may not be achieved because the impulse, and thus the current, through the pixel required for any particular greyscale-to-greyscale transition is substantially constant. This is, however, only quite approximately the case, and it has been found empirically that, at least in the case of particle-based electrophoretic media (and as it appears to other electro-optic media), the effect of applying (for example) five pulses of 50 ms apart to a pixel is not the same as applying a 250 ms pulse of the same voltage. Thus, there is some flexibility in the current through the pixel to achieve a given transition, and this flexibility can be used to help achieve DC balance. For example, a lookup table used by the invention may store a number of impulses for a given transition and the value of the total current provided by each of these impulses, and for each pixel, the controller may maintain a register configured to store the algebraic sum of the impulses applied to the pixel since some previous time (e.g., since the pixel was last in the black state). When a particular pixel is to be driven from a white or grey state to a black state, the controller may examine the register associated with that pixel, determine the current required for DC balancing the entire sequence of transitions from the previous black state to the upcoming black state, and select one of a plurality of stored impulses for the desired white/grey to black transition, which will reduce the associated register to exactly zero or at least to as small a remainder as possible (in which case the associated register will hold the value of this remainder and add it to the current applied during the subsequent transition). It is clear that repeated application of this process can achieve accurate long-term DC balancing of the pixels.

The following discussion of the various aspects of the present invention will assume familiarity with the entire contents of the above-mentioned WO03/044765, and in particular the various waveforms disclosed herein. Those skilled in the display art will appreciate that the various methods of the present invention may be modified to include various optional features of the basic look-up table method described in the above-mentioned WO03/044765 (e.g., temperature compensation, operating life compensation, humidity compensation, etc.). The various methods of the present invention may also utilize the method described in the above-mentioned WO03/044765 for reducing the amount of data that must be stored for the look-up table. Furthermore, since the data comprising the look-up table can be viewed as a general multi-dimensional dataset, any standard function, algorithm, and encoding known to those skilled in the art of data storage and processing can be used to reduce one or more of the following: (a) the size of the storage area required for the data set, (b) the computational effort required to extract the data, or (c) the time required to find and extract a particular element from the data set. These storage techniques include, for example, hash functions, lossless and lossy compression, and representations of data sets that are combinations of basis functions.

Discontinuous addressing method

Fine-tuning control of the grey level levels in the method of the invention can be achieved by using the discontinuous addressing method of the invention. As mentioned above, the discontinuous addressing method has two main variants, namely a DC-unbalanced variant and a DC-balanced variant. The DC-unbalanced variant uses an output signal with a non-zero net impulse (i.e., the lengths of the positive and negative segments are not equal) for at least one transition between gray levels, and is therefore not internally DC-balanced, and is discontinuous (i.e., the pulse contains zero voltage or portions of opposite polarity). The output signal used in the discontinuous addressing method may or may not be aperiodic (i.e. may or may not be composed of repeating units such as +/-/+/-or + +/-).

Such discontinuous waveforms (which may be referred to hereinafter as "trim" or "FT" waveforms) may not have frames of opposite polarity for the effective front plane voltage of the display, and/or may comprise only the three voltage levels + V, 0 and-V (as is typically the case, assuming an active matrix display, typically an entire display, having a pixel electrode associated with each pixel and a common front electrode extending over a plurality of pixels, so that the electric field applied to any pixel of the electro-optic medium is determined by the voltage difference between its associated pixel electrode and the common front electrode). Alternatively, the FT waveform may contain more than three voltage levels. The FT waveform may include any of the above waveform types (e.g., n-pre-pulse, etc.) with the addition of a discontinuous waveform.

The FT waveform may (and typically will) depend on one or more previous image states and may be used in order to achieve smaller changes in optical state than can be achieved with standard Pulse Width Modulation (PWM) techniques. (thus the precise FT waveform employed differs from look-up table for each transition, for example, as compared to certain prior art waveforms that are said to employ pulses of alternating polarity to prevent electrophoretic particles from sticking to a surface such as a capsule wall.) in a preferred variant of the discontinuous addressing method, there is provided a combination of all waveforms required to achieve all allowed optical transitions ("transition matrix") in the display, at least one of which is the FT waveform of the present invention, and the combination of waveforms is DC balanced. In another preferred variant of the discontinuous addressing method, the length of all voltage segments is an integer multiple of a single interval ("frame time"); the voltage segment is a waveform portion where the voltage is kept constant.

The discontinuous addressing method of the present invention is based on the following findings: in many impulse driven electro-optic media, waveforms having zero net impulse, and which in theory would not be expected to cause an overall change in the grey level of a pixel, may actually cause a small change in grey level due to some non-linear effect in the properties of such media, which may be used to achieve a finer adjustment of grey level than is possible with a simple PWM drive scheme or a driver with limited ability to vary the width and/or height of the pulse. The pulses that can achieve such a "fine-tune" waveform can be separate from the "main drive" pulses that cause the main change in grey scale and can be either before or after such main drive pulses. Alternatively, in some cases, the fine adjustment pulses may be mixed with the main drive pulses, or be individual blocks of fine adjustment pulses at a single point in the sequence of main drive pulses, or be individually interspersed or divided into small groups at multiple points in the sequence of main drive pulses.

Although the discontinuous addressing method has very general applicability, it is described mainly using as an example a driving scheme employing a source driver with three voltage outputs (positive, negative and zero) and a waveform consisting of the following three types of waveform elements (since we consider that the necessary modifications to the invention for use with other types of drivers and waveform elements will be readily apparent to those skilled in the art of electro-optic displays):

1) saturation pulse: a sequence of frames having one symbol or one symbol and zero volts, driving the reflectivity close to one extreme optical state (the optical rail, either the darkest state, referred to herein as the black state, or the brightest state, referred to herein as the white state);

2) setting a pulse: a sequence of frames with one symbol or one symbol and zero volt voltage to drive the reflectivity close to the desired grey level (black, white or intermediate grey levels); and

an FT sequence: a sequence of frames with voltages individually selected to be positive, negative, or zero causes the optical state of the ink to move much less than a single sequence of symbols of the same length. An example of an FT drive sequence having a total length of five scan frames is: [ + - + - - - - - ] (here, the voltages of the frames are expressed in the following manner: + represents a positive voltage, 0 represents a zero voltage, and-represents a negative voltage), [ - -0+ + ], [00000], [00+ -0], and [0- +00 ]. These sequences are schematically represented in fig. 1A-1E of the drawings, respectively, where the circles represent the starting and ending points of the FT sequence, with five scan frames in between.

The FT sequence may be used to allow fine control of the optical states as described above, or to produce a change in optical states similar to that of a unipolar (single sign) voltage sequence, but with a different net voltage impulse (where impulse is defined as the integral of the applied voltage over time). The FT sequence in the waveform can thus be used as a tool to achieve DC balance.

The use of an FT sequence to achieve fine control of the optical states will first be described. In fig. 2, the optical states that can be achieved with zero, one, two, three or more frames of unipolar voltages are schematically represented as points on the reflectivity axis. As can be seen from this figure, the length of the unipolar pulse can be selected to achieve the reflectivity represented by its corresponding point on this axis. However, it may be desirable to achieve gray levels such as those indicated by the "target" in FIG. 2, which cannot be appropriately approximated by any of these gray levels. The FT sequence can be used to fine-tune the reflectivity to a desired state either by fine-tuning the final state obtained after the unipolar drive pulses or by fine-tuning the initial state and then re-using the unipolar drive sequence.

As shown in fig. 3, the first example of the FT sequence represents the FT sequence used after the double pulse unipolar driving. The FT sequence is used to fine tune the final optical state to the target state. Similar to fig. 2, fig. 3 shows optical states that can be achieved with various scan frame numbers, as shown by the solid dots. The target optical state is also indicated. The optical change by applying two scan frames is shown as the optical offset caused by the FT sequence.

A second example of an FT sequence is shown in fig. 4; in this case, the FT sequence is first used to fine-tune the optical state to a position where the unipolar drive sequence can be used to achieve the desired optical state. The optical states achievable after the FT sequence are represented by the open circles in fig. 4.

The FT sequence may also be used with a Rail Stabilized Gray Scale (RSGS) waveform, such as shown in fig. 11A and 11B of WO03/044765, discussed above. The essence of the RSGS waveform is that a given pixel is only allowed to make a limited number of grey to grey transitions before being driven to one of its extreme optical states. Thus, such waveforms take advantage of frequent driving to extreme optical states (called optical balustrades) to reduce optical errors while maintaining DC balance (where DC balance is a net voltage impulse of zero, described in more detail below). By selecting fine adjustment voltages for one or more scan frames, properly resolved gray levels can be achieved with these waveforms. However, if these fine tuning voltages are not available, another method must be used to achieve the fine tuning, preferably while also maintaining DC balance. FT sequences may be used to achieve these objectives.

First, consider a cyclic version of the rail-stabilized gray level waveform, where each transition consists of zero, one, or two saturation pulses (the pulses that drive the pixels to the optical rail) followed by a set pulse as described above (to bring the pixels to the desired gray level). To illustrate how the FT sequence can be used for this waveform, the notation will be used for the waveform elements: "sat" denotes a saturation pulse; "set" represents a set pulse; and "N" represents an FT drive sequence. Three basic types of cyclic balustrade stabilized gray level waveforms are:

set (e.g., transition 1104 in FIG. 11A of WO 03/044765)

sat-set (e.g., transition 1106/1108 in FIG. 11A of WO 03/044765)

sat-sat' -set (e.g., transition 1116/1118/1120 in FIG. 11A of WO 03/044765)

Where sat and sat' are two different saturation pulses.

The modification of the first of these types with the FT sequence gives the following waveform:

N-set

set-N

that is, the FT sequence is followed by a set pulse or the reverse order of the same elements.

The modification of the second of these types with one or more FT sequences gives, for example, the following FT-modified waveforms:

N-sat-set

sat-N-set

sat-set-N

sat-N-set-N’

N-sat-set-N’

N-sat-N’-set

N-sat-N’-set-N”

where N, N' and N "are three FT sequences, which may or may not be different from each other.

A modification of the second of these types can be achieved by interspersing the FT sequence between three waveform elements, primarily in the form described above. A non-exhaustive list of examples includes:

N-sat-sat’-set

N-sat-sat’-set-N’

sat-N-sat’-N’-set-N”

N-sat-N’-sat’-N”-set-N”’

another basic waveform that can be modified with the FT sequence is to have a single pulse slide gray level driven to black (or white). In this waveform, the optical state is first brought to the optical rail and then to the desired image. The waveform of each transition may be symbolically represented by either of two sequences:

sat-set

set。

this waveform can be modified by including FT drive sequence elements in substantially the same manner as already described for the rail-stabilized gray level sequence, resulting in the following sequence:

sat-set-N

sat-N-set

and so on.

The above two examples describe the insertion of FT sequences before or after the saturation of the waveform and setting of the pulse elements. It may be advantageous to insert an FT sequence in between the saturation or set pulses, i.e. the base sequence:

sat-set

will be modified to form, for example:

{ sat, part I } -N- { sat, part II } -set

Or

sat- { set, fraction I } -N- { set, fraction II }

As described above, it has been found that the optical states of many electro-optic media achieved after a series of transitions are sensitive to previous optical states and to the time elapsed in those previous optical states, and some approaches have been described for compensating for previous states and previous dwell time sensitivities by adjusting the transition waveforms accordingly. The FT sequence may be used in a similar manner to compensate for previous optical states and/or previous dwell times.

To describe this concept in more detail, consider a sequence of gray levels to be represented on a particular pixel; these grades are denoted R₁、R₂、R₃、R₄And the like, wherein R₁Representing the next expected (final) grey level, R, of the considered transition₂Is the initial gray level of that transition, R₃Is the first previous gray level, R₄Is the second previous gray level, and so on. The gray scale sequence can then be expressed as:

R_nR_n-1R_n-2...R₃R₂R₁

the dwell time before the grey level i is denoted as D_i。D_iThe number of frame scans that stay in the gray level i can be represented.

The FT sequence described above may be selected to suit the transition from the current to the desired gray level. In the simplest form, these FT sequences are then functions of the current and expected gray levels, as symbolized by:

N＝N(R₂，R₁)

thus, it was shown that the FT sequence N depends on R₂And R₁。

To improve device performance, and in particular to reduce the residual gray scale shift associated with previous images, small adjustments to the transition waveform are advantageous. The selection of the FT sequence may be used to achieve these adjustments. The various FT sequences produce various final optical states. Different FT sequences may be selected for different optical histories of a given pixel. For example, to compensate for the first previous image (R)₃) Optionally dependent on R₃Is represented as:

N＝N(R₃，R₂，R₁)

that is, the FT sequence may be based not only on R₁And R₂Is selected and can be based on R₃To select.

Generalizing this concept, the FT sequence can be made to depend on any number of previous gray levels and/or on any number of previous dwell times, as symbolized by:

N＝N(D_m，D_m-1，...D₃，D₂；R_n，R_n-1，...R₃，R₂，R₁)

wherein the symbol D_kExpressed in grey level R_kThe elapsed dwell time, and the number of optical states n, need not be equal to the number of dwell times m required in the FT determination function. Thus, the FT sequence may be a function of previous images and/or previous and current gray scale dwell times.

As a special case of this general concept, it has been found that the insertion of a zero voltage scan frame into a further unipolar pulse can change the resulting final optical state. For example, the optical states obtained after the sequence of fig. 5, in which a zero voltage scan frame has been inserted, will be slightly different from the optical states obtained after the corresponding unipolar sequence of fig. 6 without a zero voltage scan frame but with the same total impulse as the sequence of fig. 5.

It has also been found that the effect of a given pulse on the final optical state depends on the length of the delay between this pulse and the preceding pulse. Thus, a zero voltage frame may be inserted between pulse elements to achieve fine tuning of the waveform.

The invention extends to the use of FT drive elements and the insertion of zero volt scan frames in unipolar drive elements of other waveform configurations. Other examples include, but are not limited to, double pre-pulse (including three pre-pulses, four pre-pulses, etc.) slide gray scale waveforms in which two optical balustrades are accessed (more than once with a higher number of pre-pulses) in going from one optical state to another, and other forms of balustrade stabilization gray scale waveforms. The FT sequence can also be used for general image stream gray scale waveforms, where direct transitions are made between gray levels.

Although the insertion of a zero voltage frame may be considered a special case of an FT sequence insertion, in the case of FT sequences all being zero, this special case is interesting because it has been found to be effective in modifying the final optical state.

The above has focused on using FT sequences to achieve fine-tuning of the grey levels. The use of such FT sequences to achieve DC balance will now be considered. The FT sequences may be used to alter (preferably reduce or eliminate) the degree of DC imbalance in the waveform. DC-balancing means that all full-circuit grey level sequences (sequences starting and ending with the same grey level) have zero net voltage impulse. By using one or more FT sequences, the waveform can be DC balanced or less strongly DC unbalanced, taking advantage of the fact that: the FT sequence may either (a) change the optical state in the same way as the saturation or set pulse but with a substantially different net voltage impulse; or (b) produces insubstantial changes in optical state but with a net DC imbalance.

The following description shows how FT sequences can be used to achieve DC balance. In this example, the set pulse may be of variable length, i.e., one, two, three or more scan frames. The final gray scale achieved for each of the number of scan frames is shown in fig. 7, where the number next to each point represents the number of scan frames used to achieve the gray scale.

Fig. 7 shows the optical states that can be obtained for a scan frame with positive voltage, unipolar driving, where the numerical references designate the number of unipolar frames used to produce the final grey scale. It is assumed in this example that in order to maintain DC balance, a net voltage impulse of two positive voltage frames needs to be applied. The desired (target) gray level may be achieved by three scan frames with an impulse; however, in doing so, the system will maintain the DC imbalance by one frame. In contrast, DC balance can be achieved by scanning the frame with two positive voltages instead of three, but the final optical state will deviate greatly from the target.

One way to achieve DC balance is to drive the electro-optic medium around the desired gray level with two positive voltage frames and also to use a DC-balanced FT sequence (an FT sequence with zero net voltage impulse) to bring the final adjustment sufficiently close to the target gray level, as symbolized in fig. 8, where the target gray level is achieved with two scan frames followed by an FT sequence of zero net voltage impulse selected to provide the appropriate change in optical state.

Alternatively, three positive voltage scan frames of unipolar drive may be employed to bring the reflectivity to the target optical state, followed by an FT sequence with a net DC imbalance equivalent to one negative voltage scan frame. If an FT sequence is selected that produces substantially unchanged optical states, the final optical states will remain correct and DC balance will be restored. An example of this is shown in figure 9. It will be appreciated that the use of FT sequences will typically involve some adjustment of the optical states and some effect on DC balance, and the two examples above illustrate extreme cases.

The following examples are provided by way of illustration only to illustrate experimental use of FT sequences according to the present invention.

Example (c): use of FT sequences in cyclic RSGS waveforms

This example illustrates the use of an FT sequence in improving the optical performance of waveforms designed to achieve 4 gray scale (2-bit) addressing of a single pixel display. Such a display employs an encapsulated electrophoretic medium and is constructed substantially as described in paragraphs 0069 to 0076 of 2002/0180687 above. The single pixel display is monitored by a photodiode.

The waveform voltages are applied to the pixels according to a transition matrix (look-up table) in order to achieve a sequence of grey levels in a 2-bit (4-state) grey level. As mentioned above, the transition matrix or look-up table is just one set of rules for applying voltages to the pixels for making a transition from one grey level to another within a grey level.

The waveform is subject to voltage and timing limitations. Only three voltage levels-15V, 0V and +15V are applied to the pixel. In addition, to simulate active matrix driving with a 50Hz frame frequency, voltages were applied in 20ms increments. The tuning algorithm is used iteratively in order to optimize the waveform, i.e. to achieve the condition that the dispersion in the actual optical states for each of the four grey levels over the test sequence is minimal.

In initial experiments, the cyclic balustrade stable gray level (cRSGS) waveform was optimized with simple saturation and set pulses. Consideration of previous states is limited to initial (R) in determining the transition matrix₂) And the desired final (R)₁) Grey scale. The waveform is globally DC balanced. Due to the coarseness of the minimum impulse available for tuning (15V, 20ms) and R in the transition matrix₂The lack of a previous state, from which a rather poor performance is expected.

The performance of the transition matrix is tested by converting the test pixels through a "full quintuple" grey scale sequence comprising all grey scale quintuple sequences arranged randomly. (a quintuple sequence element is a sequence of five gray levels, e.g., 0-1-0-2-3 and 2-1-3-0-3, where 0, 1, 2 and 3 represent the four gray levels available.) for a perfect transition matrix, the reflectivity of each of the four gray levels is identical for all occurrences of that gray level in the random sequence. The reflectivity of each of the grey levels will be significantly different for the actual transition matrix. The bar graph of fig. 10 actually represents the poor performance of the voltage and timing limited transition matrix. The measured reflectance for each occurrence of each of the target gray levels is very different. The cRSGS waveform optimized without the FT sequence developed in this part of the experiment is hereinafter referred to as the base waveform.

The FT sequence is then added to the cRSGS waveform; in this experiment, the FT sequence was limited to five scan frames and contained only DC balanced FT sequences. The FT sequence is set at the end of the base waveform for each transition, i.e. the waveform for each transition has one of the following forms:

set-N

sat-set-N

sat-sat’-set-N

successful incorporation of FT sequence elements into waveforms requires two steps; first, the effect of the various FT sequences on the optical state of each gray scale is determined, and second, the FT sequences appended to the various waveform elements are selected.

To determine the effect of various FT sequences on the optical states of various gray levels, an "FT efficacy" experiment was performed. First, a uniform starting point is established by repeatedly switching the electrophoretic medium between black and white optical rails. The film is then set to one of four gray levels (0, 1, 2, or 3), referred to herein as the optical state R₂. Subsequently, the application is suitably carried out from R₂To one of the other grey levels (herein referred to as R)₁) With the basic waveform of the additional FT sequence. This step is repeated using a full 51 DC balanced 5 frame FT sequence. The final optical state is recorded for each FT sequence. The FT sequences are then ordered according to their associated final reflectivities. This procedure is for initial (R)₂) And finally (R)₁) All combinations of gray levels are repeated. Final gray level 1 (R)₁1) and current gray levels 0, 2 and 3 (R)₂Ordering of FT sequences 0, 2, 3) is shown in tables 2-4, respectively, where the columns labeled "frame 1" through "frame 5" represent the potentials applied in volts during five consecutive frames of the relevant FT sequence. The final optical states achieved for the waveforms with the various FT sequences are shown in fig. 11. It can be seen from this figure that the FT sequence can be used to achieve large changes in the final optical state, and that the selection of the five scan frame FT sequence provides fine control over the final optical state, all without a net voltage impulse difference.

Table 2: final optical states of grey levels 0 to 1 for the various FT sequences.

Table 3: final optical states of grey levels 2 to 1 for the various FT sequences.

Index number	Optics (L)	Frame 1	Frame 2	Frame 3	Frame 4	Frame 5
							1	34.85	0	15	15	-15	-15
2	34.91	15	0	15	-15	-15
							3	35.07	15	15	-15	-15	0
4	35.15	15	15	0	-15	-15
							5	35.35	15	15	-15	0	-15
6	35.43	0	15	-15	15	-15
							7	35.46	15	-15	0	15	-15
8	35.51	0	0	15	-15	0
							9	35.52	0	15	-15	0	0
10	35.52	0	0	0	15	-15
							11	35.61	15	-15	15	-15	0
12	35.62	0	0	15	0	-15
							13	35.63	15	-15	0	0	0
14	35.65	-15	15	0	15	-15
							15	35.67	0	15	0	-15	0
16	35.70	-15	0	15	15	-15
							17	35.75	15	-15	15	0	-15
18	35.76	0	15	0	0	-15
							19	35.77	15	0	-15	0	0
20	35.78	15	0	-15	15	-15
							21	35.80	-15	15	15	-15	0
22	35.97	-15	15	15	0	-15
							23	35.98	15	0	0	-15	0
24	36.00	0	-15	15	15	-15

25	36.06	0	0	0	0	0
							26	36.09	-15	0	0	15	0
27	36.10	-15	0	0	0	15
							28	36.10	15	0	0	0	-15
29	36.14	-15	0	15	0	0
							30	36.28	-15	15	0	0	0
31	36.38	15	-15	-15	0	15
							32	36.40	0	15	-15	-15	15
33	36.41	0	-15	0	0	15
							34	36.44	0	-15	0	15	0
35	36.45	15	-15	-15	15	0
							36	36.49	-15	15	-15	0	15
37	36.49	0	-15	15	0	0
							38	36.55	-15	0	15	-15	15
39	36.57	-15	15	-15	15	0
							40	36.59	0	0	-15	0	15
41	36.63	0	0	-15	15	0
							42	36.72	15	-15	0	-15	15
43	36.72	15	0	-15	-15	15
							44	36.77	0	0	0	-15	15
45	36.81	-15	15	0	-15	15
							46	36.89	0	-15	15	-15	15
47	36.98	-15	-15	15	0	15
							48	37.16	-15	-15	15	15	0
49	37.19	-15	-15	0	15	15
							50	37.42	-15	0	-15	15	15
51	37.51	0	-15	-15	15	15

Table 4: final optical states of grey levels 3 to 1 for the various FT sequences.

Index number

Optics (L)

Frame 1

Frame 2

Frame 3

Frame 4

Frame 5

1	36.86	0	15	15	-15	-15
							2	36.92	15	0	15	-15	-15
3	37.00	15	15	-15	-15	0
							4	37.13	15	15	0	-15	-15
5	37.39	15	15	-15	0	-15
							6	37.47	0	15	-15	15	-15
7	37.48	15	-15	0	15	-15
							8	37.50	0	15	-15	0	0
9	37.52	0	0	15	-15	0
							10	37.53	0	0	0	15	-15
11	37.60	15	-15	15	-15	0
							12	37.62	15	-15	0	0	0
13	37.63	0	0	15	0	-15
							14	37.65	0	15	0	-15	0
15	37.67	-15	15	0	15	-15
							16	37.71	-15	0	15	15	-15
17	37.76	0	15	0	0	-15
							18	37.77	15	-15	15	0	-15
19	37.79	15	0	-15	15	-15
							20	37.80	15	0	-15	0	0
21	37.82	-15	15	15	-15	0
							22	37.96	15	0	0	-15	0
23	38.01	-15	15	15	0	-15
							24	38.03	0	-15	15	15	-15
25	38.04	0	0	0	0	0
							26	38.09	-15	0	0	15	0
27	38.09	15	0	0	0	-15
							28	38.15	-15	0	0	0	15
29	38.16	-15	0	15	0	0
							30	38.24	-15	15	0	0	0

31	38.40	15	-15	-15	0	15
							32	38.43	0	-15	0	0	15
33	38.44	0	-15	0	15	0
							34	38.44	0	15	-15	-15	15
35	38.46	15	-15	-15	15	0
							36	38.51	-15	15	-15	0	15
37	38.52	0	-15	15	0	0
							38	38.59	-15	0	15	-15	15
39	38.61	-15	15	-15	15	0
							40	38.65	0	0	-15	0	15
41	38.66	0	0	-15	15	0
							42	38.74	15	0	-15	-15	15
43	38.74	15	-15	0	-15	15
							44	38.82	0	0	0	-15	15
45	38.89	-15	15	0	-15	15
							46	38.95	0	-15	15	-15	15
47	39.02	-15	-15	15	0	15
							48	39.21	-15	-15	15	15	0
49	39.22	-15	-15	0	15	15
							50	39.44	-15	0	-15	15	15
51	39.53	0	-15	-15	15	15

Subsequently, the cRSGS waveform is constructed using FT sequences (specifically, sequence 33 from table 2, sequence 49 from table 3, and sequence 4 from table 4) and their analogs of other final gray levels, which are selected using the results shown in tables 2 to 4 and fig. 11. It is to be noted that the region between-36.9 and-37.5L on the y-axis in fig. 11 indicates the same final (R)₁) The state is different from the initial (R) that becomes available by balancing the FT sequence with DC₂) Overlap between the optical reflectivities of the states. Thus, R₁The target grey level of 1 is chosen at 37.2L and each R providing the final optical state closest to this target is chosen₂The FT sequence of (1). This process is for the other final optical states (R)₁0, 2 and 3) were repeated.

Finally, the resulting waveform is tested using the previously described pseudo-random sequence containing the history of all five depth states. This sequence contained 324 transitions of interest. The cRSGS waveform modified by the selected FT sequence is used to achieve all transitions in this sequence and to record the reflectivity of each optical state achieved. The optical states achieved are plotted in fig. 12. As is apparent by comparing fig. 12 with fig. 10, the dispersion of the reflectance per gray level is greatly reduced by combining the FT sequence.

In summary, the discontinuous addressing aspect of the invention provides an FT sequence that either (i) allows for a change in optical state, or (ii) allows for a method of achieving a DC balance of the waveform, or at least a change in the degree of DC imbalance. As described above, for example, for the DC-unbalanced variant of the method, a more mathematical definition of the FT sequence can be provided:

(a) application of a DC unbalanced FT sequence resulting in an optical state change substantially different from that of its DC reference pulse. "DC reference pulse" is a voltage V₀Pulse of (2), wherein V₀Is a voltage that corresponds to the maximum voltage amplitude applied during the FT sequence, but has the same sign as the net impulse of the FT sequence. The net impulse of the sequence is the area under the voltage versus time curve and is represented by the symbol G. The duration of the reference pulse is T ═ G/V₀. This FT sequence is used to introduce a DC imbalance that is very different from the net DC imbalance of its reference pulse.

(b) The application of a DC unbalanced FT sequence resulting in an optical state change of much smaller amplitude than the optical change obtained with its temporal reference pulse. A "temporal reference pulse" is defined as a single sign voltage pulse of the same duration as the FT sequence, but with the sign of the reference pulse selected to provide the largest change in optical state. That is, a negative voltage pulse may drive the electro-optic medium only slightly whiter when the electro-optic medium is near its white state, while a positive voltage may drive the electro-optic medium to substantially black. The sign of the reference pulse is positive in this case. The purpose of this type of FT pulse is to adjust the net voltage impulse (e.g. for DC balancing) while not greatly affecting the optical states.

The discontinuous addressing aspect of the invention also relates to the concept of using one or more FT sequences between or interposed between the pulse elements of the transition waveform, and to the concept of using FT sequences to balance the effect of previous grey levels and previous dwell times. One particular example of the invention is a zero voltage frame inserted in the middle of a certain pulse element of the waveform or between some pulse elements of the waveform for changing the final optical state.

The discontinuous addressing aspect of the present invention also allows for fine tuning of the waveform to achieve a desired gray scale with a desired accuracy, and for a way to make the waveform closer to DC balance (i.e., zero net voltage impulse to any cyclic drift of various gray scales) with source drivers that do not allow for fine tuning of the voltage, particularly source drivers that have only two or three voltage levels.

DC balance addressing method

It should be noted that the sawtooth drive scheme shown in figures 11A and 11B of WO03/044765 described above is well suited for DC balancing, as this sawtooth drive scheme ensures that only a limited number of transitions can pass between successive passes of any given pixel through the black state, and indeed on average a pixel will pass through the black state half as much as it passes.

However, as mentioned above, according to the DC-balanced addressing method of the present invention, the DC-balancing according to the present invention is not limited to balancing the total amount of impulses applied to the electro-optic medium during a series of transitions, but extends also to "internal" DC-balancing at least a part of the transitions through which the pixels of the display are subjected; this method will now be described in detail.

The DC-balanced addressing method of the present invention involves DC-balanced transitions of encapsulated electrophoretic and other impulse-driven electro-optic media that are advantageously used to drive display applications. This method is applicable, for example, to active matrix displays having source drivers capable of outputting only two or three voltages. The following detailed description focuses largely on an example employing a source driver having three voltage outputs (positive, negative, and zero), although other types of drivers may be used.

In the following description of the DC balanced addressing method of the invention, the grey levels of the electro-optical medium will be represented as 1 to N, where 1 represents the darkest state and N represents the brightest state, as in the previous description of other aspects of the invention. The intermediate states are numbered incrementally from dark to light. A drive scheme for impulse driving an imaging medium utilizes a set of rules to effect a transition from an initial gray level to a final gray level. The drive scheme may be expressed as a voltage as a function of time for each transition as shown in table 5 for each of the 16 possible transitions for a 2-bit (4 gray scale) gray scale display.

TABLE 5

In table 5, vij (t) indicates a waveform for making a transition from gray level i to gray level j. The DC-balance transition is a transition where the time integral of the waveform vij (t) is zero.

The term "optical rail" has been defined above to mean the extreme optical states of the electro-optic medium. The phrase "pushing the media toward or to the optical balustrade" will be used below. "to" means that a voltage is applied to move the optical state of the medium toward one of the optical balustrades. "push" means that the voltage pulse has sufficient duration and amplitude to cause the optical state of the electro-optic medium to substantially approach one of the optical rails. It is important to note that "pushing to the optical rail" does not mean that the optical rail state is necessarily achieved at the end of the pulse, but rather that an optical state that is substantially close to the final optical state is achieved at the end of the pulse. For example, consider an electro-optic medium with optical rails at 1% and 50% reflectivity. It was found that a 300ms pulse changed the final optical state (from 1% reflectivity) to 50% reflectivity. It may be mentioned that a 200 millisecond pulse pushes the display to the high reflectivity optical rail even though it achieves a final reflectivity of only 45% reflection. This 200 millisecond pulse is believed to push the media to one of the optical balustrades because the 200 millisecond duration is longer than the time required to traverse most of the optical range, such as the middle third of the optical range (in this case, 200 milliseconds longer than the pulse required to cause the media to reflect through the middle third of the reflectivity range, here from 17% to 34%).

Three different types of DC-balanced transitions of the DC-balanced addressing method according to the invention will now be described, as well as hybrid driving schemes employing DC-balanced and DC-unbalanced transitions. For ease of explanation, in the following description, pulses will be represented by numerical values, the magnitude of which represents the duration of the pulse. If the value is positive, the pulse is positive, and if the value is negative, the pulse is negative. Thus, for example, if the available voltages are +15V, 0V and-15V, and the pulse duration is measured in milliseconds (msec), then a pulse characterized as x-300 represents a 300 msec, 15V pulse, and x-60 represents a 60 msec, -15V pulse.

Type I:

in the first simplest type of DC-balancing transition of the present invention, a voltage pulse ("x") is preceded by a pulse ("-x") of equal length but opposite sign, as shown in fig. 13. (Note that the value of x may itself be negative, so that positive and negative pulses may occur in reverse order to that shown in FIG. 13.)

As mentioned above, it has been found that in accordance with the discontinuous addressing method of the present invention, the effect of the waveform used to effect the transition is modified by the presence of a period of zero voltage (in effect a time delay) in or before any of the pulses in the waveform. Fig. 14 and 15 illustrate a modification to the waveforms of fig. 13. In fig. 14 the delay is inserted between the two pulses of fig. 13, while in fig. 15 the delay is inserted within the second pulse of fig. 13, or the result is the same, the second pulse of fig. 13 is divided into two separate pulses separated by a delay. As described above, time delays may be added to the waveform to achieve optical states that are not attainable without these delays. The time delay can also be used to fine tune the final optical state. This fine tuning capability is important because in active matrix driving the time resolution of each pulse is defined by the scan rate of the display. The time resolution provided by the scan rate may be too inaccurate to achieve an accurate final optical state without some additional fine-tuning method. The time delay provides a small degree of fine tuning of the final optical state, while the additional features as described below provide additional ways of coarse and fine tuning of the final optical state.

Type II:

type II waveforms consist of the type I waveforms described above in which a positive and negative pulse pair (denoted as "y" and "-y" pulses) is inserted into the type I waveform at some point, as symbolized in fig. 16. The y and-y pulses need not be continuous, but may occur in the original waveform at different locations. There are two particularly advantageous forms of type II waveforms.

Type II: special example A:

in this particular form, the "-y, y" pulse pair is disposed before the "-x, x" pulse pair. It has been found that when y and x are of opposite sign, the final optical state can be fine-tuned by even a moderately coarse adjustment of the duration y, as shown in fig. 17. Thus, the value of x can be adjusted for coarse control and the value of y can be adjusted for fine tuning of the final optical state of the electro-optic medium. We believe this occurs because the y pulse augments the-x pulse, thus changing the degree to which the electro-optic medium is pushed to one of its optical rails. The degree of push to one of the optical rails is known to provide a fine adjustment of the final optical state after the pulse (in this case provided by the x-pulse) leaving that optical rail.

Type II: special example B:

for the reasons described above, it has been found advantageous to employ a waveform having at least one pulse element that is long enough to drive the electro-optic medium substantially to one of the optical balustrades. In addition, for a more visually pleasing transition, it is also desirable to reach the final optical state from the closer optical balustrade, since only a short final pulse is required to achieve a gray scale close to the optical balustrade. This type of waveform requires at least one long pulse to drive to the optical rail and one short pulse to reach the final optical state close to the optical rail, and thus cannot have the type I structure described above. However, a specific example of a type II waveform may implement this type of waveform. Fig. 18 shows an example of such a waveform, where the y pulse is placed after the-x, x pulse pair and the-y pulse is placed before the-x, x pulse pair. In this type of waveform, the final y pulse provides coarse tuning because the final optical state is extremely sensitive to the amplitude of y. The x-pulses provide fine tuning because the final optical state is generally not strongly dependent on the amplitude driven to the optical balustrade.

Type III:

a third type (type III) of DC-balanced waveform of the present invention introduces yet another DC-balanced pulse pair (denoted "z", "-z") into the waveform, as schematically shown in fig. 19. A preferred example of this type III waveform is shown in fig. 20; this type of waveform is useful for fine tuning of the final optical state for the following reasons. Consider the case without z and-z pulses (i.e., the type II waveform described above). The x pulse elements are used for fine tuning and the final optical state can be decreased by increasing x and increased by decreasing x. However, it is not desirable to reduce x beyond a certain point, since then the electro-optic medium is not brought close enough to the optical balustrade as required for the stability of the waveform. To avoid this problem, instead of decreasing x, the-x pulse can be (actually) increased without changing the x pulse by adding a-z, z pulse pair, as shown in fig. 20, where z has the opposite sign to x. The z pulse increases the-x pulse while the-z pulse keeps the transition at zero net impulse, i.e. keeps the DC-balanced transition.

The type I, II and III waveforms described above can of course be modified in various ways. Additional pulse pairs may be added to the waveform to achieve a more general configuration. The advantage of such additional pairs decreases with increasing number of pulse elements, but such waveforms are a natural extension of the type I, II and III waveforms. Additionally, as described above, one or more time delays may be inserted at various locations in any of the waveforms in the same manner as shown in fig. 14 and 15. As mentioned earlier, the time delay of the pulses affects the final optical state achieved and is therefore useful for fine tuning. In addition, by changing the position of a transition element relative to other elements in the same transition and to transition elements of other transitions, the setting of the time delay can change the visual appearance of the transition. The time delay may also be used to align certain waveform transition elements, and this may be advantageous for some display modules with certain controller capabilities. Furthermore, recognizing the fact that small changes in the sequencing of the applied pulses may substantially change the optical state after the pulses, the output signal may also be formed by swapping all or part of one of the above-mentioned pulse sequences or by repeated swapping of all or part of one of the above-mentioned sequences, or by inserting one or more 0V periods into one of the above-mentioned sequences at any position. Additionally, these swap and insert operators may be combined in any order (e.g., insert the 0V portion, then swap, then insert the 0V portion). It is important to note that all such pulse sequences formed by these transforms retain the basic property of having zero net impulse.

Finally, DC balanced transitions can be combined with DC unbalanced transitions to form a complete driving scheme. For example, co-pending application serial No. 60/481053, filed on 2.7.2003, describes a preferred waveform of type-TM (R1, R2) [ IP (R1) -IP (R2) ] TM (R1, R2), where [ IP (R1) -IP (R2) ] represents the difference in impulse potential between the final and initial states of the transition under consideration, while the remaining two terms represent DC balanced pulse pairs. For ease of illustration, this waveform is hereinafter referred to as the-x/Δ IP/x waveform and is shown in FIG. 21. While satisfactory for transitions between different optical states, this waveform is less satisfactory for zero transitions where the initial and final optical states are the same. For these zero transitions, a type II waveform such as that shown in fig. 17 and 18 is employed in this example. This full waveform is symbolized as in table 6, from which it will be seen that the-x/Δ IP/x waveform is used for non-zero transitions, and the type II waveform is used for zero transitions.

TABLE 6

		Final grey scale
								1	2	3	4

Initial gray scaleGrade	1	Type II	-x/ΔIP/x	-x/ΔIP/x	-x/ΔIP/x
							2	-x/ΔIP/x	Type II	-x/ΔIP/x	-x/ΔIP/x
	3	-x/ΔIP/x	-x/ΔIP/x	Type II	-x/ΔIP/x
							4	-x/ΔIP/x	-x/ΔIP/x	-X/ΔIP/X	Type II

The DC-balanced addressing method is certainly not limited to this type of transition matrix where the DC-balanced transitions are limited to "main diagonal" transitions where the initial and final grey levels are the same; to produce the greatest improvement in gray scale control, it is desirable to maximize the number of transitions belonging to the DC balance. However, depending on the particular electro-optic medium used, it may be difficult to make a DC-balanced transition involving a transition to or from extreme grey levels, for example to or from black and white, i.e. grey levels 1 and 4 respectively. Furthermore, in selecting which transitions to DC-balance, it is important not to unbalance the entire transition matrix, i.e. to generate a transition matrix in which closed cycles starting and ending at the same grey level are DC-unbalanced. For example, a transition to DC-balanced, but other transition to DC-unbalanced rules involving only 0 or 1 cell changes in gray scale levels are not required, as this would unbalance the entire transition matrix, as shown in the following example; pixels that go through the sequence of gray levels 2-4-3-2 will go through transitions 2-4(DC imbalance), 4-3 (balance), and 3-2 (balance), making the entire cycle unbalanced. A practical compromise between these two conflicting desires may be to employ DC-balanced transitions if only intermediate grey levels (levels 2 and 3) are involved, and DC-unbalanced transitions if the transition starts or ends at extreme grey levels (levels 1 or 4). Obviously, the intermediate gray levels chosen for such rules may vary depending on the particular electro-optic medium and controller used; for example, in a three-bit (8 gray scale) display, it may be possible to employ DC balanced transitions in all transitions starting or ending with gray scales 2-7 (or perhaps 3-6), and DC unbalanced transitions in all transitions starting or ending with gray scales 1 and 8 (or 1, 2, 7, and 8).

It will be seen from the above that the DC-balanced addressing method of the present invention allows trimming of the waveform to achieve a desired grey level with high accuracy, and the manner in which a waveform transition can be made to have zero net voltage with source drivers that do not allow voltage trimming, particularly source drivers having only two or three voltage levels. We believe that DC balanced waveform transitions provide better performance than DC unbalanced waveforms. The invention is applicable to displays in general and is particularly, but not exclusively, applicable to active matrix display modules having source drivers providing only two or three voltages. The invention is also applicable to active matrix display modules having source drivers providing more voltage levels.

The DC balanced addressing method of the present invention may provide certain additional advantages. As described above, in some driving methods of the present invention, the transition matrix is a function of a variable different from the previous optical state, such as the length of time since the last update or the temperature of the display medium. Maintaining DC balance in these cases with unbalanced transitions is quite difficult. For example, consider a display with repeated transitions from white to black at 25 ℃ and then from black to white at 0 ℃. Slower response at low temperatures is generally indicated by longer pulse lengths. Thus, the display will experience a net DC imbalance towards white. On the other hand, if all transitions are internally balanced, the different transition matrices can be freely mixed without introducing DC imbalance.

Method for defining area

As mentioned above, the unpleasant effect of the reset step can be further reduced by employing local rather than global updates, i.e. by overwriting only those parts of the display that change between successive images, the parts to be overwritten being selected on a "local area" or pixel-by-pixel basis. For example, as in a diagram illustrating parts of a mechanical device or a diagram for accident reconstruction, it is not uncommon to find a series of images of smaller objects moving over a larger static background. To use local updates, the display controller needs to compare the final image with the initial image and determine which area(s) differ between the two images and thus need to be overwritten. The controller may identify a rectangular area, typically having an axis aligned with the pixel grid, one or more local areas containing pixels that need to be updated, or simply individual pixels that need to be updated. Any of the drive schemes described above may then be used to update only the local areas or individual pixels identified in this way as requiring overwriting. Such a local update scheme may substantially reduce the power consumption of the display.

Furthermore, as described above, the defined area method of the present invention provides a defined area method that allows different update methods to be employed in different areas of the display to update the bistable electro-optic display.

Electro-optic displays are known in which the entire display can be driven in a one-bit or gray scale mode. When the display is in one-bit mode, the update is implemented using a one-bit general picture flow (GIF) waveform, whereas when the display is in grayscale mode, the update is implemented using a multi-previous pulse slide waveform or some other slow waveform, even if only one bit of information is updated in a particular area of the display.

Such an electro-optic display can be modified to perform the defined area method of the present invention by defining two additional commands in the controller, namely a "define area" command and a "clear all area" command. The "define region" command typically takes as arguments the positions of the rectangular region sufficient to completely define the display, e.g., the positions of the upper right and lower left corners of the defined region; this command may also have an additional argument specifying the bit depth to which the defined area is set, but this last argument is not necessary in the simple form of defining an area method where the defined area is always monochrome. The bit depth set by the last argument clearly exceeds any bit depth previously set for the defined region. Alternatively, the "define region" command may specify a series of points that define the vertices of the polygon. The "clear all regions" command may not take an argument, but simply reset the entire display to a single predefined bit depth, or may take a single argument that specifies which of the various possible bit depths was taken by the entire display after the clear operation.

It will be appreciated that the defined area method of the present invention is not limited to the use of only two areas, and that more areas may be provided as desired. For example, in an image editing program, it may be helpful to have a main area that displays the edited image at full bit depth, as well as an information display area (e.g., a box that displays the current cursor position) and a dialog area (providing a dialog for the user to enter text) that operate in a one-bit mode. The invention is described below primarily in two-region form, as the necessary modifications to allow the use of more than two regions will be readily apparent to those skilled in the art of construction of display controllers.

To track the depth of the different regions, the controller may maintain an array of storage elements, one element associated with each pixel in the display, and each element storing a value representing the current bit depth of the associated pixel. For example, an XVGA (800x600) display capable of operating in either a 1-bit or 2-bit mode may employ an 800x600 array of 1-bit elements (each containing a 0 for a 1-bit mode and a 1 for a 2-bit mode). In such a controller, the "define area" command sets elements in a defined area of the display to the requested bit depth, while the "clear all area" command resets all elements of the array to the same value (either a predetermined value or a value defined for an argument of the command).

Alternatively, when a region is defined or cleared, the controller may perform an update sequence on the pixels in that region to shift the display from one mode to another in order to ensure DC balance or to adjust the optical state of the relevant pixels, for example by employing an FT sequence as described above.

When the display is operating in the defined area mode, a new image is sent to the controller and the display must be refreshed, there are three possible situations:

1. only the pixels in a defined, for example one bit area have changed. In this case, a one-bit (fast) waveform can be used to update the display;

2. only the pixels in the undefined (gray level) area have changed. In this case, the grey scale (slow) waveform must be used to update the display (note that the legibility of a defined region, e.g. a dialog box, is not an issue during a screen refresh, since by definition no pixels are changed in the defined region); and

3. the pixels in both the defined and undefined regions have changed. In this case, the grayscale pixels are updated with a grayscale waveform, and the one-bit pixels are updated with a one-bit waveform (the shorter one-bit waveform must be properly zero-padded to match the length of the grayscale update).

The controller may determine which of these conditions exist by performing the following logic tests (assuming a one-bit value associated with each pixel and storing the pixel pattern, as described above) before scanning the display:

(Old _ imageXORnew _ image) > 0: pixel changes in a display

(Old _ imagexonnew _ image) anddmode _ array > 0: the gray scale pixels are changed

(Old _ imagexonnew _ image) AND (NOTMode _ array) > 0: the monochrome pixel is changed

When the controller scans the display, one waveform lookup table can be used for all pixels, either case 1 or case 2, since the unchanged pixels will receive 0V, assuming that the zero transition in the one-bit mode is the same as in the grayscale mode (in other words, both waveforms are locally updated). Conversely, if the gray scale waveform is a global update (all pixels are updated each time the display is updated), the controller needs to test to see if the pixels are within the appropriate region in order to determine whether to apply the global update waveform. For case 3, the controller must check the value of the mode bit array for each pixel while scanning to determine which waveform to use.

Alternatively, if the brightness values of the black and white states implemented in the one-bit mode are the same as those implemented in the gray scale mode, then in case 3 above, the gray scale waveform is available for all pixels in the display, thereby eliminating the need for a transfer function between the one bit and the gray scale waveform.

The defined area method of the present invention may utilize any of the optional features of the basic lookup table method, as described above.

The main advantage of the region-defining method of the present invention is that it allows the use of a fast one-bit waveform on a display that is displaying a previously written gray scale image. Prior art display controllers typically only allow the display to be either in grayscale or in a one-bit mode at any one time. Although it is possible to write a one-bit image in the grayscale mode, the correlation waveform is rather slow. In addition, the defined area method of the present invention is substantially transparent to the host system (the system is typically a computer) that provides the image to the controller, as the host system does not need to suggest to the controller which waveform to use. Finally, the define region approach allows one bit and gray level waveforms to be used on the display at the same time, while other solutions require two separate update events if both waveforms are used.

Further general waveform discussion

The above-described drive schemes may vary in many ways depending on the characteristics of the particular electro-optic display being used. For example, in some cases, it may be possible to eliminate many of the reset steps in the above-described driving schemes. For example, if the electro-optic medium used is bistable over long periods (i.e. the grey level written to a pixel varies only very slowly over time), and the impulse required for a particular transition does not vary greatly with the period over which the pixel is in its initial grey state, the look-up table may be arranged to bring the grey state directly to a grey state transition without any intermediate return to the black or white state, wherein a reset of the display is only performed when, after a sufficient period has elapsed, the gradual "drift" of the pixel from its nominal grey level has resulted in a significant error in the rendered image. Thus, for example, if a user uses the display of the present invention as an electronic book reader, it may be possible to display many information screens before a reset of the display is required; it has been found empirically that with appropriate waveforms and drivers, up to 1000 information screens can be displayed before a reset is required, so that in practice a reset is not necessary in the typical reading phase of an ebook reader.

It will be apparent to those skilled in the display arts that a single device of the present invention can effectively be configured with a plurality of different drive schemes for use under different conditions. For example, since in the drive schemes shown in figures 9 and 10 of the above-mentioned WO03/044765 the set pulse consumes a significant portion of the total energy consumption of the display, it is possible for the controller to configure a first drive scheme which resets the display frequently, thereby minimising grey scale errors, and a second scheme which resets the display only at longer intervals, thereby allowing for larger grey scale errors but reducing energy consumption. The conversion between the two schemes can be done manually or according to external parameters; for example, if the display is used in a laptop computer, a first drive scheme may be employed when the computer is operating on mains power, and a second drive scheme may be employed when the computer is operating on internal battery power.

Voltage compensation method

Another variation on the basic lookup table method and apparatus of the present invention is provided by the offset voltage method and apparatus of the present invention, which will now be described in detail.

As described above, the offset voltage method and apparatus of the present invention seek to achieve similar results to the basic lookup table method described above, but without the need to store a very large lookup table. The size of the lookup table grows rapidly with the number of previous states indexed with respect to the lookup table. To this end, as described above, there are practical limitations and cost considerations to increase the number of previous states used in selecting the impulse to achieve the desired transition of the bi-stable electro-optic display.

In the compensation voltage method and apparatus of the present invention, the size of the lookup table required is reduced and compensation voltage data is stored for each pixel of the display, this compensation voltage data being calculated from at least one impulse previously applied to the relevant pixel. The voltage finally applied to the pixel is the sum of the drive voltage selected from the look-up table in the usual manner and a compensation voltage determined from the compensation voltage data for the relevant pixel. In effect, the compensation voltage data applies a "correction" to the pixel, which correction would have been applied, for example, by indexing a look-up table to one or more additional previous states.

The lookup table used in the compensation voltage method may be of any of the types described above. Thus, the look-up table may be a simple two-dimensional table that only takes into account the initial and final states of the pixels during the associated transition. Alternatively, the look-up table may take into account one or more time and/or grey level previous states. The compensation voltage may also take into account only the compensation voltage data stored for the associated pixel, but may alternatively take into account one or more temporal and/or grey scale previous states. The compensation voltage may be applied to the relevant pixel not only during the period in which the drive voltage is applied to the pixel, but also in a so-called "hold" state in which no drive voltage is applied to the pixel.

The exact manner in which the compensation voltage data is determined may vary greatly depending on the characteristics of the bistable electro-optic medium used. The compensation voltage data is typically modified periodically in a manner determined by the drive voltage applied to the pixel for the current and/or one or more scan frames. In a preferred form of the invention, the compensation voltage data comprises a single number (register) value associated with each pixel of the display.

In a preferred embodiment of the invention, the scan frames constitute a superframe in the manner described above, such that display updates only start at the beginning of the superframe. For example, a superframe may consist of ten display scan frames, such that for a display having a 50Hz scan rate, the display scan is 20ms long, while the superframe is 200ms long. During each super-frame period when the display is rewritten, the compensation voltage data associated with each pixel is updated. The update includes two parts in the following order:

(1) modifying the previous value using a fixed algorithm independent of the pulses applied during the relevant superframe; and

(2) the value from step (1) is increased by an amount determined by the impulse applied during the relevant superframe.

In a particularly preferred embodiment of the invention, steps (1) and (2) are performed in the following manner:

(1) dividing the previous value by a fixed constant, preferably two; and

(2) the value from step (1) is increased by an amount proportional to the total area under the voltage/time curve applied to the electro-optic medium during the relevant superframe.

In step (2), the increment may be exactly or only approximately proportional to the area under the voltage/time curve during the relevant superframe. For example, as described in detail below with reference to fig. 22, the delta may be "quantized" into a finite set of classes of all possible applied waveforms, each class containing all waveforms having a total area between two limits, and the delta determined in step (2) by the class to which the applied waveform belongs.

The following examples are now given. The display used is a two-bit gray scale encapsulated electrophoretic display and the driving method employed uses a two-dimensional look-up table as shown in table 7 below, which only considers the initial and final states of the intended transitions; in the table, the column headings represent the desired final state of the display, and the row headings represent the initial state, while the numbers in each cell represent the voltage in volts that will be applied to the pixel over a predetermined period.

TABLE 7

To allow for implementation of the compensation voltage method of the present invention, a single value register is associated with each pixel of the display. The various impulses shown in table 7 are classified and a pulse class is associated with each impulse as shown in table 8 below.

TABLE 8

Pulse voltage (V)	-15	-9	-6	0	+6	+9	+15
								Pulse class	-30	-18	-12	0	12	18	30

During each superframe, the value register associated with each pixel is divided by 2 and then incremented by the value shown in table 13 for the pulse applied to the relevant pixel during the same superframe. The voltages applied to the pixels during the super frame are the driving voltages shown in Table 12 and the compensation voltage V given by the following equation_compAnd (3) the sum:

V_compeither as a (pixel register)

Wherein the pixel register value is read from a register associated with the associated pixel, and "a" is a predefined constant.

In laboratory proof of this preferred compensated voltage method of the present invention, a single pixel display employing an encapsulated electrophoretic medium sandwiched between parallel electrodes (the front one of which is comprised of ITO and is transparent) is driven between its black and white states by a 300 millisecond +/-15V square wave pulse. The display, starting with its white state, is driven black and then redriven to white after a dwell time. It has been found that the brightness of the final white state is a function of the dwell state, as shown in figure 22 of the accompanying drawings. Thus, such encapsulated electrophoretic media are sensitive to dwell times, where L of the white state changes by about 3 units depending on the dwell time.

In order to show the effect of the compensation voltage method of the present invention, experiments were repeated, but a compensation voltage consisting of an exponentially decaying voltage starting at the end of each driving pulse was added to each pulse. The applied voltage is the sum of the drive voltage and the compensation voltage. As shown in fig. 22, the white state for each dwell time is much more uniform with the compensated voltage than with the uncompensated pulses. Thus, this experiment shows that the use of such compensation pulses according to the present invention can greatly reduce the dwell time sensitivity of the encapsulated electrophoretic medium.

The compensation voltage method of the present invention may utilize any of the optional features of the basic look-up table method described above.

From the foregoing it will be seen that the present invention provides a method for controlling the operation of an electro-optic display which is well suited to the characteristics of bistable particle-based electrophoretic displays and similar displays.

It will also be seen from the above description that the present invention provides methods for controlling the operation of electro-optic displays which allow for accurate control of the grey level without the need for the entire display to inconveniently flash frequently to one of its extreme states. The invention also allows for precise control of the display despite variations in its temperature and operating time, while reducing the power consumption of the display. These advantages can be achieved at low cost because the controller can be constructed from commercially available components.

DTD integral reduction method

As mentioned above, it has been found that the impulse required for a given transition in a bistable electro-optic display varies, at least in some cases, with the residence time of the pixel in its optical state, a phenomenon which previously appears to be unaddressed in the literature is referred to hereinafter as "dwell time dependence" or "DTD". It may therefore be desirable, or even in some cases actually necessary, to change the impulse applied for a given transition as a function of the residence time of the pixel in its initial optical state.

The phenomenon of dwell time dependence, which expresses the reflectivity of a pixel as denoted by R, will now be described in more detail with reference to FIG. 23 of the drawings₃→R₂→R₁A function of time of the represented transition sequence, wherein R_kEach of the terms represents a gray level in a gray level sequence, with R having a larger index occurring before R having a smaller index. And also represents R₃And R₂And R₂And R₁To be changed over. DTD is optical state R₂Final optical state R generated by changes in elapsed time₁The elapsed time is referred to as the residence time. The DTD integration reduction method of the present invention provides a method for reducing dwell time dependence when driving a bistable electro-optic display.

Although the present invention is in no way limited by any theory as to its origin, DTD appears to be caused, to a large extent, by residual electric fields experienced by the electro-optic medium. These residual electric fields are the remnants of the drive pulses applied to the media. It is generally said that the residual voltage is generated by the applied pulses, and that the residual voltage is simply a scalar potential corresponding to the residual electric field in the usual manner suitable for electrostatic theory. These residual voltages can cause the optical state of the display film to drift over time. They may also change the efficacy of the subsequent drive voltage, thereby changing the final optical state obtained after that subsequent pulse. In this way, the residual voltage from one transition waveform can cause the final state after the subsequent waveform to be different than when the two transitions are very independent of each other. By "very independent" is meant separated sufficiently in time that the residual voltage from the first transition waveform has substantially decayed before the second transition waveform is applied.

Measurements of the residual voltage resulting from the transition waveform and other simple pulses applied to the electro-optic medium show that the residual voltage decays over time. The decay appears to be monotonic, but not merely exponential. However, quite approximately, the decay may be approximately exponentially, with a decay time constant of about one second in most cases of encapsulated electrophoretic media tested, and other bistable electro-optic media are expected to exhibit similar decay times.

Accordingly, the DTD integration reduction method of the present invention provides a method of driving a bistable electro-optic display having at least one pixel comprising applying a waveform v (t) to the pixel such that:

(where T is the length of the waveform, integration is over the duration of the waveform, V (T) is the waveform voltage as a function of time T, and M (T) is a memory function that characterizes a reduction in remnant voltage efficacy that causes a dwell time dependence at time zero from a short pulse) is less than about 1 volt-second. Desirably, J is less than about 0.5 volt-seconds, and most desirably less than about 0.1 volt-seconds. In practice, J should be set as small as possible, ideally zero.

The waveforms can be designed such that they provide a very low J value and thus a very small DTD by generating complex pulses. For example, a long negative voltage pulse (having voltage amplitudes of the same magnitude but opposite sign) before a shorter positive voltage pulse may result in a more reduced DTD. It is believed, but in no way limited by this view, that the two pulses provide a residual voltage having opposite signs. When the ratio of the lengths of the two pulses is set correctly, the residual voltages from the two pulses can be made to cancel each other out to a large extent. The appropriate ratio of the lengths of the two pulses can be determined by a memory function of the residual voltage.

In a currently preferred embodiment of the present invention, J is calculated from the following formula:

where τ is the decay (relaxation) time that is optimally determined empirically.

For certain encapsulated electrophoretic media, it has been empirically found that waveforms that produce small J values also produce exceptionally low DTDs, while waveforms with exceptionally large J values produce large DTDs. In practice, a good correlation can be found between the J values calculated by equation (2) above, where τ is set to one second, approximately equal to the measured decay time of the residual voltage after the applied voltage pulse.

Thus, it is advantageous to apply the methods described in the above patents and applications by waveforms in which each transition from one gray level to another (or at least most of the transitions in the look-up table) is accomplished with a waveform that provides a small J value. This J value is preferably zero, but it has been found empirically that, at least for the encapsulated electrophoretic media described in the above-mentioned patents and applications, the resulting residence time dependence is rather small as long as J has an amount of less than about 1 volt-second at ambient temperature.

Accordingly, the present invention provides a waveform for effecting transitions between a set of optical states, wherein for each transition the calculated value of J has a fractional value. J is calculated by evaluating a memory function that may be monotonically decreasing. This memory function is not arbitrary, but can be estimated by observing the dwell time dependence of the display film on simple voltage pulses or composite voltage pulses. For example, a voltage pulse may be applied to the display film to effect a transition from a first to a second optical state, waiting for a dwell time, and then a second voltage pulse applied to effect a transition from a second to a third voltage pulse. By monitoring the shift of the third optical state as a function of the dwell time, the approximate shape of the memory function can be determined. The memory function has a shape as a function of dwell time that is substantially similar to the difference between the third optical state and its value at long dwell times. The memory function is then given this shape and has a singular amplitude when its argument is zero. This method only yields an approximation of the memory function and the measured shape of the memory function is expected to change slightly for various final optical states. However, the characteristic time of the total characteristic, such as the decay of the memory function, should be similar for various optical states. However, if there is a significant difference in shape for the final optical state, the optimal memory function shape to be employed is obtained when the third optical state is in the middle third of the optical range of the display medium. The overall characteristics of the memory function should also be estimable by testing the decay of the residual voltage after the applied voltage pulse.

However, the method described herein for estimating the memory function is not accurate, and it has been found that even J values calculated from approximate memory are a good guide for waveforms with low DTD. A useful memory function represents the overall characteristics of the time dependence of the DTD as described above. For example, it has been found that an exponentially, memory function with a decay time of one second is well suited for predicting waveforms that provide low DTD. Changing the decay time to 0.7 or 1.3 seconds does not destroy the effectiveness of the resulting J value as a predictor for low DTD waveforms. However, memory functions that do not decay but remain indefinitely singular are clearly less useful as predictors, and memory functions with very short decay times, such as 0.05 seconds, are not good predictors of low DTD waveforms.

One example of a waveform that provides a small J value is the waveforms shown in fig. 19 and 20 above, where the x, y, and z pulses all have durations much less than the characteristic decay time of the memory function. This waveform works well when this condition is met because it is composed of pulse elements that are opposite in order whose residual voltages tend to cancel out approximately. For values of x and y that are not much smaller than the characteristic decay time of the memory function, but not larger than this decay time, it has been found that waveforms where x and y have opposite signs tend to provide lower values of J, and it has been found that the x and y pulse durations actually allow for extremely small values of J, because the various pulse elements provide residual voltages that cancel each other out, or at least largely cancel each other out, after the waveforms are applied.

It will be appreciated that the J value of a given waveform may be controlled by inserting periods of zero voltage into the waveform, or by adjusting the length of any period of zero voltage already present in the waveform. In this way, a wide variety of waveforms can be used while still keeping the J value close to zero.

The DTD integral reduction method of the present invention has general applicability. The waveform structure may be designed to be described by the parameters, its J values calculated for various values of these parameters, and appropriate parameter values selected to minimize the J value, thereby reducing the DTD of the waveform.

Claims (15)

1. A method of driving a bistable electro-optic display having a plurality of pixels each capable of displaying at least three grey levels, the method comprising applying to each pixel of the display an output signal effective to change the pixel from an initial state to a final state, wherein, for at least one transition in the pixel where the initial and final states are different, the output signal consists of a pulse having a voltage level of 0 and at least two pulses, before and after the pulse, having the same one of + V and-V.

2. A method of driving a bistable electro-optic display having a plurality of pixels, wherein each pixel is capable of displaying at least three gray levels, the method comprising:

storing a look-up table containing data representing an impulse required to convert the initial gray level to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

generating an output signal representing the impulse required to convert the initial state of said one pixel to its intended final state, as determined from said look-up table;

wherein for at least one transition from an initial state to a final state, the output signal comprises a DC balance trim sequence that:

(a) has a substantially zero net impulse; and

(b) at any point in the trimming sequence, the grey level of the pixel is not made to differ from its grey level at the beginning of the trimming sequence by more than about one third of the difference in grey level between the two extreme optical states of the pixel.

3. The method of claim 2, wherein for the at least one transition, the output signal contains at least one unipolar drive pulse in addition to the fine tuning sequence.

4. A method of driving a bistable electro-optic display having a plurality of pixels each capable of displaying at least three grey levels, the method comprising applying to each pixel of the display an output signal effective to change the pixel from an initial state to a final state, wherein, for at least one transition, the output signal is non-zero but DC balanced.

5. The method of claim 4, wherein for the at least one transition, the output signal comprises a first pair of pulses comprising a voltage pulse and a pulse of equal length but opposite sign preceding it.

6. The method of claim 5, wherein the output signal further comprises a zero voltage period between the two pulses.

7. The method of claim 5, wherein at least one of the pulses is interrupted by a period of zero voltage.

8. The method of claim 5, wherein for the at least one transition, the output signal further comprises a second pair of pulses of equal length but opposite sign.

9. The method of claim 8, wherein the second pair of pulses has a different length than the first pair of pulses.

10. The method of claim 8, wherein a first one of the second pair of pulses has an opposite polarity to a first one of the first pair of pulses.

11. The method of claim 8, wherein the first pair of pulses occurs between a first and a second of the second pair of pulses.

12. The method of claim 4, wherein for the at least one transition, the output signal comprises at least one pulse element effective to drive the pixel substantially to one optical rail.

13. A method as claimed in claim 4, wherein the output signal is non-zero but DC balanced for each transition where the initial and final states of the pixel are the same, and is not DC balanced for each transition where the initial and final states of the pixel are not the same.

14. A method according to claim 13, wherein for each transition where the initial and final states of the pixel are not the same, the output signal has the form-x/Δ IP/x, where Δ IP is the difference in impulse potential between the initial and final states of the pixel, and-x and x are a pair of pulses of equal length but opposite sign.

15. The method of claim 4, further comprising:

storing a lookup table containing data representing an impulse required to convert an initial gray level of a pixel to a final gray level;

storing data representing at least an initial state of each pixel of the display;

receiving an input signal representing an expected final state of at least one pixel of the display; and

an output signal is generated representing the impulse required to convert the initial state of the one pixel to its intended final state, determined from the look-up table.