HK1118371A1 - Methods for driving electro-optic displays - Google Patents

Abstract

An electro-optic display is driven using a plurality of different drive schemes. The waveforms of the drive schemes are chosen such that the absolute value of the net impulse applied to a pixel for all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 20 per cent of the characteristic impulse (i.e., the average of the absolute values of the impulses required to drive a pixel between its two extreme optical states).

Description

Method for driving electro-optic display

This application relates to U.S. patent application publication No.2006/0280626, U.S. patent No.7,012,600, U.S. patent No.6,531,997, U.S. patent No.6,504,524, U.S. patent application publication No.2005/0001812, U.S. patent application publication No.2005/0024353, U.S. patent application publication No.2005/0270261, U.S. patent application publication No.2005/0179642, and U.S. patent application publication No. 2002/0180687.

Technical Field

The present invention relates to a method for driving an electro-optic display, in particular a bistable electro-optic display, and to a device for use in such a method. More particularly, the present invention relates to driving methods in which it is desirable to update an electro-optic display with multiple driving schemes simultaneously. The invention has particular, but not exclusive, application to particle-based electrophoretic display devices in which one or more charged particles are suspended in a liquid and pass through the liquid under the influence of an electric field, thereby affecting the appearance of the display.

Background

The term "electro-optic" as used herein in the context of materials or displays is its conventional meaning in the imaging arts, and refers to a material having first and second display states differing in at least one optical property, the material being transitioned from the first display state to the second display state by application of an electric field to the material. Although this optical property is typically a color that is perceptible to the human eye, other optical properties are possible, such as optical transmission, reflectance, luminescence, or in the case of a display for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

The term "gray state" as used herein is its conventional meaning in the imaging arts, and refers to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-and-white transition between the two extreme states. For example, electrophoretic displays in which the extreme states are white and deep blue, and thus the intermediate "gray state" will actually be pale blue, are described in the various patents and published applications referred to below. In fact, as mentioned before, the transition between the two extreme states may also not be a change in color at all.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art 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 to assume either its first or second display state by an addressing pulse of finite duration, that state will last for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. Shown in the aforementioned application No. 2002/0180687: some particle-based electrophoretic displays capable of displaying gray levels are stable not only in their extreme black and white states, but also in their intermediate gray states, as are some other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bi-stable, but the term "bi-stable" as used herein covers both bi-and multi-stable displays for convenience.

The term "pulse" as used herein is taken in its conventional sense: integration of voltage with respect to time. However, some bistable electro-optic media act as charge sensors, with which another definition of a pulse, the integral of current with respect to time (equal to the total charge applied) can be used. Appropriate definitions for pulses should be used depending on whether the medium acts as a voltage-time pulse sensor or a charge pulse sensor.

Much of the discussion below will focus on methods of driving one or more pixels of an electro-optic display by transitioning from an initial gray level to a final gray level. The term "waveform" is used herein to denote the complete voltage versus time curve used to effect a transition from a particular initial gray level to a particular final gray level. Typically, such waveforms comprise a plurality of waveform elements, wherein the elements are substantially rectangular (i.e., a given element comprises a constant voltage applied over a period of time); these cells may be referred to as "pulses" or "drive pulses". The term "drive scheme" herein denotes a set of waveforms sufficient to effect all possible transitions between gray levels for a particular display.

Various types of electro-optic displays are known. One type of electro-optic display is the rotating bichromal member type (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is more accurate since the rotating member is not spherical in some of the above patents), such as disclosed in U.S. patent nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467 and 6,147,791. Such displays use a large number of small bodies (typically spherical or cylindrical) having two or more portions with different optical properties, and internal doublers. These bodies are suspended in liquid-filled vacuoles in a matrix, which vacuoles are filled with liquid so that the bodies can rotate freely. An electric field is applied to the display, changing the appearance of the display, thus rotating the bodies to various positions and changing the location of those bodies as seen through the viewing surface. This type of electro-optic medium is typically bistable.

Another type of electro-optic display uses an electrochromic medium, such as in the form of a nanochromic film (nanochromic film), which includes an electrode formed at least in part of a semiconducting metal oxide and a plurality of reversibly color-changeable dye molecules attached to the electrode. See, e.g., O' Regan, B. et al, Nature, 1991, 353, 737; wood, d., Information Display, 18(3), 24 (3.2002), and see Bach, u.et al, a dv. mater.2002, 14(11), 845. Color shifting films of this type are also described, for example, in U.S. patent nos. 6,301,038 and 6,870,657 and U.S. patent application publication No. 2003/0214695. This type of media is also typically bistable.

Another type of electro-optic display is the electro-wetting display, developed by philips, titled "performing pixels: the article "Video-Speed Electronic Paper Based on electrowetting" ("Video-Speed Electronic Paper Based on electrowetting"), by Hayes, R.A., et al, Nature, 425, 383-. It is shown in us patent application publication No.2005/0151709 that such electrowetting displays can be made bistable.

Another type of electro-optic display that has been extensively studied and developed over the years is a particle-based electrophoretic display, in which a plurality of charged particles are passed through a fluid under the influence of an electric field. Electrophoretic displays contribute to good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, long-term image quality issues of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in inadequate service life for these displays.

As indicated above, the presence of a fluid in an electrophoretic medium is desirable. In most prior art electrophoretic media this fluid is referred to as a liquid, but the electrophoretic medium may be made of a gaseous fluid; see, for example, "movement of electronic toner in electronic Paper-like display" by Kitamura, t. et al, "toner display using electrostatically charged insulating particles" by IDW japan, 2001, Paper HCS 1-1 and Yamaguchi, y. et al, "toner display using electrostatically charged insulating particles", IDW japan, 2001, Paper AMD 4-4. See also U.S. patent application publication No. 2005/0001810; european patent applications 1,462,847, 1,482,354, 1,484,635, 1,500,971, 1,501,194, 1,536,271, 1,542,067, 1,577,702, 1,577,703, 1,598,694; and international applications WO 2004/090626, WO2004/079442, WO 2004/001498. Such gas-based electrophoretic media are susceptible to the same types of problems associated with particle settling as liquid-based electrophoretic media when the media is used in an orientation that allows such settling, for example for signage, in which the media lies in a vertical plane. In fact, the problem of particle settling is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of gaseous fluids compared to liquid fluids causes the electrophoretic particles to settle more quickly.

A number of patents and applications, assigned to the institute of technology and technology (MIT) and the eink corporation, or both, have recently been published which describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form an adhesive layer between two electrodes. For example, U.S. Pat. Nos. 5,930,026, 5,961,804, 6,017,584, 6,067,185, 6,118,426, 6,120,588, 6,120,839, 6,124,851, 6,130,773, 6,130,774, 6,172,798, 6,177,921, 6,232,950, 6,262,833, 6,232,950, 36, 2004/0075634, 2004/0094422, 2004/0105036, 2004/0112750, 2004/0119681, 2004/0136048, 2004/0155857, 2004/0180476, 2004/0190114, 2004/0196215, 2004/0226820, 2004/00239614, 2004/0252360, 2004/0257635, 2004/0263947, 2005/0000813, 2005/0001812, 2005/0007336, 2005/0012980, 2005/0017944, 2005/0018273, 2005/0024353, 2005/0062714, 2005/0067656, 2005/0078099, 2005/0099672, 2005/0105159, 2005/0122284, 2005/0122306, 2005/0122563, 2005/0122564, 2005/0122565, 2005/0134554, 2005/0146774, 2005/0151709, 2005/0152018, 2005/0152022, 2005/0156340, 2005/0168799, 2005/0179642, 2005/0190137, 2005/0212747, 2005/0213191, 2005/0219184, 2005/0253777, 2005/0270261, 2005/0280626, 2006/0007527, 2006/0023296, 2006/0024437 and 2006/0038772, and international application publications nos. WO 00/38000, WO 00/36560, WO00/67110 and WO 01/07961, and european patent nos. 1,099,207B1 and 1,145,072B1 all describe encapsulated media of this type.

Many of the above patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium can be replaced with a continuous phase, thus producing a so-called dispersed polymer (polymer-dispersed) electrophoretic display, wherein 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 the electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even if no discrete capsule membrane is associated with each individual droplet; see, for example, the aforementioned U.S. patent No.6,866,760. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

Encapsulated electrophoretic displays typically do not suffer from the aggregation and settling failure modes of conventional electrophoretic display devices and have additional advantages such as the ability to coat or print the display on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include, but is not limited to, various forms of printing and coating such as pre-measured coating such as patch die coating, slot or die 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, curved surface (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 manufactured display may be flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and the fluid are not encapsulated in microcapsules but are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, international application publication No. wo 02/01281 and U.S. patent application publication No.2002/0075556 (both assigned to Sipix Imaging, inc.).

Although electrophoretic media are typically 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, electrophoretic displays may operate in a so-called "shutter mode" having a substantially opaque display state and a light-transmissive display state. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on changes in electric field strength, can also operate in a similar mode; see U.S. patent No.4,418,346.

The bi-or multi-stable performance of particle-based electrophoretic displays, and the like of other electro-optic displays (for convenience, such displays may be referred to hereinafter as "impulse-driven displays"), is in sharp contrast to the performance of conventional Liquid Crystal (LC) displays. The properties of twisted nematic liquid crystals are not bistable or multistable but act as voltage sensors, so 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, the LC display is driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), and the reverse transition from the lighter state to the 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 indeed for technical reasons commercial LC displays typically reverse the polarity of the driving electric field at frequent intervals. In contrast, as a first approximation, a bistable electro-optic display acts as a pulse sensor, so that the final state of a pixel depends not only on the applied electric field and the time it takes to apply the field, but also on the state of the pixel before the field is applied.

In order to achieve a high resolution display, the electro-optic medium used, whether bistable or not, must be capable of being addressed to individual pixels of the display without interference from adjacent pixels. To achieve this, one approach is to provide an array of non-linear elements, such as transistors or diodes, in which at least one non-linear element is associated with each pixel, resulting in an "active matrix" display. The addressing or pixel electrode, which addresses a pixel, is connected to a suitable voltage source via an associated non-linear element. In general, when the nonlinear element is a transistor, a pixel electrode is connected to a drain of the transistor, and this arrangement is also assumed in the following description, although this arrangement is basically arbitrarily selected, and the pixel electrode may be connected to a source of the transistor. Conventionally, in high resolution arrays, pixels are arranged in a two-dimensional array of rows and columns such that for any particular pixel it is uniquely defined by the intersection of a particular row and a particular column. The sources of all transistors in each column are connected to a single column electrode and the gates of all transistors in each row are connected to a single row electrode; also this layout of connecting the sources to the rows and the gates to the columns is common, however this connection is basically arbitrary and can be reversed if desired. Connecting the row electrodes to the row driver substantially ensures that only one row is selected for any given time, i.e. applying a voltage to the selected row electrode ensures that all transistors on the selected row electrode are conductive, while applying a voltage to all other rows ensures that the transistors on these non-selected rows remain non-conductive. The column electrodes are connected to a column driver for applying selected voltages to the different column electrodes for driving the pixels in the selected row to the desired optical state (these voltages being associated with the common front electrode, which is usually located on the opposite side of the electro-optic medium from the non-linear array and extends across the display). After a preselected interval known as the "linear addressing time", the selected row is deselected, the next row is selected, and the voltage on the column driver is changed to write the next row of the display. This process is repeated to write the entire display in a row-by-row fashion.

Seemingly the ideal method for addressing such a pulse-driven electro-optic display is the so-called "general gray scale image flow", in which the controller arranges each write of the image so that each pixel goes directly from its initial gray scale to the final gray scale. However, errors inevitably occur when writing images on impulse-driven displays, which in practical applications are encountered include:

(a) previous state dependencies; for at least part of the electro-optic medium, the pulse required to transition the pixel to a new optical state depends not only on the current and the desired optical state, but also on the previous optical state of the pixel.

(b) Residence time dependency; for at least part of the electro-optic medium, the pulse required to transition a pixel to a new optical state depends on the time the pixel stays in the respective optical state. Although the exact nature of this dependence is not clear, in general the longer a pixel stays in its current optical state, the more pulses are required.

(c) Temperature dependence; the pulse required to transition the pixel to a new optical state is strongly temperature dependent.

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

(e) Mechanical homogeneity; the pulse required to transition the pixel to a new optical state can be affected by mechanical changes in the display, such as changes in the thickness of the electro-optic medium or the viscosity of the associated stack. Other types of mechanical non-uniformities can also result from unavoidable variations in different manufacturing batches, manufacturing tolerances and material variations of the media.

(f) A voltage error; due to the slight inevitable errors in the voltages delivered by the drivers, the voltages actually applied to the pixels will inevitably also differ slightly from the theoretically applied voltages.

Typically, grayscale image streams suffer from the phenomenon of "error accumulation". For example, suppose that during each transition, a temperature dependence in the positive direction results in 0.2L^*(wherein L^*With the general commission internationale for illumination (CIE) definition:

L^*＝116(R/R₀)^1/3-16，

wherein R is the reflection coefficient and R₀Is the standard reflectance value). After 50 transitions, the error will accumulate to 10L^*. Or more practically, assuming an average error of + -0.2L for each transition expressed in terms of the difference between the theoretical and actual reflectivity of the display^*. After 100 consecutive transitions, the pixel will show a deviation from the expected state of 2L^*Average deviation of (d); on certain types of images, these deviations are apparent to the average viewer.

The error accumulation phenomenon applies not only to errors caused by temperature, but also to all types of errors listed above. As described in the aforementioned U.S. patent No.7,012,600, compensation for the above-mentioned errors is possible, but only to a limited degree of accuracy. For example, temperature errors may be compensated by using a temperature sensor and a look-up table, but the temperature sensor has a limited resolution and a slight difference between the read temperature and the temperature of the electro-optical medium. Similarly, previous state dependencies can be compensated for by storing previous states and employing multi-dimensional transition matrices, but the controller memory limits the number of states that can be recorded and the size of transition matrices that can be stored, thus limiting the accuracy of this type of compensation.

Thus, the general grayscale image stream needs to be very precisely controlled for the applied pulses to give good results, and empirically it has been found that in the current state of electro-optic display technology, it is not feasible to use the general grayscale image stream in commercial displays.

In some cases, it is desirable to employ multiple drive schemes in a single display. For example, for displays having more than two gray levels, a Gray Scale Drive Scheme (GSDS) may be employed which enables transitions between all possible gray levels, and a Monochrome Drive Scheme (MDS) which enables transitions only between two gray levels, typically two extreme optical states of each pixel. The MDS provides faster rewriting of the display than the GSDS. The MDS is employed when all pixels to be changed transition between only two gray levels used by the MDS during a display rewrite. For example, the aforementioned 2005/0001812 describes a display in the form of an electronic book or similar device that is capable of both displaying grayscale images and displaying monochrome dialogs that allow a user to enter text related to the displayed images. When a user enters text, the rapid MDS is employed to cause the dialog box to be rapidly updated, thus providing the user with rapid confirmation of the entered text. On the other hand, when the entire gray scale image displayed on the display changes, a slower GSDS is employed.

The display may effectively utilize more than two drive schemes. For example, a display may have one GSDS for updating a small area of the display and a second GSDS used when the entire image on the display needs to be changed or refreshed. For example, a user editing a small portion of a drawing on a display may use a first GSDS (which does not require flashing of the display) to view the results of the editing, but may also use a second "clean" GSDS (which does include flashing of the display) for more accurately displaying the final edited drawing, or displaying a new drawing on the display. In this scheme, the second GSDS is referred to as the "gray scale clear" drive scheme or "GSCDS".

As described in detail in 2005/0001812 above, for at least some types of electro-optic displays, it is desirable that the drive scheme employed be DC balanced in the sense that, for any series of transitions beginning and ending at the same gray level, the algebraic sum of the pulses applied during the series of transitions is bounded. It was found that a DC-balanced driving scheme may provide more stable display performance and reduced image artifacts (artifacts). Ideally all of the individual waveforms in the drive scheme are DC balanced, but in practice it is difficult to make all waveforms DC balanced, and therefore even if the drive scheme as a whole is DC balanced, the drive scheme is typically a mixture of DC balanced and DC unbalanced waveforms.

Using two such mixed DC balanced drive schemes in the same display can result in DC imbalance of the drive scheme as a whole due to the transition cycles using transitions from both drive schemes. For example, consider a display employing MDS and GSDS and a simple transition cycle, white-black-white. GSDS has one net impulse (I) for white-to-black (W → B) transition₁And (because it is DC balanced) has a net pulse-I for the B → W (black → white) transition₁. Similarly, MDS has a net pulse I for a white-to-black (W → B) transition₂(not equal to I)₁) And (because it is DC balanced) has a net pulse-I for the B → W (black → white) transition₂. If a pixel is driven from white to black with GSDS and then from black to white with MDS, the net pulse for a cycle is I₁-I₂Which is not equal to 0. Moreover, by repeating this same cycle indefinitely, the net impulse of the cycle accumulates, so the net impulse is unlimited and the overall drive scheme is no longer DC balanced.

Disclosure of Invention

The present invention provides an electro-optic display and a method for operating the same that allows two different drive schemes to be employed simultaneously in a manner that ensures that the overall drive scheme is DC balanced or very close to DC balanced.

The invention provides an electro-optic display employing a plurality of different drive schemes having waveforms selected so that the absolute value of the net impulse (net impulse) applied to all pixels of the same and different classes of irreducible loops divided by the number of transitions in the loop is less than 20% of the characteristic impulse.

Wherein:

homogeneous irreducible loops are sequences of gray levels starting at the initial gray level, passing through zero or more gray levels, and ending at the initial gray level, wherein all transitions are effected using the same drive scheme, and wherein the loop visits no more than one gray level other than the initial gray level;

a non-uniform irreducible loop is a sequence of gray levels beginning at an initial gray level, passing through one or more gray levels, and ending at the initial gray level, wherein the loop includes transitions employing at least two different drive schemes, the drive scheme for effecting a last transition in the loop is the same as the drive scheme for effecting a transition to the initial gray level immediately prior to the beginning of the loop, and the loop does not include a shorter irreducible loop; and

the characteristic pulse is the average of the absolute values of the pulses required to drive the pixel between its two extreme optical states.

Ideally, the net impulse applied to a pixel across all homogeneous and heterogeneous irreducible loops (as defined below) divided by the number of transitions in the loop is less than 10% of the characteristic impulse, and preferably less than 5%. More desirably, the net impulse for all irreducible loops, homogeneous and heterogeneous, is substantially zero, i.e., all of the loops are DC balanced.

In the method, the plurality of driving schemes may include a gray scale driving scheme and a monochrome driving scheme, or two gray scale driving schemes and a monochrome driving scheme. In the latter example, one of the two grayscale driving schemes may employ local updating of the image while the other employs global updating. Alternatively, one of the two gray scale drive schemes may provide more accurate gray scale than the other, but this may result in a more flickering display.

The present method may be used with any of the types of electro-optic media discussed above. Thus, for example, an electro-optic display may comprise a rotating bichromal element, an electrochromic or an electrowetting display medium. Alternatively, electro-optic displays may comprise a particle-based electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field. The charged particles and the fluid may be encapsulated in a plurality of capsules or microcells, or may be present as a plurality of separate droplets in a continuous phase comprising a polymeric binder. The fluid may be gaseous.

The invention comprises an electro-optic display comprising a layer of an electro-optic medium, at least one electrode arranged to apply an electric field to the layer of electro-optic medium, and a controller arranged to control the electric field applied to the electro-optic medium via the at least one electrode, the controller being arranged to implement the method of the invention.

The displays of the present invention can be used in essentially any application previously used for electro-optic displays, such as electronic book readers, portable computers, tablets, mobile phones, smart cards, signs, watches, shelf labels, and flash drives.

As mentioned above, the present invention provides a method of driving an electro-optic display using a plurality of different drive schemes having waveforms selected so that the absolute value of the net impulse applied to all pixels of the same and different classes of irreducible loops divided by the number of transitions in the loop is less than 20% of the characteristic impulse.

Detailed Description

The invention is based on the concept of homogeneous and heterogeneous irreducible loops. For these purposes, a gray level cycle is a sequence of gray levels having the same initial and final gray levels. For example, assuming a gray level of four gray levels (2-bits), the gray levels are represented as 1, 2,3 and 4 from darkest to brightest, examples of such gray level cycles are:

1→1

2→3→2

1→4→3→2→1。

homogeneous irreducible loops are a sequence of gray levels starting at the initial gray level, passing through zero or more gray levels and ending at the initial gray level, wherein all transitions are effected by the same drive scheme (typically a gray level drive scheme or "GSDS"). While a typical gray level loop can access any gray level multiple times, the same kind of irreducible loop accesses any gray level no more than once, except for accessing the final gray level, which must be the same as the initial gray level, as already described. For example, assuming the same gray scale of four gray levels (2-bit), the homogeneous irreducible loops have:

1→1

2→2

1→2→1

3→2→1→3

1→2→3→4→1。

the first cycle simply transitions from gray level 1 to gray level 1 and the second from gray level 2 to gray level 2. The third example starts at gray level 1, transitions to gray level 2, and then transitions back to gray level 1.

The gray scale loop may be homogeneous (i.e., all transitions are implemented using the same drive scheme), but not irreducible. Examples of non-irreducible homogeneous loops are:

1→2→3→2→1。

1→2→2→1

3→2→3→2→3

because these loops all contain repeated accesses to the same gray level that is not the initial and final gray levels, all of these loops are non-irreducible and can be reduced to multiple irreducible loops.

It is apparent that for any number of gray levels within a gray scale there is a limited number of homogeneous irreducible loops.

The different classes of cycles are similar to the same class of cycles except for the inclusion of transitions employing at least two different drive schemes. As with the same type of cycle, in different types of cycles, the initial and final gray levels must be the same; also, in different classes of loops, the driving scheme used to effect the last transition of a loop must be the same as the driving scheme used previously to effect the transition to the original gray level. By way of example, assuming that in the aforementioned four gray levels, the transition from gray level 1 to gray level 4 using drive scheme a can be symbolized as:

1→(a)→4

the reverse transition from gray level 4 to gray level 1 using drive scheme B is symbolized as:

4→(b)→1

heterogeneous loops can be constructed by two transformations, namely:

1→(a)→4→(b)→1

as shown at the end of the loop, where the initial gray scale 1 state is implemented using drive scheme B.

It will be apparent that various other heterogeneous cycles can each be constructed using multiple drive schemes. Irreducible heterogeneous loops can be defined as having the following two properties:

(a) the initial gray scale and the final gray scale are the same, and the driving scheme for realizing the final gray scale is the same as the driving scheme for realizing the initial gray scale; and

(b) heterogeneous loops do not contain irreducible loops themselves.

Examples of irreducible heterogeneous loops are as follows:

1→(a)→4→(b)→1→(b)→2→(a)→1

1→(a)→4→(b)→1→(c)→4→(d)→1

examples of non-irreducible heterogeneous loops are as follows:

1→(a)→4→(a)→1→(b)→4→(a)→1

1→(a)→2→(b)→3→(b)→2→(a)→1

because they include irreducible loops: the first loop includes two consecutive irreducible loops 1 → (a) → 4 → (a) → 1, and the second loop includes two nested irreducible loops.

It will be appreciated that complex homogeneous loops can be deconstructed in a similar manner into a finite set of irreducible loops and irreducible loops embedded within other irreducible loops. For example, such a homogeneous cycle:

1→4→3→2→3→2→3→2→1→2→1

can be deconstructed into two successive loops nested 2 → 3 → 2 in the loop 1 → 4 → 3 → 2 → 1, followed by the loop 1 → 2 → 1.

Since both homogeneous and heterogeneous loops can be deconstructed into combinations of irreducible loops in this way, it follows that if all irreducible loops are DC balanced, all possible loops (i.e. all possible sequences starting and ending with the same gray level) are DC balanced.

As already mentioned, in a single display employing multiple drive schemes, it is advantageous that the entire drive scheme, as well as the individual drive schemes, be DC balanced (or, less ideally, substantially DC balanced in the sense that the sum of pulse generations for any given cycle is small). In accordance with the present invention, the selection of the drive scheme enables all homogeneous and heterogeneous irreducible loops to be DC balanced, or in a less preferred form of the invention, all homogeneous and heterogeneous irreducible loops are substantially DC balanced. Substantial DC-balancing is to allow small DC imbalances in some or all of the homogeneous and heterogeneous cycles.

As already mentioned, a preferred form of the method is to use a monochrome drive scheme and at least one greyscale drive scheme as the plurality of drive schemes. As is well known to those skilled in the art of electro-optic displays, a Gray Scale Driving Scheme (GSDS) may be used to effect a transition from any gray level to any other gray level within a gray scale. An example of a gray level sequence obtained by the effect of GSDS gray level update is as follows:

2 → (G)3 → (G)1 → (G)4 → (G)3 → (G)1 → (G)3 → (G)2 wherein "→ (G)" means that the associated transition is effected by the GSDS. This example assumes that the aforementioned four gray levels (2-bit) have gray levels represented by 1, 2,3, and 4 from the darkest to the lightest.

Transitions between gray levels belonging to a monochrome subset of gray levels, including two gray levels within the aforementioned gray levels, may be implemented using a Monochrome Drive Scheme (MDS). In this example, the monochrome subset is {1, 4}, i.e., the darkest and brightest gray levels (typically black and white, respectively). In any given gray scale sequence, some transitions may be effected by MDS and others by GSDS. For example, the gray scale sequence may be:

2→(G)3→(G)1→(M)4→(M)1→(M)4→(G)3→(G)1→(M)4→(G)2

where "→ (M)" indicate the associated transition effected by the MDS. This sequence indicates different classes of updates, i.e. updates that use a combination of GSDS and MDS.

A particularly preferred embodiment of the present invention employs three different drive schemes, namely a grey scale drive scheme, a grey scale clear drive scheme and a monochrome drive scheme. The gray scale drive scheme and the gray scale clear drive scheme may differ in various respects, for example, the gray scale drive scheme may employ local updating (i.e., overwriting only pixels that need to be changed), while the gray scale clear drive scheme may employ global updating (i.e., overwriting all pixels regardless of whether the gray scale of the pixels is changed). Alternatively, a gray scale clean drive scheme may provide more accurate gray levels during transitions than a gray scale drive scheme, but at the cost of more flicker.

After adjusting the various waveforms of the drive scheme in the present invention to substantially or fully DC balance, any of the techniques described in the various patents and applications mentioned in the first paragraph of this application can be employed to achieve all homogeneous and heterogeneous irreducible loops. These techniques include changing the waveform according to various previous states of the display (so that, for example, the same type of cycle 1 → 2 → 1 and 1 → 3 → 2 → 1 both end at the 2 → 1 transition, while the waveform for the 2 → 1 transition may be different in both cases), insertion of balanced pulse pairs, and other waveform elements that can achieve some change in gray levels but with zero net pulse.

Claims (15)

1. A method of driving an electro-optic display using a plurality of different drive schemes, comprising: the waveforms of the drive scheme are selected such that the absolute value of the net impulse applied to pixels of all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 20% of the characteristic impulse;

wherein:

a homogeneous irreducible loop is a sequence of gray levels starting at an initial gray level, passing through zero or more gray levels, and ending at the initial gray level, wherein all transitions are effected using the same drive scheme, and wherein the loop accesses no more than one gray level other than the initial gray level;

a non-irreducible loop of a different class is a sequence of gray levels beginning at an initial gray level, passing through one or more gray levels, and ending at the initial gray level, wherein the loop includes transitions employing at least two different drive schemes, the drive scheme for effecting a last transition in the loop being the same as the drive scheme for effecting a transition to the initial gray level immediately prior to the beginning of the loop, and the loop does not include a shorter irreducible loop; and

the characteristic pulse is the average of the absolute values of the pulses used to drive the pixel between its two extreme optical states.

2. The method of claim 1, wherein the absolute value of the net impulse applied to pixels of all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 10% of the characteristic impulse.

3. The method of claim 2, wherein the absolute value of the net impulse applied to pixels of all homogeneous and heterogeneous irreducible loops divided by the number of transitions in the loop is less than 5% of the characteristic impulse.

4. A method according to claim 3, wherein the net impulse applied to pixels of all homogeneous and heterogeneous irreducible loops is substantially zero.

5. The method of claim 1, wherein the drive scheme comprises a grayscale drive scheme and a monochrome drive scheme.

6. The method of claim 1, wherein the drive scheme comprises a two gray scale drive scheme and a monochrome drive scheme.

7. A method as claimed in claim 6, wherein one of the two grey scale drive schemes employs a local update of the image and the other employs a global update.

8. A method as claimed in claim 6, wherein one of the two grey scale drive schemes is capable of providing more accurate grey scales than the other drive scheme but results in more flicker of the display.

9. The method of claim 1, wherein the electro-optic display comprises a rotating bichromal member, electrochromic or electrowetting display medium.

10. The method of claim 1, wherein the electro-optic display comprises a particle-based electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field.

11. The method of claim 10, wherein the charged particles and the fluid are encapsulated in a plurality of capsules or microcells.

12. The method of claim 10, wherein the charged particles and the fluid are present as a plurality of separate droplets in a continuous phase comprising a polymeric binder.

13. The method of claim 10, wherein the fluid is gaseous.

14. An electro-optic display comprising a layer of an electro-optic medium, at least one electrode arranged to apply an electric field to the layer of electro-optic medium, and a controller arranged to control the electric field applied to the electro-optic medium by at least one of the electrodes, the controller being arranged to implement a method according to any preceding claim.

15. An electronic book reader, portable computer, tablet computer, mobile phone, smart card, sign, watch, shelf label or flash drive comprising a display according to claim 14.