WO2005048237A1

WO2005048237A1 - Sequential display technique that displays the color green second

Info

Publication number: WO2005048237A1
Application number: PCT/US2003/035084
Authority: WO
Inventors: Donald Henry Willis
Original assignee: Thomson Licensing SAS
Current assignee: Thomson Licensing SAS
Priority date: 2003-11-04
Filing date: 2003-11-04
Publication date: 2005-05-26
Anticipated expiration: 2006-05-04
Also published as: AU2003291723A1; AU2003291723A8

Abstract

The sequential display of each of three primary colors by a sequential display system (100) occurs by first establishing a color display order in which the brightest primary color appears second. Thus, in a system using red, green and blue as the primary colors, the color green (13', 16') having the highest brightness, will appear second, as between red (12' and 15') and blue (14', 17'). The primary colors then appear in sequence in the established color display order at least once during a frame interval. Establishing a color display order in which green appears second will cause a moving white object to appear with leading and trailing edges of red and blue, or blue and red, respectively, which is visually less objectionable than if either of the leading or trailing edges was green.

Description

SEQUENTIAL DISPLAY TECHNIQUE THAT DISPLAYS THE COLOR GREEN SECOND

TECHNICAL FIELD This invention relates to a sequential color display system and a method of operating the same to reduce the incidence of motion artifacts.

BACKGROUND ART Presently, there exist color television projection systems that provide a color display by projecting red, green, and blue pictures either simultaneously or in sequence during each frame interval. The frame interval constitutes the time interval between the display of successive images. The duration of the frame interval depends on the selected television standard. The NTSC standard currently in use in the North America specifies a frame interval of 1/60 second whereas certain European television standards employ a frame interval of 1/50 second. Sequential color television projection systems often employ a motor-driven color wheel interposed in the light path between a light source and a light controller to sequentially project different primary color pictures. The color wheel has a plurality of separate primary color windows, typically red, green and blue, so that during successive intervals, red, green, and blue light, respectively, falls on the light controller. The light controller can take the form of a Digital Micromirror Device (DMD) comprised a plurality of individually movable micromirrors, each pivoting about a limited arc for selectively reflecting light onto a display screen to create a color picture. Alternatively, the light controller could take the form of a Liquid Crystal display that acts as an electronic shutter to selectively control the transmission of light onto a mirror for reflection onto the display screen. Sequential display systems of the type described above typically make an individual picture of one primary color followed by another picture of another primary color. Usually, after three primary colors are shown, the colors start over again, repeating the previous sequence. Often each color appears more than once per frame interval. For example, each color of each picture can appear as many as four times or more during each frame interval. Thus, a total of 12 color pictures can appear during each frame interval. In such a sequential display in which red, green, and blue pictures appear at least once for each frame interval, a moving white object will have a leading edge of the first color and a trailing edge of a last color, as seen by an observer visually tracking the moving object. In the past, sequential display systems reduced the visibility of the edge by increasing the color display frequency. However, present day sequential display systems typically make no effort to control the color display order. When each color picture has a moving object in the same position on the screen, a viewer's eye will have imprinted upon the retina each color picture in a different position on the retina as the viewer's eye moves to follow the object. As a result, a moving white object will appear with a leading edge of the first primary color, and a trailing edge of the last primary color of the three-color sequence. In a sequential display system that employs red, green and blue as its primary colors, displaying the color green, as either the leading or trailing color will make the leading or trailing edges, respectively, more objectionable since green has the highest brightness among red, green, and blue. Thus, there is need for a technique for further reducing motion artifacts in sequential display system.

BRIEF SUMMARY OF THE INVENTION Briefly, in accordance with a preferred embodiment of the present principles, there is provided a method for reducing motion artifacts in a sequential display system that displays each of three primary colors in sequence. The method commences by establishing a color display order for the sequential display of the primary colors such that the primary color having greatest brightness among the three primary colors appears second in the display order. Thereafter, the primary colors are displayed in sequence in the display order at least once. By establishing a color display order such that the brightest primary color appears second (i.e., in between the other two primary colors of lesser brightness), then a moving white object will appear with its leading and trailing edges of the first and last primary colors, respectively which are of lower brightness, and thus appear less objectionable. Thus for example, in system that utilizes red, green and blue as the three primary colors, establishing a display order so green appears second will cause a moving white object to appear with leading and trailing edges of red and blue, or blue and red, respectively, which is visually less objectionable than if either of the leading or trailing edges was green. BRTEF DESCRIPTION OF THE FIGURES FIGURE 1 illustrates the diagonal movement of a white square whose motion will provide an insight in understanding the principles of the present invention; FIGURE 2 illustrates a display of color pictures that depict the moving white square of FIG. 1 during a given frame wherein the color pictures have a color display order in accordance with the prior art; FIGURE 3 illustrates a display of color pictures that depict the moving white square of FIG. 1 during a given frame wherein the color pictures have a color display order in accordance with the present principles; and FIGURE 4 depicts an exemplary sequential display system for practicing the sequential display method of the present principles.

DETAILED DESCRIPTION FIGURE 1 depicts the sequential movement of a white square 10 that travels diagonally in the direction illustrated by arrow 11 , so as to move from lower right position to an upper left position. Thus, the square 10 in the upper left of FIG. 1 represents its position subsequent to the square 10 in the lower right of the Figure. In a television display, the square 10 will appear in each frame the duration of which depends on a selected television standard. In an NTSC television system, the duration of each frame (hereinafter referred to as a frame interval) is l/60th of a second, whereas the PAL television standard practiced in some European Countries prescribes a frame interval of 1/50 of a second. Within a given frame, the square 10 is displayed in a fixed position. However, the square 10 will be displayed in a different fixed position in each subsequent frame, reflecting the motion of the square. When a viewer watches successive television images of the moving square 10, the viewer's eyes will move in the same direction as the square. Thus, the motion of the viewer's eye will coincide with the motion of the square 10 as indicated by arrow 11 in FIG. 1. In a sequential color television system, the image of the square 10 in a given frame is displayed as a sequence of primary color pictures. Thus, in a sequential color television system that employs red, green and blue as its primary colors, the image of the square 10 will appear as a sequence of red green and blue pictures, as exemplified by the sequence of color pictures 12, 13, 14, 15, 16, and 17 in FIG. 2 which has a color display order of green, red, and blue. In other words, pictures 12 and 15 are green, pictures 13 and 16 are red, and pictures 14 and 17 are blue. This is but one example and there could be other multiples of primary color pictures besides the two sets of green, blue, and red pictures shown in FIG. 2. The six pictures 12-17 occur in sequence during each frame so that in a NTSC sequential color television system, the interval between each color picture is 1/360th of a second for the example of FIG. 2. As the viewer's eye follows the movement of the white square 10 in FIG. 1, each of the color pictures 12-17 of FIG. 2 will imprint on the retina of the viewer's eye. From the reference perspective of the retina, the picture 12 will imprint first, followed in rapid succession by pictures 13-17. In other words, the pictures 12-17 will imprint in a direction opposite the direction of motion of the eye as represented by arrow 11 in FIG. 2. As a result, each picture imprints on the viewer's retina in a different position than the succeeding pictures. This is because the viewer's eye moves in the same direction as the motion of the square 10 of FIG. 1 and at the same speed. For each succeeding frame of the television image, the pictures 12-17 will imprint in the same sequence and at the same positions on the retina as shown in FIG. 2. Heretofore, no attempt was made to prescribe a specific color display order for the color pictures 12-17 within each frame. With a color display order of green, red and blue as depicted in FIG. 2, the leading edge of the moving white square 10 of FIG. 1 will appear green, since picture 12 of FIG. 2, which imprints first on the viewer's retina, is green. As among the primary colors, red green and blue, green has the highest brightness. Thus a leading (or trailing edge) that is green will have a more objectionable appearance as compared to a leading or trailing edge of blue or red, especially in light of the relatively short duration of each color picture within each frame interval. In accordance with the present principles, a less objectionable artifact results from a color display order established such that the primary color having the highest brightness appears second in the color display order with the other two primary colors of lower brightness appearing first and last, respectively. FIGURE 3 depicts a sequential display of color pictures 12'-17' that collectively represent the image of the square 10 of FIG. 1 in a given frame. The picture sequence of FIG. 3 differs from FIG. 2 because the established color display order of the pictures 12'-17' in FIG. 3 has the color green appearing second. As seen in FIG. 3 the pictures 12' and 15' are both red, whereas pictures 13' and 16' are both green, and pictures 14' and 17' are both blue. With a established color display order of red, green and blue as depicted in FIG. 3, the picture 12' which is red will imprint first on the viewer's retina, followed by pictures 13'-17' . Since the first picture 12' imprinting of the viewer's retina is red, the square 10 of FIG. 1 will appear a leading edge that appears red, followed yellow and white. The trailing edge will appear blue, led by a cyan portion. Since the color red has a lower brightness than green, the red leading edge will have a less noticeable appearance, as compared to green leading edge. While the color pictures 12'-17' in FIG. 3 have an established a color display order in which red appears first, followed by green and blue, a similar efficacious result occurs with an established color display order in which blue appears first followed by green followed by red (not shown). As long as the highest brightness primary color (e.g., green in a primary color set of red, green and blue) appears second in the color display order, rather than first or last, the leading and trailing edges of the square 10 of FIG. 1 will appear less noticeable than if the highest brightness color appeared first or last in the color display order. FIGURE 4 depicts a present-day pulse width modulated sequential display system 100 having the capability of providing a sequential color display in accordance with the present principles wherein the highest brightness primary color (e.g., green) appears second in the color display order. The system 100 takes the form of the display system disclosed in the Application Report "Single Panel DLP™ Projection System Optics" published by Texas Instruments, June 2001. As part of the display system 100, a lamp 120 is situated at the focus of an elliptical reflector 130 that reflects light from the lamp through a color wheel 140 and into an integrator rod 150. A motor 160 rotates the color wheel 140 to place a separate one of red, green and blue primary color windows between the lamp 120 and the integrator rod 15. In an exemplary embodiment, the color wheel 14 has diametrically opposed red, green and blue color windows. Thus, as the motor 160 rotates the color wheel 140, red, green and blue light will strike the integrator rod 150 of FIG. 3. In practice, the motor 160 rotates the color wheel 140 at a sufficiently high speed so that during a frame interval of a 1/60 second, red, green and blue light each strikes the integrator rod four times, yielding twelve color pictures within the frame interval, four red, four green and four blue that are interleaved. Referring to FIG. 4, the integrator rod 150 collects the light from the lamp 120, as it passes through a successive one of the red, green and blue color windows of the color wheel 140, and directs the collected light onto a set of relay optics 180. The relay optics 180 spread the light into a plurality of parallel beams that strike a fold mirror 200, which reflects the beams through a set of lenses 220 and onto a Total Internal Reflectance (TIR) prism 230. The TIR prism 230 reflects the parallel light beams onto a Digital Micromirror Device (DMD) 24, such as the DMD device manufactured by Texas Instruments, for selective reflection into a projection lens 260 and onto a screen 280. The DMD 240 takes the form of a semiconductor device having a plurality of individual micromirrors (not shown) arranged in an array. By way of example, the DMD manufactured and sold by Texas Instruments has a micromirror array of 1280 columns by 720 rows, yielding 921,600 pixels in the resultant picture projected onto the screen 280. Other DMDs can have a different arrangement of micromirrors. Each micromirror in the DMD 24 pivots about a limited arc under the control of a corresponding driver cell (not shown) in response to the state of a binary bit previously latched in the driver cell. Each micromirror rotates to one of a first and a second position depending on whether the latched bit applied to the driver cell, is a "1" or a "0", respectively. When pivoted to its first position, each micromirror reflects light into the lens 260 and onto the screen 280 to illuminate a corresponding pixel. While each micromirror remains pivoted to its second position, the corresponding pixel appears dark. The total duration in which each micromirror reflects light through the projection lens 260 and onto the screen 280 (the micromirror duty cycle) determines the pixel brightness. The individual driver cells in the DMD 24 receive drive signals from a driver circuit 300 of a type well known in the art and exemplified by the circuitry described in the paper " High Definition Display System Based on Micromirror Device", R.J. Grove et al. International Workshop on HDTV (October 1994). The driver circuit 300 generates the drive signals for the driver cells in the DMD 240 in accordance with sequences of pulse width segments applied to the driver circuit by a processor 310. Each pulse width segment comprises a string of pulses of different time duration, the state of each pulse determining whether the micromirror remains on or off for the duration of that pulse. The shortest possible pulse (i.e., a 1-pulse) that can occur within a pulse width segment (some times referred to as a Least Significant Bit or LSB) typically has a 15-microsecond duration, whereas the larger pulses in the segment each have a duration that is larger than onelSB. In practice, each pulse within a pulse width segment corresponds to a bit (hereinafter described as a "pixel control" bit) within a digital bit stream whose state determines whether the corresponding pulse is turned on or off. A "1" bit represents a pulse that is turned on, whereas a "0" bit represents a pulse that is turned off. The total sum (duration) of the actuated pulses in all the pulse width segments for a given color controls the brightness of a corresponding pixel for that color. The driver circuit 310 generates each of four separate pulse width segments per color for every pixel per frame. Thus, during each frame interval, the driver circuit 310 generates pixel control bits for the pulses of twelve segments, four red, four blue and four green. The transmission of the pixel control bits to the DMD 24 is synchronized with the rotation of the color wheel so that each segment for a given color corresponds to the appearance of that color for illumination on the DMD 240. To increase pixel brightness, the driver circuit 301 causes one or more pulses previously de-actuated at lower brightness levels to become actuated. In accordance with present principles, the driver circuit 310 achieves close coordination between the pulses becoming actuated (getting turned on) and de-actuated (getting turned off) with the appearance of each of the primary colors on the DMD 240. In particular, the driver circuit 31 coordinates the, pulses that become actuated and de-actuated so each green segment appears second in the color display order. As described, having each green appear second in the color display order causes the leading edge of a moving white object to appear as either blue or .red (depending on the display order), with the trailing edge appearing either red or blue, respectively, thereby making the edges less noticeable. The foregoing describes a technique for reducing motion artifacts in a pulse width modulated display system by establishing a color display order in which the primary color having the highest brightness appears second.

Claims

CLAIMS 1. A method for reducing motion artifacts in a sequential display system that sequentially displays each of three primary colors, comprising the steps of: establishing a color display order for sequential display of the primary colors such that the primary color having greatest brightness appears second in the display order among the three primary colors; and sequentially displaying each of the primary colors in the established display order at least once during a frame interval. 2. The method according to claim 1 wherein the step of establishing the color display order further comprises the step of establishing a color display order wherein red, green and blue are the primary colors, and wherein the color green appears second in the display order. 3. The method according to claim 2 further comprising the step of establishing the color display order wherein red appears first and blue appears last in the color display order. 4. The method according to claim 2 further comprising the step of establishing the color display order wherein blue appears first and red appears last in the display order. 5. The method according to claim 1 further comprising the step of repeating the sequential display of each of the primary colors in the established display order so each color appears at least twice. 6. A sequential pulse width modulated display system comprising: a light source; a projection lens for focusing incident light onto a screen; a Digital Micromirror Device having a plurality of individual micromirrors arranged in an array, each micromirror pivotal about an arc in response to receipt of a drive signal applied to a driver cell associated with the micromirror to reflect light from the light source into the projection lens and onto the screen to illuminate a picture element (pixel) therein; a rotating color wheel interposed between the light source and the digital micromirror to successively impart each of three primary colors to the light striking the digital micromirror device and reflected thereby into the projection lens; a processor for forming sequences of pulse width segments in synchronism the rotation of the color wheel to establish a color display order such that a highest brightness primary color appears second in a color display order; a driver circuit responsive to the sequences of pulse width segments formed by the processor for driving the digital micromirror device to illuminate the corresponding pixel sequentially with each of the three primary colors in the established color display order. 7. The system according to claim 6 wherein the color wheel has diametrically opposed red, green and blue color windows and wherein the processor establishes the color display order wherein the color green appears second in the display order. 8. The system according to claim 7 wherein the processor establishes the color display order such that the color red appears first and the color blue appears last in the display order. 9. The system according to claim 7 wherein the processor establishes the color display order such that the color blue appears first and the color red appears last in the display order. 10. The system according to claim 6 further comprising the step of repeating the he sequential display of each of the primary colors in the established display order so each color appears at least twice.