WO2007099466A1

WO2007099466A1 - Color-sequential operation of a multi-color display device

Info

Publication number: WO2007099466A1
Application number: PCT/IB2007/050447
Authority: WO
Inventors: Roel Van Woudenberg
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-02-28
Filing date: 2007-02-12
Publication date: 2007-09-07

Abstract

A method (10) of color-sequential operation of pixels of a color display device is provided. The pixels are capable of modulating light of JV different colors, (N≥2). The method comprises a step of addressing (11) a value for a first color to one of the pixels, a step of illuminating (12) the pixel with light of the first color, and a step of repeating the steps of addressing (11) and illuminating (12) for all further colors of the N colors. All steps of illuminating (12) the pixel with the light of the N colors are performed in less than T*(N-I)ZN, T being a predetermined time period between two consecutive addressings (11) of values for the first color to the pixel, and wherein the pixel is not illuminated for the remaining duration of the predetermined time period.

Description

Color-sequential operation of a multi-color display device

The invention relates to a method of color-sequential operation of pixels of a color display device, the pixels being capable of modulating light of JV different colors, N being an integer of at least 2, the method comprising the following sequence of steps: addressing a value for a first color to one of the pixels; illuminating the pixel with light of the first color; and repeating the steps of addressing and illuminating for all further colors of the N colors.

The invention also relates to a multi-color display device and to a computer program product.

Such a method is, for example, known from US 2005/0062708 Al. According to US 2005/0062708 Al, three colors, red, green and blue, are applied to a pixel consecutively. During a frame period of 1/60 second all three colors are applied to the pixel. The frame period is divided into three sub-frame periods of 1/180 second for addressing the color value and illuminating the pixel with light of a respective one of the three colors.

The method of US 2005/0062708 Al has two major disadvantages. When objects move, the time-sequentially displayed colored sub-frames are imaged on different positions on the retina, causing the colors of the moving object to break apart. This effect is called color breakup and also occurs for stationary objects with eye saccade (fast eye movements). As a result of the color breakup, a white moving object shows red and blue edges. The other problem with the method of US 2005/0062708 Al is called 'motion blur'. Motion blur occurs because an image held on the screen for the duration of a frame period blurs on the retina as the eye tracks the (average) motion from one frame to the next. Motion blur is a common problem of LCD displays and does not only occur with color sequential operation methods.

It is an object of the invention to provide a method of color-sequential operation of pixels of a multi-color display device, with reduced color break up. According to a first aspect of the invention, this object is achieved by performing all steps of illuminating the pixel with the light of the N colors in less than T * (N-I)ZN, T being a predetermined time period between two consecutive addressings of values for the first color to the pixel, whereas the pixel is not illuminated for a remaining duration of the predetermined time period.

By condensing the illumination with the light of all colors within a fraction of the predetermined time period, the time between illuminating the pixel with light of the first color and illuminating the pixel with light of the last (N*) color is reduced and consequently also color breakup is reduced. Advantageously, the reduction of the total time used for illumination of the pixel also results in reduced motion blur. For a display device using, for example, three colors RGB, the illumination is condensed within 2/3 of the frame period. In the prior art method of US 2005/0062708 Al, after 2/3 of the frame period the third color value is still to be addressed and the pixels is still to be illuminated with light of the third color. Due to this difference, the method according to the invention shows less color breakup and motion blur.

It is to be noted that it is known to prevent color breakup in RGB display devices by displaying the sub-frames at a 3 or more times higher frequency, e.g. at 540 Hz (RGB x 60 x 3). Such frequencies are, for example, used in the so-called LCoS system. Because the display of the three different color is less separated in time, color breakup is reduced. However, the LCoS system does have two major disadvantages. The pixels have to be addressed with a three times higher data rate and motion blur is not reduced.

In such 540 Hz systems the display is addressed with the same data three times. For each image, the data needs to be retrieved three times instead of once as in 180 Hz systems. This results in a significant cost penalty for the electronics system, especially when signals have to be distributed over long distances as from the Timing Controller to the column drivers in large-panel LCD-TV systems. In a 540 Hz system motion blur is not reduced, because the total duration of the light generation is not reduced with respect to regular 180 Hz systems. The method according to the invention does not need the high data rate of the 540 Hz system, because each color is only to be addressed to each pixel once per image. The method according to the invention does not show as much motion blur as the 540 Hz system, because the illumination is condensed within a fraction of the frame time. During the remaining duration of the predetermined period no color is allocated to the pixel and the pixel is not illuminated. A known method for reducing motion blur in one color displays or non color sequential color displays is to reduce the duty cycle (ON-time) of the light generation means. By reducing the ON-time of the light generation means, the sample and hold time of the image is reduced. The reduction of the sample and hold time results in less motion blur. Unfortunately, in color-sequential operating displays, motion blur is not (significantly) reduced by reduction of the duty cycle of the light generation means.

In the color-sequential method of US 2005/0062708 Al the duty cycle of the light generating means is also reduced. However, because the 1/60 second frame period is divided into three consecutive 1/180 second sub frame periods, the total time between the start of the illumination with the first color and the end of the illumination with the third color is still more than two third of the total frame period. Thus, with the method of US 2005/0062708 Al, color breakup and motion blur are not significantly reduced.

In an embodiment the repeating the steps of addressing and illuminating does not allocate a color to the remaining duration of the predetermined time period. So, in addition to not illuminating the pixel, the pixel is addressed with a value which corresponds to "no color" during the remaining duration of the predetermined time period. This further ensures that there is no light emitted by the pixel during this remaining period.

In a further embodiment, the pixels are arranged in rows and columns and the pixels are addressed row by row. With such a systematic pixel addressing, the electronics system can be kept the most simple.

In an embodiment, the illuminating of the pixel with the light of the first color starts after the pixels of all rows have been addressed their respective first color values and the pixels of all rows are illuminated simultaneously. For simultaneously illuminating all pixels a less complicated electronics system is required, than when all pixels or rows have to be illuminated at different moments in time. For example, one backlight may be used for simultaneously illuminating all pixels.

In another embodiment, the addressing of the second color value to the pixel is performed before the pixels of all rows have been addressed their respective first color values and the addressing of the pixels of all rows is stretched over the whole or almost the whole frame period. Generally, a stream of image data to be displayed will be gradually received row by row over the duration of the whole frame period. If the addressing of the pixels of the first row starts as early as possible in the frame period and the addressing of the pixels of the last row starts near the end of the frame period, the addressing of each pixel is performed relatively close to the receiving of the image data for said pixel. Consequently not much memory is required for buffering the image data. For the first color value 'as early as possible' means directly after the start of the frame period. For all other color values 'as early as possible' means directly after the illumination following the addressing of the previous color value. Preferably, the illumination is performed by Light Emitting Diodes (LEDs).

LEDs can be driven at high peak outputs at very short duty cycles. Using LEDs makes it possible to use only a very small fraction of the predetermined time period between two consecutive addressings of values for the first color to the pixel. Thereby color breakup and motion blur is minimized. According to a second aspect of the invention, a multi-color display system for displaying video images is provided. This display system comprises pixels being capable of modulating light of JV different colors, N being an integer of at least 2, an input for receiving values for the colors to be applied to the pixels for displaying the video images, illumination means for illuminating the pixels and a processor, being operative to address the values for a first color to one of the pixels, illuminate the pixel with light of the first color, and repeat the addressing and illuminating for all further colors of the N colors. According to the invention the processor is further operative to perform the illumination of the pixel with the light of the N colors in less than T * (N-I)ZN, T being a predetermined time period between two consecutive addressings of values for the first color to the pixel, whereas the pixel is not illuminated for a remaining duration of the predetermined time period.

Such a display system may be a direct-view display system or a projection system. Each of these systems may be realized by reflective display panels, transmissive panels or a combination thereof. The illumination means preferably are backlights that can be driven at high peak outputs when driven at short duty cycles, such as LED-systems.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. 1 shows a flow diagram of a method according to the invention,

Fig. 2 schematically shows a multi-color display system according to the invention,

Figs. 3a-d show timing schedules for addressing and illuminating pixels using a prior art method, Figs. 4a and 4b show timing schedules for addressing and illuminating pixels according to the invention, and

Fig. 5a, 5b and 5c show more timing schedules for addressing and illuminating pixels according to the invention.

Fig. 1 shows a flow diagram of a method 10 of color-sequential operation of pixels of a multi-color display device, according to the invention. The method 10 is only shown for the operation of one pixel. In reality this method 10 will also be applied to all other pixels of the display. The operation of another pixel may be performed simultaneous with the operation of the first pixel. The operation of the other pixels may also start after the start or completion of the operation of the first pixel. Various timing schemes for operation of a plurality of pixels will be described below with reference to other figs.

In a first step, a first color value is addressed to the pixel in addressing step 11. For standard 3 color displays (Red, Green, Blue) the first color will typically be red. When the correct color value has been addressed, the pixel is illuminated in illumination step 12. This illumination is performed with red light. The red light may, for example, come from a red LED or from a combination of white light and a red filter. After the illumination step 12, it is checked whether all colors, for example RGB, have already been applied to the pixel in control step 13. For the standard RGB display, the addressing step 11 and the illumination step 12 are first repeated for green and then for blue. For displaying one image, the loop 15 with the addressing step 11, illumination step 12 and control step 13, is performed once for every color that can be provided by the pixel. In a three color RGB display, the loop 15 is performed three times. In a four color RGBX display, the loop 15 is performed four times. Here X is a fourth color field, e.g. using illumination with yellow light or with white light. In prior art color-sequential operation methods for display devices, the illumination 12 with light of the last color is immediately followed by the addressing 11 of a color value for the first color, to display the next image.

According to the invention a pause step 14 is performed after the loop 15 has been executed for all colors. By condensing the illumination with light of all colors within a fraction of the predetermined time period, the time between illuminating the pixel with light of the first color and illuminating the pixel with light of the last color is reduced and consequently also color breakup is reduced. Furthermore, the reduction of the total time used for illumination of the pixel results in reduced motion blur. For a display device using, for example, the three colors RGB, the illumination is condensed within 2/3 of the frame period. Preferably, an even smaller fraction of the predetermined time period is used for the illumination. When, for example, LEDs are used, these LEDs may be driven at high peak outputs at very short duty cycles (< 1 ms). Fig. 2 schematically shows a multi-color display system 20 according to the invention. The display system 20 comprises a screen with a multitude of pixels 21, preferably arranged in a rows and columns. A processor 24 is arranged for addressing color values to the pixels 21. When considering one of the pixels 21, for each image that is displayed on the display screen, a color value is addressed to this pixel, for each available color. Generally, the pixels 21 are capable of modulating red, green and blue light such that all colors in the image are composed by combining these three colors in the right amounts. Also other combinations of colors may be used for composing all other colors. It is known, for example, to use a four- color system RGBW with red, green, blue and white light, as well as with red, green, blue and yellow light, also called RGBY. These three or more colors are addressed to the pixel time sequentially. The addressing may be performed for one pixel at a time, or may be performed for groups of pixels simultaneously. For example, the pixels are addressed row by row. The invention may be applied to displays using active matrix addressing (e.g. TFT) as well as to screens using passive matrix addressing.

At an input 25 the color values to be applied to the pixels 21 are received, for example, from a computer program, TV tuner or video card. A memory 22 may store the received color values until the processor 24 reads the values from the memory and applies them to the respective pixels 21. This memory 24 does not necessarily need to be in the display device itself. The color values could also be sent in the wanted order and multiplicity from a TV-receiver/TV-chassis to the processor in a display device. After the addressing of a color value to a pixel, the illumination means 23 illuminates said pixel. The illumination means 23 may illuminate one pixel at a time, or may illuminate groups of pixels simultaneously. The illumination means 23 may, for example provide white light that is filtered by color filters for providing the colors, or the illumination means 23 may be arranged for providing the colored light by itself. The operation of the illumination means is managed by the processor 24. The illumination means may, for example, comprise LEDs (Light Emitting Diodes), laser diodes or OLEDs (Organic Light Emitting Diodes).

According to the invention, after addressing and illuminating the pixel for a first image, no color is applied to the pixel for some time, until a color value is addressed to the pixel for the next image. Consequently, the operation of the pixels according to the invention may be regarded as an RGB-black operation method. By condensing the illumination with the light of all colors within a fraction of the predetermined time period, the time between illuminating the pixel with red light and illuminating the pixel with blue light is reduced and consequently color breakup and motion blur are reduced.

Fig. 3a-d show timing schedules for addressing and illuminating pixels using a prior art method. All figs. 3a-d relate to the operating of a display wherein the pixels are arranged in rows and columns. The pixels are addressed and illuminated row by row. The figs, show with an arrow how the rows of the display are addressed from top to bottom (in direction of the arrow) over time (time axis from left to right in the Figs.). The thick dotted lines 33 indicate the start/end of a frame time. During a frame one image is displayed on the screen. During the next frame the next image is displayed. For providing the correct color to each pixel for showing said image, the three colors RGB are applied to the pixel in the right proportions. According to the invention, the colors are applied time sequentially. The arrows 31 indicate when the pixels of a row are addressed. The shaded areas 32 indicate when the pixels are illuminated with light of one of the three colors RGB.

For example, in fig. 3a, first the red values of the pixels in the uppermost row are addressed and thereafter the red values of the pixels in the underlying rows are addressed. After a row of pixels has been addressed, the pixels in that row are illuminated with red light. The color values that have been addressed to the pixel determine the gray value of the light that is observed by the user. After the illumination of the pixels with the red light, the addressing and illuminating are repeated for the other colors. After one frame time, new color values are addressed to the pixels for displaying the next image.

According to the prior art, the frame is partitioned in three sub-frames, which are separated by the thin dotted lines 34 in figs. 3a-d. The addressing of color values for each color has its own sub-frame and it takes a whole frame period to apply the three desired colors to a pixel. When display moving objects, the time-sequentially displayed colored sub- frames are imaged on different positions on the retina as the eye tracks the motion of the objects, causing the colors of the moving object to break apart. This effect is called color breakup and also occurs for stationary objects with eye saccade (fast eye movements). As a result of the color breakup, a white moving object shows red and blue edges. Motion blur is another problem of the prior art method is. Motion blur occurs because an image held on the screen for the duration of a frame period blurs on the retina as the eye tracks the (average) motion from one frame to the next. In figs. 3b and 3c, the color break-up and motion blur are reduced, because the illumination of the pixels with light of all three colors takes up less time. In fig. 3b, the light is turned on row by row like in fig. 3a, but the light is turned on at a later moment and for a shorter time span. In fig. 3a, most of the time more than one color is present on the screen, which may result in backlight color cross-talk between regions that are simultaneously illuminated with different colors. In fig. 3b, only one color is present at a time and consequently backlight color cross-talk is reduced. In fig. 3c, for each color, first all pixels are addressed and then the whole screen is illuminated at once during a limited time span. However the total time span of the illumination and the separation of the illumination with light of different colors are still relatively large and color break up and motion blur still occur. In both fig. 3b and fig. 3c, the frame period comprises three sub-fields; one sub-field for every color (R,G,B). Illumination with the third color (green) will start 2/3 of the frame period after the start of illumination with the first color (red). Consequently, no matter how short the light of each color is provided, the total time span of illumination will always be larger than 2/3 of the frame period. Similarly, when using N colors, the methods of figs. 3b and 3c will always require a total illumination time span of more than T * (N-I)ZN for each image. The prior art methods of fig. 3b and 3c, therefore result in more color break up and motion blur than the method according to the invention.

In fig. 3d, a known method for further reducing color break up is shown. The further reduction is obtained by displaying the sub-frames at a 3 or more times higher frequency, e.g. at 540 Hz (RGB x 60 x 3). Such frequencies are, for example, used in the LCoS system. Because the display of the three different colors is less separated in time, color breakup is reduced. However, the LCoS system does have two major disadvantages. The pixels have to be addressed with a three times higher data rate because all pixels have to be addressed three times as much as with the methods shown in figs. 3a-c. Furthermore motion blur is not reduced because the light generation takes place during the whole of the frame period.

Figs. 4a and 4b show timing schedules for addressing and illuminating pixels according to the invention. In fig. 4a the addressing of the pixels with the second (green) color value is performed directly after the illumination of the pixel with light of the first color (red). The illumination time of the pixels for a particular color is shorter than 1/3 of the frame period. Because the addressing of the second color value of the pixel is already performed well before the start of the second sub-frame, the total time span during which the pixel is illuminated is reduced to less than 2/3 of the frame period. During the remaining 1/3 of the frame period the pixel is not illuminated. For this black period, no addressing of color values to the pixel is needed. The timing scheme according to fig. 4a results in a significant reduction of color breakup and motion blur, without requiring the high data rate of the method shown in fig. 3d. The color values are only addressed to each pixel once per color and per image.

In fig. 4b, the illumination time is even further reduced. Preferably, the illumination is performed by Light Emitting Diodes (LEDs). LEDs can be driven at high peak outputs at very short duty cycles. Using LEDs makes it possible to use only a very small fraction of the predetermined time period between two consecutive addressings of values for the first color to the pixel.

Fig. 5a, 5b and 5c show more timing schedules for addressing and illuminating pixels according to the invention. The timing scheme of fig. 5a is similar to the timing schedule of fig. 4b. In fig. 5a the illumination period is however further reduced. Furthermore cross talk is reduced, because at any moment only one color is present at the screen. Fig. 5b shows a timing scheme according to the invention, wherein first all color values are addressed to a pixel and then the whole screen is illuminated at once.

Fig. 5 c shows a preferred embodiment of the method according to the invention. In this embodiment the addressing and illuminating of all pixels is spread over the whole frame period. For one pixel or a small region of pixels, the three colors are still applied in only 1/3 of the frame period. Color breakup and motion blur is thus prevented, just like with the timing schemes described above. By spreading the operation of the pixels over the whole frame period the required size of the memory 22 is reduced. Generally, the memory will receive the color values to be applied in a continuous stream. Receiving all color value data for one image takes around one frame period. In the embodiment according to fig. 5c, the operation is almost pure streaming operation between the input video and the output

R/G/B subfields. The received data can be applied to the pixel almost directly after arrival at the memory. After applying the color value to the corresponding pixel, the memory can be used again for other color values. Moreover, this embodiment has its memory accesses evenly distributed over the full frame period, thus reducing its (peak) speed requirements. As the total memory cost (for the memory itself and for implementing it in the electronics system) reduces with memory size and memory speed, this reduces the system cost and complexity.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method (10) of color-sequential operation of pixels (21) of a color display device (20), the pixels (21) being capable of modulating light of JV different colors, TV being an integer of at least 2, the method comprising the following sequence of steps: addressing (11) a value for a first color to one of the pixels (21), illuminating (12) the pixel with light of the first color, and repeating the steps of addressing (11) and illuminating (12) for all further colors of the N colors, wherein all steps of illuminating (12) the pixel with the light of the N colors are performed in less than T * (N-I)ZN, T being a predetermined time period between two consecutive addressings (11) of values for the first color to the pixel, and wherein the pixel is not illuminated for a remaining duration of the predetermined time period.

2. A method (10) as claimed in claim 1, wherein the repeating the steps of addressing (11) and illuminating (12) does not allocate a color to the remaining duration of the predetermined time period.

3. A method (10) as claimed in claim 1 or 2, wherein the pixels (21) are arranged in rows and columns and wherein the pixels are addressed row by row.

4. A method (10) as claimed in claim 3, wherein the illuminating (12) of the pixel with the light of the first color starts before the pixels of all rows have been addressed their respective first color values.

5. A method (10) as claimed in claim 3, wherein the illuminating (12) of the pixel with the light of the first color starts after the pixels of all rows have been addressed their respective first color values.

6. A method (10) as claimed in claim 5, wherein the pixels (21) of all rows are illuminated simultaneously.

7. A method (10) as claimed in claim 3, wherein the addressing (11) of a second color value to the pixel is performed before the pixels (21) of all rows have been addressed their respective first color values.

8. A method (10) as claimed in claim 7, wherein the addressing (11) to the pixels (21) of all rows is stretched over more than 50% of the predetermined time period.

9. A method (10) as claimed in claim 1 or 2, wherein the illumination (12) is performed by Light Emitting Diodes, laser diodes or OLEDs.

10. A multi-color display system (20) for displaying video images, the display system comprising pixels (21) being capable of modulating light of JV different colors, N being an integer of at least 2, an input (25) for receiving values for the colors to be applied to the pixels (21) for displaying the video images, illumination means (23) for illuminating the pixels (21), a processor (24), being operative to address the values for a first color to one of the pixels, illuminate the pixel with light of the first color, and repeat the addressing and illuminating for all further colors of the N colors, wherein the processor (24) is further operative to perform the illumination of the pixel with the light of the N colors in less than T * (N-I)ZN, T being a predetermined time period between two consecutive addressings of values for the first color to the pixel, whereas the pixel is not illuminated for a remaining duration of the predetermined time period.

11. A multi-color display system (20) as claimed in claim 10, wherein the illumination means (23) are Light Emitting Diodes, laser diodes or OLEDs.

12. A computer program product, which program is operative to cause a processor (24) to perform a method (10) as claimed in claim 1.