CN100396105C - Method of operating a pulse width modulated display system with hybrid coding - Google Patents

Abstract

A field pulse width modulated display system (10), comprising: a Digital Micromirror Device (DMD) (24) having a plurality of micromirrors, each micromirror selectively reflecting light to illuminate a corresponding pixel. The driver circuit (30) controls the DMD (24) in response to a sequence of pulse width segments formed by a processor (31). The processor (31) increases the brightness of the pixel by actuating selected pulses such that a first large duration pulse (or combination of pulses) becomes actuated to reach a second pixel brightness boundary within a first range of brightness levels between the first and second pixel brightness boundaries, and the first large duration pulse (or combination of pulses) remains actuated within a second range of pixel brightness levels between the second and third pixel brightness boundaries. When the third pixel brightness boundary is reached, the second large duration pulse (or combination of pulses) now becomes actuated, with the first large duration pulse remaining actuated. The pulse width segments are formed in this manner for reducing the motion profile.

Description

Method of operating a pulse width modulated display system with hybrid encoding

This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application Serial No. 60/404,156, filed August 13, 2002.

technical field

The present invention relates to a pulse width modulated light projection system, and more particularly, to techniques for operating a pulse width modulated light projection system to minimize motion contouring.

Background technique

In recent years, there has been a type of semiconductor device, known as a digital micromirror device (DMD), comprising a plurality of independently movable micromirrors arranged in a rectangular array. Each micromirror is rotated along a limited arc, typically on the order of 10-12°, under the control of a corresponding driver unit in which a bit is latched. Upon application of the previously latched bit "1", the driver turns its associated micromirror unit to the first position. Conversely, a previously latched bit "0" applied to the driver unit causes the driver unit to turn its associated micromirror to the second position. By properly positioning the DMD between the light source and the projection lens, when turned to the first position by its corresponding driver unit, each individual micro-mirror of the DMD reflects light from the light source through the lens onto the display screen, thereby illuminating the An individual picture element (pixel) in a display. When rotated to its second position, each micromirror reflects light away from the display screen so that the corresponding pixel appears dark. An example of such a DMD device is the DMD from the DLP ^™ Projection System from Texas Instruments, Dallas Texas.

Presently, projection systems employing DMDs of the type described above operate by controlling the time period during which individual micromirrors remain "on" (i.e., turned to their first position) and during which the micromirrors remain "off" (i.e., turned to their second position). The duty factor compared to the time interval is used to control the brightness (illuminance) of each pixel. . To this end, such current DMD-type projection systems use pulse width modulation to control pixel brightness by varying the duty cycle of each micromirror according to the state of the pulses in a sequence of pulse width segments. Each pulse width segment includes pulse trains of different durations. The state of each pulse in the pulse width segment (ie, whether each pulse is on or off) determines whether the micromirror remains on or off for the duration of that pulse. In other words, the larger the sum of the pulse widths in the turned-on (activated) pulse width segment, the longer the duty cycle of each micromirror.

In television projection systems using DMDs, the frame interval, the time between displaying successive images, depends on the television format selected. The NTSC system currently used in the United States requires a frame interval of 1/60 second, while the European television system uses a frame interval of 1/50 second. Currently, DMD-type television projection systems typically achieve color displays by projecting red, green, and blue images simultaneously or sequentially during each frame time interval. A typical sequential DMD-type projection system uses a motor-driven color wheel inserted in the optical path of the DMD. The color wheel has multiple discrete primary color windows, typically red, green and blue, so that during successive time intervals, red, green and blue light respectively fall on the DMD. In order to realize a color picture, red, green and blue light must fall on the DMD at least once in each continuous frame time interval. If only one red, one green and one blue image is produced and each takes 1/3 of the frame time interval, there will be a time delay between producing colors with perceptible color breaks with motion. Currently, DMD systems solve this problem by dividing each color into several time intervals and interleaving the time intervals in time, thereby reducing the delay between colors.

Pulse width modulated projection systems of the type described above with multiple images of each primary color produced during each frame time interval to produce a color picture are often impaired by small amplitude transient motion profiles, such as those associated with motion in the scene or the viewer's Those outlines associated with the eyes. Artifacts of this type result from variations in the distribution of light pulses across different portions of the display cycle.

US Patent 5,986,640 discloses a scheme to reduce motion contours by segmenting the most significant bits in a sequence of pulse width segments between two or more temporally adjacent segments (time intervals). Although this approach works to reduce contours, it cannot eliminate contours on all transitions. Furthermore, partitioning the bits in a manner sufficient to reduce the outline increases the number of times each pixel must be addressed, thereby increasing the bandwidth required to achieve such addressing.

Therefore, there is a need for a technique for operating a pulse width modulated display that reduces motion contours while overcoming the above-mentioned problems of the prior art.

Contents of the invention

According to the present principles, a method for operating a pulse width modulated display system, such as a pulse width modulated display system employing a digital micromirror device (DMD) to selectively reflect light from a light source through a projection lens is proposed to the display screen. In such display systems, the illuminance of each pixel for a given color is controlled in response to pulses in a sequence of pulse width segments. The state of each pulse in each segment determines whether or not the pixel is illuminated during the time interval associated with that pulse. In order to reduce the incidence of motion contours, the brightness of the pixel is increased by activating (actuate) selected pulses such that within a first range of brightness levels between the first and second pixel brightness boundaries, the first maximum A large-duration pulse (or combination of pulses) becomes active to reach the second pixel brightness limit. Within a second range of pixel intensity levels between the second and third pixel intensity limits, the first large duration pulse (or combination of pulses) remains active, and when the third pixel intensity limit is reached, the first The second pulse of maximum duration (or a combination of pulses) also becomes active, with the first pulse of maximum duration remaining active.

As the pixel brightness increases, another still inactive large-duration pulse (or combination of pulses) becomes active when a successively higher pixel-brightness limit is reached, where each of the already activated large-duration pulses ( or a combination of pulses) remains active. Each large-duration pulse (or combination of pulses) that becomes active at the brightness limit of each pixel is called a "thermometer code" pulse because, once activated, the pulse (or combination of pulses) combination) remains active while also increasing pixel brightness above this brightness limit in a manner similar to temperature levels on a mercury thermometer. A given segment can include more than one such thermometer code pulse, depending on the width (ie, duration) of each pulse in each segment. However, only a single thermometer code pulse that was previously deactivated changes state (ie, becomes active) when increasing pixel brightness to reach a given pixel brightness limit. Conversely, when reducing the pixel brightness to a given pixel brightness limit, only the single thermometer code pulse that had been activated now becomes deactivated, whereby the other thermometer code pulses that are still to be deactivated remain activated.

Description of drawings

Fig. 1 shows a schematic block diagram of a current pulse width modulation display system;

Figure 2 shows a front view of a color wheel comprising part of the display system of Figure 1; and

Figures 3-7 collectively illustrate pulse diagrams of the present principles showing each of a plurality of sequences of pulse width segments used to control the image for a given color in the display system of Figure 1. The brightness of the corresponding color of one of the pixels to reduce motion contours.

Detailed ways

FIG. 1 shows a current pulse width modulated display system 10 of the type disclosed in Application Report "Single-Plate DLP ^™ Projection System Optics," Texas Instruments, June 2001, which is hereby incorporated by reference. System 10 includes a lamp 12 at the focus of a parabolic reflector 13 that reflects light from the lamp through a color wheel 14 to an integrator rod 15 . A motor 16 rotates the color wheel 14 to provide an independent red, green and blue primary color window between the lamp 12 and the integrator rod 15 . In the exemplary embodiment shown in FIG. 2, the color wheel 14 has diametrically opposed red, green and blue color windows 17 ₁ and 17 ₄ , 17 ₂ and 17 ₅ , and 17 ₃ and 17 ₆ , respectively. Therefore, when the motor 16 rotates the color wheel 14 of FIG. 2 counterclockwise, red, green and blue light will reach the integrator rod 15 of FIG. 1 in the order of RGBRGB. In practice, the motor 16 rotates the color wheel 14 at a speed high enough that each of the red, green and blue light reaches the integrator rod five times during the frame time interval of 1/60 second. 15 color images were generated. Other mechanisms exist for providing each of the three primary colors in succession. For example, a color scrolling mechanism (not shown) could also perform this task.

Referring to FIG. 1 , the integrator rod 15 concentrates the light from the lamp 12 onto a set of relay optics 18 as it passes through one of the successive red, green and blue colored windows of the color wheel 14 . Relay optics 18 disperse the light into multiple parallel beams that reach fold mirror 20 , which reflects the beam through a set of lenses 22 onto a total internal reflectance (TIR) prism 23 . TIR prism 23 reflects the collimated light beam to digital micromirror (DMD) 24 , such as a DMD device manufactured by Texas Instruments, for selective reflection into projection lens 26 and onto screen 28 .

DMD 24 takes the form of a semiconductor device having a plurality of individual mirrors (not shown) arranged in an array. As an example, a DMD manufactured and sold by Texas Instruments has a mirror array of 1280 columns by 720 rows, resulting in 921,600 pixels of the associated picture projected onto the screen 28 . Other DMDs may have different mirror settings. As previously described, each micromirror in the DMD rotates an associated finite arc under the control of the corresponding driver unit (not shown) in response to the state of the binary bit previously latched in the driver unit. Each micromirror is rotated respectively to first and second positions depending on whether the latch bit applied to the driver unit is a "1" or a "0". When turned to its first position, each micromirror reflects light into lens 26 and onto screen 28 to illuminate the corresponding pixel. When each micromirror remains turned to its second position, the corresponding pixel appears dark. The time interval during which each micromirror reflects light through projection lens 26 onto screen 28 (micromirror duty cycle) determines the brightness of the pixel.

The individual driver units in the DMD 24 receive drive signals from a driver circuit 30 of the type known in the art, described in the paper "High Definition Display System Based on Micromirror Device", R.J. Grove et al. International Workshop on HDTV (October 1994) (herein incorporated by reference) as an example. Driver circuit 30 generates drive signals for the driver units in DMD 24 based on a sequence of pulse width segments 31 applied to the driver circuit by the processor. Each pulse width segment includes a pulse train of different duration, and the state of each pulse determines whether the micromirror remains on or off for the duration of the pulse. Typically, the shortest possible pulse (i.e., a 1-pulse) that can occur within a pulse width segment (sometimes referred to as the least significant bit or LSB) has a duration of 15 microseconds, with each larger The pulses have a duration that is an integer multiple of the LSB time interval. In effect, each pulse within a pulse width segment corresponds to a bit in the digital bit stream whose state determines whether the corresponding pulse is on or off. A "1" bit indicates an activated (on) pulse, and a "0" bit indicates a deactivated (off) pulse.

The motion contour minimization method of the present principles is best understood by the following example of the field sequential system of FIG. 1 in which each primary color is displayed in five pulse width segments. Each pulse width segment has a total width of 51 LSBs, so each sequence of five pulse width segments has a total pulse width of 255 LSBs, thus typically having a duration of 15 milliseconds for each LSB (1-pulse) For a given color of , each pixel can be made to have one of 256 brightness levels. Thus, each 51 LSB pulse width segment has a duration of 765 milliseconds.

Table 1 shows the setting of the LSB in each of the five segments comprising the pulse width sequence.

Table 1

part Pulse Width (LSB) segment 1 7 4 13 2 13 6 6 segment 2 7 7 13 1 1 13 4 6 segment 3 7 4 13 2 13 6 6 segment 4 7 7 13 1 1 13 4 6 segment 5 7 4 13 2 13 6 6

According to this principle, motion is minimized by minimizing the number and width of pulses that become inactive for one adjacent least significant bit (i.e., 1-pulse) of a brightness change when one or more other pulses become active. Silhouettes are minimal. In practice, to increase pixel brightness, selected pulses in one or more segments are activated such that at each successive pixel brightness boundary, another pulse of large duration remains inactive (i.e., in the demonstrated 13-pulse in an embodiment, or a combination of pulses such as 7-pulse and 6-pulse) becomes active. In addition, each long-duration pulse (or combination of pulses) that was previously activated remains active when the previous pixel intensity limit is reached. Each large duration pulse (or combination of pulses) that becomes active to reach a given pixel brightness limit is called a "thermometer code" pulse because once activated, such thermometer code pulses remain active while still Pixel brightness is increased above the pixel brightness limit in a manner similar to mercury in a mercury thermometer. (When a particular temperature level is reached, the mercury continues to rise above that level in response to an increase in temperature). Depending on the width (ie, duration) of each pulse in each segment, a given pulse width segment can have multiple thermometer code pulses.

Figures 3-6 show in combination pulse diagrams for a sequence of pulse width segments representing the corresponding pixel for a given color at each of brightness levels #0-#255. In the illustrated embodiment, segment 3 is selected as the first segment whose thermometer code pulses are activated, with each thermometer code that has been activated to reach the pixel brightness limit as the pixel brightness increases above this limit. Pulse remains active. As shown in Figure 3, an active 1-pulse is required to achieve brightness level #1. Since there is no 1-pulse in segment 3 in this example, the 1-pulse in segment 2 is activated. To reach pixel brightness level #2 at which the 2-pulse in segment 3 becomes active and the 1-pulse in segment 2 becomes deactivated. To achieve pixel brightness level #3, the 1-pulse in segment 2 and the 2-pulse in segment 3 become active.

To reach brightness level #4 at which the 4-pulse in segment 3 becomes active and the previously active pulse is deactivated. To achieve each of pixel brightness levels #5 to #12, one of 4-pulse, 2-pulse, and 6-pulse (first) in segment 3 is selected, and 1-pulse in segment 2 becomes active . Intensity level 13# (constituting the first pixel intensity limit) is reached by activating the 13-pulse (first) in segment 3, deactivating all other pulses at that pixel intensity level.

To reach brightness level #14, the 1-pulse in segment 3 is activated, the 13-pulse (first) in segment 3 remains active. Thus, above the first pixel intensity limit (intensity level #13), the 13-pulse (first) in segment 3 remains active. Thus, the 13-pulse (first) in segment 3 constitutes the first thermometer code pulse in the segment that becomes active. Brightness level #14 is achieved by keeping 13-pulse active in segment 3 and activating a selected one from 4-pulse, 2-pulse and 6-pulse (first) in segment 3 and 1-pulse in segment 2 One of #25 each. At brightness level #26 (constituting the second pixel brightness limit), the 13-pulse (second) in segment 3 becomes active and the 13-pulse (first) in the same segment remains active. At pixel brightness level #27, both 13-pulses (first and second) in segment 3 remained active, and now the 1-pulse in segment 2 is active at this brightness level. Thus, the 13-pulse (second) in segment 3 constitutes the second thermometer code pulse in the segment that becomes active.

By keeping the two 13-pulse (first and second) active in segment 3, and activating one of the 7-pulse, 4-pulse, 2-pulse and 6-pulse in segment 3 and from segment 2 One of 7-pulse, 1-pulse and 4-pulse selected in, each of brightness level #28-#61 is realized. At brightness level #37, the 7-pulse and 6-pulse (second) in segment 3 are activated, and these pulses remain active as the pixel brightness level increases. Thus, the 7-pulse and 6-pulse (second) in segment 3 together form the combined thermometer code pulse. Note that at pixel intensity level #51, all pulses in segment 3 become active. As the pixel brightness level increases above brightness level #51, all pulses in this segment except the 2-pulse in segment 3 remain active.

Referring to Figure 4, at pixel intensity level #62 (which constitutes the adjacent higher pixel intensity boundary), the 13-pulse (first) in segment 2 and all but 2-pulse in segment 3 are becomes active. To achieve pixel brightness level #63, the 1-pulse in segment 2 is activated, the 13-pulse (first) in segment 2 and all other pulses in segment 3 except the 2-pulse remain active. Thus, the 13-pulse (first) in segment 2 becomes the first thermometer code pulse in that segment that is activated.

A further increase in pixel brightness to one of the brightness levels #63-#74 of Figure 4 is achieved by activating the selected pulse in segment 2 and the 2-pulse in segment 3 with the previously activated thermometer code pulse in segment 2 (ie 13-pulse (first)) remains active, all other pulses in segment 3 remain active. To reach brightness level #75, the 13-pulse (second) in segment 2, the previously active thermometer code pulse in that segment, and all but 2-pulse in segment 3 become active. Thus, the 13-pulse (second) in segment 2 constitutes the second thermometer code pulse in that segment that is activated when the associated pixel brightness limit is reached, and for pixels whose brightness is above the pixel brightness limit increase, keep active.

As is now understood, once activated to reach the associated pixel intensity limit, for adjacent higher pixel intensity levels, each large pulse (e.g. 13-pulse) or set of pulses comprising large duration pulses in each segment Combinations of pulses (eg 7-pulse and 6-pulse (second) in segment 3) remain active. Thus, according to the present principles, each such pulse (or combination of pulses) constitutes a thermometer code pulse. In practice, each thermometer pulse is of a sufficiently large nature (i.e., has a sufficiently long duration) so that once activated to reach a pixel brightness limit, the pulse remains active at brightness levels above the pixel brightness limit, while Limits the total number of pulses in a segment. In other words, at a given pixel brightness limit, a single thermometer code pulse (or such combination of pulses including thermometer code pulses) becomes active and remains active as the pixel brightness increases above that limit. Conversely, for a pixel brightness decrease to a given pixel brightness limit, only a single thermometer code pulse becomes deactivated, and thermometer code pulses that have not been deactivated remain active until an adjacent lower pixel brightness limit is reached. However, each thermometer code pulse in each segment should not be so large that when activated or deactivated, there is an incremental change in pixel brightness (i.e., the pixel brightness increases to the next higher level, or the pixel Brightness decreases to the next lower level) with a noticeable transition.

Furthermore, the selection of thermometer code pulses should be used to limit the "swapping" of pulses to essentially a single pulse width segment (ie, the selection of activated pulses) to achieve a particular brightness state. However, it is not necessary to limit the "swapping" of pulses to substantially a single pulse width segment (ie, a modified binary pulse arrangement) to gain the advantages of the present invention. Pulse swapping to achieve a particular brightness level can occur between multiple segments as long as a single thermometer code pulse (or combination of pulses) in a single segment becomes active or becomes deactivated between adjacent pixel brightness boundaries between.

Techniques for minimizing motion contours in pulse width modulated displays have been described above.

Claims (14)

1. the method for an operating impulse width modulated display system, described display system has a plurality of pixels, for given color, pulse in the response pulse duration section sequence in each section, control the illumination of each pixel, wherein the state of each pulse in each section has determined whether described pixel is illuminated during the time interval related with this pulsion phase, and described method comprises step:

Sensitizing pulse is to increase the brightness of pixel, thereby in first scope of the pixel brightness level between first and second pixel brightness boundary, first duration pulse element becomes activation, to reach second pixel brightness boundary, and in second scope of the pixel brightness level between the second and the 3rd pixel brightness boundary, first duration pulse element keeps activating, and when reaching the 3rd pixel brightness boundary, second largest duration pulse element becomes activation, and simultaneously first duration pulse element keeps activating.

2. method according to claim 1, it is characterized in that the first and second big duration pulse elements one of them comprises individual pulse at least.

3. method according to claim 1, it is characterized in that the first and second big duration pulse elements one of them comprises the combination of pulse at least.

4. method according to claim 1, it is characterized in that selecting the duration of each big duration pulse element, so that increase pixel brightness and reach the relevant moment minimum of pixel brightness boundary with increasing progressively, and make the pulse number in each section minimum.

5. method according to claim 1 is characterized in that at least one pair of big duration pulse element is arranged in same section.

6. method according to claim 1 is characterized in that at least one pair of big duration pulse element is arranged in different sections.

7. method according to claim 1 is characterized in that the pulse that will be activated to increase pixel brightness is limited in the same section in fact.

8. the method for an operating impulse width modulated display system, described display system has a plurality of pixels, for given color, pulse in the response pulse duration section sequence in each section, control the illumination of each pixel, wherein the state of each pulse in each section has determined whether described pixel is illuminated during the time interval related with this pulsion phase, and described method comprises step:

Reduce the brightness of pixel by the deexcitation pulse, thereby in given pixel brightness boundary, the first duration pulse element that has been activated becomes deexcitation now, and in adjacent lower pixel brightness boundary, first duration pulse element keeps deexcitation, second largest duration pulse element becomes deexcitation, and still each big duration pulse element that is activated before of un-activation keeps activating.

9. method according to claim 8, it is characterized in that the first and second big duration pulse elements one of them comprises individual pulse at least.

10. method according to claim 8, it is characterized in that the first and second big duration pulse elements one of them comprises the combination of pulse at least.

11. method according to claim 8, it is characterized in that selecting the duration of each big duration pulse element, so that reach the relevant moment minimum of pixel brightness boundary, and make the pulse number in each section minimum with the little pixel brightness of monotone decreasing.

12. method according to claim 8 is characterized in that described big duration pulse element is arranged in same section.

13. method according to claim 8 is characterized in that described big duration pulse element is arranged in different sections.

14. method according to claim 8 is characterized in that and will be limited in the same section in fact by the pulse of deexcitation.

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