KR20050050644A - Pulse width modulated display with hybrid coding - Google Patents

Abstract

A field sequential pulse width modulated display system (10) comprises a digital micromirror device (DMD) (24) having a plurality of micromirrors that each selectively reflect light to illuminate a corresponding pixel. A driver circuit (30) controls the DMD (24) responsive to sequences of pulse width segments formed by a processor (31). The processor (31) increases the pixel brightness by actuating selected pulses such that within a first range of brightness levels between first and second pixel brightness boundaries, a first large-duration pulse (or combination of pulses) becomes actuated to reach the second pixel brightness boundary, and within a second range of pixel brightness levels between second and third pixel brightness boundaries, the first large duration pulse (or combination of pulses) remains actuated. Upon reaching the third pixel brightness boundary, a second large duration pulse (or combination of pulses) now becomes actuated with the first large duration pulse (or combination of pulses) remaining actuated. Forming the pulse width segments in this manner serves to reduce motion contouring.

Description

Pulse width modulated display with hybrid coding

Cross Reference of Related Application

This application is incorporated by reference in U.S.C. 35, US Provisional Patent Application No. 60 / 404,156, filed August 13, 2002. Claim priority under 119 (e).

Technical field

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

Currently, there is a type of semiconductor device known as a digital micromirror device (DMD) and comprising a plurality of individually movable micromirrors arranged in a rectangular array. Each micromirror orbits a limited arc, typically 10 to 12 degrees, under the control of the corresponding driver cell latching the bit. Upon application of the previously latched " 1 " bit, the driver cell causes the associated micromirror cell to pivot the first position. Conversely, the application of the "0" bit previously latched into the driver cell causes the driver cell to orbit the micromirror associated with the second position. By properly placing the DMD between the light source and the projection lens, each individual micromirror of the DMD device reflects light from the light source through the lens and is individual picture element in the display when it is pivoted to the first position by the corresponding driver cell. It will reflect onto the display screen to illuminate (pixel). When pivoted to the second position, each micromirror reflects light away from the display screen and causes the corresponding pixel to darken. One example of such a DMD device is the DMD of the DLP ^™ projection system available from Texas Instruments, Dallas Texas.

Conventional projection systems incorporating the DMD of the described type allow the micromirrors to be "off" while the individual micromirrors remain "on" (ie, they are pivoted to their first position). The luminance (lighting) of the individual pixels is controlled by controlling the duty cycle while remaining (ie, turning to their second position). To this end, such conventional DMD-type projection systems use pulse width modulation to control pixel brightness by changing the duty cycle of each micromirror according to the state of the pulses in the sequence of pulse width segments. Each pulse width segment includes a string of pulses of different time periods. The state of each pulse in the pulse width segment (ie, whether each pulse is on or off) determines whether the micromirror is on or off for the duration of the pulse. In other words, the larger the sum of the widths of the pulses in the pulse width segment that is on (actuated), the longer the duty cycle of each micromirror.

In television projection systems using DMD, the frame period, ie the time between displaying consecutive images, depends on the selected television standard. The NTSC standard currently in use in the United States requires a frame interval of 1/60 seconds, while some European television standards use a frame interval of 1/50 seconds. Conventional DMD-type television projection systems generally perform color display by simultaneously or successively projecting red, green, and blue images during each frame period. A typical sequential DMD-type projection system uses a motor-driven color wheel inserted into the DMD's light path. Between successive periods, the color wheel has a plurality of separate three primary windows, typically red, green and blue, such that red, green, and blue light fall on the DMD, respectively. To achieve a color picture, red, green and blue light must be emitted at each successive frame interval at least once in the DMD. If one red, one green and one blue image is made and each consumes one third of the frame interval, the large duration between colors will produce perceptible color separation in motion. Conventional DMD systems address this problem by splitting each color into several intervals and interleaving the intervals in time, thus reducing the delay between colors.

Pulse width modulated projection systems of the type described above with the ability to create multiple images of each of three primary colors during each frame interval to produce a color picture, have small amplitude transients such as those associated with movement in the scene or the viewer's eyes. On the phenomena they are often outlined in motion. This type of artifact is due to changes in the distribution of light pulses across different parts of the display period.

U. S. Patent No. 5,986, 640 discloses a method of reducing motion contours by dividing the most significant bits in a sequence of pulse width segments between two or more time-adjacent segments (periods). This approach is intended to reduce the contour, but does not remove the contour on all changes. In addition, dividing the bits in a manner sufficient to reduce the contour will increase the number of times each pixel must be addressed, thus increasing the bandwidth required to achieve such addressing.

Thus, there is a need for a technique to operate a pulse width modulated display to reduce motion contour while overcoming the above mentioned disadvantages of the prior art.

1 is a block schematic diagram of a conventional pulse width modulated display system.

FIG. 2 shows a frontal view of a color wheel that includes a portion of the display system of FIG. 1. FIG.

3-7 show a pulse map illustrating each of a sequence of a plurality of pulse width segments controlling the luminance of one of the pixels within the display system of FIG. 1 for a given color to reduce motion contours in accordance with the present principles. Drawings showing collective examples.

Summary of the Invention

In accordance with the present principles, a pulse width, such as a pulse width modulated display system incorporating a Digital Micromirror Device (DMD) to selectively reflect light from a light source through a projection lens and to a display screen There is a method provided for operating a modulated display system. In such display systems, the illumination of each pixel for a given color is controlled in response to the pulses in the sequence of pulse width segments. The state of each pulse in each segment determines whether the pixel is illuminated for the period associated with that pulse. In order to reduce the occurrence of motion contours, pixel luminance is increased by operating selected pulses so that within a first range of luminance levels between the first and second pixel luminance boundaries, the pulse of the first versus sustained period (or Combination of pulses) is operative to reach the second pixel luminance boundary. Within the second range of pixel luminance levels between the second and third pixel luminance boundaries, the first large-duration pulse (or combination of pulses) remains in operation and upon reaching the third pixel luminance boundary, Along with the pulse of the first large-lasting period that is in the activated state, the pulse (or combination of pulses) of the second large-long period is also activated.

As the pixel brightness increases, with each already activated large-duration pulse (or combination of pulses) remaining in operation, other yet inactive large-duration pulses (or combinations of pulses) are continuous. This will activate upon reaching a higher pixel luminance boundary. Each large-duration pulse (or combination of pulses) that is to be operated at each pixel luminance boundary is referred to as a "thermometer code" pulse because it is operated once, and the pulse (or combination of pulses) is referred to as the temperature level of the mercury thermometer. In a manner similar to the above, it remains in operation on other increases in pixel luminance on the luminance boundary. Depending on the width (ie, duration) of each of the pulses within each segment, a given segment may include one or more such thermometer code pulses. However, upon an increase in pixel luminance to reach a given pixel luminance boundary, only a single previously unactivated thermometer code pulse changes state (i.e., becomes active). Conversely, when the pixel luminance is reduced to a given pixel luminance boundary, only a single thermometer code pulse that has been activated should not yet be activated and therefore with no other thermometer code pulses that are still active, it is now not activated.

details

Figure 1 June 2001, referring to this specification is published by Texas Instruments ever integrated into the application report "single panel DLP ^TM projection system optics (Single Panel DLP ^TM Projection System Optics)" conventional pulse of the type shown in a wide The modulation display system 10 is shown. The system 10 includes a lamp 12 in focus of a parabolic reflector 13 that reflects light from the lamp through the color wheel 14 and into the integrator rod 15. The motor 16 rotates the color wheel 14 to place a separate one of the red, green and blue primary color windows between the lamp 12 and the integrator rod 15. In the exemplary embodiment shown in FIG. 2, the color wheel 14 has the opposite red, green and blue color windows 17 ₁ and 17 ₄ , 17 ₂ and 17 ₅ , and 17 ₃ and 17 ₆ , respectively. Thus, as the motor 16 rotates the color wheel 14 of FIG. 2 counterclockwise, red, green, and blue light will strike the integrator rod 15 of FIG. 1 in an RGBRGB sequence. In particular, the motor 16 rotates the color wheel 14 at a sufficiently high speed so that for a 1/60 second frame period, the red, green and blue light hits the integrator rod five times, respectively, and within the frame period 15 Generate color images. Different mechanisms exist to confer successively each of the three primary colors. For example, a color scrolling mechanism (not shown) can also do this.

Referring to FIG. 1, the integrator rod 15 relays light from the lamp 12 when the light passes through successive ones of the red, green and blue color windows of the color wheel 14. focus on a set of optics 18. Relay optics 18 diffuse the light into a plurality of parallel beams impinging the folded mirror, reflecting the beams through the set of lenses 22 and total internal reflectance (TIR) prism 23 To reflect. The TIR prism 23 reflects parallel light beams into a Digital Micromirror Device (DMD) 24, such as a DMD device from Texas Instruments, for selective reflection onto the projection lens 26 and the screen 28.

DMD 24 takes the form of a semiconductor device with a plurality of individual mirrors (not shown) arranged in an array. As an example, a DMD manufactured and sold by Texas Instruments Inc. has a micromirror array of 720 rows and 1280 columns, producing 921,600 pixels in the resulting picture projected on screen 28. Different DMDs can have different arrangements of micromirrors. As discussed previously, each micromirror in the DMD orbits around a defined call 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 the first and second positions depending on whether the latched bit applied to the driver cell is "1" or "0", respectively. When pivoted to its first position, each micromirror reflects light to lens 26 and to screen 28 to illuminate the corresponding pixel. Each micromirror remains pivoted to its second position, but the corresponding pixel is darkened. The period between each micromirror reflecting light through the projection lens 26 and to the screen 28 (micromirror duty cycle) determines the pixel brightness.

Individual driver cells within the DMD 24 are well known in the art and are described in detail in R.G. Grove et al. At an international workshop on HDTV (in October 1994) (incorporated by reference herein). Receive drive signals from a driver circuit 30 of the type illustrated by the circuit described in the document “High Definition Display System Based on Micromirror Device”. Driver circuit 30 generates driver signals for driver cells in DMD 24 in accordance with sequences of pulse width segments applied by processor 31 to the driver circuit. Each pulse width segment includes a string of pulses of different time periods, and the state of each pulse determines whether the micromirror is on or off for the duration of the pulse. The shortest possible pulse (i.e., 1-pulse) that can occur in a pulse width segment (referred to as the least significant bit or LSB several times) has a 15-microsecond period, while longer pulses in the segment have a duration of LSB period. Each has a duration that is an integer. In practice, each pulse in the pulse width segment corresponds to a bit in the digital bit stream and its state determines whether the corresponding pulse is on or off. The "1" bit represents a pulse that is active (on), while the "0" bit represents a pulse that is not active.

The motion contour minimization method of the present principles may be best understood by the following example for the field sequential system of FIG. 1 in which three primary colors are each displayed in a sequence of five pulse width segments. Each pulse width segment has a total pulse width of 51 LSBs, and each sequence of five pulse width segments has a total pulse width of 255 LSBs, resulting in each pixel having one of 256 luminance levels for a given color. Make it possible. Each LSB (1-pulse) typically has a duration of 15 microseconds. Thus, each 51 LSB pulse width segment has a duration of 765 microseconds. Table 1 describes an exemplary arrangement of LSBs in each of the five segments including a pulse width sequence.

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

The motion profile is minimized in accordance with the present principles by minimizing the number and width of pulses that are not working when one or more other pulses are activated for a continuous one-minimum significant bit (ie, one-pulse) change in luminance. In particular, to increase pixel brightness, pulses selected in one or more segments are activated, resulting in pulses of large duration that have not yet been activated (ie, 13 in the exemplary embodiment) at each successive pixel brightness boundary. Pulses, or combinations of pulses, such as 7-pulse and 6-pulse) are activated. In addition, the pulse (or combination of pulses) of each large-duration period previously operated upon reaching the expected pixel luminance boundary remains activated. Each versus sustained-pulse (or combination of pulses) that is operated to reach a given pixel luminance boundary is referred to as a "thermometer code" pulse because it is operated once, and each such thermometer code pulse is associated with mercury in a mercury thermometer. In a similar manner it remains operational on other increases in pixel brightness on the pixel brightness boundary. (When a certain temperature level is reached, the mercury continues to rise on the level in response to the temperature increase.) Depending on the width (i. E. Duration) of each of the pulses in each segment, a given pulse width segment may have multiple thermometer code pulses. Can be.

3-6 collectively illustrate a pulse map of a sequence of pulse width segments that illuminate a corresponding pixel for a given color at each of luminance levels # 0-# 255. In the illustrated embodiment, the segment 3 is the first segment in which the thermometer code pulses are operated with each thermometer code pulse activated to reach a pixel luminance boundary which is in an operating state when the pixel luminance increases above the boundary. Is selected. As shown in FIG. 3, reaching luminance level # requires the action of one-pulse. Segment 3 does not have 1-pulse in this example, so 1-pulse in segment 2 is activated. To reach pixel luminance level # 2, the two-pulse in segment 3 is operated with one-pulse in segment 2 which is not operated at this luminance level. To reach pixel luminance level # 3, 1-pulse in segment and 2-pulse in segment 3 are activated.

To achieve luminance level # 4, the 4-pulse in segment 3 is operated with previously activated pulses that have not been operated at this luminance level. In order to achieve the respective pixel luminance levels # 5 to # 12, a selected one of four-pulse, two-pulse and six-pulse (first) in segment 3 and one-pulse in segment 2 is activated. Luminance level # 13 (which constitutes the first pixel luminance boundary) Luminance level # 13 is reached by operating the 13-pulse (first) in segment 3 with all of the other pulses that do not operate at this pixel luminance level.

To reach luminance level # 14, the 1-pulse in segment 2 is operated with 13-pulse (first) in segment 3 that is in the operating state. Thus, on the first pixel luminance boundary (luminance level # 13), the 13-pulse (first) in segment 3 remains activated. Therefore, the 13-pulse (first) in segment 3 constitutes a first thermometer code pulse in that segment being operated. Each of the luminance levels # 14- # 25 maintains 13-pulse in segment 3 activated and actuates 4-pulse, 2-pulse and 6-pulse in segment 3 (first) and 1-pulse in segment 2. Is achieved. At luminance level # 26 (which constitutes the second pixel luminance boundary), the 13-pulse (second) in segment 3 is activated and the 13-pulse (first) in 13-pulse (first) in the same segment Stay on. At pixel luminance level # 27, all 13-pulses (first and second) in segment 3 remain activated, and one-pulse in segment 2 is operated at this luminance level. Thus, the 13-pulse (second) in segment 3 constitutes a second thermometer code pulse in that segment to be activated.

Luminance levels # 28- # 61 each hold two 13-pulses (first and second) in the segment 3 that is operated, and 7-pulse, 4-pulse, 2- in the segment 3 And a selected one of pulses and 6-pulses (first and second) and selected ones of 7-pulse, 1-pulse and 4-pulse in segment 2. At luminance level # 37, both the 7-pulse and the 6-pulse (second) in segment 3 are activated and all of these pulses remain activated as the pixel luminance level increases. Thus, the 7- and 6-pulses (second) in segment 3 collectively constitute a combination thermometer code pulse. Except for the 2-pulse in segment 3, all of the other pulses in that segment remain active as the pixel luminance level increases above luminance level # 51.

Referring to FIG. 4, at luminance level # 62 (which constitutes a continuously higher pixel luminance boundary), the 13-pulse (first) in segment 2 is equal to all in segment 3 except for 2-pulse. It will work with the pulses. To reach luminance level # 63, one-pulse in segment 2 is activated and all pulses in segment 3 except for 13-pulse (first) and two-pulse in segment 2 Stay on. Thus, the 13-pulse (first) in segment 2 becomes the first thermometer code pulse in the segment in which it is actuated.

Also, the increase in pixel luminance to reach one of the luminance levels # 63- # 74 of FIG. 4, along with all other pulses in the segment 3 that are in operation, causes the segment 2 to remain in operation. Is performed by operating one of the pulses in segment 2 and the two pulses in segment 3 together with the previously actuated thermometer code pulse (i.e. 13-pulse (first)). To reach luminance level # 75, the 13-pulse (second) in segment 2 is to be operated with all previously activated thermometer code pulses in that segment and with all pulses in segment 3 except the 2-pulse. do. Above luminance level # 75, the 13-pulse (second) in segment 2 remains activated. Thus, the 13-pulse (second) in segment 2 is activated upon reaching an associated pixel luminance boundary and constitutes a second thermometer code pulse in this segment that remains activated to increase to pixel luminance above that pixel luminance boundary. .

As can be appreciated at present, a pulse of each large-duration period (eg 13-pulses) in each segment, or a combination of pulses collectively comprising a pulse of large-duration period (eg , 7-pulse and 6-pulse (second) in segment 3, once operated to reach an associated pixel luminance boundary, are in a state of being operated for successively higher pixel luminance levels. Thus, each such pulse (or combination of pulses) constitutes a thermometer code pulse in accordance with the present principles. In practice, each thermometer code pulse has a sufficiently large characteristic (i.e. of sufficiently long duration), so that when operated to reach a pixel luminance boundary, the pulse will limit its number of pixels while limiting the total number of pulses in the segment. It remains activated at the luminance levels above the boundary. That is, at a given pixel luminance boundary, a single thermometer code pulse (or a combination of such pulses including a thermometer code pulse) is activated and remains activated as the pixel luminance increases above that boundary. Conversely, to reduce the pixel luminance to a given pixel luminance boundary, only a single thermometer pulse is deactivated and the thermometer code pulses that have not yet been operated remain active until the next successive lower pixel luminance boundary is reached. However, each thermometer code pulse in each segment must not be too large so that, when activated and deactivated, there is a noticeable transition in incremental change in pixel brightness (i.e. increase in pixel brightness to the next higher level or next lower level). Reduction of pixel brightness to level).

Moreover, the selection of thermometer code pulses must be limited to substantially "swapping" (ie, selection of activated pulses) of pulses into a single pulse width segment to reach a specific luminance state. However, there is no need to limit the swapping of pulses to a substantially single segment (ie, modified binary pulse arrangement) in order to obtain the advantages of the present invention. Swapping of pulses that reach a particular luminance level can occur between several segments, as long as a single thermometer code pulse (or combination of pulses) in a single segment is enabled or disabled between consecutive pixel luminance boundaries.

The foregoing describes a technique for minimizing motion contours in pulse width modulated displays.

Claims (20)

A method of operating a pulse width modulated display system having a plurality of pixels, wherein illumination of each said pixel for a given color is controlled in response to pulses within each segment of a sequence of pulse width segments, each pulse in each segment Wherein the state of determines whether the pixel is illuminated for a period associated with the pulse: Within a first range of pixel luminance levels between first and second pixel luminance boundaries, a pulse element of a first versus sustained period is operated to reach the second pixel luminance boundary, and second and third pixel luminances Operating selected pulses to increase pixel brightness such that within the second range of pixel brightness levels between boundaries, the pulse element of the first large-lasting period remains in operation; And upon reaching the luminance boundary, the pulse elements of the second large-lasting period are operated while the pulse elements of the first large-lasting period are in an activated state. The method of claim 1, Wherein at least one of the first and second large-duration pulse elements comprises a single pulse. The method of claim 1, And wherein at least one of the first and second large-duration pulse elements comprises a combination of pulses. The method of claim 1, The period of the pulse element of each large-duration period is chosen to minimize the transient associated with the incremental increase in pixel luminance to reach the pixel luminance boundary and minimize the number of pulses in each segment. How a width modulated display system works. The method of claim 1, And at least one pair of said large-duration pulse elements are in the same segment. The method of claim 1, And at least one pair of said large-duration pulse elements are in different segments. The method of claim 1, And wherein said pulses operated to increase pixel brightness are confined to substantially the same segment. A method of operating a pulse width modulated display system having a plurality of pixels, wherein illumination of each said pixel for a given color is controlled in response to pulses within each segment of a sequence of pulse width segments, each pulse in each segment Wherein the state of determines whether the pixel is illuminated for a period associated with the pulse: Within a given pixel luminance boundary, the pulsed element of the first large-duration period that is activated is not currently active, and at successively lower pixel luminance boundaries, the pulse element of the first large-duration period remains inoperative, The pulse element of the second large-duration period is not activated and includes reducing the pixel brightness by not operating the selected pulses, wherein each previously activated large-duration pulse element that is not yet activated is A method of operating a pulse width modulated display system that remains operational. The method of claim 8, Wherein at least one of the first and second large-duration pulse elements comprises a single pulse. The method of claim 8, And wherein at least one of the first and second large-duration pulse elements comprises a combination of pulses. The method of claim 8, The duration of the pulse element of each large-duration period is selected to minimize the transient associated with unit reduction in pixel luminance to reach the pixel luminance boundary and minimize the number of pulses in each segment. How modulation system works. The method of claim 8, Wherein the pulse elements of the large-duration period are in the same segment. The method of claim 8, Wherein the pulse elements of the large-duration period are in different segments. The method of claim 8, And wherein said inoperative pulses are confined to substantially the same segment. In a pulse width modulated display system, Light source, A projection lens for focusing incident light on the screen; A digital micromirror device having a plurality of individual micromirrors arranged in an array, each micromirror surrounding an arc in response to receiving a drive signal applied to a driver cell associated with the micromirror. The digital micromirror device that illuminates a picture element (pixel) by pivoting to reflect light from the light source to the projection lens and to the screen; Means for continuously imparting each of the three primary colors to the digital micromirror device for reflection into the projection lens; Within a first range of luminance levels between first and second pixel luminance boundaries, a pulse element of a first large-lasting period is operated to reach the second pixel luminance boundary, and second and third pixel luminances A processor that forms a sequence of pulse width segments to increase pixel brightness by operating selected pulses within a second range of pixel brightness levels between boundaries, the pulse element of the first large-period period to be in operation; And wherein upon reaching the third pixel luminance boundary, the pulse element of the second large-duration period is operated while the pulse element of the first large-duration period is in an activated state; And driver circuitry responsive to the sequences of pulse width segments formed by the processor for driving the digital micromirror device to illuminate the corresponding pixel. The method of claim 15, And at least one of the first and second large-duration pulse elements comprises a single pulse. The method of claim 15, And at least one of the pulse elements of the first and second large-duration periods comprises a combination of pulses. In a pulse width modulated display system, Light source, A projection lens for focusing incident light on the screen; A digital micromirror device having a plurality of individual micromirrors arranged in an array, each micromirror pivoting around an arc in response to receiving a drive signal applied to a driver cell associated with the micromirror and projecting the lens from the light source. The digital micromirror device, which illuminates a picture element (pixel) by reflecting light into and to the screen; Means for continuously imparting each of the three primary colors to the digital micromirror device for reflection into the projection lens; At the first pixel luminance boundary, the pulse element of the first large-duration period that is now activated is deactivated, and the second consecutive lower pixel luminance boundary, the pulse element of the first large-duration period is not activated. In a state in which it has been activated and other previously operated large-duration pulse elements that have not yet been activated, the second large-duration pulse element is deactivated by deactivating the pulses selected to be inoperative. A processor for forming sequences of pulse width segments to reduce pixel brightness; And driver circuitry responsive to said sequence of pulse width segments formed by said processor that drives said digital micromirror device to illuminate said corresponding pixel. The method of claim 18, And at least one of the first and second large-duration pulse elements comprises a single pulse. The method of claim 18, And at least one of the pulse elements of the first and second large-to-last periods comprises a combination of pulses.