Abstract:
A pulse width modulation driven display such as a spatial light modulator, which updates pixel data between PWM periods of consecutive frames, to avoid tearing artifacts in the perceived display image.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to spatial light modulators, and more specifically to a spatial light modulator having a mechanism for performing data refresh without causing tearing artifacts in the display.  
           [0003]    2. Background Art  
           [0004]    [0004]FIG. 1 illustrates a conventional display system  10  having a spatial light modulator (SLM)  12  which modulates a light beam from a light source  14  such as an ultra-high-pressure (UHP) lamp according to pixel value data from a pixel source  16  such as a graphics engine, for display on a display device  18  such as a rear projection television screen. The SLM has a frame buffer memory  20  which stores the pixel values and provides them to a pixel array  22  of independently-controllable pixel cells  24  under control of control logic  26 . The pixel cells typically operate like liquid crystal display cells, in that they can be controlled to transmit or not transmit light, depending on a control voltage or current applied to them. In some SLMs, a pulse width modulation (PWM) scheme is used, giving a pixel any of a number of digital values. The longer the PWM signal is on, the longer the pixel cell is transmissive, making the pixel brighter; the longer the PWM signal is off, the longer the pixel cell is opaque, making the pixel darker.  
           [0005]    [0005]FIG. 2 illustrates a typical PWM transfer curve of a pixel cell, and represents perceived brightness (vertical axis) over time (horizontal axis). Time can be expressed in terms of percentages of the duty cycle of the PWM signal during respective frames of display. Two frames (frame0 and frame1) are illustrated. For a first period of time, from the start until “A0”, having the signal on does not give any perceivable brightness to the pixel. Then, from “A0” to “B0”, the longer the signal is held on, the brighter the pixel is perceived to be. Then, after “B0” until the end of the frame, continuing to hold the PWM signal on does not yield any perceived additional brightness beyond that achieved at “B0”. Then, the same function is repeated for the next frame.  
           [0006]    Commonly, the perceived brightness is not a purely linear function between “A0” and “B0”, and techniques such as lookup tables are used to compensate for the particular transfer curve of the pixel technology at hand. The desired brightness (typically expressed as a digital value from 0 to 255) is used as an index into the lookup table, which outputs a value indicating how long the PWM signal should be held on to achieve the specified brightness.  
           [0007]    FIGS.  3 A-C illustrate tearing in the display. Each image may be interpreted to represent either the display, the display image itself, the pixel array which produces that display image, or the contents of a frame buffer memory which drives the pixel array. FIG. 3A shows a display image showing the inventor&#39;s initials “SH” and a triangle. This may be a static image which has been displayed for any period of time. Then, the pixel engine causes the image to shift one or more pixels to the right, such as if the image were to begin scrolling to the right. If the SLM is not properly controlled, tearing may occur, as shown in FIG. 3B. The new pixel values for the upper portion of the image have been updated (causing that portion of the image to shift to the right), but the new pixel values for the lower portion of the image have not been updated before the image has been committed or refreshed to the display. FIG. 3C represents the desired outcome, in which the entire image has been shifted to the right and all pixels have received new values before the image is committed to the display. Ideally, the display pixel values will be completely updated to the pixel array before the pixel array is committed to the display, so the user does not see tearing. In some embodiments, this may mean that the pixel values should be completely updated to a back buffer before the back buffer is committed to the front buffer, or that the pixel values should be completely updated to a first ping-pong buffer before ping-ponging the display to the first buffer from a second buffer that held the prior frame. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.  
         [0009]    [0009]FIG. 1 shows a conventional SLM display system according to the prior art.  
         [0010]    [0010]FIG. 2 shows an exemplary PWM transfer curve of an SLM pixel array cell.  
         [0011]    FIGS.  3 A-C show tearing in a raster display.  
         [0012]    [0012]FIG. 4 shows two consecutive frames of operation of the invention.  
         [0013]    [0013]FIG. 5 shows a display system according to the invention.  
         [0014]    [0014]FIG. 6 shows a method of operation of the invention.  
         [0015]    [0015]FIG. 7 shows another method of operation of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]    While the invention will be described with reference to an exemplary embodiment of an SLM display system, the invention is not necessarily limited to the embodiment shown, nor even to SLM systems. The invention may find applicability in a wide variety of display or other applications.  
         [0017]    [0017]FIG. 4 illustrates two consecutive frames (“frame 0” and “frame 1”) of display time. Each includes an initial “on” period during which the PWM signal should be held on, a “PWM” period during which the pixel&#39;s appearance is determined by the PWM signal, and an “off” period during which the PWM signal is held off. Most suitably, the “A” point, the end of the time in which the PWM signal is forced to be “on”, will coincide generally with the earliest time at which the PWM signal begins to cause a perceived change in the brightness of the pixel, while the “B” point, the latest point at which the PWM signal is allowed to be on, will coincide generally with the earliest time at which the PWM signal stops causing perceived change in the brightness of the pixel.  
         [0018]    In the first frame, the PWM period is from a first time “A0” at the start of the PWM period to a second time “B0” at the end of the PWM period. In the second frame, the PWM period is from a first time “A1” at the start of the PWM period to a second time “B1” at the end of the PWM period.  
         [0019]    This invention makes use of the period from “B 0 ” to “A 1 ” to update the frame buffer memory contents. This invention makes use of the fact that the PWM signal has a deterministic waveform from “B” to “A”, which is not dependent upon the pixel data value.  
         [0020]    [0020]FIG. 5 illustrates an SLM display system  40  according to one embodiment of this invention. The SLM  42  receives pixel values from a pixel source  16 , modulates light from a light source  14 , and provides the modulated light to a display  18 . The SLM includes a pixel display array  22 , which is controlled by control logic  44  and whose values come from a frame buffer  20 . The control logic includes a frame detector  46  which detects the boundary between consecutive frames. The control logic also includes logic  48  for detecting the start of a PWM period, a pulse width modulator  50 , and logic  52  for detecting the end of the PWM period. The control logic further includes an update controller  54  which governs the transfer of pixel values from the pixel source into the frame buffer. In some embodiments, various of these logic systems may be simple comparators which compare respective predetermined values against a counter  56  value which increments from 0 to N during each frame and which is reset by the frame detector at the start of each new frame. Other embodiments are, of course, within the ability of the skilled reader of this disclosure. The pulse width modulator may, in some embodiments, include a lookup table (not shown) for implementing a non-linear transfer curve function for the PWM signal.  
         [0021]    [0021]FIG. 6 illustrates one embodiment  60  of a method of operation of the SLM display system, which will be explained with reference also to FIGS. 4 and 5. After the end-of-PWM detector  52  detects ( 61 ) the end of the current frame&#39;s PWM period (“B0” in FIG. 4), the update controller  54  enables the frame buffer to begin ( 62 ) updating its pixel value contents from the pixel source  16 . The frame buffer completes ( 63 ) updating the pixel value contents before the start-of-PWM detector  48  detects ( 64 ) the start of the next frame&#39;s PWM period (“A1” in FIG. 4). At the start of the next frame&#39;s PWM period, the pulse width modulator  50  begins ( 65 ) performing the PWM signal&#39;s transfer curve function, and the pixel cell is illuminated accordingly. The PWM signal function completes ( 66 ) at or before the end of the PWM period ( 61 ).  
         [0022]    [0022]FIG. 7 illustrates another embodiment  70  of a method of operation of the SLM display system, and will be explained with reference also to FIGS. 4 and 5. The frame detector  46  detects ( 71 ) the start of a new frame period, and the counter  56  is reset ( 72 ). The pulse width modulator  50  forces ( 73 ) the PWM signal on. When the start-of-PWM detector  48  detects that the counter has reached ( 74 ) a value corresponding to the “A” start of the PWM period, the pixel data update will have ended ( 75 ), and the pulse width modulator  50  begins executing the non-deterministic portion of the PWM signal waveform. When the counter reaches ( 76 ) a value corresponding to the desired brightness of the pixel, the pulse width modulator turns the PWM signal off, and the final brightness of the pixel has been reached. When the end-of-PWM detector  52  detects that the counter has reached ( 77 ) a value corresponding to the “B” end of the PWM period, the PWM signal is forced ( 78 ) off (although it should already have been turned off at the end of its PWM period, per its brightness value), and the update controller  54  enables the frame buffer to read ( 79 ) in the new pixel data values from the pixel source, which will be completed before the counter reaches ( 74 ) the “A” value at the start of the next frame&#39;s PWM period, but not necessarily before the start of the next frame. The frame buffer can, in some embodiments, take advantage of the entire deterministic period from “B0” to “A1” in updating its pixel data value contents.  
         [0023]    The reader will appreciate that these are but exemplary methods, and that other methods will be appreciated in light of this disclosure and are within its scope.  
         [0024]    The reader should further appreciate that, although the invention has been described in terms of a spatial light modulator, the principles of this invention can readily be applied in other pulse width modulation applications, such as liquid crystal displays, flat panel plasma displays, and so forth.  
         [0025]    The reader should appreciate that drawings showing methods, and the written descriptions thereof, should also be understood to illustrate machine-accessible media having recorded, encoded, or otherwise embodied therein instructions, functions, routines, control codes, firmware, software, or the like, which, when accessed, read, executed, loaded into, or otherwise utilized by a machine, will cause the machine to perform the illustrated methods. Such media may include, by way of illustration only and not limitation: magnetic, optical, magneto-optical, or other storage mechanisms, fixed or removable discs, drives, tapes, semiconductor memories, organic memories, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW, Zip, floppy, cassette, reel-to-reel, or the like. They may alternatively include down-the-wire, broadcast, or other delivery mechanisms such as Internet, local area network, wide area network, wireless, cellular, cable, laser, satellite, microwave, or other suitable carrier means, over which the instructions etc. may be delivered in the form of packets, serial data, parallel data, or other suitable format. The machine may include, by way of illustration only and not limitation: microprocessor, embedded controller, PLA, PAL, FPGA, ASIC, computer, smart card, networking equipment, or any other machine, apparatus, system, or the like which is adapted to perform functionality defined by such instructions or the like. Such drawings, written descriptions, and corresponding claims may variously be understood as representing the instructions etc. taken alone, the instructions etc. as organized in their particular packet/serial/parallel/etc. form, and/or the instructions etc. together with their storage or carrier media. The reader will further appreciate that such instructions etc. may be recorded or carried in compressed, encrypted, or otherwise encoded format without departing from the scope of this patent, even if the instructions etc. must be decrypted, decompressed, compiled, interpreted, or otherwise manipulated prior to their execution or other utilization by the machine.  
         [0026]    Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments.  
         [0027]    If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.  
         [0028]    Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.