Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to display systems, and more particularly to projection display systems. 
   2. Description of the Background Art 
   A two-dimensional projection image may be formed by using one or more linear arrays of light-modulating pixels. The light-modulating pixels may comprise, for example, GRATING LIGHT VALVE (GLV) pixels. Publications describing GLV devices and their application to display systems include: “Grating Light Valve Technology: Update and Novel Applications,” by D. T. Amm and R. W. Corrigan of Silicon Light Machines in Sunnyvale, Calif., a paper presented at the Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; “Optical Performance of the Grating Light Valve Technology,” David T. Amm and Robert W. Corrigan of Silicon Light Machines, a paper presented at Photonics West-Electronics Imaging, 1999; “An Alternative Architecture for High Performance Display,” R. W. Corrigan, B. R. Lang, D. A. LeHoty, and P. A. Alioshin of Silicon Light Machines, a paper presented at the 141st SMPTE Technical Conference and Exhibition, Nov. 20, 1999, New York, N.Y.; and U.S. Pat. No. 6,215,579, entitled “Method and Apparatus for Modulating an Incident Light Beam for Forming a Two-Dimensional Image,” and assigned at issuance to Silicon Light Machines. The above-mentioned publications are hereby incorporated by reference in their entirety. 
   In such display systems, the linear array modulates an incident light beam to display pixels along a column (or, alternatively, a row) of the two-dimensional (2D) image. A scanning system is used to move the column across the screen such that each light-modulating pixel is able to generate a row of the 2D image. In this way, the entire 2D image is displayed. 
   There are challenges, however, in implementing a scanning system that efficiently renders a high quality video image. For example, one measure of the efficiency of the scanning system is its duty cycle. The duty cycle indicates the fraction of time during which the image is being actively rendered on-screen and the fraction of time during which no image is being actively rendered. The higher the duty cycle is; the higher the efficiency is. Hence, achieving a high duty cycle is one challenge. 
   As another example, the quality of the video image depends on a number of factors. One factor is the refresh rate. The refresh rate is the rate at which an image is displayed upon the screen. Low refresh rates result in a video image that appears to “flicker” to a viewer. Sufficiently high refresh rates are desirable to reduce or eliminate flicker in the video image, and achieving a high refresh rate is another challenge. 
   SUMMARY 
   The above-described challenges may be overcome by the present invention. One embodiment of the invention relates to a method for bi-directional progressive scanning in a display system. The method includes receiving image data for an image to be displayed, forward scanning the image data in a first direction using a linear array of controllable light elements, and reverse scanning the image data in a second direction opposite to the first direction using the linear array. Another embodiment of the invention relates to an apparatus for bi-directional progressive scanning. The apparatus includes a linear array of controllable light elements, and a scanner driver that drives a scanner apparatus using a drive signal that is applied to drive both forward and reverse optical scanning of an image by the linear array. 
   These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a timing diagram depicting a sawtooth drive signal and corresponding scan mirror response for progressive scanning. 
       FIGS. 2A and 2B  are illustrations depicting an optical scan and a dark retrace scan in progressive scanning. 
       FIG. 3  is a flow chart depicting a conventional method of progressive scanning. 
       FIG. 4  is a timing diagram depicting a triangular drive signal and corresponding scan mirror response in accordance with an embodiment of the present invention. 
       FIGS. 5A and 5B  are illustrations depicting a forward optical scan and a reverse optical scan in accordance with an embodiment of the present invention. 
       FIG. 6  is a flow chart depicting a method for bi-directional progressive scanning in accordance with an embodiment of the present invention. 
       FIG. 7  is a graph depicting scan mirror position as a function of time near a “tip” of the triangular scan in accordance with an embodiment of the present invention. At the tip, the direction of motion of the scan mirror changes. 
       FIG. 8  is a block diagram depicting an apparatus for bi-directional progressive scanning in accordance with an embodiment of the present invention. 
     The use of the same reference label in different drawings indicates the same or like components. Drawings are not to scale unless otherwise noted. 
   

   DETAILED DESCRIPTION 
   Various types of scanners may be used to move the column (or row) of light across the screen. For example, galvonometer-based scanners, resonant scanners, polygon scanners, rotating prisms, or other types of scanners may be used. 
   A drive signal is applied to the scanner to control (“drive”) the movement of the column (or row) of light. For example, to achieve a progressive scan of the column across the screen (e.g., from left to right), a sawtooth drive signal may be used. 
     FIG. 1  is a timing diagram depicting a sawtooth drive signal and corresponding scan mirror response for progressive scanning. As shown in the bottom portion of FIG.  1 , a sawtooth drive signal (also called a sawtooth scan signal) may be generated and used to drive the scan mirror. During each cycle of the sawtooth signal, a first segment goes from a first voltage (for example, zero volts) to a second voltage (for example, one volt), and a second segment goes from the second voltage back to the first voltage. As depicted, the first segment is typically longer and less steep in slope than the second segment. 
   The scan mirror response to the sawtooth scan signal is shown in the top portion of  FIG. 1 . The scan mirror response generally follows the sawtooth scan signal and so the scan mirror response also has a first (less steep) segment and a second (more steep) segment in each cycle. However, because of physical limitations of the scan mirror mechanisms, the slope of the scan mirror response cannot change as quickly as the slope of the scan signal can change. Hence, the tips of the scan mirror response are rounded in comparison to the tips of the sawtoooth scan signal. 
   The first (less steep) segment of the scan mirror response provides a usable display time during which the column of light moves across the screen (for example, from left to right) to paint the 2D image. Meanwhile, the second (more steep) segment of the scan mirror response is used to move (for example, from right to left) the scan mirror&#39;s position back to the starting point of the first segment. During this second segment, the column of light is turned off so that the retracement to the starting point is not visible on the screen. 
     FIGS. 2A and 2B  are illustrations depicting an optical scan and a dark retrace scan in progressive scanning. The example of  FIG. 2A  corresponds to the first (usable) segment of the scan signal. This first segment may be referred to as the optical scan segment as during this segment the 2D image is formed by sweeping the column of pixels across the screen.  FIG. 2A  indicates a position of the column of pixels provided by the linear GLV (or other similar) array as the column sweeps across the 2D image, for example, from left to right. 
   The example of  FIG. 2B  corresponds to the second (unusable) segment of the scan signal. This second segment may be referred to as the retrace scan segment as during this segment the column of pixels is turned off (made dark) as the column returns to the starting position of the optical scan.  FIG. 2B  indicates a position of the column of pixels provided by the linear GLV (or other similar) array as the now dark column returns across the 2D image, for example, from right to left. 
     FIG. 3  is a flow chart depicting a conventional method of progressive scanning. The method  300  as depicted includes five steps ( 302 ,  304 ,  306 ,  308 , and  310 ). 
   In the first step  302 , the first segment of the scan signal is provided. This step  302  corresponds to providing the less steep (longer) segment of the sawtooth drive signal as illustrated in  FIG. 1 . 
   In the second step  304 , the column (or row) of pixels is optically scanned to display the 2D image in response to the first segment of the scan signal. This step  304  corresponds to the optical scan illustrated in  FIG. 2A . 
   In the third step  306 , the second segment of the scan signal is provided. This step  306  corresponds to providing the more steep (shorter) segment of the sawtooth drive signal as illustrated in  FIG. 1 . In the fourth step  308 , the column (or row) of pixels is retraced (without illumination) back to the starting position of the optical scan in response to the second segment of the scan signal. This step  308  corresponds to the retrace scan illustrated in  FIG. 2B . 
   Finally, in the fifth step  310 , the method  300  proceeds to a next image. The next image may comprise, for example, a next frame of a video sequence. Alternatively, the next image may be a refresh of the same frame of the video sequence. Subsequently, the method  300  begins again starting with the first step  302 . 
   The efficiency or duty cycle of the progressive scanning depicted in  FIGS. 1–3  is given by the usable display time divided by the cycle time. The shorter the unusable retrace time in comparison to the usable display time, the greater the duty cycle will be. A typical duty cycle for such a system may be about 75%. Even with a very expensive scanner system, a duty cycle of 90% may be difficult to achieve. This is because physical limitations of the scanner systems (for example, maximum speed and minimum turnaround time limitations) make further reduction of unusable time difficult to achieve. Note that the unusable time in progressive scanning is not only due to the retracing per se, but also due to the unusable time as the scanner slows and changes direction between the optical scan and the retracing. 
     FIG. 4  is a timing diagram depicting a triangular drive signal and corresponding scan mirror response in accordance with an embodiment of the present invention. As shown in the bottom portion of  FIG. 4 , a triangular drive signal (also called a triangular scan signal) may be generated and used to drive the scan mirror. During each cycle of the triangular signal, a first segment goes from a first voltage (for example, zero volts) to a second voltage (for example, one volt), and a second segment goes from the second voltage back to the first voltage. As depicted, the first segment is typically the same length and the same steepness as the second segment. 
   The scan mirror response to the triangular scan signal is shown in the top portion of  FIG. 4 . The scan mirror response generally follows the triangular scan signal and so the scan mirror response also has a triangular shape. However, because of physical limitations of the scan mirror mechanisms, the slope of the scan mirror response cannot change as quickly as the slope of the scan signal can change. Hence, the tips of the scan mirror response are rounded in comparison to the tips of the triangular scan signal. 
   The first segment of the scan mirror response provides a first usable display time during which the column of light moves across the screen in a “forward” direction (for example, from left to right). Meanwhile, the second segment of the scan mirror response provides a second usable display time during which the column of light moves across the screen in a “reverse” direction (for example, from right to left). In between the first and second segments (and between the second and first segments), an unusable turn-around time exists. The unusable turnaround time is due to the physical limitations of the scanner system. 
     FIGS. 5A and 5B  are illustrations depicting a forward optical scan and a reverse optical scan in accordance with an embodiment of the present invention. The example of  FIG. 5A  corresponds to the forward optical scan.  FIG. 5A  indicates a position of the column of pixels provided by the linear GLV (or other similar) array as the column sweeps in a forward direction across the 2D image, for example, from left to right. This forward optical scan corresponds to the first usable display time. 
   The example of  FIG. 5B  corresponds to the reverse optical scan.  FIG. 5B  indicates a position of the column of pixels provided by the linear GLV (or other similar) array as the column sweeps in a reverse direction across the 2D image, for example, from right to left. This reverse optical scan corresponds to the second usable display time. 
     FIG. 6  is a flow chart depicting a method for bi-directional progressive scanning in accordance with an embodiment of the present invention. The method  600  as depicted includes five steps ( 602 ,  604 ,  606 ,  608 ,  610 ,  612 ,  614 , and  616 ). 
   In the first step  602 , the first segment of the scan signal is provided. This step  602  corresponds to providing one segment (for example, the positively-sloped segment) of the triangular drive signal as illustrated in  FIG. 4 . 
   In the second step  604 , the column (or row) of pixels is optically scanned in the forward direction to display the 2D image in response to the first segment of the scan signal. This step  604  corresponds to the forward optical scan illustrated in  FIG. 5A . 
   In the third step  606 , turnaround occurs such that the motion of the scanner slows, stops, and then reverses direction. This step  606  corresponds to the unusable turnaround time after the first segment (and before the second segment) as illustrated in  FIG. 4 . 
   In the fourth step  608 , the method  600  proceeds to a next image. The next image may comprise, for example, a next frame of a video sequence. Alternatively, the next image may be a refresh of the same frame of the video sequence. 
   In the fifth step  610 , the second segment of the scan signal is provided. This step  610  corresponds to providing the other segment (for example, the negatively-sloped segment) of the triangular drive signal as illustrated in  FIG. 4 . 
   In the sixth step  612 , the column (or row) of pixels is optically scanned in the reverse direction to display the 2D image in response to the second segment of the scan signal. This step  612  corresponds to the reverse optical scan illustrated in  FIG. 5B . 
   In the seventh step  614 , turnaround again occurs such that the motion of the scanner slows, stops, and then reverses direction. This step  614  corresponds to the unusable turnaround time after the second segment (and before the first segment) as illustrated in  FIG. 4 . 
   Lastly, in the eighth step  616 , the method  600  proceeds to a next image. Again, the next image may comprise, for example, a next frame of a video sequence. Alternatively, the next image may be a refresh of the same frame of the video sequence. Subsequently, the method  600  begins again starting with the first step  602 . 
   The efficiency or duty cycle of the bi-directional progressive scanning depicted in  FIGS. 4–6  is given by the usable display time divided by the cycle time. The shorter the turnaround time in comparison to the usable display time, the greater the duty cycle will be. Because the method  600  of  FIG. 6  accomplishes bi-directional progressive scanning, the need for retracing is avoided. This results in higher achievable efficiencies. Duty cycles of greater than 95% may be achievable with bi-directional progressive scanning in accordance with the present invention. 
   Note that while the drive (scan) signal is depicted as a sharp triangle in  FIG. 4 , in other embodiments, the drive signal may not be as sharp. Hence, an approximate triangular scan signal may also be used. If such an approximate triangular scan signal does not have a constant slope in the (usable) optical scanning portions, then the non-constant slope may be compensated for by adjusting the brightness of the illumination. The lower the slope, the longer time a column is displayed, so the less bright the illumination needed. Conversely, the steeper the slope, the shorter time a column is displayed, so the more bright the illumination needed. 
     FIG. 7  is a graph depicting scan mirror position (y-axis) as a function of time (x-axis) near a “tip” of the triangular scan in accordance with an embodiment of the present invention. At the tip, the direction of motion of the scanner changes. In order to change direction, the scanner needs to decelerate (slow down) until it stops for an instant, then accelerate (speed up) in the new direction. 
   In one embodiment of the invention, when the scanner starts to slow substantially, then the usable optical scanning ends and the unusable turnaround time begins. In an alternate embodiment of the invention, the usable optical scanning time may be extended into the period where the scanner slows substantially. In order to do this, the increasing slowness of the scanning must be compensated for because the longer the scanner remains at a particular position, the brighter that column will appear. One way to compensate for the increasing slowness of the scanning would be to proportionally decrease the brightness of the incident light illuminating the linear array. 
     FIG. 8  is a block diagram depicting an apparatus for bi-directional progressive scanning in accordance with an embodiment of the present invention. As depicted in  FIG. 8 , the apparatus  800  includes two data buffers  802  and  804 , a multiplexor  806 , array drivers  808 , a linear array of light-modulating pixels  810 , a scanner  812 , and a scanner driver  814 . 
   The two data buffers  802  and  804  receive image data. In one embodiment of the invention, the first data buffer  802  receives image data that corresponds to a first image. The second data buffer  804  receives image data that corresponds to a second image. Image data for the third image is received by the first data buffer  802 . Image data for the fourth image is received by the second data buffer  804 . And so on, such that the first and second data buffers receive image data for alternate images to be presented. 
   The multiplexor  806  selects either the image data from the first data buffer  802  or the image data from the second data buffer  804 . The multiplexor  806  transfers the selected data to array drivers  808 . Array drivers  808  drive the linear array of light-modulating pixels  810  using the selected data to drive the linear array. In accordance with one embodiment, while the array drivers  808  use the image data from the first data buffer  802 , the second data buffer  804  may be filling with the next image data. And, while the array drivers  808  use the next image data from the second data buffer  804 , the first data buffer  802  may be filling with subsequent image data. 
   The linear array  810  transmits the modulated light to the scanner  812 . The scanner  812  moves the column of modulated light across the screen in accordance with the scan signal provided by the scan driver  814 . 
   As described above, the scan signal may comprise a triangular scan signal that is utilized to perform bi-directional progressive scanning. In that case, the scanner driver  814  provides a control signal to the array drivers  808  such that the array drivers  808  provide the image data in a “forward order” to the linear array during forward optical scans and provide the image data in a “reverse order” to the linear array during reverse optical scans. 
   The scanner driver  814  may also provide a control signal to the multiplexor  806 . This control signal controls the multiplexor  806  and so controls how many times an image is refreshed before moving onto the next image. For example, if each image is refreshed three times before a next image is displayed, then the multiplexor  806  would be controlled to switch between the first  802  and second  804  data buffers only once every one-and-a-half cycles of the triangular scan signal. As another example, if each image is refreshed four times before a next image is displayed, then the multiplexor  806  would be controlled to switch between the first  802  and second  804  data buffers only once every two cycles of the triangular scan signal. As yet another example, if each image is refreshed five times before a next image is displayed, then the multiplexor  806  would be controlled to switch between the first  802  and second  804  data buffers only once every two-and-a-half cycles of the triangular scan signal. As yet another example, if each image is refreshed six times before a next image is displayed, then the multiplexor  806  would be controlled to switch between the first  802  and second  804  data buffers only once every three cycles of the triangular scan signal. 
   Now let us discuss image “flicker” and its impact on desirable refresh rates for bi-directional progressive scanning. It turns out that bi-directional progressive scanning using a system in accordance with the present invention may use a “bi-directional” screen refresh rate of about 120 hertz or more (refreshing about every 8.3 milliseconds or less) to make flicker unnoticeable to the typical viewer. These bi-directional screen refresh rates are about double what they would need to be for a uni-directional progressive scanning system. This is because for uni-directional scanning each scan starts from the same side of the screen (for example, the left side). This means that it only takes one scan cycle in time before any particular column is “re-painted.” However, for bi-directional scanning, a scan starts from the side at which the previous scan ended. For example, if the prior scan ends at the right side, the following scan begins at the right side. This means that it may take up to almost two scan cycles in time before a particular column (for example, the left-most column) is “re-painted.” 
   In one specific embodiment of the invention, the images received correspond to source material from film that is produced at about twenty-four hertz rate (24 images per second or one image about every 42 milliseconds). In this case, if the images were “bi-directionally” refreshed only once per image, then the viewer likely notice a significant amount of flicker. This is because the bi-directional screen refresh rate would be 24 hertz, and the effective uni-directional screen refresh rate would be merely 12 hertz. In order to minimize flicker for a typical viewer, such 24-image-per-second video should be bi-directionally refreshed at least five times per image. This corresponds to a bi-directional screen refresh rate of about 120 hertz (60 hertz triangular wave into the scan mirror) and an effective uni-directional screen refresh rate of about 60 hertz. Alternatively, the 24 image-per-second video should be bi-directionally refreshed at least six times per image. This corresponds to a bi-directional screen refresh rate of about 144 hertz (72 hertz triangular wave into the scan mirror) and an effective uni-directional screen refresh rate of about 72 hertz. In other implementations, higher bi-directional refresh rates (168 hertz, 192 hertz, 216 hertz, etc.) may be used to further reduce flicker. 
   In another specific embodiment of the invention, the images received correspond to source material from a television signal that is produced at about thirty hertz (30 images per second or one image about every 33 milliseconds). Again, if the images were “bi-directionally” refreshed only once per image, then the viewer would likely notice a significant amount of flicker. This is because the bi-directional screen refresh rate would be 30 hertz, and the effective uni-directional screen refresh rate would be merely 15 hertz. In order to minimize flicker for a typical viewer, such 30-image-per-second video should be bi-directionally refreshed at least four times per image. This corresponds to a bi-directional screen refresh rate of about 120 hertz (60 hertz triangular wave into the scan mirror) and an effective uni-directional screen refresh rate of about 60 hertz. In other implementations, higher bi-directional refresh rates (150 hertz, 180 hertz, 210 hertz, etc.) may be used to further reduce flicker. 
   The multiple refreshes of a single image may further be used to increase the displayable information per pixel. This may be accomplished by dithering of the image data between different refreshes of the image. For example, if four refreshes are used per image, then the displayable grayscale resolution per pixel may be increased by two bits, say from 8-bit resolution to 10-bit resolution. Note that the dithering will introduce a lower frequency to the screen refresh rate, but it turns out that noticeable image flicker will generally not be introduced by such dithering. This is because the intensity changes between dithered images are very small (less than 1% in 8-to-10 bit dithering), so detectable flicker is generally not introduced. 
   In the present disclosure, numerous specific details are provided such as examples of apparatus, process parameters, materials, process steps, and structures to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
   While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure. Thus, the present invention is limited only by the following claims.

Technology Category: g