Abstract:
The disclosed embodiments relate to a system and method for optical calibration of a picture modulator. More specifically, there is provided a video unit ( 10 ) comprising a modulator ( 18 ) configured to modulate a projection lens assembly ( 16 ) between a first position and a second position, a photodiode assembly ( 22 ) configured to produce a first voltage corresponding to a first pixel pattern generated when the projection lens assembly ( 16 ) is in the first position and to produce a second voltage corresponding to a second pixel pattern when the projection lens assembly ( 16 ) is the second position, and a video control system ( 26 ) configured to adjust the location of the second position based on the first and second voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to provisional U.S. Application No. 60/613,068, filed on Sep. 24, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to projecting video images onto a screen. More specifically, the present invention relates to a system for optically calibrating a pixel-shift modulator in a video display unit.  
       BACKGROUND OF THE INVENTION  
       [0003]     This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.  
         [0004]     Projection-based video units create video images by varying the color and shade of projected light. One example of a projection-based video unit is a digital light processing (“DLP”) system, which employs an optical semiconductor, known as a digital micromirror device (“DMD”) to create video images. Another example of a projection-based video unit is a liquid crystal display (“LCD”) projection system, which projects light through one or more LCD panels to create video images. Many first generation DLP and LCD systems employed a 1:1 correspondence between the resolution of the imaging system and the display resolution. However, it can be expensive to produce DMDs and LCD panels that maintain this 1:1 correspondence while providing higher resolution programming, such high definition television (“HDTV”). For this reason, several techniques have been developed to facilitate the display of video images at resolutions above those natively available from a DMD or LCD panel.  
         [0005]     Pixel-shifting is one such resolution-enhancing technique. In pixel-shifting, the light generated by a video imaging system within a video unit, such as a DMD or and LCD, is shifted to focus on more than one pixel locations on a screen. For example, in a DLP system, the light reflected off of one of the micromirrors may be directed at a first pixel location, then at a second pixel location, then back to the first pixel location, and so forth to increase the resolution of the DLP system beyond what is available natively from the DMD. Typically, pixel-shifting is performed by a mechanically modulated projection lens or mirror that can shift between two or more different positions. For example, in a DLP-based system, the projection lens assembly may first direct light from one of the micromirrors on the DMD to the display screen at a first pixel location. After the first pixel has been displayed for a given period of time, the projection lens assembly may be actuated to shine light from the same DMD micromirror at a second pixel location. The projection lens assembly alternates rapidly between the two positions to display each respective pixel. The result is a first and second pixel displayed in separate positions on the display screen.  
         [0006]     As will be appreciated, one of the challenges in designing pixel-shifting systems is calibrating the mechanical modulator such that the shifted pixels are displayed in the proper location. Conventional calibration systems either employed an open-loop system wherein the user adjusted the pixel-shifting using a test pattern or employed a closed-loop system that measured the physical movement of the modulator. Disadvantageously, these conventional systems are either unreliable or relatively expensive.  
         [0007]     Embodiments of the present invention may relate to an improved system and method for calibrating a pixel shift modulator in a video unit.  
       SUMMARY OF THE INVENTION  
       [0008]     Certain aspects commensurate in scope with the disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.  
         [0009]     The disclosed embodiments relate to a system and method for optical calibration of a picture modulator. More specifically, there is provided a video unit comprising a modulator configured to modulate a projection lens between a first position and a second position, a photodiode assembly configured to produce a first voltage corresponding to a first pixel pattern generated when the projection lens is in the first position and to produce a second voltage corresponding to a second pixel pattern when the projection lens is the second position, and a video control system configured to adjust the location of the second position based on the first and second voltages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0011]      FIG. 1  is a block diagram of a video unit configured to calibrate a pixel shift modulator in accordance with embodiments of the present invention;  
         [0012]      FIG. 2  illustrates a modulator calibration assembly in combination with a non-offset pixel pattern in accordance with embodiments of the present invention;  
         [0013]      FIG. 3  illustrates a modulator calibration assembly in combination with an offset pixel pattern in accordance with embodiments of the present invention; and  
         [0014]      FIG. 4  is a flow chart illustrating an exemplary technique for optical calibration of a modulator in accordance with embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.  
         [0016]     Turning initially to  FIG. 1 , a block diagram of a video unit configured to calibrate a pixel-shift modulator in accordance with one embodiment is illustrated and generally designated by a reference numeral  10 . In one embodiment, the video unit  10  may comprise a Digital Light Processing (“DLP”) projection television or projector. In another embodiment, the video unit  10  may comprise a liquid crystal display (“LCD”) projection television or projector. In still other embodiments, the video unit  10  may comprise another suitable form of projection television or display.  
         [0017]     The video unit  10  may include a light engine  12 . The light engine  12  is configured to generate white or colored light that can be employed by an imaging system  14  to create a video image. The light engine  12  may include any suitable form of lamp or bulb capable of projecting white or generally white light. In one embodiment, the light engine  12  may be a high intensity light source, such as a metal halide lamp or a mercury vapor lamp. For example, the light engine  12  may include an ultra high performance (“UHP”) lamp produced by Philips Electronics. The light engine  12  may also include a component configured to convert the projected white light into colored light, such as color wheels, dichroic mirrors, polarizers, and filters. Moreover, in alternate embodiments, the light engine  12  may include components capable of generating color light, such as light emitting diodes.  
         [0018]     As described above, the light engine  12  may be configured to project, shine, or focus colored light at the imaging system  14 . The imaging system  14  may be configured to employ the colored light to create images suitable for display on a screen  24 . As described further below, the imaging system  14  may be configured to generate one or more pixel patterns that can be used to calibrate pixel shifting in the video unit  10 . In one embodiment, the imaging system  14  comprises a DLP imaging system that employs one or more DMDs to generate a video image using the colored light. In another embodiment, the imaging system may employ an LCD projection system. It will be appreciated, however, that the above-described exemplary embodiments are not intended to be exclusive, and that alternate embodiments, any suitable form of imaging system  14  may be employed in the video unit  10 .  
         [0019]     As illustrated in  FIG. 1 , the imaging system  14  may be configured to project images into a projection lens assembly  16 . The projection lens assembly  16  may include one or more lenses and/or mirrors that project the image created by the imaging system  14  onto the screen  24 . In one embodiment, the projection lens assembly  16  includes a folded mirror. The projection lens assembly may also be coupled to a modulator  18  capable of shifting the projection lens assembly  16  about an axis to facilitate pixel-shifting within the video unit  10 . In one embodiment, the modulator  18  may be configured to shift the projection lens assembly  16  between two positions. In alternate embodiments, the modulator  18  may be configured to shift the projection lens assembly between three, four, or more different positions.  
         [0020]     As illustrated, light exiting the projection lens assembly  16  may be directed to either the screen  24  or to a modulator calibration assembly  19 . In one embodiment, the modulator calibration assembly  19  may be located in an overscan region of the video unit  10 . The modulator calibration assembly  19  may include an optical target plate  20  and a photodiode assembly  22 . The optical target plate  20  is configured to filter or block light projected by the projection lens assembly to facilitate calibration of the modulator  18 , as will be described below. In one embodiment, the optical target plate includes a grating comprising a series of transparent and opaque stripes (see  FIGS. 2 and 3 ) which are etched on the optical target plate  20  at half the pixel pitch of the imaging system  14  at a 45 degree angel to the pixel pattern and oriented orthogonal to the major axis of the modulator  18 . It will be appreciated, however, that the exact pattern on the optical target plate  20  may be different in alternate embodiments. Moreover, in some embodiments, the optical target plate may be omitted from the video unit  10  and the modulator  18  may be calibrated using the photodiode assembly  22  without the optical target plate  20 .  
         [0021]     As illustrated in  FIG. 1 , the optical target plate  20  may be oriented between the projection lens assembly  16  and the photodiode assembly  22 . The photodiode assembly  22  may be comprised of a series of photo-transistors or other light sensitive sensors that may be configured to convert light projected from the projection lens assembly  16  into voltages. As described further below, the video unit  10  may use voltages generated by the photodiode assembly  22  to calibrate the modulator  18 .  
         [0022]     The light engine  12 , the imaging system  14 , the modulator  18 , and the photodiode assembly  22  may each be communitively coupled to a video control system  26 , which is configured to control the calibration of the modulator  18 . The video control system  26  may also include one or more processors, associated memory, and/or other suitable control system components. The video control system  26  may also include an on-screen display (“OSD”) pattern generator that is configured to generate one or more video images or pixel patterns that can facilitate calibration of the modulator  18 , as described below. Further, the video control system  26  may also include an analog-to-digital (“A/D”) converter or other component suitable for converting voltages generated by the photodiode assembly  22  into digital signals, which the video control system  26  can use to calibrate the modulator  18 . In one embodiment, the video control system  26  may be configured to execute software or instructions to calibrate the modulator  18 .  
         [0023]     As will be described further below, the video unit  10  may be configured to calibrate the modulator  18  by comparing voltages generated by the photodiode assembly  22  when the modulator  18  is in a non-offset position with voltages generated when the modulator  18  is in an offset position. Accordingly,  FIG. 2  illustrates the modulator calibration assembly  19  in combination with a non-offset pixel pattern from the perspective of the projection lens assembly  18  in accordance with one embodiment. For simplicity, like reference numeral have been used to designate those features previously described in relation to  FIG. 1 .  FIG. 2  illustrates the optical target plate  20  (only the opaque stripes are visible) in front of the photodiode assembly  22 . Projected onto the photodiode assembly  22  are four rows of pixels (the illustrated diamond shaped boxes) labeled as rows  30  and rows  32 . The pixels in the rows  30  (shaded with diagonal lines) are mostly visible through the opaque stripes of the optical target plate  20  when the modulator is in a non-offset position; whereas the pixels in the rows  32  (shaded with dashed horizontal lines) are mostly obscured by the opaque stripes when the modulator is in the non-offset position.  
         [0024]     On the other hand,  FIG. 3  illustrates the modulator calibration assembly  19  in combination with an offset pixel pattern from the perspective of the projection lens assembly  18  in accordance with one embodiment. As with  FIG. 2 , like reference numeral have been used to designate those features previously described in relation to previous figures.  FIG. 3  illustrates the movement of the rows  30  and  32  with an offset of one-half a pixel by the modulator  18 . As illustrated, when the modulator  18  is offset by one-half a pixel, the pixel rows  30  become mostly obscured and the pixel rows  32  become mostly visible through the optical target plate  20 . As described further below, the video unit  10  may employ this difference in visibility through the optical target plate in the offset and non-offset positions of the modulator  18  to calibrate the modulator  18 .  
         [0025]      FIG. 4  is a flow chart illustrating an exemplary technique for optical calibration of the modulator  18  in accordance with one embodiment. In one embodiment, the video control system  26  may perform the technique  40  in conjunction with the light engine  12 , the imaging system  14 , the modulator  18 , and the photodiode assembly  22  to calibrate the modulator to offset pixels by one-half of a pixel width. As indicated by block  42 , the technique  40  may begin with the imaging system  14  directing light at the pixel locations that are mostly visible through the optical target plate  20  when the modulator is in the non-offset position. In one embodiment, directing light at the mostly visible non-offset pixel locations includes generating a pixel pattern where the pixels in the row  30  are illuminated (e.g., white or another color) and the row  32  is not illuminated (i.e., black). Once the pixels in the row  30  have been illuminated, the video control system  26  may measure the voltage generated by the photodiode assembly  22  while the pixels in the row  30  are illuminated.  
         [0026]     Next, the video control system  26  may direct the modulator  18  to move the projection lens assembly  16  to the offset position, as indicated by block  46 . Once the modulator  18  has moved, the video control system  26  may direct the imaging system  14  to illuminate the pixel locations that should be mostly visible when the modulator  18  is in the offset position. In one embodiment, directing light at the pixel locations that should be mostly visible when the modulator is at the offset position includes illuminating the pixels in the row  32  and not illuminating the pixel locations in the row  30 . After illuminating the pixels in the row  32 , the video control system  26  may measure the voltage generated by the photodiode assembly  22 , as indicated in block  50 . If the voltage measured in block  44  and the voltage measured in block  50  match within a predetermined degree of error, the video control system  26  may determine that the offset of the modulator  18  is properly calibrated to shift pixels by one-half of a pixel, as indicated in block  54 .  
         [0027]     If, however, the voltage measured in block  44  and the voltage measured in block  50  do not match within a predetermined degree of error, it may indicate that the modulator  18  is not properly calibrated. As such, the video control system  26  may adjust the offset value of the modulator  18 , as indicated by block  56 . In one embodiment, adjusting the offset value of the modulator  18  may include either increasing or decreasing the movement of the projection lens assembly  16 . After the offset value of the modulator  18  has been adjusted, the technique  40  may cycle back to block  46  to determine whether the adjusted offset value is the correct calibration. The technique  40  continues in this manner until the voltage generated by the photodiodes when the modulator is in the offset position matches the voltage generated by the photodiodes when the modulator is in the non-offset position within a predetermined margin of error. It will be appreciated, however, that it may take multiple adjustments for the modulator  18  to be calibrated.  
         [0028]     While the technique  40  was described above it terms of calibrating the modulator  18  with one offset position of one-half pixel, in alternate embodiments, the video unit  10  may also be configured to calibrate multiple offset positions. For example, once the video control system  26  has determined the correct modulator position for a one-half pixel shift, the video control system  26  may use this position to determine intermediate positions for the modulator  18 . Alternatively, the video control system  26  can be programmed with target photodiode voltages that correspond to various positions of the modulator  18  and calibrate the movement of the modulator  18  to generate the target voltages in the photodiode assembly. Moreover, while the technique  40  is described using the optical target plate  20 , in alternate embodiments, the video control system can be configured to calibrate the modulator  18  based on the pixel patterns alone.  
         [0029]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.