Patent Document

FIELD OF THE INVENTION 
   The present invention relates generally to projecting video images onto a screen. More specifically, the present invention relates to a system for reducing dithering noise in a video projection unit, and a method for manufacturing a video unit that exhibits reduced dithering noise when projecting video. 
   BACKGROUND OF THE INVENTION 
   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. 
   Digital Light Processing (“DLP”) is a display technology that employs an optical semiconductor, known as a Digital Micromirror Device (“DMD”) to project video onto a screen. DMDs typically contain an array of at least one million or more microscopic mirrors mounted on microscopic hinges. Each of these mirrors is associated with a point on the screen, known as a pixel. By varying the amount of light that is reflected off each of these mirrors, it is possible to project video onto the screen. Specifically, by electrically actuating each of these hinge-mounted microscopic mirrors, it is possible to either illuminate a point on the screen (i.e., “turn on” a particular micromirror) or to leave that particular point dark by reflecting the light somewhere else besides the screen (i.e., “turn off” the micromirror). Further, by varying the amount of time a particular micromirror is turned on, it is possible to create a variety of gray shades. For example, if a micromirror is turned on for longer than it is turned off, the pixel that is associated with that particular micromirror, will have a light gray color; whereas if a particular micromirror is turned off more frequently than it is turned on, that particular pixel will have a darker gray color. In this manner, video can be created by turning each micromirror on or off several thousand times per second. Moreover, by shining colored light at the micromirrors instead of white light, it is possible to generate millions of shades or color instead of shades of gray. 
   As stated above, the shading of a particular pixel may be partially determined by the length of time that the micromirror corresponding to that pixel is either turned on or turned off. However, as described above, the micromirrors are hinge mounted. As such, there are electrical and mechanical constraints inherent to turning on or turning off a particular micromirror. For example, in current generation DMDs, the minimum amount of time that one of the micromirrors can be turned on is approximately fifteen microseconds. This minimum amount of time is known as the least significant bit (“LSB”). While succeeding generations of DMD technology may reduce this minimum time threshold, it cannot be eliminated, and there are a variety of shades of color that correspond to turning the micromirror on for somewhere between zero seconds and this minimum time threshold (currently fifteen microseconds). These particular shades present special challenges for DLP-based video units. 
   Dithering is one method to overcome the above described challenges. Dithering involves fooling the eye of the viewer by rapidly turning a micromirror on and then off, such that the mind of viewer blurs the turned on shade and the turned off shade (i.e., black) together to form a shade that corresponds to turning on one of the micromirrors for less than fifteen microseconds. While dithering does increase the number of shades that the DLP system can display, it may cause additional noise, known as either dithering noise or error diffusion noise. This dithering noise reduces the quality of the television picture and in some cases may even be overtly visible to a viewer. Reducing this dithering noise is desirable. 
   SUMMARY OF THE INVENTION 
   The disclosed embodiments relate to a system and method for projecting video onto a screen. A video unit ( 10 ) may comprise a color wheel ( 14 ), a light source ( 12 ) configured to project light through the color wheel ( 14 ) during a spoke time, and a digital micromirror device ( 18 ) configured to reflect the projected light ( 30 ) onto a screen ( 26 ). The method may comprise the acts of shining a generally white light ( 28 ) through a region on a color wheel ( 14 ) to create a colored light ( 30 ), the region comprising two color filters, reflecting the colored light ( 30 ) off of a micromirror disposed on a digital micromirror device ( 18 ), and projecting the reflected colored light ( 34 ) onto a screen ( 26 ). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a block diagram of a video unit configured to reduce dithering noise in accordance with embodiments of the present invention; and 
       FIG. 2  is a diagram of a color wheel configured to reduce dithering noise in accordance with embodiments of the present invention; and 
   

   DETAILED DESCRIPTION 
   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. 
   Turning initially to  FIG. 1 , a block diagram of a video unit configured to reduce dithering noise in accordance with embodiments of the present invention 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. In another embodiment, the video unit  10  may comprise a DLP-based video or movie projector. 
   The video unit  10  may comprise a light source  12 . The light source  12  may comprise any suitable form of lamp or bulb capable of projecting white or generally white light  28 . Lamps or bulbs of this type are well known in the art and need not be described in further detail. In one embodiment, the light source  12  is configured to project, shine, or focus the white light  28  into one static location as described further below. 
   As illustrated in  FIG. 1 , the exemplary video unit  10  also comprises a color wheel  14  aligned in an optical line of sight with the light source  12 .  FIG. 2  is a diagram of the color wheel  14  configured to reduce dithering noise in accordance with embodiments of the present invention. The color wheel  14  may comprise a variety of color filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  arrayed as arcuate regions on the color wheel  14 . Specifically, in the illustrated embodiment, the color wheel  14  comprises color filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  configured to convert the white light  28  into one of the three primary colors of light: red, green, or blue. In particular, the illustrated embodiment of the color wheel  14  comprises two red color filters  40   a  and  40   b , two green color filters  42   a  and  42   b , and two blue color filters  44   a  and  44   b . The skilled in the art will appreciate that in alternate embodiments, the specific colors of the filters  40   a ,  40   a ,  42   a ,  42   b ,  44   a , and  44   b  may be altered or the number of filters may be altered. For example, in one alternate embodiment, the color wheel  14  may comprise only one red color filter  40   a , one green color filter  42   b , and one blue color filter  44   a . In this embodiment, the arcuate regions occupied by the color filters  44   a ,  44   b , and  44   c  may be approximately twice as long (as measured along the circumference of the color wheel  14 ) than the color filters  40   a ,  42   b , and  44   a  depicted in  FIG. 2 . In still other embodiments, the color filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  may occupy either more or less of the surface area of the color wheel depending on the configuration and function of the video unit  10 . 
   In addition, the color wheel  14  may comprise boundaries between each of the filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b . These boundaries are known as spokes  46   a ,  46   b ,  48   a ,  48   b ,  50   a , and  50   b  due to their resemblance to the spokes of wheel. For example,  FIG. 2  illustrates three types of spokes: the yellow (i.e., red-green) spokes  46   a  and  46   b , the cyan (i.e., green-blue) spokes  48   a  and  48   b , and the magenta (i.e., blue-red) spokes  50   a  and  50   b.    
   Turning next to the operation of the color filter  14 , each of the filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  is designed to convert the white light  28  generated by the light source  12  into colored light  30 . In particular, the color wheel  14  may be configured to rapidly spin in a counterclockwise direction  51  around its center point  52 . In one embodiment, the color wheel  14  rotates 60 times per second. As described above, the light source  12  may be configured to focus the white light  28  at the color wheel  14 . On the opposite side of the color wheel from the light source  12 , there may be an integrator  15 , which is also referred to as a light tunnel. In one embodiment, the integrator  15  is configured to the evenly spread the colored light  30  across the surface of a Digital Micromirror Device (“DMD”)  18 . As such, those skilled in the art will appreciate that most, and possibly all, of the light that will be reflected off the DMD  18  to create video will pass through the integrator  15 . 
   Because the integrator  15  is fixed and the color wheel  14  rotates, the light that will enter the integrator  15  can be illustrated as a fixed area  54  that rotates around the color wheel  14  in the opposite direction from the color wheel&#39;s direction of rotation. For example, as the color wheel  14  rotates in the counterclockwise direction  51 , the fixed area  54  rotates through each the filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  in the clockwise direction  53 . As such, those skilled in the art will recognize that the colored light  30  entering the integrator  15  will rapidly change from red to green to blue to red to green to blue with each rotation of the color wheel  14  as the fixed area  54  passes through each of the color filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b . In other words, because the light source  12  is stationary, the counterclockwise rotation of the color wheel  14  causes the fixed area  54  to rotate in a clockwise direction  53  through the colors of the color wheel. In alternate embodiments, the color wheel  14  itself may rotate in the clockwise direction  53 . Those skilled in the area will appreciate that the size and shape of the fixed area  54  is merely illustrative. In alternate embodiments, the size and shape of the fixed area  54  may be different depending on the optical design of the system. 
   However, as the fixed area  54  passes though each of the spokes  46   a ,  46   b ,  48   a ,  48   b ,  50   a , and  50   b , the color of the colored light  30  entering the integrator  15  will not be consistent. In particular, as the fixed area  54  crosses the edge of one particular spoke  46   a ,  46   b ,  48   a ,  48   b ,  50   a , and  50   b , the colored light  30  entering the integrator  15  will comprise two different colors of light. These times (when two different colors of light are entering the integrator  15 ) are referred to as spoke times. In further example, the percentage of red light will decrease and the percentage of green light will increase as the fixed area  54  moves across the spoke  46   a  into the green filter  42   a  until the colored light  30  entering the integrator  15  consists entirely of green light (i.e., the fixed area  54  crosses completely out of the red filter  40   a  wholly into the green filter  42   a ). The color of the colored light  30  will then remain a consistent green color until the fixed area  54  crosses the spoke  48   a.    
   Because the color of the colored light  30  entering the integrator  15  is not consistent during the spoke times, conventional DLP processors are configured to turn-off all of the micromirrors on the DMD  18  during the spoke times. As will be described further below, embodiments of the present invention comprise turning on one of more of the micromirrors of the DMD  18  during one of the spoke times to reduce dithering noise. 
   Returning now to  FIG. 1 , the video unit  10  may also comprise a digital light processing (“DLP”) circuit board arrayed within an optical line of sight of the integrator. The DLP circuit board may comprise the DMD  18  and a processor  20 . As described above, the DMD  18  may comprise up to one million or more micromirrors mounted on microscopic, electrically-actuated hinges that enable the micromirrors to tilt between a turned on position and turned off position. In the illustrated embodiment, the DMD  18  is also coupled to the processor  20 . In one embodiment, the processor  20  may receive a video input and, as described in greater detail below, direct the micromirrors on the DMD  18  to turn on or off, as appropriate to create the video image. In alternate embodiments the processor  20  may be located elsewhere in the video unit  10 . 
   The colored light  30  that reflects off a turned on micromirror (identified by a reference numeral  34 ) is reflected to a projecting lens  24  and then projected on to a screen  26  for viewing. On the other hand, the colored light that reflects off of a turned off micromirror (identified by a reference numeral  32 ) is directed somewhere else in the video besides the screen  26 , such as a light absorber  22 . In this way, the pixel on the screen  26  that corresponds to a turned off micromirror does not receive the projected colored light  30  while the micromirror is turned off. 
   As described above, the colored light  30  rapidly changes from red to green to blue and then back to red many times per second. When the DMD  18  receives this stream of rapidly changing colored light  30 , the micromirrors on the DMD  18  are directed rapidly turn on or off to create the video images. In one embodiment, this direction is provided by the processor  20 . This rapid turning on and off of the micromirrors is coordinated to match the sequence of colors in the colored light  30 . For example, when the colored light  30  is red, the micromirrors turn on or off as appropriate to generate the shades of red for a particular frame of video. For example, one micromirror may turn on for 25 microseconds to contribute one shade of red to its associated pixel while another one of the micromirrors may turn on for 30 microseconds to contribute another shade of red to a different pixel while still another micromirror may turn off completely for an entire rotation of color wheel  14 , if no red light is to projected to a particular one of the pixels during that frame. In a similar fashion, the micromirrors generate shades of green and blue, if needed, for when the colored light  30  is green or blue, respectively. 
   Because these different colors of light are rapidly changing (e.g. 60 times per second), the viewer sees a cohesive image formed from the three colors of light on the screen  26 . For example, to create a particular shade for a particular pixel, the micromirror corresponding to that particular pixel may turn on for 20 microseconds of red light, 22 microseconds of green light, and 17 microseconds of blue light. Alternately, the micromirror may turn on for 20 microseconds of red light and 20 microseconds of blue light, but remain turned off for green light. Those skilled in the art will appreciate that millions of color combinations can be projected by varying the lengths of time that the micromirrors are turned on. 
   However, as mentioned above, if the particular shade of color corresponds to turning on a micromirror for less than 15 microseconds (i.e., the shade is darker than 15 microseconds worth of light), that particular shade of color cannot be projected in the manner described above. One technique to produce these darker shades, however, is by dithering. 
   In dithering, the particular micromirror that corresponds to the darker shaded pixel may be turned on for only a portion of the time that the colored light  30  is a particular color. For example, if the video unit  10  wants to project a pixel with a shade that corresponds to 8 microseconds of red light, the micromirror corresponding to that particular pixel may be configured to turn on for 16 microseconds of red light during a video frame and to turn off for the following video frame. As such, the 16 microsecond shade of red and the absence of red (from the turned off period of time) may average out and appear to the viewer as a single shade of red that corresponding to 8 microseconds of red light. While this technique for simulating a particularly shade is effective, turning pixels on and off both temporally and spatially in order to obtain a lower average brightness can generate dither noise. As mentioned above, this dithering noise can degrade the quality of the picture. 
   One technique to reduce this dithering noise is to use the colored light  30  produced during one of the spoke times to create the dithered color. Those skilled in the art will appreciate that higher luminance light creates more dithering noise than lower luminance light. As such, in one embodiment, the colored light  30  entering the integrator  15  during a particular spoke time is used in place of the colored light  30  projected solely through one of the color filters  40   a ,  40   b ,  42   a ,  42   b ,  44   a , and  44   b  to dither if the luminance of the color being transitioned to during the spoke time is lower than the luminance of the color being transitioned from. In this way, by mixing a percentage of lower luminance light into the colored light  30  used for dithering, it is possible to reduce the dithering noise because the average luminance of the colored light used for dithering is less than the luminance for color of light being transitioned from. In another embodiment, the colored light  30  generated during a spoke time is only used to dither shades of the green light because green light has a luminance greater than both red light and blue light. In particular, if a shade of green light is desired that corresponds to turning on a micromirror for less than fifteen microseconds, the colored light  30  produced when the fixed area  54  (see  FIG. 2 ) is passing through either the spoke  48   a  or  48   b  is used for dithering. In one embodiment, the colored light  30  used for dithering is cyan light comprising approximately 70% green light and 30% blue light. 
   Returning again to  FIG. 1 , the video unit  10  may also comprise a projection lens  24  to project the light reflected from the DMD  18  onto the screen  26 . In one embodiment, the projecting lens  24  facilitates the projection of turned-on light  34  by expanding the turned-on light  34  to cover the relatively large area of the screen  26 . In an alternate embodiment, the screen  26  may not be a part of the video unit  10 . For example, the screen  26  may be mounted on a wall and the video unit  10  may comprise a projector configured to project video across a room to the screen  26 . 
   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.

Technology Category: h