Abstract:
Systems and methods for filtering to comply with copy-protection regulations set forth for HDTV signals by the Motion Picture Association of America (“MPAA”) are presented. A copy-protection filter constrains the resolution of the HDTV signal when copy-protection bits are present in a video signal. The copy-protection filter may be placed in an analog data stream before the video signal is converted from a digital to an analog signal. A second copy-protection filter is optionally placed in a digital data stream. The copy-protection filter may be combined with other filters in a video encoder, or with a scaler before the input video data stream enters a compositor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to filtering of video signals, and more particularly to filtering performed in a set top box or digital television decoder box. 
   2. Related Art 
   The basic concept behind high-definition television (“HDTV”) is to increase the visual field contained by the image, that is, the resolution. An average cable television signal is transmitted at a resolution of approximately 640×480. In this lexicon, 640 corresponds to the number of active lines per picture, and 480 corresponds to the number of pixels per line. HDTV can include resolutions up to 1920×1080. 
   In response to this increase in available resolution, a set of standards was released by the Motion Picture Association of America (“MPAA”) to regulate proprietary HDTV transmissions. These regulations were meant to protect proprietary transmissions from being copied by users. The protected transmissions are marked with copy-protection bits that may be recognized by a receiver. What are needed, therefore, are methods and systems for implementing the MPAA standards. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to systems and methods for reducing a resolution of a digital video signal to comply with the MPAA copy-protection standards. The system comprises a compositor for mixing a video data stream and a graphics data stream to produce a video signal, a video encoder coupled to the compositor, and a copy-protection filter to constrain the resolution of the video signal when copy-protection bits are present in the video signal. 
   In an embodiment, the copy-protection filter is a low-pass horizontal filter. In another embodiment, the copy-protection filter is a low-pass vertical filter. In yet another embodiment, a combination horizontal and vertical filter are used. For each filter, any number of taps may be used. 
   The copy-protection filter may be placed in an analog data path in the video encoder before the signal is converted to an analog signal. In another embodiment, a digital data path is coupled to the analog data path. The copy-protection filter may be placed on the analog data path after the connection between the analog and digital data paths. In yet another embodiment, a second copy-protection filter may be placed in the digital data path. The second copy-protection filter may be operated independently of the first copy-protection filter. 
   In a further embodiment, the copy-protection filter may be combined with other filters in the system. In still another embodiment, the copy-protection filter may be combined with a video scaler before the initial video data stream enters the compositor. 
   Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
       FIG. 1  is a block diagram representing a portion of a video system. 
       FIG. 2  is a flowchart of an example method of constraining a resolution of a digital video signal. 
       FIG. 3  illustrates tapping of a video scan line using a horizontal filter. 
       FIG. 4  illustrates tapping of a video scan line using a vertical filter. 
       FIG. 5  is a frequency domain illustration of the present invention. 
       FIG. 6  is a block diagram of an embodiment of the present invention implementing a copy-protection filter. 
       FIG. 7  is another block diagram of an embodiment of the present invention, in which a copy-protection filter is combined with other system filters. 
       FIG. 8  is another block diagram of an embodiment of the present invention, in which a copy-protection filter is combined with a scaler in the input video signal stream. 
       FIG. 9  is a flowchart of an example method of constraining a resolution of a digital video signal. 
       FIG. 10  is a flowchart of another example method of constraining a resolution of a digital video signal. 
   

   The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
   DETAILED DESCRIPTION OF THE INVENTION 
   While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications. 
   An HDTV signal may have one of two different resolutions. These resolutions, also referred to as frame formats, are 1920×1080 and 1280×720. The purpose of the present invention is to filter a video signal down to the visual equivalent of 520,000 pixels per frame, or a frame format of 960×540. This means that the 1920×1080 signal is to be filtered to ¼ resolution, and the 1280×720 is to be filtered to ½ resolution. However, it is desired only to filter those signals that contain copy-protection bits in the video signal. 
     FIG. 1  is a block diagram of a system  100  included in, for example, a set-top box or a TV-on-a-chip system. System  100  includes a compositor  102  and a video encoder (“VEC”)  104 . Compositor  102  accepts an input video signal  106  and an input graphics signal  108 . Compositor outputs composite video signal  110 . 
   VEC  104  includes a sync timing circuit  112 , a bandpass filter  114 , and a digital-to-analog converter (“DAC”)  116 , along a first data path  118 . In one embodiment, VEC  104  also includes a digital visual interface (“DVI”) transmitter  120  on a second data path  122 . 
   Sync timing circuit  112  inserts a sync timing signal into the composite video signal  110 . The sync timing signal may include both horizontal and vertical sync, and is used to determine the correct position of an image produced from the composite video signal  110  on a display device. 
   Bandpass filter  114  limits the bandwidth of the luminance and/or chrominance of composite video signal  110 . The bandwidth is limited to comply with additional standards for analog HDTV required by the Society of Motion Pictures and Television Engineers (SMPTE). This requirement limits the bandwidth of the chrominance signals so that a television does not ring when a sharp chrominance edge is input. This requirement is unrelated to the copy-protection requirements promulgated by the MPAA. 
   In one embodiment, bandpass filter  114  is controlled in such a way that it does not limit the bandwidth when a sync timing signal is inserted into video signal  110 . The sync timing signal inserted into the video signal  110  should not be filtered. Otherwise, the timing signal would line up the pixels incorrectly. For this reason, in an embodiment, when processing a part of the signal with an inserted sync timing signal, the bandpass filter  114  is disabled to allow the full bandwidth to pass through. Then, during the active video portion of the video signal, where no timing signals have been inserted, the bandpass filter  114  is enabled to provide the final resolution. 
   After the bandpass filter  114 , composite video signal  110  proceeds along first data path  118  to DAC  116 . In an embodiment, DAC  116  is a triple DAC, including three individual 10-bit DACs. DAC  116  converts the composite video signal  110  to analog format to create an analog video signal  124 . Analog video signal  124  is output through analog output  126 . 
   DVI transmitter  120 , when implemented, transmits video signal  110  in a digital format through digital output  128 . 
   In order for the video system to produce MPAA-compliant signals, a copy-protection filter is inserted into one or more of the video signal data paths  106 ,  110 ,  118 , and  122 . In one embodiment, the copy-protection filter is responsive to copy-protection bits inserted into the input video signal by the distributor. In another embodiment, a control system responsive to copy-protection bits turns the filter on and off as needed. 
   A variety of one or more types of low-pass filters may be used in the copy-protection filter. For example, a finite impulse response (“FIR”) filter may be implemented. Alternatively, or additionally, an infinite impulse response (“IIR”) filter is used. One of skill in the art will recognize that other alternative filters may also be used. 
     FIG. 2  is a flowchart of an example method  200  of constraining a resolution of a digital video signal. In step  202 , a raster-formatted video signal is received by a low-pass filter having some cutoff frequency. The low-pass filter may be any of the filters discussed above. In the example of FIG.  1 , the video signal is one or more of video signals  106 ,  110 ,  118 , and  122 . Method  200  then proceeds to step  204 . 
   In step  204 , a set of pixels is filtered from the raster-formatted video signal. In one embodiment, the low-pass filter is a horizontal filter, meaning that it extracts points horizontally across a scan line via a series of taps. In another embodiment, the low-pass filter is a vertical filter, meaning that it extracts points from successive scan lines instead of the same scan line. In yet another embodiment, a combination vertical and horizontal filter may be used. In each embodiment, a tap from the filter extracts one pixel from the video signal. In one embodiment, the filter includes a sync function. 
     FIG. 3  illustrates how a video scan line is tapped using a horizontal filter. In an embodiment, 11 taps are used. One of skill in the art will recognize that any number of taps may be used depending on the level of signal quality desired. In this embodiment, during step  204 , an 11-tap horizontal filter extracts a set of pixels  302 , here including 11 adjacent pixels, from a scan line  304  of a video signal. 
     FIG. 4  illustrates how a scan line is tapped using a vertical filter. In one embodiment, a 4-tap vertical filter is used, although one of skill in the art will recognize that any number of taps may be implemented. In this embodiment, during step  204 , the filter extracts a set of pixels  402 , here including 4 pixels, from a set of successive scan lines  404 . Each pixel in the set of pixels  402  occupies the same relative position on its respective scan line with respect to all other pixels in the set of pixels  402 . This conformity gives the filter its vertical nature. 
   Returning to  FIG. 2 , method  200  then proceeds to step  206 . In step  206 , a filtered video signal is output. 
     FIG. 5  shows the effect that a filter of the present invention has on an example received signal  502 . When signal  502  is a 1920×1080 signal, with a sample rate F s  equal to 74.25 MHz, and a passband F s /2 equal to 37 MHz, signal  502  is filtered down to approximately F s /8. This is approximately ¼ of the passband signal (shaded area  504 ). When the received signal  502  is a 1280×720 signal, signal  502  is filtered down to approximately F s /4. This is approximately ½ of the passband signal. 
     FIG. 6  is a block diagram of the system  100 , including a copy-protection filter  602 . Copy protection filter  602  is of the type(s) described in the discussion of  FIGS. 2-5 , with a cut-off frequency of, for example, F s /8 or F s /4. Copy-protection filter  602  can be placed anywhere along data path  118  before DAC  116 . In one embodiment, copy-protection filter  602  is placed in data path  118  between sync timing circuit  112  and bandpass filter  114 . 
   In the example of  FIG. 6 , copy-protection filter  602  is placed after a connection  604 . In this manner, video signals sent to an analog output would be filtered, while video signals sent to a digital output would retain their high resolution characteristics. 
   Alternatively, a second copy-protection filter  606  is optionally placed in second data path  122  between connection  604  and DVI transmitter  120 . A benefit of this embodiment is that the signal in the analog path may be filtered to limit resolution independently of the signal on the digital path. For example, the analog output signal may be filtered, while the digital signal retains its resolution. Alternatively, the digital signal may be filtered, while the resolution of the analog signal remains unfiltered. Each filter may also be active at the same time as the other. 
     FIG. 7  is a block diagram of yet another embodiment of the present invention. In the example of  FIG. 7 , copy-protection filter  602  is combined with bandpass filter  114 . This combination is referred to as combination filter  702 . By implementing both functions in combination filter  702 , both cost and required hardware space are decreased. 
     FIG. 8  is a block diagram of a further embodiment of the present invention. In the example of  FIG. 8 , system  100  includes a scaler  802  for scaling a video source into a standard video signal. In this embodiment, copy-protection filter  602  is implemented within scaler  802 . This allows for input video signal  106  to be filtered without affecting input graphics signal  108 . Since copy-protection filter  602  is implemented before connection  604 , both the digital and analog outputs are constrained. 
     FIG. 9  is a flowchart of a method  900  for implementing the present invention. Method  900  begins with step  902 , in which a video signal is input into, for example, copy-protection filter  602 . Method  900  then proceeds to step  904 . 
   In step  904 , a decision is made as to whether copy-protection bits are present in the video signal. If copy-protection bits are present, method  900  proceeds to step  906 . If copy-protection bits are not present, method  900  advances to step  908 . 
   In step  906 , the video signal is constrained to a fraction of its initial resolution. Step  906  may be performed, for example, by the method discussed above with respect to  FIG. 2 . As discussed above with respect to  FIG. 3 , the fraction of resolution required depends on the frame format of the video signal. 
   In step  908 , the luminance and chrominance of the signal are limited by, for example, bandpass filter  114 . Method  900  proceeds to step  910 . 
   In step  910 , the video signal is converted to an analog video signal in, for example, DAC  116 . The analog video signal is then output in step  912 . 
   Other steps may be added to method  900  in accordance with the present invention. For example,  FIG. 10  is a flowchart of alternate method  1000 . Method  1000  begins with step  1002 , in which a video signal is input into, for example, system  100  as illustrated in  FIG. 6 . Method  1000  then proceeds to step  1004 . 
   In step  1004 , the video signal is split into a digital path signal and an analog path signal. The analog path signal follows the same steps as discussed with respect to  FIG. 9 , namely, steps  904 - 912 . The digital path signal follows steps  1004 - 1010 . 
   In step  1004 , a decision is made as to whether copy-protection bits are present in the digital video signal. If copy-protection bits are present, method  1000  proceeds to step  1008 . If copy-protection bits are not present, method  100  advances to step  1010 . 
   In step  1008 , the digital video signal is constrained to a fraction of its resolution. The constraining may be performed, for example, by the method discussed above with respect to  FIG. 2 . 
   In step  1010 , the constrained digital video signal is output. It will be apparent to one of skill in the art that if no copy-protection filter is present in the digital path, or if it is otherwise desired that the digital video signal remain unfiltered, steps  1006  and  1008  may be eliminated. 
   CONCLUSION 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.