Abstract:
The present invention provides an apparatus and method for an adaptive Y/C separator that selects between input filters responsive to features of video signals. The adaptive Y/C separator can monitor the results of numerous filters and choose a combination of separators dependent upon signal properties. The adaptive Y/C separator can even blend results of the Y/C separators to improve signal quality.

Description:
This application claims priority from U.S. Provisional Application No. 60/472,280 filed May 20, 2003, which we incorporate by reference. 
   The present invention relates generally to a filter for a composite video signal, and particularly to a method and filter for separating luminance and chrominance signals in a composite video signal. 

   BACKGROUND OF THE INVENTION 
   Two predominant television standards for video signals are PAL (Phase Alternation by Line) and NTSC (National Television Systems Committee). Encoded according to one of these standards, color television broadcasts are typically transmitted as composite video signals that include a brightness signal (luminance, luma or Y) and a color signal (chrominance, chroma or C). 
   To produce the composite video signal, a modulated color signal (color sub-carrier) is added to the luminance signal prior to transmission. The chrominance signal occupies the same frequency spectrum as the high frequency luma signals, so color television receivers include an Y/C separator circuit for separating the composite video signal into its luminance and chrominance components. However, Y/C separators often permit crosstalk from the luminance into the chrominance (cross color) and of chrominance into luminance (cross luminance). Y/C crosstalk generally degrades the quality of displayed video pictures. 
   Many techniques for separating composite video signals into luminance and chrominance components exist. For instance, a band-pass filter used in conjunction with a notch or comb filter can separate the chrominance component from a composite signal. However, in addition to passing the chrominance signal, a notch/band-pass filter will pass high frequency luminance energy in the chrominance pass band. Therefore, some luminance appears as cross-color in a displayed picture. For instance, a vertical striped pattern on a displayed article of clothing can have a spatial frequency creating a high frequency luminance component, which appears as cross-color in a separated chrominance signal. This cross-color usually appears in a video picture as a colored rainbow superimposed on the reduced-amplitude striped pattern. 
   Although the notch/band-pass filter combination has drawbacks, it is useful under certain conditions. For example, the filter combination is effective in picture regions lacking high luminance frequencies in the horizontal direction while containing high frequency chroma information in the vertical direction. In these regions, the comb filter described below degrades pictures with cross luma. 
   Comb filtering is another technique for Y/C separation. Comb filtering usually provides a considerably better component separation compared to the above-described notch/band-pass filter combination. Although conventional comb filters provide improved separation, crosstalk between the chrominance and luminance still occurs. Comb filters are therefore most effective in pictures having a flat field of color or high luminance frequencies in the horizontal direction. However, when spatial discontinuities in the vertical direction occur in a video picture, a conventional comb filter may inadequately separate chrominance and luminance components, causing undesirable artifacts in the displayed picture. 
   Accordingly, there exists a need for an improved filter for separating the chrominance and luminance components of a composite video signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features, and advantages of the present invention will be readily understood with reference to the following detailed description in conjunction with the accompanying drawings, where reference numerals correspond to structural elements. 
       FIG. 1  is a diagram representing outputs of different filters for a vertical transition. 
       FIG. 2  is a diagram representing outputs of different filters for a horizontal transition. 
       FIG. 3  is block diagram of an Y/C separator adaptive to characteristics of a video signal. 
       FIG. 4  is a block diagram representing a notch filter to average one input with the input delayed by ½ a sub carrier. 
       FIG. 5  is block diagram representing the band-pass filter match and comb filter match blocks of the Y/C separator in  FIG. 3 . 
       FIG. 6  is block diagram representing circuitry for summing two 2 line comb filters with the sum of a band-pass and comb filter output. 
       FIG. 7  is a flow diagram of an Y/C separator adaptive to characteristics of a video signal. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For convenience, like numerals in the description refer to like structures in the drawings. The invention described herein provides a system and method to adaptively separate Y and C signals, advantageously matching the strength of various Y/C separations with the corresponding signal characteristics. 
   Adaptive Y/C separation improves video signal quality. As discussed above, prior art notch/band-pass filters are each inadequate under certain video signal characteristics. Changes in video signals therefore occasionally present separators with the very conditions each separator is ill equipped for, causing undesirable artifacts in the displayed picture. Therefore video signal quality will improve by using an adaptive Y/C separator to adapt Y/C separation to signal features. For example, in picture regions lacking high luminance frequencies in the horizontal direction while containing high frequency chroma information in the vertical direction, adaptive Y/C separation may advantageously use a notch/band-pass filter combination, while in regions with a flat field of color adaptive Y/C separation may use better-suited comb filters. 
   In general, any feature of a video or other signal may be detected and used in an adaptive filter to determine a favorable filter configuration. For example, a plurality of Y/C separators may be coupled with a filter to select a subset of the separators in response to a video signal. The filter may then select separators to match filter advantages with features in the video signal. Furthermore, adaptive Y/C separation may combine the results of different separators or filters in order to improve displayed pictures. 
   Referring now to  FIG. 3 , numeral  300  illustrates a block diagram of an adaptive Y/C separator in accordance with an embodiment of the present invention. Other configurations may benefit from the present invention. 
   The adaptive Y/C separator  300  includes a plurality of filters including a band-pass filter  301 , a top two-line filter  302   a , a bottom two-line filter  302   b , a narrow band comb filter  303  and a wide band comb filter  304  that filter a plurality of input signals, as we explain in more detail below. The filters separate the Y and C components from the input video signals and thus, are also termed Y/C separators. The filters may filter any number of lines of input video signals. The adaptive Y/C separator  300  further includes a diagonal edge detector  305  to detect a diagonal edge between composite video signal and a signal delayed two horizontal lines (2H). 
   The Y/C separator  300  includes a band-pass filter leakage block  306 , a narrowband comb filter leakage block  309 , and a wide band comb filter leakage block  310 . The leakage blocks  306 ,  309  and  310  calculate any energy outside of the sub-carrier frequency band. Put differently, blocks  306 ,  309  and  310  detect the presence of horizontal transitions (leakage). The band pass filter leakage block  306  filters the output of the band pass filter  301 . The narrow band filter leakage block  309  filters the output of the narrow band comb filter  303 . The wide band filter leakage block  310  filters the output of the wide band comb filter  304 . A minimum comb filter leakage block  311  picks a minimum amount of leakage between the outputs of the narrow band comb filter leakage  309  and the wide band comb filter leakage  310 . The output of the minimum comb filter leakage  311  reflects a relative mixture of the two comb filter inputs. To improve results, the minimum comb filter leakage block  311  adds a fraction of wideband comb filter leakage (error). The added fraction of wideband comb filter leakage helps because horizontal luma edges typically occur at the same location as horizontal chroma edges (for which the notch/band pass filter is the best match) and the horizontal luma edge causes a large amount of leakage in the wideband comb filter  304 . Biasing the error upwards helps to bias the band-pass vs. comb filter decision towards a band-pass filter and preserve the horizontal edge. 
   The adaptive Y/C separator  300  additionally includes a band-pass filter match block  307  that compares the outputs of the band pass filter  301 , top two line filter  302   a , and bottom two line filter  302   b . A comb filter match block  308  compares the outputs of the top two line filter  302   a , bottom two line filter  302   b , and narrow band comb filter  303 . 
   A comb selector  312  produces a chroma output (subsequently provided to the chroma selector  313 ) based on outputs from comb filters  303  and  304  responsive to the outputs of the summation block  320   c  and the minimum comb filter leakage  311 . A chroma selector  313  produces the Chroma signal responsive to the output of comb select block  312 , the output of the band-pass filter  301 , and the band pass and comb selection error signals, the comb selection error signal being summed at summation block  320   d  with the output to of the comb filter match block  308  to produce comb filter error signal. And summation block  320   e  generates the Luma signal by summing the Chroma signal to the 1H signal. Additionally, the chroma selector  313  may provide a signal for downstream processing that indicates the filters selected for chroma signal generation. 
   In the present embodiment, the filters  301 ,  302   a ,  302   b ,  303 , and  304  receive a signal delayed one horizontal line (1H), two horizontal lines (2H), and composite inputs. A one-line delay can either be equal to the average line length or a multiple of the color sub carrier period. The latter delay will give better Y/C separation, but the choice is typically driven by the decoder&#39;s overall design so that it is a constant number of samples. In an embodiment, a delay line or any other circuitry capable of delaying the video signal, e.g., a logic circuit, may generate the 1H and 2H signals. More specifically, the band-pass filter  301  receives the 1H signal as an input. The band-pass filter  301  outputs the signal to band-pass filter leakage  306 , band pass filter match  307 , and chroma select  313 . The top two-line filter  302   a  receives as inputs 1H signal and 2H signal. The filtered outputs of top two-line filter  302   a  are input to band-pass filter match  307  and comb filter match  308 . The bottom two-line filter  302   b  receives as inputs the 1H and composite signals. The filtered outputs of bottom two-line filter  302   b  are input to band-pass filter match  307  and comb filter match  308 . 
   Furthermore, the narrow band comb filter  303  receives composite, 1H and 2H signals as inputs. The filtered outputs of narrow band comb filter  303  are input to comb filter match  308 , narrow band comb filter leakage  309 , and comb select  312 . The next Y/C separator, wide band comb filter  304 , receives composite, 1H and 2H signals as inputs. Wide band comb filter  304  then filters the signals for output to wide band comb filter leakage block  310  and comb select block  312 . 
   In the present example, the diagonal edge detector  305  receives a composite and a 2H signal, filters them, and sends the filtered result to the summation block  320   c . The summation block  320   c  sums the output of the wide band comb filter leakage  310  with the output of the diagonal edge detector  305 . 
   A summation block  320   a  sums the outputs of the band pass filter leakage block  306  and the band pass filter  307 . A summation block  320   b  generates a band pass error signal by summing the outputs of the summation block  320   a  with the output of the diagonal edge detector  305 . Summing the output of block  312  with the output of block  308  produces a final comb filter error signal. 
   The selection between the two comb filters is based on which filter has the lowest error signal. The error signal for the narrowband comb filter is simply the leakage found above (represented by blocks  306 ,  309 , and  310 ). For the wideband comb filter  304 , it is the leakage plus the diagonal edge detector  305  output. The chroma output can be a binary selection or some blending based on the size of the error signals. In the latter case, the result is a comb filter bandwidth that changes on a pixel-by-pixel basis. To improve results, increase narrowband comb filter error by adding a fraction of wideband comb filter leakage (error) as shown in minimum comb filter leakage block  311 . The added fraction of wideband comb filter leakage helps because horizontal luma edges occurring at the same location as horizontal chroma edges cause a large amount of leakage in the wideband comb filter  304 . Biasing the error upwards helps to bias the band-pass vs. comb filter decision towards a band-pass filter and preserve the horizontal edge. 
   Generally, as shown in the present embodiment in  FIG. 3 , the filters  301 - 304  may operate over 1, 2 and 3 lines of video. In an alternate PAL implementation, the filters  301 - 304  may operate over 1, 3 and 5 lines of video. The outputs of filters  301 - 304  are then compared to each other to detect vertical transitions in the chroma signal. To detect horizontal transitions in the chroma signal, Y/C separator  300  subtracts a sub carrier frequency from the output of the different filters and compares the magnitude of the resultant signals to each other. Finally, a diagonal edge detection algorithm  305  may bias the decision towards narrow bandwidth comb filter  303  because of its ability to preserve edges. 
   Detection of vertical transitions is possible, e.g., by comparing two line filters,  302   a  and  302   b , to one line band-pass filter  301  and three-line narrow bandwidth comb filter  303 . Two-line filters  302   a  and  302   b  represent an average of the chroma signal over two lines above or below the middle line. One line band pass filter  301  represents the middle line chroma signal. The narrow band comb filter (e.g., a three line filter)  303  represents the average chroma signal over three lines. 
   With no vertical transition, the outputs of the two line filter  302   a  and  302   b , should equal the output of 3-line Y/C separator  303 . If there is a transition then one of the two line filters  302   a  and  302   b  should be close to the one line band pass filter  301 . 
     FIG. 1  shows different filters&#39; outputs for a vertical transition. The errors shown represent a minimum difference between two 2-line comb filters  302   a  and  302   b  and band-pass  301  and 3-line comb filter  303 . On either side of the transition, band-pass filter  301  has a smaller error than 3-line comb filter  303 . More particularly, consider the case where Color A (block  1001 ) abruptly transitions to Color B ( 1002 ), as shown in  FIG. 1 . The 3-line comb filter  303  outputs the color sequence A, (3A+B)/4 approaching the transition between  1001  and  1002 , and outputs the color sequence (3A+B)/4 leaving the transition. Over the same transition, the top 2-line comb filter  302   a  outputs the color sequence A, A approaching the transition, and outputs the color sequence (A+B)/2, B leaving the transition. The bottom 2-line comb filter  302   b  outputs the color sequence A, (A+B)/2 approaching the transition, and outputs the color sequence B, B leaving the transition. Over the same transition the band pass filter outputs the color sequence A, A, B, B. Therefore the error, as explained above, is 0, 0, 0, 0 over the transition for the band pass filter and 0, (A−B)/4, (A−B)/4, 0 for the 3-line comb filter. If there were no other features in the signal, then use of band-pass filter  301  favorably preserves the vertical chroma transition. 
   For a 3-line NTSC or 5-line PAL comb filter, two line filters  302   a  and  302   b  may be narrow bandwidth comb filters such that the average of the two filters equals a 3-line comb filter. 
   For a 3-line PAL comb filter, two line filters  302   a  and  302   b  may use a PAL modifier. A PAL modifier may add or subtract a 90-degree phase shift to the composite or 2H input so the same 2-line comb filter used in the NTSC version can be applied. Unfortunately, the PAL modifier also affects high frequency luma signals so the output represents an average of a true 2-line comb filter and a notch filter. To compensate for the corrupted 2 line filter, a 2-line to 3-line NTSC comparison is replaced with a comparison made between the average of the 3-line comb filter and band-pass filter  301  and the average of the top and bottom 2 line filters  302   a  and  302   b.    
   To detect horizontal chroma transitions the decision logic looks at the energy in the chroma signal outside of the color sub-carrier frequency band. To preserve the transition it is desirable to pick the filter with the widest bandwidth, or pick the filter with the least amount of change in its output. Picking the least amount of change works as it prefers to pick the wider bandwidth filter before and after the transition while in the middle of the transition the bandwidth will have little impact. 
   Referring now to  FIG. 2 , narrow bandwidth comb filter  303  spreads out a transition while the wide bandwidth comb filter  304  preserves the edge of the transition. More particularly, Color A (block  2001 ) transitions to Color B (block  2003 ) through a horizontal transition Color (A+B)/2 (block  2002 ). The wide bandwidth comb filter  304  outputs the color sequence A, −A, A, −A as it approaches the transition  2002 . At the transition  2002 , the wide bandwidth comb filter  304  outputs (A+B)/2. After the transition  2002 , the wide bandwidth comb filter  304  outputs the color sequence −B, B, −B, and B. Over the same transition, narrow bandwidth comb filter  303  outputs the color sequence A, −A, A, (−7A−B)/8 as it approaches transition  2002 . At the transition  2002 , the narrow bandwidth comb filter  303  outputs (A+B)/2. After the transition  2002 , the narrow bandwidth comb filter outputs the color sequence (−7B−A)/8, B, −B, B. 
   In  FIG. 2 , the transition is also spread out in the narrow bandwidth comb filter leakage  309  in comparison with the wide bandwidth comb filter leakage  310 . More particularly, the wide bandwidth comb filter leakage  310  is the color sequence 0, 0, 0, (−A+B)/2 as it enters the transition  2002 . Leaving the transition  2002 , the wide bandwidth comb filter leakage  310  is the color sequence (A−B)/2, 0, 0, and 0. Over the same transition, narrow bandwidth comb filter leakage  309  is the color sequence 0, 0, (A−B)/8, (−3A+3B)/8 as it enters transition  2002 . Leaving the transition  2002 , the narrow bandwidth comb filter leakage  309  is the color sequence (3A−3B)/8, (B+A)/8, 0, 0. Therefore, in rejecting the chroma sub carrier, the residual signal is greater for the narrow bandwidth comb filter  303  at the beginning and end of the transition while it is slightly smaller in the middle. In one embodiment, the wide bandwidth comb filter leakage  310  is divided by two, in which case the wide bandwidth comb filter  304  always has the smallest leakage. 
   Referring back to  FIG. 3 , we now illustrate example filters in more detail. The band-pass filter of block  301  may be represented by a finite impulse response filter (FIR) with the following coefficients: 
   [−1, 0, 0, 0, 2, 0, 0, 0, −1]/4 
   These coefficients are not required to the overall design of the filter. For example, one implementation of the PAL version of the circuit uses a longer FIR filter to reduce cross-color without changing any decision logic. 
   [1, 0, 0, 0, −4, 0, 0, 0, 6, 0, 0, 0, −4, 0, 0, 0, 1]/16 
   The wide bandwidth comb filter, block  304 , may use the following convolution kernel: 
   
     
       
         
           
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   Again, the exact coefficients are not important to the overall design of the filter. For example, one 3-Line PAL implementation of the circuit may use the following convolution kernel: 
   
     
       
         
           
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   The narrow bandwidth comb filter  303  may use the following convolution kernel: 
   
     
       
         
           
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   As above, the exact type of filter is unimportant and a 3-Line PAL implementation may use the following: 
   
     
       
         
           
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   The two 2-line Y/C separators, blocks  302   a  and  302   b , use slightly different filters depending on whether the overall filter is an NTSC/5-line PAL or a 3-Line PAL design. For the NTSC case, the 2-line comb filters may be constructed by first applying the band-pass filter described for block  301  to each of the inputs. The output of the band-pass filters are then processed by the following filters: 
   Top 2-line comb filter  302   a  may be: 
   
     
       
         
           
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   Bottom 2-line comb filter  302   b  may be: 
   
     
       
         
           
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   For the 3-line PAL filter case, the filter is slightly more complex in order to shift the phase of the top and bottom lines by 90 degrees: 
   
     
       
         
           
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   The bottom 2-line Y/C separator may have a similar modification. The +/−90 degree shift caused by the filter may be controlled based on whether the sample will be decoded to a U or a V sample and whether or not the V sample has been inverted (as determined by the PAL switch generated elsewhere in the video decoder). 
   The detection of horizontal transitions is done with the leakage filters in blocks  306 ,  309  and  310 . The leakage filters may consist of a notch filter to remove the sub carrier frequency followed by a minimum/maximum function, as shown in more detail in  FIG. 4 .  FIG. 4  shows a notch filter implemented as the average of the input and the input delayed by ½ the sub carrier period. The minimum of the absolute values  403  of the output of averager  402  and the same signal delayed by ½ sub carrier period is taken. The maximum  404  of the minimum function and the absolute value of the averager output delayed by ¼ the sub carrier period is then taken, creating an output representing the peaks of the leakage without spreading the envelope out too far from a region of interest. 
   The details for blocks  307  and  308  are shown in  FIG. 5 . As discussed above, the first step is to take the difference between the two 2-line comb filters  302   a  and  302   b  and the band-pass  301  or 3-line comb filter  303  results at deltas  501  and  511 . However, before finding the minimum difference between the two 2-line Y/C separators,  302   a  and  302   b , and the band-pass filter  301 , more robust results may be obtained by first filtering the differences using a maximum function  504  and  514 , and then taking the minimum  520  of the two results. The same maximum/minimum function may be applied to the 3-line comb filter error measurement for PAL as shown in  FIG. 6 . For the PAL 3-line comb filter error measurement, only half the circuitry is used and the delta  603  is of the sum of the two 2-line comb filters  601  and the sum of the band-pass and comb filter outputs  602 . 
     FIG. 7  is a flow diagram of a method of Y/C separation adaptive to features of a signal, as explained in detail above. An output signal from a plurality of filters is received at box  705 . Features of the received signal are then used to determine or select a subset of the Y/C separators in box  710 . In block  715  the subset is then used as an adaptive filter. 
   Referring back to the embodiment in  FIG. 3 , the diagonal edge detection block  305  may comprise the following filter: 
   
     
       
         
           
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   The filter is essentially the change in slope over one period of the sub carrier. Because the change is done over one full period, the change effectively cancels out the sub carrier frequency for both PAL and NTSC video. 
   The present embodiment selects the two comb filters with the lowest error signal. A single value can represent the error signal, thus simplifying filter selection logic. The error signal for the narrowband comb filter may be the leakage found above as represented by blocks  306 ,  309  and  310 . For the wideband comb filter, the error signal may then be the leakage plus a diagonal edge detector output. The chroma output can be a binary selection or some blending based on the size of the error signals. 
   In one embodiment, the blending is accomplished by using the difference in the error signals times a gain value to limit the difference in the comb/band pass filter. For example, to blend between the wide and narrow band comb filters, you would first compute the difference equal to the wide band comb filter minus the narrow band comb filter. You would then limit its value from zero by the amount of the narrowband comb filter error less the wideband comb filter error. If the resulting difference in the errors were less than zero, then the limit would be set to zero. The limited difference in the comb filter errors would then be added back to the narrowband comb filter resulting in a comb filter bandwidth that changes on a pixel-by-pixel basis. Even more sophisticated blending can be obtained by applying the concepts of fuzzy sets and fuzzy logic. 
   The selected error signal should reflect the relative mixture of the two comb filter inputs. As discussed above, better results can be obtained by increasing the narrowband comb filter error by adding a fraction of the wideband comb filter error  310  to narrow band comb filter leakage  309  in minimum comb filter leakage block  311 . The added fraction helps because horizontal luma edges that tend to occur at the same location as horizontal chroma edges cause a large amount of leakage in the wideband comb filter. Biasing the error upwards helps to bias the band-pass vs. comb filter decision towards a band-pass filter and preserve the horizontal edge. In the present embodiment the output of block  312  is then added to the output of block  308  to produce the final comb filter error signal. 
   Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the appended claims.