Datasheet

Functional Description
9
May 2006 SLES043A
The filter frequency plots show that both 4-line and 3-line (with filter coefficients [1,3,3,1]/8 and [1,2,1]/4) comb
filters have zeros at 1/2 of the horizontal line frequency to separate the interleaved Y/C spectrum in NTSC.
The 4-line comb filter has less cross-luma and cross-chroma noise due to slightly sharper filter cutoff. The
4-line comb filter with filter coefficients [1,1,1,1]/4 has three zeros at 1/4, 2/4, and 3/4 of the horizontal line
frequency. This is to be used for PAL only because of its 90_ U/V phase shifting from line to line. The comb
filter can be selectively bypassed in the luma or chroma path. If the comb filter is bypassed in the luma path,
then chroma trap filters are used. TI’s patented adaptive comb filter algorithm reduces artifacts such as
hanging dots at color boundaries and detects and properly handles false colors in high frequency luminance
images such as a multiburst pattern or circle pattern. Adaptive comb filtering is the recommended mode of
operation. The complete comb filter selection is shown in the chrominance control #1 register (see Section
3.22.25).
f − Frequency − MHz
0.0
0.2
0.4
0.6
0.8
1.0
012345
Amplitude − dB
4 line (1,1,1,1)/4
Figure 3−2. Comb Filters Frequency Response
4 line (1,3,3,1)/8
3 line (1,2,1)/4