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The comb filter achieves the clearer separation by subtracting the color signal taken from adjacent lines in the picture, so to 'comb' out the color from the luminance. A regular TV uses a notch filter, that simply cancels the frequency at which the color signal is found. That leaves some smearing, and limits the overall resolution.
Comb filters come in a variety of types, for instance a 2-line comb filter will subtract the color signal taken from each previous line, a 3-line comb filter will subtract the sum of the previous and subsequent lines, and a 3D comb filter will adapt its processing depending on motion. They can also be analog or digital with various pro's and con's to either. 3D comb filters are digital.
A comb filter permits both a reduction in interference between color and luminance, as well as a slightly higher resolution. In practice I think it is quite subtle, you are likely to see the most difference in a test card but not as much in typical video.
You usually cannot turn the comb filter off, but it will not activate in S-video or RGB signals since it is not applicable there. It will only be functional with composite signals (from an external device or the off-air RF feed).
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