In article . com,
wrote:
In a TV, the comb filter is made by summing adjacent lines using a
delay element. The phase of the color subcarier alters between lines,
so summing adjacent lines cancels the chroma. You can invert the signal
and cancel the luminance.

Another way to say this is that the Comb filter works with NTSC because
both the lumiance and chroma have their high frequency (fine detail)
information in harmonics of the horizontal scan rate. But the color
subcarrrier frequency was deliberatly picked to have an offset of 1/2
the horizontal frequency so that the frequency spectrums of the two
signals occupy the gaps in each other's signal. By having a filter
response with a notch for every harmonic of the horizontal scan rate,
you can get the chroma information without the lumiance interfering.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)