What is a 3D comb filter and what does it do?

Broadcast and cable television transmissions are a combination of two separate but intermingled signals: a luminance (black and white) signal and a chrominance (colour) signal.

A 3D comb filter is an electronic filter that separates the luminance signal from the chrominance signal three horizontal scan lines of video at a time. The results eliminate or reduce video noise (cross-colour interference or colour bleeding) and dot crawl (dot interference) and gives the viewer a clearer, sharper video picture.

The comb filter got its name from the graph of its frequency response (which looks like the teeth of a comb when displayed on a graph).