@resonator:
A "comb filter" simply separates the Y (luma) and C (chroma) data from a composite video signal. It does not deinterlace. (Sometimes interlacing artefacts are referred to as "combing" - confusingly, this is in no way related to a comb filter!)

Since digital video is stored as YUV or RGB, an analogue composite signal must be separated into Y/C before it can be digitized.

@Orinoco_Womble:
This might be a useful read:
http://www.videohelp.com/forum/archive/hardware-video-processors-and-filters-t239205.html