A bandpass filter used to process audio and video signals by mixing the original with a delayed signal. Comb filters allow selected frequencies to pass while blocking their harmonics and all other frequencies. Their most common application is to sharpen a composite video image by eliminating residual color (chroma) from the brightness (luma) signal that was not entirely filtered out in an earlier stage. Early analog and two-line digital comb filters mix one line of video with the next. Since color signals are 180 degrees out of phase in alternating lines, the mixing removes most remaining color from the luma. A three-line digital comb filter averages the signal from the line before and after it to obtain a more accurate cancellation. The better a TV's comb filter, the less artifacts in the image (see dot crawl). See composite video.