Comb Filters and Notch Filters in a Display

Inside a TV, either a comb filter or a notch filter separates the color and brightness parts of a video signal into their component parts. If the signals aren’t properly separated, you see moving dots of color around the edges of images, a phenomenon called dot crawl.

Many video sources (such as broadcast NTSC television or composite video connections from other source devices) send both the color (chrominance) and brightness (luminance) parts of the video signal as a single combined signal. (The signal carried over a composite video cable is combined in this way.)

Notch filters are the least effective at separating the signals, whereas the various kinds of comb filters do a better job. You’ll find three kinds of comb filters in direct-view displays:

  • 2-D comb filter: This is the least effective (and least expensive) kind of comb filter, though it’s better than a notch filter.

  • 3-D comb filter: This type of comb filter is better than 2-D but not as good as the digital comb filter.

  • Digital comb filter (sometimes called 3-D Y/C): Found in more expensive direct-view sets, the digital comb filter uses more sophisticated (and digital!) signal processing to separate chrominance and luminance. This is the king of the hill when it comes to comb filters.

When you connect sources to your display with S-video, component video, or HDMI cables, chrominance and luminance are separated in the source device, not in your TV. The same is true for HDTV signals coming into your TV over the air or on a cable system (without a set-top box). In these cases, your display’s built-in comb filter is bypassed. In other words, the comb filter is important mostly for nondigital TV broadcasts and for older source components such as VCRs.

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