Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!ll-xn!ames!amdcad!sun!falk From: falk@sun.uucp (Ed Falk) Newsgroups: comp.graphics Subject: Re: Comb filters Message-ID: <23250@sun.uucp> Date: Sun, 12-Jul-87 03:01:43 EDT Article-I.D.: sun.23250 Posted: Sun Jul 12 03:01:43 1987 Date-Received: Mon, 13-Jul-87 00:37:10 EDT References: <8707110357.AA14175@unisoft.UNISOFT> Organization: Sun Microsystems, Inc. - Mtn View, CA Lines: 54 In article <8707110357.AA14175@unisoft.UNISOFT>, doug@certes.UUCP writes: > I keep hearing references to "comb filters" in reference to both video > and to audio. What are they? > > Part two of the question: some years ago, I read an interview with someone > at Atari, who said that they had a new video game that used comb filters > to remove phase information from sound output, which removed directional > cues that the human ear/brain uses, so that the sound from this arcadge > game seemed to be coming from all directions at once ... I don't know how audio comb filters work, but here's what they do in the video domain... A TV picture is 525 scan lines at 30 frames per second. This comes to 15750 scan lines per second. TV pictures are amplitude-modulated. If you look at the spectrum of a TV video signal, you will see spikes at 15750 hz intervals. In general, most of the information in a B&W TV signal lies at 15750 hz intervals.. I believe that color sync rates are slightly different than B&W in order to minimize interference with the 60-hz line frequency (I may be wrong about this part). When color was invented, there was a question of how you could put as much power into the color subcarrier while minimizing interference with the B&W image. The simple answer was to choose a subcarrier frequency that was an odd half-multiple of the ~15750 hz sync rate. This causes the sideband spikes of the color subcarrier to lie nicely between the horizontal sync spikes. The color subcarrier is approximately 3.58 Mhz. Most cheapo color monitors simply set up a filter that chops the video signal at some arbitrary point (around 2.5 Mhz typically), calling everything below the cutoff the B&W signal and everything above the cutoff the color signal. In other words, the B&W signal is passed through a low-pass filter and the color signal is passed through a high-pass filter. The better color monitors use what's known as a "comb filter". The comb filter looks like a series of bandpass filters with the bands corresponding to the spikes in the signal being filtered. The idea being to capture as much information and power for each signal as possible, thus improving the image. If you draw a chart of the frequency response of this kind of filter, it looks a little bit like a comb; hence the name. How this works for audio I can only guess. Possibly the resonant frequencies of your ear canal are removed from the audio thus destroying information you need to judge direction. -- -ed falk, sun microsystems, falk@sun.com terrorist, cryptography, DES, drugs, cipher, secret, decode, NSA, CIA, NRO.