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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.