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Path: utzoo!mnetor!uunet!seismo!gatech!hao!boulder!sunybcs!cald80!bob
From: bob@cald80.UUCP (bob)
Newsgroups: comp.graphics
Subject: Re: Comb filters
Message-ID: <1226@cald80.UUCP>
Date: Sun, 12-Jul-87 22:43:11 EDT
Article-I.D.: cald80.1226
Posted: Sun Jul 12 22:43:11 1987
Date-Received: Mon, 13-Jul-87 05:17:35 EDT
References: <8707110357.AA14175@unisoft.UNISOFT>
Reply-To: bob@cald80.UUCP (bob)
Organization: Calspan Advanced Technology Center
Lines: 33
Keywords: sound games

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 ...

Comb filters are basically a group of very narrow bandpass filters
that are spaced at some preset intervals.  The effect is to only pass
a certain group of desired frequencies (this method is sometimes used
in frequency-hop transmission).

The fun with audio comes in when you set up 2 comb filters that pass
different frequency sets (alternate the frequencies), then pass the
output from each filter set to a different speaker.  The result is
that the sound appears to be coming from all directions at the same
time because your hearing can't tell that the frequencies are broken
up.  This is especially true if the sound fed to the filters just
happens to be white noise.

I remember a friend of mine trying to use this method to simulate
stereo on a mono system.  He gave up when he figgered out that he'd
have to build 20 1KHz filters and keep them from ringing :-).

________________________________________________________________________________
   Thinking quickly, the IBM System Jock     # Bob Meyer
uttered an incantation in EBCDIC and made    # Calspan Advanced Tech. Center
the sign of the Terminated Fork.             # seismo!kitty!sunybcs!cald80!bob
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him in a recursive SED script.