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From: don@oakhill.UUCP (Don Weiss)
Newsgroups: net.analog,net.audio
Subject: Re: Frequency Shifter with no practical application makes funny noises!
Message-ID: <251@oakhill.UUCP>
Date: Thu, 29-Nov-84 13:28:53 EST
Article-I.D.: oakhill.251
Posted: Thu Nov 29 13:28:53 1984
Date-Received: Sat, 1-Dec-84 06:49:28 EST
References: 
Reply-To: don@oakhill.UUCP (Don Weiss)
Distribution: net
Organization: Motorola Inc. Austin, Tx
Lines: 21
Xref: genrad net.analog:95 net.audio:3897
Summary: 

[post no bills]

In 1968 or 1969 Dr. Manfred Schroeder of Bell Labs lectured at the University
of Illinois on his recent research in digitally simulating acoustic
environments.  A result of his early simulations suggested that there was a
"fine-grained" structure to the frequency response of a typical room that
had not previously been discovered by traditional room-measurment methods.
Upon investigating with new kinds of measurments, this fine grained
"sawtooth" in the room response was found in some real rooms.

Now, this frequency-domain sawtooth had a periodicity on the order of a
few hertz, which explained why it hadn't been remarked on before.  Dr.
Schroeder then proposed that this characteristic could be exploited in sound
reinforcement systems to increase gain-before-feedback by freqency shifting
the input from all live microphones by half of the room periodicity, which
would amount to a shift of only a hertz or three.  The idea, of course, was
to effectively cut down the loop gain of the feedback mechanism, without
cutting down the actual forward gain of the sound reinforcement system.

Interesting idea; I haven't heard any more about it since the lecture.