Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site oakhill.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!wjh12!talcott!harvard!seismo!ut-sally!oakhill!don 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.