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Path: utzoo!linus!philabs!cmcl2!seismo!harvard!sullivan
From: sullivan@harvard.ARPA (John Sullivan)
Newsgroups: net.audio,net.music
Subject: speeding up sound
Message-ID: <318@harvard.ARPA>
Date: Sun, 15-Jul-84 13:16:04 EDT
Article-I.D.: harvard.318
Posted: Sun Jul 15 13:16:04 1984
Date-Received: Tue, 17-Jul-84 05:45:48 EDT
Organization: Aiken Computation Lab, Harvard
Lines: 25

Recently net.music has had some discussion of devices (Harmonizers)
which will change the pitch but not the speed (or vice-versa) of a signal.
Now of course this is impossible theoretically, but I can think of a couple
of ways this might work in practice for limited bandwidth signals like most
sound.  For example, one could pick a time constant delta-t, and get fourier
transforms of each interval of that length.  Then these could be shifted
in frequency space, and then transformed back.  Of course they successive
intervals would no longer match up right, but I suppose with an appropriate
time constant (maybe ~.05 sec?  We need it short enough to have essentially
fixed frequency spectrum from one interval to the next, but long enough so
we don't mess up bass notes.) it might work.

Another way to do what really amounts to the same thing, but with less
computation would be to sample the signal for short intervals (say delta-t=.05
sec long each) which either overlap slightly or have small gaps between them,
depending on whether it is to be speeded up or slowed down.  Then you
can output these at a corrected rate so they just fit back to back.

Is this how the devices actually work?  Does anyone know the real time
constant used?  How do they avoid loud clicks going from one sampling
period to the next?


	John M. Sullivan
	{allegra,ihnp4,decvax,genrad}!harvard!sullivan