Path: utzoo!attcan!uunet!samsung!sdd.hp.com!uakari.primate.wisc.edu!aplcen!zeus.ece.jhu.edu
From: weimin@zeus.ece.jhu.edu (weimin liu)
Newsgroups: sci.electronics
Subject: Re: Need  circuit to drop music an octave
Message-ID: <6204@aplcen.apl.jhu.edu>
Date: 7 Aug 90 20:58:04 GMT
References: <1990Jul20.223615.4305@portia.Stanford.EDU> <1332@fs1.ee.ubc.ca> <58975@lanl.gov> <32424@cup.portal.com>
Sender: andreou@aplcen.apl.jhu.edu
Organization: Johns Hopkins University
Lines: 18

In article <32424@cup.portal.com> R_Tim_Coslet@cup.portal.com writes:
>The only way I know of that will definitely do what you want is a
>Digital Signal Processor doing a Fast Fourier Transform on the input,
>divide the "frequencies" obtained by 2, and inverse Fourier Transform
>to generate the output.

This will change the "color" of the notes, i.e., the resonant properties
of a particular instrument.  What you really want is to change the 
fundamental frequency (f0) but not the resonant frequencies.  I don't
know much about music but for speech there are several methods, such as
linear predictive coding (LPC) and cepstrum (not spectrum) analysis, to
separate f0 from the higher resonant stuff.

They did something like this once on Late Night with David Letterman in
which Paul Shaefer's voice pitch was significantly lowered through the
microphone.

weimin@zeus.ece.jhu.edu