Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!tekbspa!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: comp.dsp
Subject: Re: Adjust-Speed CD player?
Summary: pitch shift... adding offset freq doesn't work
Message-ID: <2389@optilink.UUCP>
Date: 25 Sep 89 17:53:41 GMT
References: <61860@tut.cis.ohio-state.edu> <4653@orca.WV.TEK.COM>
Organization: Optilink Corporation, Petaluma, CA
Lines: 32

In article <4653@orca.WV.TEK.COM>, mhorne@ka7axd.WV.TEK.COM (Michael T. Horne) writes:
> 
> All this said, I don't think this is the optimal method for tone shifting,
> however it might work for `fast/slow-forward' effects.  If you wish to shift
> the tones while retaining the same sample rate, I would suggest some sort of
> frequency scaling algorithm, perhaps by doing a digital mix with a reference
> (digital) carrier (i.e. ref = 100 Hz for a 100 Hz shift upward in frequency),
> followed by a carrier and lower sideband suppression (Hilbert transform filters
> are very easy to implement digitally).  At a fast glance, I think this might
> work well for moving the spectra of an audio source up/down some arbitrary
> frequency, and should be doable with some of the common DSP chips currently
> available.

Pitch shifting like this doesn't work (well), since it alters all the harmonic 
relationships.  For example: a 1000 Hz / 2000 Hz octave, after shifting 100 Hz
becomes a 1100 Hz / 2100 Hz pair -- definitely not an octave.  If you have the
opportunity, listen to single-sideband voice (ham radio is a good place).  You
can only shift the pitch a small amount (by mis-tuning the receiver) before
virtually destroying the intellegibility of the voice, where with true pitch
shifting (all frequencies multiplied by n), voice remains reasonably 
understandable over several octaves.

Of course if you think shifting-by-adding messes up voice, you should hear what
it does to music!  Definitely not Hi-Fi -- more like atonal bagpipes :-)

... Paul


-- 
Paul M. Elliott      Optilink Corporation     (707) 795-9444
         {pyramid,pixar,tekbspa}!optilink!elliott
"I used to think I was indecisive, but now I'm not so sure."