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From: sandell@ils.nwu.edu (Greg Sandell)
Newsgroups: comp.music
Subject: Re: Books on massaging waveforms...
Keywords: samples sound
Message-ID: <987@anaxagoras.ils.nwu.edu>
Date: 28 Jun 90 00:02:56 GMT
References: <4090@uqcspe.cs.uq.oz.au>
Sender: news@anaxagoras.ils.nwu.edu
Reply-To: sandell@ils.nwu.edu (Greg Sandell)
Distribution: comp
Organization: The Institute for the Learning Sciences
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In article <4090@uqcspe.cs.uq.oz.au>, tonyg@batserver.cs.uq.oz.au (Tony
Gedge) writes:
> Can someone point me towards a good starting book in manipulating
> sampled sound?  I want to do things like:
> 
>   * Raise and lower the pitch of samples without changing the speed
>         that samples are played.
>   * Mix two or more samples together.
>   * Change the sampling rate.
> 

I would get the new book on Computer Music by F. R. Moore which is out
in Prentice-Hall, 1990.  The title is something like (sorry, I don't have it
here in my office) THE ESSENTIALS OF COMPUTER MUSIC.  

The task of changing the pitch of a soundfile without changing its apparant
speed or duration (which is what I think you are trying to describe above)
is a problem which requires large amounts of number crunching; it takes 
quite a long time to make such a conversion (like, maybe 60 seconds for 
a one second soundfile on a powerful computer).  The
algorithm I know of which does this is the Phase Vocoder, for which
C code is provided in Moore's book.

The second two questions I don't know quite as much about, but I believe
if you simply add samples together and then normalize them so you don't
get samples out of range, you get mixed samples.  For changing the
sampling rate, if you are going from a higher sampling rate to a lower
one, you can keep one sample, delete the next (new_samp_rate/old_samp_rate) 
samples, and keep doing this for the rest of the soundfile.  I have done this
in going from 22000k to 8000k and the results sound just fine.  Somehow
I suspect this is not the most HiFi way to do this task, though.  As for going
from a smaller sampling rate to a larger one, the strategy of duplicating
samples in the equivalent way will *definitely* not work!