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From: doug-merritt@cup.portal.com
Newsgroups: comp.sys.amiga
Subject: Re: Sampling at 29KHz
Message-ID: <6060@cup.portal.com>
Date: 30 May 88 06:51:14 GMT
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I wrote:
> Do you know whether my idea about doing the same technique
>via DMA (period modulation of another channel) would work?

Phil Stone wrote:
>I thought it depended on *random* fluctuations in sampling frequency - 
>wouldn't this just introduce a higher-order periodicity? 

I'm not sure. Ordinarily you have problems with a sample that is not
an integral multiple of its constituents, because the discontinuity where
it wraps from end to start introduces a high frequency click. I was
thinking it might not be perceptible in the case of a sample filled with
random values, especially one that was simply period-modulating the
actual output. You could certainly detect it via autocorrelation, though,
so likely your ear could, too (for some waveforms, anyway).

Intuitively I would expect that generating a random sample that is
windowed as usual (rising from a zero amplitude envelope at the beginning,
and falling to zero again at the end) would give perfectly adequate
results. That's the usual trick with getting finite samples to behave
the way that you expect the theoretical model of an infinite Fourier
series to work.

Mainly my question is whether or not the Amiga audio subsystem works
the way I'm inferring it does...I.e. is it possible to even try this
idea using DMA? It *looks* like it is.

>BTW - sampler hardware would also have to do this on the input end wouldn't it?
>If it *could* be worked out, it might be a great way of doing hi-fi sampling.

Yes, you do want it on input as well. As I recall from the driver
available for the Applied Visions Future Sound digitizer, this is quite
possible via a software mod. I wouldn't know about other sound digitizers.
	Doug
      Doug Merritt        ucbvax!sun.com!cup.portal.com!doug-merritt
                      or  ucbvax!eris!doug (doug@eris.berkeley.edu)
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