Newsgroups: comp.music
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!van-bc!cynic!curt
From: curt@cynic.wimsey.bc.ca (Curt Sampson)
Subject: Re: Oversampling: Re: DSP for Audio Marketing
Organization: Mad Artists' Technological Hangout
Date: Fri, 12 Apr 1991 06:06:57 GMT
Message-ID: <1991Apr12.060657.19964@cynic.wimsey.bc.ca>
References: <1991Apr5.172338.11258@vaxa.strath.ac.uk> <31771@usc>

In article <31771@usc>
  alves@calvin.usc.edu (William Alves) writes:

> Oversampling is not used because the analog filters are more expensive,
> but because their roll-offs, even though they're very steep, will cut 
> well into the audio spectrum. Not only will this effect the frequency 
> response, but, as it turns out, more importantly, the phase response. 

Actually, the roll-offs, being very steep, stay out of the audio
spectrum.  The problem here is solely due to phase shift.

> ...Philips Corporation came up with a way of, in effect, shifting the
> sampling rate up 4 or 8 times upon playback. This isn't exactly interpo-
> lating 3 samples in between every two because that would cause secondary
> aliasing and (depending on the interpolation algorithm) more noise. 
> Nevertheless, it's convenient to think about it that way. 
> 
> These extra samples do not add any information that wasn't recorded, of
> course, but they do shift the unwanted "images" four or eight times further
> up into the audio spectrum. There they can be easily filtered out by an
> analog filter with a much shallower roll-off. 

This is not completely correct.  When the sample is converted from
44.1 to, say, 176.4 KHz, no interpolation or other changing of the
data is done.  The data is simply duplicated.  If you started with the
following sequence:

	3 7 11 15

it would be turned into this when 4x oversampled:

	3 3 3 3 7 7 7 7 11 11 11 11 15 15 15 15

Then a digital filter is applied, which is essentially a way of
"smoothing" the samples.  The "edges" you see (because it is
essentially a sqare wave before filtering) are what cause the high
frequences to be present.

cjs
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