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From: alves@calvin.usc.edu (William Alves)
Newsgroups: comp.music
Subject: Oversampling: Re: DSP for Audio Marketing
Message-ID: <31771@usc>
Date: 9 Apr 91 02:36:32 GMT
References: <1991Apr5.172338.11258@vaxa.strath.ac.uk>
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In article <1991Apr5.172338.11258@vaxa.strath.ac.uk> cnbs30@vaxa.strath.ac.uk writes:
>
>How much of this is just marketing gimmicks?  4 x's versus 8 x's
>oversampling, can we hear the difference?  Oversampling is described
>in some Hifi magazines as simply inserting extra samples between
>adjacent samples in order to increase the resolution?  I always
>thought that oversampling was to avoid having to use expensive
>analogue filters at the input (anti aliasing) and outputs??

Oversampling is, to oversimplify, "inserting extra samples," and it is also
a way to avoid having to use steep roll-offs analog filters on the outputs.
First of all, output filters are used because a digital sampling system
produces artifacts known as "images," or high-frequency reflections of the
input signal at multiples of the Nyquist frequency. Low-pass analog filters
are the most straight-forward way to get rid these images (hence "anti-
imaging filters).

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. 
Early CD players suffered from severe phase distortion in the high fre-
quencies because of these filters. This was the probably culprit in early
audiophile complaints of "graininess" or "harshness" in the high-end.

Because digital-to-analog converters are generally n times faster than
a comparably-priced analog-to-digital converter where n is the number of
bits, 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. 

For more info, see:

Wayne Schott, "Philips Oversampling System for Compact Disc Decoding," Audio,
April 1984, 32-35.

Ken C. Pohlmann, Principles of Digital Audio, (Indianapolis, IN: Howard W. Sams
& Co., 1985). There's a second edition out now.

Can one tell the difference between 4x and 8x oversampling? I couldn't on the
CD-players I tested. Differences can be the result of a lot of things. The
bottom line is trust your ears and not the numbers.

Electronics manufacturers found out long ago that consumers respond over-
whelmingly to big numbers, even when totally meaningless. Things that can-
not be quantified on a spec sheet, on the other hand, get the short end.
My personal pet peeve is the poor quality of analog output amps in CD players
and synths which often totally obliterate the advantages of high-tech digital
filters and such.

Bill Alves