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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.audio
Subject: Re: cd oversampling
Message-ID: <431@petrus.UUCP>
Date: Tue, 30-Jul-85 16:59:48 EDT
Article-I.D.: petrus.431
Posted: Tue Jul 30 16:59:48 1985
Date-Received: Thu, 1-Aug-85 07:34:56 EDT
References: <707@charm.UUCP> <1167@tekgvs.UUCP>
Organization: Bell Communications Research, Inc
Lines: 34

> In article an earlier article someone wrote:
> 1>A second thought:  why they call
> 2>the output filters "anti-alias filters" I'll never know.
> 
> 3>Oversampling introduces new aliases, but they are
> 4>above the original Nyquist frequency and are removed by output
> 5>filters.
> 
> <3-5 restated>: the (new) aliases are removed by the output filters.
> 
> And that's why they're called anti-alias filters...

Negative. The output of the D/A and sample-and-hold on a CD player contains
multiple IMAGE SPECTRA. This is entirely distinct from the phenomenon of
"aliasing", which is unique to A/D conversion, i.e., digital recording.

Aliasing refers to the possibility of a signal above half the sampling rate
"masquerading" or "aliasing" (hence the name) as a lower frequency.  Such
high frequencies MUST be filtered out before A/D conversion, because once
they reach the A/D converter there is absolutely nothing that can be done
afterwards to separate aliases from the desired audio signals.

This is in contrast to the problem of removing the high frequency "image
spectra" which are found in the "stairsteppy" output of a D/A converter.
This is done by a "reconstruction" filter which can be anywhere between the
D/A converter and the speaker, or even omitted entirely if you don't care
about feeding all that supersonic crap to your tweeters and fetching all of
the dogs in the neighborhood.  Unlike aliases in the A/D process, the image
spectra produced in D/A conversion do NOT directly affect the audible sound,
unless your hearing is so good that you can hear well past the CD
half-sampling rate.  However, they can cause intermodulation distortion,
burn out tweeters and waste amplifier power.

Phil