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Anti-aliasing on low-cost audio cards, how done?

        [ Apologies for two postings in one day on a related subject...
      I promise I won't flood the airwaves any further!  - Greg ]

Subject: Anti-aliasing on low-cost audio cards, how done?

The are lots of audio cards for the PC and Mac that are very low cost,
meaning they can't possibly have very sophisticated lowpass filters on
them.  What do these cards do, if anything, to avoid aliasing?

Is there an assumption that use of a high (44.1 kHz) sampling rate
obviates the need for the filters, i.e. "the frequency products are
beyond the range of hearing".  This is folklore, isn't it?  My
understanding is having no filtering at all can result in unpredictable
phase rotation, meaning that stereo effects (among other things) could
be hosed.

A DSP book I own describes how CD players typically do it.  They oversample
the 44.1 kHz output 4 times, and then apply *digital* filtering the
samples.  This simplifies the anti-aliasing requirements to the point
where very simple (low cost) analogue filters are required.

Do PC/Mac sound cards employ this technique?  Is this sort of thing done
all within a DSP chip (like the MC56000), or with separate processors on
the sound card?

Thanks in advance,

Gregory J. Sandell, Research Associate, sandell@sparky.parmly.luc.edu
Parmly Hearing Institute, Loyola University Chicago
6525 N. Sheridan Chicago IL 60626 USA voice:312-508-3976 FAX:312-508-2719
WWW: http://www.parmly.luc.edu/sandell/