Anti-aliasing on low-cost audio cards, how done? ("Gregory J. Sandell" )

Subject: Anti-aliasing on low-cost audio cards, how done?
From:    "Gregory J. Sandell"  <sandell(at)SPARKY.PARMLY.LUC.EDU>
Date:    Thu, 2 Nov 1995 11:50:39 -0600

[ 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, Greg -- Gregory J. Sandell, Research Associate, sandell(at) Parmly Hearing Institute, Loyola University Chicago 6525 N. Sheridan Chicago IL 60626 USA voice:312-508-3976 FAX:312-508-2719 WWW:

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