Re: Soundcard sample rates



Robert Scott wrote:

Soundcard sample rates may not be what they claim.
I have been testing different brands of PC soundcards. 44100 and 22050 are two
of the standard sample rates that most soundcards support. Of course all sample
rates are only as accurate as the crystal oscillator on the soundcard, so when I
found several soundcards with a 22050 sample rate that was high by 0.6%, I
chalked it up to poor crystal accuracy, or maybe ceramic resonators. But then I
checked the 44100 sample rate on those cards and found it to be right on. I was
puzzled as to why the 44100 sample rate was not exactly double the 22050 sample
rate. When I made more careful measurements, it appeared that the 22050 sample
rates were really trying for 22200, which is a less common standard.

Does anyone know why they might do that (and not tell anyone!)

By the way, the soundcard sample rates were all tested by writing a custom
program to generate an audio tone of a certain frequency based on the assumption
that the sample rate was the nominal rate requested. The the audio frequencies
were measured with NIST-traceable means.

If you have requested the sample rate which is not directly supported by the sound card hardware, the Win32 API will do the resampling to the supported rate automatically, without any obvious indication. The integrated sound chipsets typically support for 44.1kHz in hardware; all other rates are actually made by the software resampling. Perhaps what you see is related to the resampling artifacts for your particular hardware/software combination.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com



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