[Speex-dev] Audio Speed Variability

James Stanton jstanton at clairvista.com
Thu Oct 4 15:14:14 PDT 2007


Thanks for the reply!  You mentioned output sample rates should be 44100 
or 48000, should I worry about input (Mic) Sample rates as well?  
(Currently I was requesting the sample rate on both ends to be 16000 
samplesPerSecond, for ease of passing into the codec)   Also, do you 
recommend any particular resampler that I should use, or are any of the 
ones out there probably okay, or should I just write my own?  Thanks 
again for your help!


John Miles wrote:

>>-----Original Message-----
>>From: speex-dev-bounces at xiph.org [mailto:speex-dev-bounces at xiph.org]On
>>Behalf Of James Stanton
>>Sent: Thursday, October 04, 2007 12:53 PM
>>To: speex-dev at xiph.org
>>Subject: [Speex-dev] Audio Speed Variability
>>I have a video conference like application that I've been working on for
>>a while now, and a recent change is causing some odd problems, and I was
>>wondering if anyone else had seen problems like this....
>Short answer: don't use output sample rates other than 44100 or 48000.
>Longer answer: Sound chips usually run at one of those rates, often either.
>Those rates are more or less guaranteed to work properly.  Most chips don't
>support other rates directly; a software resampler in the driver is used
>instead.  Unfortunately, Microsoft released a horribly-broken reference
>resampler implementation to sound hardware OEMs a few years ago, and many of
>them still use it.  On their sound cards, if you ask for 11025 Hz, for
>example you're likely to get 11100 Hz or something similarly-imprecise.
>That obviously causes cumulative latency/slippage problems.
>Bottom line: voice codec applications that need to work at lower rates
>really need to resample to 44.1K or 48K themselves in order to work robustly
>across all hardware platforms.  Neither MS nor sound-hardware OEMs have
>shown the slightest interest in fixing this bug, so that's just the way it
>-- john
>Speex-dev mailing list
>Speex-dev at xiph.org

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