[Speex-dev] Sampling Rate
david.hogan at freshtel.net
Mon Dec 11 15:47:13 PST 2006
Oops, CTRL+Enter send strikes again ...
At the other end for playback you can convert it back to
48000 (or whatever) by repeating each sample 3 times (48/16 == 3), then
running a 8000Hz lowpass over the result to remove any aliasing
> -----Original Message-----
> From: David Hogan
> Sent: Tuesday, 12 December 2006 10:44 AM
> To: 'speex-dev at xiph.org'
> Subject: RE: [Speex-dev] Sampling Rate
> I'm no DSP or audio expert by any means, but I can share what
> works for me. People in the know, I would appreciate tips on
> whether this stuff is ok.
> You could sample at 32000Hz (or 48000Hz, any AC97 card will
> support this), run a 8000Hz lowpass filter over the data
> (16000Hz sample rate can only represent frequencies up to
> 8000Hz) and then drop every second (or 2 out of 3 for
> 48000->16000) sample. The result being, 16000Hz sampled
> audio. If you omit the filter the result will contain artifacts.
> At the other end for playback you can convert it back to
> 48000 (or whatever) by
> You can use
> http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html to
> generate such a filter, choose Butterworth + lowpass, filter
> order 10, put your sample rate in, and corner frequency 1 8000Hz.
> Failing that, you could use http://www.mega-nerd.com/SRC/ to
> do the resampling for you if you're prepared to GPL your code
> or cough up for the license. This library will perform any
> arbitrary conversion for you as well (for example 44.1->16),
> not just simple ones like 32->16.
> Hope this helps!
> David Hogan
> > -----Original Message-----
> > From: speex-dev-bounces at xiph.org
> > [mailto:speex-dev-bounces at xiph.org] On Behalf Of
> khaynes at kirkgames.com
> > Sent: Tuesday, 12 December 2006 10:11 AM
> > To: speex-dev at xiph.org
> > Subject: Re: [Speex-dev] Sampling Rate
> > It seems that I only have the following values available for
> > sampling from
> > the mic.
> > "The value must be 8000, 11025, 22050, 32000, 44100, or 48000"
> > Which leaves 8000 and 32000 for use with speex. I think since
> > this is a game
> > and not a voice application, I'm stuck using the 8kHz rate.
> > What speex
> > setting would you recommend I use for the best
> > quality/performace, what
> > frame size (number of samples) to send to the encoder, etc..
> > > Kirk,
> > >
> > > Speex was designed for 8kHz, 16kHz, and 32kHz sample
> rates. If you
> > > don't use one of these sample rates, you'll be messing up
> > > assumptions deep within the codec. Why these sample rates? It's
> > > telecommunications tradition, rather than PC audio tradition.
> > >
> > > If you want an efficient and high quality format for
> voice chat, try
> > > 16kHz with VBR quality 6. You should see around a 10:1
> > > ratio when someone is talking. That is, around 25kbps would be a
> > > rough peak using these settings.
> > >
> > > If that's too much bandwidth for you, you can cut it by
> almost half
> > > using VBR quality 2. (The loss of quality will be
> noticible to most
> > > people using headsets. It is less noticible when using speakers.)
> > > For further bandwidth savings you could use 8kHz, but
> it's too much
> > > of a quality hit to be worth it in my opinion.
> > >
> > > Tom
> > _______________________________________________
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> > Speex-dev at xiph.org
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