[Speex-dev] Sampling Rate

[David Hogan]

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
artifacts.

Cheers,
David Hogan

> -----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
> 
> Hi,
> 
> 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!
> 
> Cheers,
> 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 
> important
> > > 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 
> compression
> > > 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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