[Speex-dev] Downsampling

[Jean-Marc Valin]

Hi,

For transforming stereo to mono, averaging is fine and that's what
everybody does. For sampling rate conversion, it's another matter (too
long for this email) and you should read a bit about it a perhaps grab a
library that does that.

As for echo cancellation, it will be less complex (and as good) on a
(cleanly) down-sampled signal (and certainly not on stereo).

	Jean-Marc

Le mardi 07 juin 2005 à 22:16 +0200, Thorvald Natvig a écrit :
> Ok, this is slightly offtopic, but relates to the quality of input for 
> speex :)
> 
> I'm working on echo cancellation by means of sampling the wave mix 
> of the sound card as well as the microphone. I originally had two sound 
> cards, which had some synchronization problems (now solved, more or 
> less), but I have also discovered a much better solution using ASIO 2.0, 
> which enables me to sample multiple sources on the same soundcard.
> 
> Unfortunately, ASIO is limited to the internal sampling frequency of the 
> sound card, which for my Audigy 2 is 48 khz, and only delivers stereo. As 
> I understand it, just picking every 3rd sample and averaging left and 
> right will introduce aliasing, so I added a FIR-filter to remove all 
> components above 7.5 khz before downsampling. While I can't say I really 
> notice any difference in audible audio quality, the samples seem to have 
> slightly better SNR when inspected in various audio programs.
> 
> However, I still downmix to mono by just averaging the left and right 
> samples, which (from my googling) I can tell will introduce aliasing but I 
> found no good highquality solution to this :(
> 
> The question is, is this FIR-decimator the best method to downsample? I 
> thought of converting the frame to the frequency domain, drop components 
> > 8khz and convert back, but I'm not sure if that would produce a 
> better signal or worse.
> 
> Also, would it be best to do echo cancellation and preprocessing at 48khz 
> and then decimate to 16khz, or (as I currently do) decimate first then do 
> echo cancellation and preprocessing?
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-- 
Jean-Marc Valin <Jean-Marc.Valin at USherbrooke.ca>
Université de Sherbrooke