(#487) resampling (was Speakfreely patches)

Message posted to the Speak Freely Forum
  by John H on Wed, 8 Oct 2003 02:12:46 +0200 (MEST).
  http://www.fourmilab.ch/wb/speak-freely.pl?rev=487

  Reply To: (#486) (#484) Speakfreely patches
  Author:   "Wm. Burke ....
  Date:     2003-10-07 12:41

: Forwarded from the [email protected]
: mailing list: I'm curious, are you sampling
: at 44kHz and then
: truncating that down to 8kHz?

With that mod, I was sampling at 44kHz and then downsampling before doing the
GSM etc compression. Then at the other end upsampling after decompression. I
use a polyphase filter bank on each end.
The reason I did it was that the sound card I was using at the time did a poor
job of 8kHz sampling. I'm pretty sure it had no anti-aliasing filters on it.
That meant that all the noise in the 4kHz to 22kHz+ band was getting reflected
back into the 0-4kHz band, making the recording a lot noisier than necessary.
So by sampling at 44kHz, I could add my own anti-aliasing filter to cut the hf
noise out.
But really, the only reason I can see to use this is if the audio hardware
and/or drivers aren't doing the job properly at 8kHz - otherwise you are best
off just sampling at 8kHz in the first place. I don't know how many cards have
this faulty sampling - but mine was the motherboard built-in one with a VIA
chipset.


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