[asterisk-users] Sox and bad quality when converting to 8 kHz

[Steve Underwood]

On 03/25/2011 04:58 AM, Thomas Winter wrote:
> Hi list,
> I have an 44100 Hz file with human voice, stereo with 16Bit.
> When convertig this to 8 kHz, mono I loose a lot of quality and have
> some ground noise. I tried several sox options but without success.
> Can somebody help....
>
> best regards Thomas
You really need to remove the bass end of the spectrum before 
downsampling to 8k/s. uLaw/ALaw sound pretty muddy and horrible if you 
don't do that, and the other common 8k/s codecs don't sound any better. 
Jean-Marc Valin wrote a little filtering utility for this purpose.

Steve