[Asterisk-Users] Re: Audio quality problem recording calls using gsm codec

[Tony Mountifield]

In article <42781D12.2050105 at tidalwave.net>, xlab <xlab at tidalwave.net> wrote:
> When using phones that are using G.711 codec and the calls are recorded
> with "Monitor", when played back the files sound great.
> 
> When we use gsm codec at one or both ends of the call, the recorded
> files sound very bad.  Much worse than the audio sounds during the call.
> 
> With the "Monitor" command we have tried WAV, wav, and gsm and this does
> not make any noticable difference, the sound quality is still poor
> (actually about the same each way).

This is probably because Asterisk calls sox to mix the separate incoming
and outgoing files into a single file. In order to mix two gsm files,
sox will need internally to convert them both to linear, do the mixing,
and then convert back to gsm. Since gsm is not a lossless compression,
the sound gets worse with each conversion round-trip.

I'm not sure what you can do about it. Try wav again, which is supposed
to be linear. WAV and gsm are both GSM compressed. Or possibly you could
try signed linear explicity as a format (can't remember whether it is
sln or slin).

Cheers
Tony
-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
Play: tony at mountifield.org - http://tony.mountifield.org