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

[xlab]

> 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
>  
>
We have tried it with and without the mixing option.  Sox was not used.  
The recorded sounds are garbled sounding.  There appears to be a 3-4 Hz 
signal in the audio that causes the amplitude to decrease momentarily 
for around 26ms at that rate.
As I stated previously, this only happens when gsm is used on the 
phones.  It is still present in the recording regardless of the three 
ways the recorded file is saved (gsm, wav, WAV).  The problem is not the 
normal degradation of the gsm codec, it is much worse than that.  Any 
other ideas?  I can send a sample file to listen to if that would help.
Thanks,
Tom