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

[Tony Mountifield]

In article <427928FC.6070001 at tidalwave.net>, xlab <xlab at tidalwave.net> wrote:
> 
> > 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.
> >
> >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.
> >
> 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.

Oh well, it was just a thought as I read your posting....

> 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.

Sorry, but codecs are outside my area, and lack of time is a problem...

Hope you find the solution

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