[asterisk-users] Playback 5% Too Fast?

[Tim Panton]

On 13 Mar 2007, at 00:32, David Brazier wrote:

> Hi All
>
> I have a problem with IVR scripts which consist mainly of Playback of
> audio files, driven from an AGI application.  There are clicks  
> every few
> seconds or more frequently that is audible on the remote end  
> (PSTN), but
> not on the Asterisk recording of the call.  If I record the remote end
> and compare it to the local recording, it appears to be about 5%-7%  
> too
> fast - i.e. if I synchronise the starts, the remote end finishes  
> sooner.
> I can find points in the remote recording where parts of the waveform
> have been missed out, leading to jumps in the waveform, which  
> correspond
> to the audible clicks.  These "jumps" seem like dropped packets,  
> and I'm
> deducing that Asterisk is sending data slightly too fast (i.e. more
> frequently than 50x160 sample per second) for the remote end, which  
> has
> to drop data to keep up.
>
> This is a VoIP-only set up - no Zap hardware.  Thinking this was a
> timing issue, I have installed Zaptel to get ztdummy, which is loaded
> OK, but that hasn't made any difference.  I have tried it with  
> different
> VoIP providers and observed the same problem.
>
> Behaviour has persisted from 1.2 to 1.4 and now 1.4.1.  CentOS 4.4
> (2.6.9 kernel), Dell 1950.
>
> Any ideas how to progress?  Is this a timing issue or am I wide of the
> mark?
>
> Thanks for any help
>
> David

It would be interesting to see an ethereal trace of the packets going
to your PRI gateway. Ideally the packet capture would be done by
a separate system, so that the clock of your Dell won't also be the
'reference' clock.

Do you run NTP on that system. If you do, take a look at the
skew over a day or so and see if the Dell is running fast.

It might be worth investing in a low end digium card
just to generate a clock that is independent of your CPU
clock.

Tim Panton

www.mexuar.net
www.westhawk.co.uk/