asterisk users - Apr 2007 - Linux IAX client to zaptel voice quality issue

Hi,
I've had a hard time understanding what was going on in a new * setup.

The deployment has a * box running on dual xeon RH9 stock 2.4.20-8
and some different versions of asterisk (1.2.10/1.2.16) + libpri +
zaptel + wanpipe.

Short version: audio from iaxclient clients is fine from windows
but poor from linux when going to zaptel.
E.g. Iaxcomm running on windows works fine, but the same version
of it running on linux has poor quality when going IAX->Zap->PRI
(sounds like samples are being dropped ~4 times/second).
Return audio is clear.
This happens no matter what version of linux we tried.
Finally a way of dealing with it was setting jitterbuffers in
zapata.conf up to 10 from default 4.
All audio is fine if zaptel is not used (i.e. iax to iax or iax to sip
calls sound great).

I don't really understand what the issue is, but one possibility
is that somehow iaxclient is not being able to set correct timestamps
in linux, and that in turn triggers some kind of correction in
zaptel.

FTR, clients are using DSP-400 headsets (although problem can be
reproduced with other sound devices) and the problem also happens
with analog TDM zaptel interfaces.

Keywords: linux client iax zaptel voice audio quality
-- 
Carlos G Mendioroz  <tron@acm.org>