[Asterisk-bsd] audio quality problems on IAX channels

[Michael Iedema]

Greetings everyone,
I'm experiencing some audio problems on IAX channels which I believe
are related to timing issues.

Running Asterisk 1.4.11 on FreeBSD 6.2-p7, I experience garbled /
robotic audio on IAX channels between my box and a IAX provider. GSM
is being used as it is the only choice so I have no chance to test out
another codec in my situation.

These issues were not present when I was running 1.4.9 and had no
zaptel modules loaded (except ztdummy). After upgrading to 1.4.10, the
problem appeared but was fixed in most situations by only loading
either zaptel driver modules or ztdummy. Loading both caused the
problem. Only with IAX channels do I still have bad audio.

Are there any "gotcha"s with IAX timing I should be aware of? Any
settings for the GSM codec or jitterbuffer which may remedy the
sitaution?

Thanks in advance,
-Michael I.