[asterisk-users] Digitized audio at the beginning of a call

[Andrew]

Hi,

Apologies if this has been asked before, but I don't seem to be able  
to find any info on it anywhere.

Sometimes when placing a call on hold, the caller hears digitized/ 
robotic music on hold that gradually improves over the course of  
about 20 - 30 seconds until it sounds pretty normal. The first time  
the call is placed on hold the music sounds normal. If that same call  
is resumed and placed on hold again, the quality is poorer. A third  
time usually results in what I have described. Sometimes it is bad  
straight off but this pattern is typical.

A few specifics about this problem:

- It only happens when the call originates remotely from another  
identical Asterisk PBX. It never ever happens on a local call. It  
also never ever happens on an inbound call from a local PSTN gateway  
(Cisco router with a SIP dialpeer). It is only when the call comes  
from the remote PBX. SIP is being used throughout.

- The problem happens almost every time when the called phone is a  
Polycom 601. It happens much more rarely on Cisco 7912, 7940 and 7941  
handsets, although when it does happen the symptoms are the same.

- Reinvites are completely disabled on both PBXs.

- Codec is G.711alaw end to end. Music on hold is playing as native  
sound files in alaw format.

- When transferring a call, the symptoms often carry over into the  
beginning of the transferred call so that the caller will hear  
digitized voice which gradually improves. The problem doesn't appear  
to be hold specific, but it is the easiest way to replicate the  
symptoms.

I would love to know general reasons why digitized audio improves  
over time, almost as if it is training up. I'm also scratching my  
head as to why this happens more on the Polycom, given that Asterisk  
is playing the music and not the phone. Any pointers would be greatly  
appreciated.

Regards,
Andrew