[asterisk-dev] About zitter and delta control

[Dan Austin]

Rifat wrote:
> Now my problem is that, when packets are splitted in kernel space, no
> harm is done. But when packets are merged, i see somehow large zitter
> and delta that makes the voice robotic. I am quite new to this line.
> So from my experience I can say nothing on which side it is the
> problem. I carefully observed RTP sequencing and timestamping, and
> yes, it is handled perfectly. To have a work around, I suspect the
> following things:

I suspect the jitter may follow a pattern, as I saw something similar
when packetization support was added to Asterisk.  Due to how outgoing
packets were the average jitter would approximate the smaller ptime of the
two legs.  This was easiest to see with p1=20ms and p2=30ms:

0.00 - start conversation
0.02 - p1(1) arrives, not enough data to assemble p2(1)
0.04 - p1(2) arrives, p2(1) assembled, leaving 10ms in the buffer
0.06 - p1(3) arrives, p2(2) assembled and sent 10ms early, buffer empty
0.08 - repeat

The device receiving p2 would see two 30ms packets in 40ms and then
the next packet after another 40ms.  Some devices handled the jitter, 
others would reproduce the audio with a 'robotic' quality.

If your module is sending packets as soon as they are complete you
would see this.  If you are actually using the larger ptime to
schedule the packet transmission, then this observation is not
likely to help you.

Dan