Synchronization of multiple senders using rtpBin

[andrew2412]

Dear GST-devs,

I am using gstreamer pipelines to send packets with encoded audio and to
receive those packets, decode and play/save the audio on the receiver. My
case differs from other similar applications, because I have many senders
and only one receiver. I'd like to have streams from all senders
synchronised, so I use RTPbin with property rtpjitterbuffer-mode set to "4"
- synced receiver and sender clocks. System clocks on all devices are
synchronized outside the gstreamer just before streaming (no clock update
during the streaming). 

Problem: I have observed that streams from different senders are shifted in
time by at least several microseconds. Exact number of ms changes with time
of recording - at the beginning the difference is very low, and after a
couple of minutes I see that the difference gets bigger. Also, when turning
the streaming app on and off again, the initial difference seems to be
random.  My test signal is metronom, so I can compare streams in an easier
way.

Can you please give me some advice on how to minimise the time difference
between streams from multiple senders? 
Second thing, I have a concern that the RTPJitterBuffer in RTPbin element
does not use NTP time from RTCP SR packets  - is that so?

Best regards,
Andrzej Ruminski



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