[vlc-devel] demux/live555.cpp Elementary Stream Synchronization.

Guy Bonneau gbonneau at matrox.com
Tue Feb 5 19:33:38 CET 2008

Thanks Ross for the answers. They are very helpful. I believe I have found
the cause of my synchronization problems. I switched that thread to the
live555 email list.
Guy Bonneau


From: vlc-devel-bounces at videolan.org [mailto:vlc-devel-bounces at videolan.org]
On Behalf Of Ross Finlayson
Sent: Tuesday, February 05, 2008 11:08 AM
To: Mailing list for VLC media player developers
Subject: Re: [vlc-devel] demux/live555.cpp Elementary Stream

I'm trying to play with VLC Media Player 2 elementary streams that are sent
in 2 RTP subsession with the live555 library in a testing application. One
subsession is an Mpeg2 Video Elementary Stream and the other subsession is a
2 channels audio PCM sampled at 48000 KHz. Both elementary stream are in
sync and genlocked at the source. I have much trouble getting VLC Media
player plays the 2 streams synchronized.

First, to get a/v synchronization at the client, you *must* be using RTCP,
as well as RTP.

Second, your presentation times - at the sender - must be aligned with 'wall
clock' time - i.e., the times you would get if you were to call

If your audio and video streams have correct presentation times which are
aligned with 'wall clock' time, and you are using RTCP, then - at the
receiver (VLC) end - these same presentation times *will* be delivered to
the decoding software, and the decoding software *will* do a/v
synchronization correctly.

I have looked at the code of live555.cpp in the VLC Media Player project. It
seems to me that the code is designed such that it is not possible to
receive 2 streams that doesn't have the same sample rate. In this case 90
KHz for the video and 48KHz for the audio.

No, that's not correct.  (BTW, instead of "sample rate", you mean "RTP
timestamp frequency".)  The code handles different RTP timestamp frequencies
(for different streams) just fine.

Ross Finlayson
Live Networks, Inc.

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