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

Ross Finlayson finlayson at live555.com
Tue Feb 5 17:07:33 CET 2008

>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 

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 "gettimeofday()".

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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