[vlc-devel] Re: lip sych problem

Viktor Kompaneyets vato at wnet.ua
Mon Nov 28 12:05:52 CET 2005

Ross Finlayson wrote:

>>Should add here - RTP streams are actually 2 (at least) different streams
>>with different payloads
> This is correct.
>>that are not syncronised well (look at live555
>>mailing list). I.e - system will "sync on average bitrate", not on each
>>video packet. It means, that LiveMedia free library do not contain
>>mechanisms to make A/V sync/resync.
> But this is all completely false.  See
> <http://www.live555.com/liveMedia/faq.html#separate-rtp-streams>

Theoretically - yes. But, I'm working with MPEG streams, that came from
DVB/Sattelite. So - they DO HAVE errors within it.

> Once again: If your original data source (at the server end) has (i)
> proper presentation times (corresponding to the 'wall clock' time
> generated by calling "gettimeofday()")

When do you're working with _files_ - yes. If You're handling streams - in
common - no.

> , and (ii) it properly 
> implements RTCP, then RTSP/RTP clients that use the "LIVE555
> Streaming Media" software - including VLC - *will* get correct,
> time-synchronized presentation times.

... At each I-frame? I said about true resync mechanism, that works without
frame dropping. It works with MPEG TS, but gives wrong A/V sync within RTP.
And if there is transcoding-the things goes even vorse.

> If, however, your server does not implement RTCP, and/or does not use
> properly synchronized presentation times to begin with, then you're
> SOL.  

All of my servers are well RTCP-implemented, but I should work with _bad_ or
_near-to-bad_ quality streams as sources.  It can have _any_ kind of
timestamping errors. So I'm using ony that technics, that can handle that
streams. Unfortunatelly - Live555 is not that soution.

> End of story. 
> I spent a lot of time making sure that the LIVE555 RTP/RTCP
> implementation works properly - to generate correct,
> time-synchronized presentation times at the client end. 

Try using streams from DVB budget cardg as source - and You will see a lot
of troubles with RTP.

> I wish 
> people would stop claiming that this doesn't work.

But - it works only in _clean_ environment. Try it at wild....

Viktor Kompaneyets
Wnet project manager

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