[Live-devel] track synchronization with live555

[Ross Finlayson]

***This will be my last posting on this thread.  Please do not bother 
responding, unless you don't mind hearing nothing more back from me 
about this.***

I think you're misunderstanding what it is that the "LIVE555 
Streaming Media" library is intended to deliver to higher-level code 
(such as media players).  In this case, it intends to deliver exactly 
the information that is provided by the RTP/RTCP and RTSP protocols. 
In particular, it provides:

1/ A presentation time, in the sender's clock, which is accurate 
after the first RTCP "SR" packet is received.  (Unfortunately, but 
unavoidably, it's not accurate before this time; however, the library 
also provides a Boolean function that tells you when the presentation 
times have become accurate.)

2/ The information that's delivered - by the server - in the 
"RTP-Info" header.  Note that it provides the *exact* information 
that the server delivers.  It doesn't do any translation on this 
information; that's not the job of the LIVE555 library.

It's up to the higher-level code (such as yours) that uses the 
LIVE555 libraries to make use of this raw information, however it 
sees fit.

Note, however, that these two pieces of information - the 
presentation time, and the RTP-Info header - are only loosely related 
(see section 12.33 of RFC 2326).  In particular, the RTP-Info header 
information is certainly not intended to ever replace the 
synchronized presentation time that's provided by RTCP SR.  If you 
wish, you can use the RTP-Info information to map the presentation 
time to NPT (using a fixed offset, which you can compute once the 
first RTCP SR arrives ).  But you shouldn't just ignore the 
presentation time, and think that you can just use the RTP-Info 
information, plus the RTP timestamps in each packet, to generate your 
own substitute 'presentation times'.  You can't do this, especially 
if you want to guarantee A/V synchronization over time.
-- 

Ross Finlayson
Live Networks, Inc.
http://www.live555.com/