[Live-devel] RTCP SR clock sync diff

[Ross Finlayson]

>The RTCP SR NTP timestamp is only valid in direct comparison with a 
>NTP timestamp from another stream of the same reference clock. So 
>you can use it to synchronize the presentation times of multiple 
>streams in the same session. You're not supposed to use it to 
>directly generate a presentation time since you have no guarantee 
>that you and the sender share a reference clock.

This is incorrect.  The information in incoming RTCP "SR" packets is 
used to generate presentation times from incoming RTP packets' 
timestamps.  These presentation times - like the RTP timestamps 
themselves - are (necessarily) based on the sender's clock (because 
that was the only clock available to the entity (the sender) that 
created the presentation times).

(Yes, it's true that receivers' clocks will typically drift somewhat 
relative to the sender's.  There are a number of medium-specific 
techniques (e.g dropping/repeating audio samples and/or video frames) 
that receivers can use to compensate for this, over time.  But the 
presentation times - even though based on the sender's clock - remain 
valid, and necessary.)
-- 

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