Re: [AVT] RTP: explanation of "sampling instant"

[Dave Singer <singer@apple.com>]

> > Instead, for each medium the RTP timestamp is
> >              related to the sampling instant by pairing it with a
> >              timestamp from a reference clock (wallclock) that
> >              represents the time when the data corresponding to the RTP
> >              timestamp was sampled.  The reference clock is shared by
> >              all media to be synchronized.  The timestamp pairs are not
> >              transmitted in every data packet, but at a lower rate in
> >              RTCP SR packets as described in Section 6.4.
> >
> >              Applications transmitting stored data rather than data
> >              sampled in real time typically use a virtual clock to
> >              determine when the next frame or other unit of each medium
> >              in the stored data should be presented.
> I think "determine" is the wrong word here. Its the receiver that 
> determines when to present it. The sender indicates when it should 
> be presented. So, I would propose:
>
> Applications transmitting stored data rather than data sampled in 
> real time typically use a virtual clock to indicate when the next 
> frame (or other unit of each medium) in the stored data should be 
> presented.
This has always been my problem about how this is presented;  it is 
an unusual protocol description which describes what happened at the 
source.  Usually protocols describe characteristics of the stream or 
what the receiver ought to do.  In particular, there is no 'sampling 
instant' for streams sent from disk under rtsp control, for example. 
However, there is a reference common clock which can be used to 
relate the RTP timestamps in the various streams.
--
David Singer
Apple Computer/QuickTime
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