[Live-devel] How to get NPT from RTSPClient?

[Luca Abeni]

Hi Ross,

On Tue, 2005-10-18 at 04:27 -0700, Ross Finlayson wrote:
> >How can I obtain the current NTP time instead?
> >I mean, not the presentation time of the frame, but an NTP time that I
> >can use for synchronizing the player's time and the streamer's time.
> 
> The "presentationTime" *is* time-synchronized with the sender.  See 
> <http://www.live555.com/liveMedia/faq.html#separate-rtp-streams>
I think I did not explain myself well (or I am not understanding your
answer and the FAQ - I apologize in both cases). I am not thinking about
synchronizing the audio and video streams, but I am thinking about
compensating the drift between sender clock and receiver clock.

I know presentationTime is time-synchronized with the sender, but when I
play a frame on a client I use the client time to decide when to display
it... So if I do not adjust the time to compensate for the drift between
sender time and player time I'll have some long-term stability
problems... Or are you saying that presentationTime is already
compensated?

> >If RTP packets are delivered without delays (or with a constant delay),
> >I can use the presentation time of the last received frame, but what can
> >I do if RTP packets can be delayed (for example because the streamer is
> >buffering some packets to deliver them at a constant bitrate)?
> 
> It doesn't matter.  The RTP/RTCP implementation automatically takes 
> care of this.
Again, maybe I am misunderstanding something (I apologize about that)...
My problem is that if a frame arrives (let's say) 3 seconds late I fear
I cannot use its presentationTime (as videolan does) to compensate the
drift between the sender's and the player's clock.
Or am I wrong?

In other words, I'd like to have some function telling me "the sender
time is now T", and not "the presentation time of the last received
frame is T".

			Thanks,
				Luca
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