[gst-devel] Some Basic questuions

[Wim Taymans]

On Tue, 2008-06-17 at 11:02 +0530, Manish Rana wrote:
> Hi, 
> 
> I have 1 confusion regarding RTP if someone could help.
> 1. Where does NTP sit in the current gstrtpbin arch.

gstrtpbin keeps and sets an NTP time on the session managers so that
they can generate SR RTCP packets for the receivers. Roughly, the buffer
timestamps are added to the NTP base time to get an NTP-RTP timestamp
pair that is sent to the receiver in SR packets. Since all session
managers have the same NTP base time and the timestamps (+segments)
define interstream sync at the gstreamer level, SR packets from
different sessions can be synced together at the receiver. (grep for ntp
in gstrtpbin.c and gstrtpsession.c to see how this is done).

When receiving SR packets, the NTP times are used together with the RTP
timestamps to align streams from multiple sessions. The algorithm is
explained in the RTP book by Colin Perkins, Chapter 7. The code for this
in gstrtpbin.c is in gst_rtp_bin_associate().


> 2. How do i get the clock-base time at the receiver end.
>        a. Is it through SDP?

Usually not, I have never seen that nor any RFC that defines this. It
would also require the SDP to be generated dynamically, which is usually
not the case.

>        b. Is there any other way possible ??( If yes Please let me
> know)

clock-base is mapped to the RTSP rtptime property in the RTP-Info Header
field from the RTP PLAY request. It's very RTSP specific, I've never
seen it outside of an RTSP context.

> 3. Will clock-base be used for AV sync ??

If specified it will be used to synchronize steams before the first SR
packet has arrived. It's a quick way to get synchronized streams without
having to rely on SR packets.

Regards,
Wim 

> 
> Best Regards
> Manish Kumar
> 
> 
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