Transmit absolute time

[Sebastian Dröge]

On Mi, 2015-10-14 at 23:38 +0200, Pietro Bonfa' wrote:
> Dear Sebastian,
> 
> RFC6051 looks exactly like what I need but implementing it into
> gstreamer means modifying the plugins performing the RTP payload and
> depay, am I correct? This sounds like a hard work for a newcomer of
> gstreamer like me.

It would be separate elements that you put before the depayloader and
after the payloader. Only tricky bit is to specify the RTP header
extension ID.

> As of today, the simple approach of starting the acquisitions on the
> RPis "at the same time" gives the best synchronization (since the
> acquisition frequency is fixed).

So, do you want *synchronisation* or just get the capture times
according to some network clock as metadata? :)

If you want synchronisation, see what Carlos wrote and also take a look
at
http://cgit.freedesktop.org/gstreamer/gst-rtsp-server/tree/examples/test-netclock.c
http://cgit.freedesktop.org/gstreamer/gst-rtsp-server/tree/examples/test-netclock-client.c
That's very similar to what Carlos did, but gives you not 100% accurate
synchronisation, the RTP header extension could. The synchronisation
will be up to 11 microseconds accurate with the example code there
(90kHz RTP clock rate).

These two are using the GStreamer netclock, but it's a one-line change
to make it use the PTP clock.

> For this reason a metadata stream could be a reasonable approach as
> long as I'm sure I can grab all the frames. References for this
> approach? Chapter 12 of the manual?
> 
> Finally, there's plenty of space in my mailbox! If you could send me
> (or upload) the presentation maybe I would find some keywords to
> deepen this topic.

You have mail (in a few minutes) :)

-- 
Sebastian Dröge, Centricular Ltd · http://www.centricular.com

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