Transmit absolute time

Sebastian Dröge sebastian at
Fri Oct 16 06:28:28 PDT 2015

On Do, 2015-10-15 at 11:42 +0200, Pietro Bonfa' wrote:
> Dear Sebastian and Carlos,
> thanks for this extremely stimulating discussion. Carlo's work is very
> encouraging for me, it is exactly what I want to do.
> I'm carefully reading your messages in order to come up with a solution
> that incorporates both your comments.

Let us know if you run into any problems or have further questions :)

> Just to make it clear: I have frame-grabbers working at 40 fps which are
> allowed to start on every second (all times referred to synchronized PTP
> clocks).
> So, in principle, all frames from the frame-grabbers are like
> n.000, n.025, n.050, n.075 etc etc, where n is an integer number of
> seconds since a reference time (say Epoch).
> In reality (considering only two frame grabbers for simplicity) I have
> frame-grabber 1 acquiring frames on
> n.001, n.026, n.051 etc etc
> and frame-grabber 2 acquiring frames on
> n.003, n.028, n.053 etc etc
> So the problem here are, of course, those 2 ms and not the 11
> microseconds of RTP.

So you mean that the frame grabbers are not producing a frame at 0.025
second boundaries but somewhere around that? And you want all frame
grabbers to produce a frame at exactly N*0.025 seconds, i.e. all of
them at the same time?

In that case we would need more details about how you grab frames. You
would need to modify that code to ensure that frames are grabbed at
exactly the required times. Once that happened you can then forward
that all to GStreamer as discussed before and it will all work fine.

Sebastian Dröge, Centricular Ltd ·
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