[gst-devel] live stream synchronization

[Edward Hervey]

On Thu, 2010-04-22 at 16:31 -0700, Attila RS wrote:
> I'm having some trouble getting playback of live streams to
> synchronize correctly (especially if playback is started in the middle
> of a stream). The way segments (start/stop/position time), base time
> and buffer timestamps are used in gstreamer is not completely clear to
> me. So a basic explanation of how synchronization is intended to
> operate in gstreamer would be helpful.

  The design documentation will give you all the gritty details on how
synchronization works :
http://cgit.freedesktop.org/gstreamer/gstreamer/tree/docs/design/part-synchronisation.txt 

  The other documents in that directory are also a good read to have a
deeper unnderstanding of other parts of GStreamer

> 
> 
> In the past for other applications I have done the following for
> something like transport stream encapsulated media: The demuxer would
> get PTS/DTS time for each frame and associate it with the frames
> buffer (gstreamer essentially does the same). The demuxer would also
> take the current PCR value from the stream and subtract the current
> system time to get a time-offset and associate that with the buffer as
> well (gstreamer doesn't do this). The buffer then gets passed along to
> decoders and render elements. Then the render element get a buffer it
> gets the current system time and adds the time-offset from the buffer
> to get the current PCR time. The current PCR time is then compared to
> the buffers PTS to see if it is time to render or not.
> 
> 
> Since an offset or diff time is not passed along with each buffer in
> gstreamer it is a bit unclear to me exactly how a sink element will be
> able to get the current pcr time.
> 
> 
> As it stands now in gstreamer the PTS values associated with a frame
> are passed along as expected and match the values in the stream, but
> they are compared to the running time, which ends up always starting
> from 0. I get something like the below from the display sink.
> 
> 
> basesink gstbasesink.c:1826:gst_base_sink_get_sync_times:<play_video>
> got times start: 0:13:16.422277777, stop: 0:13:16.455644443, do_sync 1
> basesink gstbasesink.c:1991:gst_base_sink_wait_clock:<play_video> time
> 0:00:02.848977777, base_time 3:51:48.648315390
> basesink gstbasesink.c:2842:gst_base_sink_render_object:<play_video>
> buffer late, dropping
> 
> 
> I'm assuming the demux plugin (mpegtsdemux) isn't setting the stream
> position correctly. The time value printed above should be equal to
> the current PCR time. 
> 
> 
> Anyway any help with what the plugins should be doing and how best to
> proceed would be appreciated.
> 
> 
> Attila
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