[gst-devel] live stream synchronization

[Attila RS]

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.

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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