[vdr] A/V out of sync for BSkyB channels Channel4, More4, E4, Film4

Günter Merz lotan_rm at hotmail.com
Sun Jul 18 16:47:16 CEST 2010

I've found a solution to my longstanding problem: Video and audio being out of
sync on my vdr system using xine-lib as output display/control application.

Having excluded basically any other component, I started debugging xine-lib.

The first thing I've found was: When video and audio become out of sync,
metronome doesn't seem to get any audio pts anymore, they are always 0.

I checked the mpeg_pes demuxer but the audio pts were read from the stream

The xine libmad adaptor seemed not to forward the pts to the metronome:
It buffers the MPEG audio packets until a threshold is reached (MAD_MIN_SIZE:
2889 bytes) and then has libmad decode the packets which is send to audio out.
The pts of the last audio packet is forwarded on to metronome which can then
sync video with audio.

For the channel4 channels MPEG audio packets have a size of 576 bytes which
means it takes five packets to fill the buffer enough for processing. In the
stream every fifth audio packet contains a pts.

The result of this is: If after a seek, the last audio packet is the one with
the pts, video and audio are in sync. If the pts is in any of the four previous
ones no pts will reach metronome and video and audio will never be synced before
a new seek and even then there's a one in five chance that video and audio are
not synced.

Other channels did not show this behaviour because e.g. BBC One has an audio
packet size of about 750 bytes and send a pts every fifth packet as well. This
means that not every pts from the stream gets through to metronome but some do.
This also means that syncing after a seek is probably not as quick as it could
be but it will sync.

My workaround to this problem is for the libmad xine adaptor (1.2 version) to
start decoding not only when a the buffer has reached a theshold but also when a
pts != 0 arrives. This does mean however that the buffer isn't always filled to
the theshold and decoding might not perform as well as it could (xine developers
are welcome to think of a better solution):

--- src/libmad/xine_mad_decoder.c.old	2010-07-17 20:00:54.000000000 +0200
+++ src/libmad/xine_mad_decoder.c	2010-07-17 20:02:10.000000000 +0200
@@ -190,7 +190,7 @@
     mad_stream_buffer (&this->stream, this->buffer,
-    if (this->bytes_in_buffer < MAD_MIN_SIZE)
+    if (this->bytes_in_buffer < MAD_MIN_SIZE && buf->pts == 0)
     if (!this->needs_more_data) {

I hope this is helpful to other people.

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