[mythtv-commits] Ticket #5265: AVSync problem with certain BBC programmes

MythTV mythtv at cvs.mythtv.org
Sun Apr 27 20:15:33 UTC 2008

#5265: AVSync problem with certain BBC programmes
 Reporter:  jackanddougal at yahoo.co.uk  |       Owner:  ijr       
     Type:  defect                     |      Status:  new       
 Priority:  minor                      |   Milestone:  unknown   
Component:  mythtv                     |     Version:  0.21-fixes
 Severity:  medium                     |     Mlocked:  0         
 Since changing the video card in my machine to one that actually supports
 interlaced-output (connected directly to a TV via an VGA->RGB-Scart lead),
 I attempted to use XvMC as the software decoding was too slow to generate
 the full resolution display (only a 800MHz PIII:)).

 This all appeared to be good except for certain BBC programmes (generally
 the high budget dramas and the like) which have very bad audio/video sync
 issues, including stuttering, pauses etc.  In fact, when I set the
 recording to start 5 minutes early, the content before the programme is
 fine, but as soon as the programme starts (within frames) the problem

 After many hours looking at the decoder, playback and audio playback
 loops, and adding addititional logging in these areas, I determined that
 the root cause is that these programmes have multiple audio streams in
 them, #1 (the main stereo feed) one running typically 260ms behind the
 video stream, #2 (audio description) ruinning around 700ms ahead of the
 video stream.

 The problem is that the 'lastapts' value is set whenever either audio
 stream is decoded (so it swings by around 1000ms total) which confuses the
 playback code.

 The solution is to only set 'lastapts' when you are actually decoding the
 audio stream that is playing.  A patch is attached (based on 0.21-fixes)
 to do this.

 I don't know why this only effects XvMC rather than the software decoders,
 but I suspect it is due to having a limited number of XvMC frame buffers

 This problem has existed ever since the 'properly support multiple audio
 streams' change went in at 4281, so it may well account for the large
 number of NVIDIA users on the web complaining about choppy playback with
 SDTV content.



Ticket URL: <http://svn.mythtv.org/trac/ticket/5265>
MythTV <http://www.mythtv.org/>

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