#5265: AVSync problem with certain BBC programmes
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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
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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
occurs.
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
available.
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.
Cheers
Daniel
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Ticket URL: <http://svn.mythtv.org/trac/ticket/5265>
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