[mythtv] Transcoding AC3/5ch and video sync

Mark Spieth mark at dclabs.com.au
Sun Sep 17 07:40:15 UTC 2006

> Hi,
> I've been working on an AC3/5ch -> stereo transcoding issue on and off
> for a while, and I'm trying to find some advice on where to focus my
> efforts.
> Here's what's happening:
> I transcode an ATSC recording with AC3/5ch audio to MPEG4, mp3 2ch.
> The resulting file has video that is about 300ms early, or, audio that
> is 300ms late, whichever way you like to look at it. This offset is
> constant, and only happens when a recording with 5 channels is
> transcoded. AC3/2ch does not create this effect at all. This may be
> difficult to notice at first, but I see it easily and it is distracting.

if you are using my patch then Im not surprised it doesnt work. only 2ch and 
6ch are curretnly supported.
5ch will generate the wrong data format and the audiooutputbase doesnt cope 
with it.
it will periodically starve and do funny things.
need to fix up audiooutputalsa to cope with the 5ch format or change the AC3 
decoder to generate 6ch format even though 5ch are only found. see a52dec in 
in 2 ch mode, a52dec is always in 2ch format so should not have a problem.
transcoding is a completely different issue and is totally unsupported for 
 >2ch if you are using internal mythtranscode support. if you are using 
external programs, then its a different question.

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