[mythtv] Transcoding AC3/5ch and video sync
jcrews at gridlox.net
Sun Sep 17 04:37:49 UTC 2006
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
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.
I'm not at all familiar with a/v processing, and I'm trying to
understand how audio and video are kept in sync while transcoding.
Looking at -v all output, it's a bit of a pain figuring out exactly
what's going on here.
This is what I think is going on:
The process of downmixing AC3/5ch to 2 channels is causing the audio
timecodes to become wrong by about 300ms. Just converting 2 channel
AC3 (no downmix) doesn't create the delay.
I'd like to develop a fix for this, but the trouble I'm having is
finding the source of the discontinuity. What I need to know is how
exactly audio and video are kept in sync while transcoding, and, where
this is happening (I think I'm on the right track looking at
Also, a great help would be if anyone at all knows why (specifically)
the downmixing process would add 300ms to the audio's timecode.
Pinpointing the responsible code or pattern would help immensely.
Other than fixing the downmix, a workaround could just be to do a
direct audio copy from ATSC -> mpeg4, removing what I suspect is
causing the delay, and improving sound quality for passthru users
(this would of course be optional).
Thanks for any help.
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