[mythtv] Transcoding AC3/5ch and video sync

Mark Spieth mark at dclabs.com.au
Mon Sep 18 02:03:38 UTC 2006

>> I have identified the problem but need to figure out a fix.
>> need some way of telling avf that max channels is less.
> Isn't avf giving us 2 channels?
>> however transcode may transcode to ac3 which means leave as 6ch if 6ch so
>> need some way of querying the nvr and telling nvp which passes it to avf.
>> so in the mean while use 2ch mode for any backend/frontend combinations.
> I'm not sure what you mean by this - the ATSC MPEG2 ts I get is 6ch, I
> can't change this. Certain programs just have 6 channels, where others
> have 2 (mostly the SD transfers on the HD transport). My
> avformatdecoder has the maximum channels set to 2 (I changed nothing).
>> frontend only machines are ok of course to leave as 6ch.
>> assuming there are no other problems :)
>> cheers
>> mark
> Now I'm just a little bit confused :) Am I missing something monumental 
> here?

I suspect you arent using my multichannel audio patch. The above discussion 
is then irrelevant.
the a52dec does the downconverting to 2ch always in a standard .20 myth 
the timestamps are calculated against blocks. you will see slight jitter 
between audio and video timestamps but they will keep in sync unless 
something is drastically wrong.
the sync adjust is in nvp and while you could do something automatically, Id 
suggest that there is probably something wrong with the stream. lavc a52dec 
does a good job at downconverting all combinations.
perhaps there needs to be some defaults for various channel/stream types for 
setting some things automatically.
or perhaps a transcode jobsched which you can shift audio by a specified 

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