[mythtv] Transcoding AC3/5ch and video sync

Mark Spieth mark at dclabs.com.au
Mon Sep 18 01:11:37 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.
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.
frontend only machines are ok of course to leave as 6ch.
assuming there are no other problems :)

----- Original Message ----- 
From: "Jesse Crews" <jcrews at gridlox.net>
To: <mythtv-dev at mythtv.org>
Sent: Monday, September 18, 2006 1:46 AM
Subject: Re: [mythtv] Transcoding AC3/5ch and video sync

> Quoting Mark Spieth <mark at dclabs.com.au>:
>>> 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.
> Using .20 11197, clarificaiton - I meant 5.1 (6ch) audio.
>> 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
>> lac.
>> 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
> Using internal, input format to mythtranscode looks to be 2ch MP3. It
> spits out a "Copying Audio and transcoding Video" which is why I think
> that it's being fed MP3 (the driving conditional statment says so).
> Just to make sure, I actually want the output to have 2 channels, so
> I'm not saying this is undesirable.
> I don't think I've seen the entire audio transcoding pipe, mostly
> because it's a little difficult to follow. I see lavc is being given
> audio samples and the codec for whatever is in the source, and it's
> spitting out the 2ch format that nvr/mythtranscode wants. I initally
> thought the MP3 was done elsewhere, but it looks like lavc is being
> given MP3/stereo as the requested output format/code, right?
> So, with this in mind, how are the timecodes controlled and kept
> synchronous after lavc has done its magic with the 6->2 channel downmix?
> More generally, how (abstractly) are the audio and video kept
> synchronous during this process? The timecodes on the -v playback
> appear to move around quite a bit in relationship to each other. How
> about those last_apts values?
> I hope that helps clarify what I'm looking at.
> Please bear with me, I'm not familiar with the finer points of a/v 
> processing.
> Regards,
> Jesse
>> external programs, then its a different question.
>> cheers
>> mark
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