[mythtv-users] nuvexport audio sync

[Joel Ebel]

After my most recent mythtv setup, I have a number of problems with 
nuvexport.  The symptoms  I have are this:

* Very slow transcoding (4 fps)
* Audio at 1/3 speed after transcoding

The slow audio and slow transcoding seem to be related.  I believe this 
is the result of the number of channels not being detected correctly. 
5.1 audio is still being detected as 2 channel audio.  Each sample 
interval reads only 2 of the 6 channels, so it takes 3 sample intervals 
to read all 6 channels.  The result is that audio is 3 times slower.  By 
changing the sample rate up to 144 kHz (48 * 3), I effectively fix the 
audio speed, and as a result, the transcoding speed increases 
dramatically, since the audio and video are then in sync.  But that's 
the wrong way to down mix 6 channels to 2.

If the source audio really is 48 kHz 5.1 channel, then why didn't 
nuvexport detect that and pass the right options on to ffmpeg?  It 
appears to be a property of the episode object which comes from mythtv 
perl bindings.  The perl bindings appear to get this information from 
mplayer.  My mplayer output shows the correct number of channels if the 
lavf demuxer is used, but the perl bindings try mpegts first and appear 
to be satisfied with the output from the mpegts demuxer.  Even if I 
force the use of the lavf demuxer, I get ffmpeg errors regarding the 
number of channels for the output stream.  I guess it's trying to 
preserve 6 channels into an mp3.  This will take some hacking to work 
around, but I'm curious if anyone else has had to resolve this problem.

I've also experience very poor video quality, but I haven't yet 
investigated causes.

Thanks,
Joel