[mythtv-users] nuvexport audio sync
Joel Ebel
mythtv at ebel.mybox.org
Sun May 1 08:08:08 UTC 2011
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
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