[mythtv-users] trandcoding messes up 6 channel audio slow audio playback

[Jean-Yves Avenard]

On 31 December 2010 21:47, Gavin Hurlbut <gjhurlbu at gmail.com> wrote:
> Define "work".  Yes, every 5.1 AC3 file I've tried successfully got 6
> channel PCM out (in LF, RF, LR, RR, C, SW order).  However, ffmpeg is
> stupid and has no concept of how to downmix PCM from 6->2 channels at
> ALL.  It can do AC3 -> 2 channel PCM, but it can't handle 6 channel
> PCM, apparently.  Therefore I have to preprocess to get stereo so
> ffmpeg can use it.  If we can do this downmixing right in
> mythtranscode, it would be better, I think.


yeah, ffmpeg doesn't do any downmixing other than with the AC3 and DTS codec.

They don't even have a downmixing muxer ; it's only done at the codec
decoding level.

That's why I put the downmixer in mythtv.
With AAC, with libfaad there used to be a downmixer but as it was then
removed in favour of the builtin ffmpeg decoder.

The only type of multi-channels audio I've ever seen are AAC, DTS and AC3.

There are some ogg file with multi-channel audio, but never saw those
with a video

It's a big downside of ffmpeg IMHO. Everything looks like it's
downmixing, all the APIs are there (you specify how many channels out
you want) but all you get are the front and left channels, center and
surround are gone


I'll see what I can come up with for mythtranscode, however, I'm away
for most of January, so that won't happen before february

JY