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Topic: How to do multichannel Vorbis encoding with channel coupling (Read 2931 times) previous topic - next topic
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How to do multichannel Vorbis encoding with channel coupling

Hi there,

as Hydrogenaudio is the site of the audio coders, i am searching for help here. Some may have noticed that we are starting to use Vorbis with our DivX/XviD movies instead of Lame, thanks to a 1st Ogg container implementation of Tobias W. ( http://www.stud.fernuni-hagen.de/q5045045/ ). Its working fine now ( 0.9.8.4 ) and you can see from the various discussions that this is in the middle of our interest for the time being, everbody is keen to drop good old AVI for one and all times ( http://www.powerdivx.com/phpbb2/viewforum.php?f=13 ; http://forum.doom9.org/forumdisplay.php?s=&forumid=54 ; http://www.everwicked.com/forums/forumdisp...p?s=&forumid=29 ).

It was perfect timing of our friend Dark Avenger to implement Vorbis support into HeadAC3he 0.21, one of the most used AC3 to MP3 transcoding tools used by DivX community. DA had to modify the Vorbis.dll to make it accept 32 and 24 bit floating point input, plus he also added multichannel capabilities, means direct 5.1 AC3 to 6 channel Vorbis transoding.

This is working fine but unfortunately we
- cant play the 6 ch Vorbis other than using PP's winamp plugin, being not very helpful for our movies, plus the channel mapping is incorrect.
- get quite huge files doing that, being about 300 kbps, so there is not much wo against using the original AC3 coming with the movies  !

We feel that with using Vorbis compared to the AC3 we could get about same quality 5.1 with average bitrates of 160 - 200 kbps. To achieve this we have to find ways to
1. Set different encoding qualities ( bitrates ) for the surround and center channels to reduce bitrates for them, compared to L/R
2. Find a way to make channel coupling feature possible for ever channel ( 'master' and 'slave' channels ).
3. Define a few basic channel mappings for different hardware/ language setups, like :
2 language Stereo ( 4 channels, 2 masters, 2 slaves )
3 language Stereo ( 6 channels, 2 masters, 4 slaves )
4 channel surround ( 4 channels, 4 masters )
5.1 one language ( 6 channels, 3 masters, 3 slaves )
7.1 one language ( 8 channels, 3 masters, 5 slaves )
5.1 two languages ( 9 channels, 3 masters, 6 slaves )
This is necessary to make sure all encoders/decoders will use same channel nrs for the mapping.


DA made clear to me that he is not as deep into the Vorbis source to be able to implement a kind of 'selectable' channel coupling feature, same for how to be able to allow different encoding qualities for the individual channels ( like 0.01 for surround/center/sub and 2 - 3 for L/R ) ....

Any input or help here is highly appreciated ... Thanks

Christian