[Vorbis] Multi-channel support

[Ralph Giles]

On Mon, Jun 26, 2006 at 02:12:15PM -0700, Daniel PEACOCK wrote:

> Are multi-channel (more than 2) formats fully supported in the OggVorbis specification ?  I couldn't
> find any information about multi-channel support on xiph.org.   I've used 'oggdropXPd' to encode a
> 5.1 wavefile and the Xiph OggVorbis libraries (vorbisfile.dll) to decode the file successfully,
> however the order of the channel interleaving is different to the original wave file (a 5.1
> WAVEFORMATEXTENSIBLE file).   Is the channel order specified for OggVorbis files, or is there a way
> I can retrieve this information from the file itself ?

Multi-channel audio is fully supported in the Vorbis format, however 
there are a number of bugs with current encoders and decoders with
respect to channel order. That's probably what you're seeing. We're 
working to get those fixed this summer; I believe recent releases of 
illi's DirectShow and Arek's Quicktime components do the correct thing, 
as well as latest gstreamer. The commandline oggenc is still ignoring 
the WAVEFORMATEXTENSIBLE channel mapping table.

The spec does specify a fixed channel order for streams with one to six 
channels (and six is 5.1 surround in the dolby configuration). See 
section 4.3.9 "output channel order". Unfortunately there is no way to 
distinguish within the stream itself between various surround 
configurations and generic multitrack audio which happen to have the 
same number of channels. The "Ogg Skeleton" enhanced metadata proposal 
is probably the best place to do so if you need this. For normal 
surround playback just getting everyone using the prescribed order is 
sufficient.

> On a related note ... do any of the encoders assume that a particular channel contains LFE only ?

All the the encoders I know treat multichannel (>2) audio as a set of
independent streams which is of course not very efficient. There is 
some low hanging fruit here in adding channel coupling and an implicit
low-pass on the LFE channel for 5.1 surround. As far as doing a properly 
tuned mode, Monty has said that his ignorance of 5.1 mixing techniques 
(and the lack of lossless masters to listen/experiment with) has kept 
him from doing much. However, both he and aoyoume-san are interested in 
working on surround modes. This is something where we really need the 
help of larger organizations and artists working in surround.

Hope that answers your question,
 -r