[Vorbis-dev] 5.1 surround channel coupling

[Sebastian Gesemann]

Sebastian Olter wrote:
> The most logical way (i.e. involving the simplest logic :P) is to map
> B-Format channels 1:1. Vorbis is a rare case of a codec that allows up
> to 255 discrete channels to be encoded, and its official encoder,
> oggenc, needed just 2 lines of code to be tweaked in order to handle
> this. My patch includes this simple modification as well.

Does your patch make use of another mapping type other than 0 ?

The mapping type refers to a a field in the codec setup packet that is 
transmitted for each "mapping", see section 1.2.3, 4.2.4.5 and 4.3.9 
(quoted below).

According the last quoted paragraph an Ambisonics format should use a 
mapping type other than 0. But these are still undefined.

According to
http://en.wikipedia.org/wiki/Ambisonics#Possible_combinations
the different Ambisonics flavours including UHJ can be uniquely 
idenfified by its channel count. So an extension to the Vorbis format to 
support Ambisonics properly could also use an implicit channel mapping 
for Ambisonics (proposed mapping type 1) like it's the case for mapping 
type 0.

Note: Current decoders will refuse to play streams with mapping types 
other than 0.


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Section 4.3.9. Output channel order:

Vorbis I specifies only a channel mapping type 0. In mapping type 0, 
channel mapping is implicitly defined as follows for standard audio 
applications:

one channel
     the stream is monophonic
two channels
     the stream is stereo. channel order: left, right
three channels
     the stream is a 1d-surround encoding. channel order: left, center, 
right
four channels
     the stream is quadraphonic surround. channel order: front left, 
front right, rear left, rear right
five channels
     the stream is five-channel surround. channel order: front left, 
front center, front right, rear left, rear right
six channels
     the stream is 5.1 surround. channel order: front left, front 
center, front right, rear left, rear right, LFE
greater than six channels
     channel use and order is defined by the application

Applications using Vorbis for dedicated purposes may define channel 
mapping as seen fit. Future channel mappings (such as three and four 
channel Ambisonics) will make use of channel mappings other than mapping 0.

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Cheers!
Sebastian