[opus] ambisonics formats and channel mappings

[Marc Lavallée]

Hi Opus list.

I subscribed because your discussion on the IETF draft ("Ambisonics in
an Ogg Opus Container") was mentioned on the sursound list.

I tried Opus for ambisonics more than a year ago. It does works with
uncoupled channels (I had to patch the encoder). I don't know what else
could be done to optimize support for ambisonics, as I'm not a codec
expert. 

So I think that the easiest (and most important for now) is to disable
channel coupling and define the metadata.

The allowed number of channels should not be restrained to a list like
1,4,9,16,etc, because ambisonics can support mixed-order schemes.
The Ambix format was adopted by Google, but it's a new format; 
the FuMa format is widely used and could easily be supported as well.
https://en.wikipedia.org/wiki/Ambisonic_data_exchange_formats

Down-mixing to stereo is a sort of ambisonic decoding; it is simple and
could be included in the Opus decoder. But decoding to binaural or 5.1
is not trivial. Usually, decoding ambisonics is the job of a dedicated
decoder, and depends on the role, number and positions of output
channels. so I don't think that the Opus decoder should be involved,
unless approximate methods could be considered good enough as default.

--
Marc