[opus] Channel Mapping Family for Ambisonics

[Jean-Marc Valin]

Hi Michael,

On 04/25/2016 05:32 AM, Michael Graczyk wrote:
> Channel Mapping Family 2
> 
> Allowed numbers of channels: (1 + l)^2 for l = 1...15.
> 
> Explicitly 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196,
> 225. Ambisonics from first to fifteenth order.

Would it make sense to allow an arbitrary number of channels and just
"truncate" the list of channels. For example, two-channel ambisonics
would be W plus X and three-channel would be W, X and Y. The idea is
that you would get these mappings for free -- if there's any use for
them anyway.

> 1. Should I be more explicit about what "ambisonics" is, what
> normalization is, and what each channel really means?

For these kinds of things, you can just use references.

> 2. Do I need to list the specific meaning of each possible channel
> count? For example, should I write "4 channels: First order
> ambisonics" similar to what is done for surround sound?

I'm not sure I understand what your asking here.

> 3. Should we define whether downmixing should occur when Ogg Opus
> players do not support the channel mapping? Since channel 1 is a mono
> channel, it is always possible to play something reasonable.

It's not absolutely required, but it would indeed be nice if you could
define a simple way that players can downmix. I guess mono is just "take
the W channel", but maybe a reasonable stereo would be nice too.

Cheers,

	Jean-Marc