[opus] Channel Mapping Family for Ambisonics

Michael Graczyk mgraczyk at google.com
Tue May 3 04:30:10 UTC 2016

On Fri, Apr 29, 2016 at 4:32 PM, Timothy B. Terriberry
<tterribe at xiph.org> wrote:
> As a general point, it's too late to add anything to the soon-to-be RFC
> 7845. It's too far along in the publication process. This would have to go
> in a separate document (which, other than having to add an abstract, brief
> introduction, and some other boilerplate sections, shouldn't be a big deal).

That makes sense. Also congrats on the RFC. Could you link me to an
example of such a document? I don't mind writing up the abstract and
such based on an example if you have one.

> You say l = 1...15 but the explicit list actually shows (l + 1) ranging from
> 1 to 15, meaning l ranges from 0 to 14.

Thanks, fixed.

> "Channel" is ambiguous. There are "internal" or "encoded channels" (signaled
> in each Opus packet using the stereo flag), "decoded channels" (based on the
> configuration applied to each decoder, as specified by the stream count and
> coupled stream count), and "output channels", which are what you wind up
> with after applying the channel mapping table. I assume you mean "output
> channels" here, and should say so.
Thanks for clarifying, I changed channels here to "output channels".

> It's also probably a good idea to explicitly say that you use the same
> channel mapping table format as channel mapping families 1 and 255. At
> least, I'm assuming you do. I'm also assuming you don't plan to support
> Ambix's "extended format" with its adaptor matrix.
Thanks, I reworded to

"This channel mapping uses the same channel mapping table format used
by channel mapping families 1 and 255. Each output channel is

You are correct, I think it is not worth the complexity to support
adaptor matrices. Should I explicitly mention that or will the choice
be clear by not mentioning it?

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