[codec] Comments on draft-ietf-codec-ambisonics-01

Jean-Marc Valin <jmvalin@jmvalin.ca> Fri, 17 February 2017 21:57 UTC

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From: Jean-Marc Valin <jmvalin@jmvalin.ca>
To: Jan Skoglund <jks@google.com>, "codec@ietf.org" <codec@ietf.org>
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Date: Fri, 17 Feb 2017 16:57:15 -0500
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Subject: [codec] Comments on draft-ietf-codec-ambisonics-01
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Hi Jan,

Here's some comments based on my review of the ambisonics draft:

3.  Ambisonics With Ogg Opus

>    Ambisonics MAY be encapsulated in the Ogg format by encoding with the

I would suggest replacing the normative MAY by just "can"

3.1.  Channel Mapping Family 2

>    This channel mapping uses the same channel mapping table format used
>    by channel mapping families 1 and 255.

I would suggest saying "mapping family 1" rather than "mapping families
1 and 255" since otherwise, you have families 2 referencing itself.
Also, we don't know what families 4 and up will look like.

3.2.  Channel Mapping Family 3

I would suggest removing the "Output Channel Numbering" field because it
is fully equivalent to simply permuting lines of the matrix. Also, I
believe that the size of the matrix was meant to be "32*(N+M)*C bits"
rather than "32*N*C bits".

About the matrix -- and as previously discussed privately -- I think it
should probably be stored as 16-bit integers in Q15 format. If any
global scaling is ever required, then the "output gain" field can always
be used.

>    Note that [RFC7845] specifies that the identification header cannot
>    exceed one "page", which is 65,025 octets.  This sets a practical
>    maximum ambisonic order of 10, if full order is utilized and the
>    number of coded streams is the same as the ambisonic order plus the
>    two non-diegetic channels.

This text is unclear about whether partial order 11 is allowed. I have
no strong opinion on the subject, but think it should be clarified.

Cheers,

	Jean-Marc