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

Drew Allen <bitllama@google.com> Mon, 13 March 2017 22:01 UTC

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References: <2f534e1b-b1af-266a-50ef-36f1739d878b@jmvalin.ca> <CAMdZqKGzdndiwpdXsYcHS7+r8Ega5LcQmAvcjiuHTHJgtTUwDg@mail.gmail.com> <CA+KMCSXhS2m4Dkous=4RkOibYWuoi+V_zBrhi1+anm-c+syQ1Q@mail.gmail.com> <CAMdZqKFDtD684HMkoO9bXi-c+g+8R+ay9kPdWSQOtHFDbC3ZLA@mail.gmail.com> <CABQ9DcuD+Et6+rBG-rCnWX-Dk-9STZMeYs-6fQWTk1kyjigRhw@mail.gmail.com> <52f5a570-e9f4-ea49-515e-498f0ed4f1bb@mozilla.com>
In-Reply-To: <52f5a570-e9f4-ea49-515e-498f0ed4f1bb@mozilla.com>
From: Drew Allen <bitllama@google.com>
Date: Mon, 13 Mar 2017 22:00:53 +0000
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To: Jean-Marc Valin <jmvalin@mozilla.com>, Mark Harris <mark.hsj@gmail.com>, Jan Skoglund <jks@google.com>, "codec@ietf.org" <codec@ietf.org>
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Subject: Re: [codec] Comments on draft-ietf-codec-ambisonics-01
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Got it. In that case, it certainly seems reasonable if I understand
correctly. Thanks for clearing that up!

On Mon, Mar 13, 2017 at 2:55 PM Jean-Marc Valin <jmvalin@mozilla.com> wrote:

> On 13/03/17 05:44 PM, Drew Allen wrote:
> > I think the issue is that the number of total channels rises
> > quadratically in respect to the ambisonic order (N + 1)^2. If a user
> > wants to use just the horizontal channels, it is only 2 * N + 1. If they
> > wish to code very high-order (>10th order) horizontal channels, they
> > would be artifically limited by all the zero channels being produced,
> > no? Or can this handled without actually creating all those empty
> channels?
>
> As far as I understand, the current draft already has all the
> limitations you're describing. The channel mapping array is basically
> equivalent to a CxC permutation matrix that multiplies the Cx(N+M)
> weight matrix. The result is still a Cx(N+M) matrix, so using the
> resulting matrix as weights can still do everything without the need for
> the channel mapping to do the permutations.
>
> Cheers,
>
>         Jean-Marc
>
> > On Mon, Mar 13, 2017 at 2:41 PM Mark Harris <mark.hsj@gmail.com
> > <mailto:mark.hsj@gmail.com>> wrote:
> >
> >     On Mon, Mar 13, 2017 at 10:38 AM, Jan Skoglund <jks@google.com
> >     <mailto:jks@google.com>> wrote:
> >     > Hey,
> >     >
> >     > Thanks for your comments
> >     >
> >     > On Mon, Mar 13, 2017 at 10:08 AM Mark Harris <mark.hsj@gmail.com
> >     <mailto:mark.hsj@gmail.com>> wrote:
> >     >>
> >     >> On Fri, Feb 17, 2017 at 1:57 PM, Jean-Marc Valin
> >     <jmvalin@jmvalin.ca <mailto:jmvalin@jmvalin.ca>>
> >     >> wrote:
> >     >> > 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".
> >     >>
> >     >> To expand on this a bit, a mapping family maps M+N decoded
> channels
> >     >> (corresponding to the actual order of the coupled and uncoupled
> >     >> channels in the bitstream) to C output channels (channels with a
> >     >> specific semantic meaning).  The additional "Output Channel
> >     Numbering"
> >     >> table confuses things by adding an additional mapping from the
> output
> >     >> channel numbers to a different set of numbers with actual semantic
> >     >> meaning, leaving the output channel numbers with no apparent
> meaning.
> >     >>
> >     >> This does have a potential benefit as a matrix compression
> technique,
> >     >> to reduce the size of the matrix when it would contain rows that
> are
> >     >> all zero.  However considering that the matrix occurs only once,
> and
> >     >> mapping family 2 already offers a way to compress the matrix, this
> >     >> alone does not seem worth the complexity of another level of
> >     >> indirection.  If matrix compression is desired it would probably
> be
> >     >> less confusing to describe it in those terms and keep the semantic
> >     >> meaning tied to the output channels.
> >     >>
> >     >>
> >     >> The description of the Output Channel Numbering also does not
> specify
> >     >> the intended behavior if the same value appears in the table
> multiple
> >     >> times.
> >     >>
> >     >> Additionally, section 4.2 describes how to perform a stereo
> >     downmix of
> >     >> mapping family 3, but makes assumptions about the output channel
> >     >> numbering.  This seems harmful and likely to promote
> implementations
> >     >> that make similar assumptions.  If it is necessary to apply the
> >     output
> >     >> channel numbering described in section 3.2 in order to implement a
> >     >> correct stereo downmix, then it would be better to simply use the
> >     >> output channels from section 3 as input to the downmix,
> consolidating
> >     >> sections 4.1 and 4.2, rather than specify new formulas that make
> >     >> assumptions about the mapping.  That would also greatly simplify
> >     >> section 4.
> >     >>
> >     >> Eliminating the Output Channel Numbering table as Jean-Marc
> suggests
> >     >> should resolve these concerns.
> >     >
> >     >
> >     > The problem is that once we allow mixed orders there is no unique
> >     way for a
> >     > receiver/decoder
> >     > to resolve the mapping to ACNs from just a number of total output
> >     channels.
> >
> >
> >     In mapping family 2, the channel count (C) is the number of channels
> >     in the fully periphonic configuration, but it is not necessary to
> >     encode them all.  The channel mapping table can map each ACN to a
> >     specific decoded channel or to silence.  For mixed order, some of the
> >     ACNs will be mapped to silence and will not be encoded.
> >
> >     In mapping family 3, the matrix can do everything that the channel
> >     mapping table can do and more.  Why not treat C in the same manner,
> as
> >     the number of channels in the fully periphonic configuration, even if
> >     some are silent?
> >
> >      - Mark
> >
> >     _______________________________________________
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> >
> >
> >
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