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>
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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 > > > > _______________________________________________ > > codec mailing list > > codec@ietf.org <mailto:codec@ietf.org> > > https://www.ietf.org/mailman/listinfo/codec > > > > > > > > _______________________________________________ > > codec mailing list > > codec@ietf.org > > https://www.ietf.org/mailman/listinfo/codec > > > >
- [codec] Comments on draft-ietf-codec-ambisonics-01 Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Mark Harris
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jan Skoglund
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jan Skoglund
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Mark Harris
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Drew Allen
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Drew Allen
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Drew Allen
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jan Skoglund
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jan Skoglund
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jean-Marc Valin
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Timothy B. Terriberry
- Re: [codec] Comments on draft-ietf-codec-ambisoni… Jan Skoglund