Re: [codec] Comments on draft-ietf-codec-ambisonics-01
Jean-Marc Valin <jmvalin@mozilla.com> Mon, 13 March 2017 21:55 UTC
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To: Drew Allen <bitllama@google.com>, Mark Harris <mark.hsj@gmail.com>, Jan Skoglund <jks@google.com>, "codec@ietf.org" <codec@ietf.org>
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From: Jean-Marc Valin <jmvalin@mozilla.com>
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Date: Mon, 13 Mar 2017 17:55:07 -0400
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Subject: Re: [codec] Comments on draft-ietf-codec-ambisonics-01
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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