Re: [codec] Comments on draft-ietf-codec-ambisonics-01
Jan Skoglund <jks@google.com> Mon, 13 March 2017 17:39 UTC
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From: Jan Skoglund <jks@google.com>
Date: Mon, 13 Mar 2017 17:38:54 +0000
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To: Mark Harris <mark.hsj@gmail.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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Hey, Thanks for your comments On Mon, Mar 13, 2017 at 10:08 AM Mark Harris <mark.hsj@gmail.com> wrote: > On Fri, Feb 17, 2017 at 1:57 PM, Jean-Marc Valin <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. On another note, unless I am missing something, the formula in figure > 2 does not appear to be correct. For example, according to the > formula, k=3 (ACN 2) corresponds to order 0 degree 2, which does not > make sense. It is also not clear why this channel index begins at 1, > when all others begin at 0. It might also be a good idea to clarify > that this is an output channel index, as other uses of channel index in RFC 7845 refer to decoded channel index (and also begin at 0). > You're completely right, and there's no reason why the channel index should start with 1. - Mark >
- [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