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....@gmail.com
> <mailto:mark....@gmail.com>> wrote:
>
> On Mon, Mar 13, 2017 at 10:38 AM, Jan Skoglund <j...@google.com
> <mailto:j...@google.com>> wrote:
> > Hey,
> >
> > Thanks for your comments
> >
> > On Mon, Mar 13, 2017 at 10:08 AM Mark Harris <mark....@gmail.com
> <mailto:mark....@gmail.com>> wrote:
> >>
> >> On Fri, Feb 17, 2017 at 1:57 PM, Jean-Marc Valin
> <jmva...@jmvalin.ca <mailto:jmva...@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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