[vorbis] AC-3 Decoding

[Michael Paine]

> Ambisonics is a way to pre-encode positions into a 4 channel format, but
it
> will allways loose some precision.

This was part of the design with ambisonics.  4 channels != 4 speakers.
Increasing the speaker array to 6 or 8 will yield the precision to near
95..99% original aim.

> Ambisonics could simply be supported by 4
> channel support and seperate post processing, the only thing vorbis would
> really need in this respect is a flag in the header to indicate its an

Why would this flag be of needed?  Ambisonics is stereo compatible.. unless
you are considering it for Dolby 5.1 compatibility too.  There is a way
(caled Ambisonic GD format or "G+2") for encoding 4 channel ambisonics (not
second order through) into Dolby 5.1 channeling, but that would waste 1.1
channels.

> ambisonic recording and perhaps a special mode to make best use of the
> correlation between the channels ala the proposed joint stereo encoding.

It will be most interesting if the join stereo encoding could be expanded to
join quad-channel encoding -- does MP4-AAC have anything like this?

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