[vorbis-dev] Channel coupling in Vorbis

[Ralph Giles]

On Mon, 14 Aug 2000, Thomas Marshall Eubanks wrote:

> I would recommend NOT using the ambisonic for surround sound. I read the specs on this - it
> uses a spherical harmonic expansion of the sound field. The trouble with that is
> that it is 3-D. Unless you plan to have speakers in your ceiling, it will waste 1/3 of
> your bandwidth and (except for the dipole terms) won't separate cleanly into
> high and low bitrate channels (or horizontal versus vertical). Also, I believe that
> ambisonic is "color blind", while clearly the separation of the monopole and other
> terms is best NOT done on a  frequency independent basis.

If I understand properly, your objection is that directional perception
varies with frequency, and the ambisonic channels don't take this into
account. Can't we leave that to the codec? I don't see why it shouldn't be
free to shift things in and out of the W channel, or is it something more
sophisticated you're worrying about?

Overall, I think the ambisonic site makes a good case that Dolby's 5.1
format is broken, and offers a clean, flexible alternative.

Dolby does have the advantage of being the current standard for film
soundtracks. It may be we can safely mix the five channels down to 3
channel ambisonic without losing much quality...but I'm not sure. (Sounds
ludicrous doesn't it? Can someone correct me?) In theory, we're also free
to either ignore the low frequency effects channel (the .1) or mix it into
the W.

In any case, 5.1 is used on DVDs because filmmakers can just compress the
theatrical soundtrack without any tweaking or remixing. We'd have to be
certain they're not losing anything before pushing this for film work.

 -ralph


--
giles at ashlu.bc.ca

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