[vorbis-dev] Calling for 5.1 Mastering experience! (vorbis ambisonics and 5.1)

[Ralph Giles]

On Tuesday, June 10, 2003, at 09:30 pm, Gregory Maxwell wrote:

> I've been doing a fair amount of work with Vorbis support for 
> Ambisonics,
> which seems to be going along nicely. It seems that there is signifant
> interest in coding 5.1 material with Vorbis esp as tarkin becomes more
> complete, so I've decided to take a break from pure ambisonic work to 
> look
> into this.

I assume you're aware of the technical documentation on dolby's site? 
(http://www.dolby.com/pro/) In particular the surround mixing guide has 
a lot of detailed guidelines. I don't have any practical experience 
with it though, so I can't vouch for it.

In particular it says that the LFE track is for option reinforcement, 
so while the idea is to encode it separately and mix it into the 
subwoofer channel, mixing it into the other 5 is acceptable, as is 
ignoring it. Don't know how safe that is in practice.

Encoding it separately breaks the symmetry of the ambisonic encoding, 
but is probably closer to the intent of the original mix. It should 
compress quite well with it's own codebook given the lowpass, but I 
think it also tends to share a lot of entropy with the W channel.

> I've decided that the best (from a pure elegance and patent avoidance) 
> way
> to handle this is to basically decode the 5.1 input into a WXY or WXYUV
> ambisonic signal, which can then be handled by the Vorbis Ambisonic 
> support.

So U and V are the planar quadupole (m=+/-2) moments? I'm glad to hear 
you can get by with the same number of channels.

Cheers,
  -r

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