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

[Gregory Maxwell]

On Tue, 10 Jun 2003, Ralph Giles wrote:

> 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.

Yes I am. I've done a lot of research to determine the 'correct' speaker
placement for my ambisonic conversion.

> 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.

I just can't figure out what the orignal logic is for 5.1 to even HAVE a
seperate subwoofer channel.  But a lot of the decisions behind the dolby
5.1 stuff are not mathmatically sound or even intutivly sane.

> 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'd like to think that users of surround Vorbis (including people who are
listening to content transcoded from 5.1 sources) will *not* be listening
on speakers configured per the dolby 5.1 specs but rather listening on an
(likely irregular) Ambisonic array sized according to their needs and
resources. Because of this, I'm slightly more concerned with compatiblity
with such setups than absoultly perfect mirror of the mix intent.

However, if it's common practice to do evil things with the LFE (say put
the same LF content thats on the main channels but 180deg out of phase)
then I will have to seriously consider using a seperate channel.

<p>> > 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.

They happen to fit nicely with the proscribed speaker placements for 5.1.

I haven't done testing to determine how well I'm really representing 5.1
yet as doing so will require me setting up a proper 5.1 speaker system
which seems horribly boring compared to my 14 channel ambisonic rig.

<p>--- >8 ----
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