[vorbis-dev] Updated Vorbis RTP I-D

[Monty]

> I think we should find a more flexible way to define mapping of channels 
> to different surround formats. how could i enocde left, right and LFE 
> channel for 5.1? or l r c LFE or l r Sl Sr, etc...

Currently 1,2,3,4,5,6 are defined, according to Dolby/ITU standard.

As for 'more flexible ways', that's open in future revisions of the
spec.  More channel mappings as well as ambisonic encoding are
intended for Vorbis II.

> and what's with ambisonics?

www.ambisonic.net full surround spherical wavefront encoding in four
channels (for first order representation).  It boasts better real
sound localization than 5.1, does not require strict speaker
placement, and uses fewer channels.  At the time of its original
introduction in the 70s, practical ambisonic decoders couldn't easily
be made from analog electronics in anyone's price range and the format
disappeared.  In this era of cheap, fast DSP, decoding complexity is
no longer an issue.

> how are surround channels automatically down-mixed to stereo or mono?

Depends on the application, and how the surround was mixed.  These are
things that are not standardized uniquely in the real world yet either.

> >Another area where the Vorbis spec and current practices of the AVT WG
> >differ is the output channel order.

You have to pick your spec, or decide to support all of them.  We
decided to pick one (ITU surround) for now :-)

> >The 3, 5 and 6 channels layout do not match.

> >The I-D does state that the channel ordering SHOULD follow the AIFF-C
> >format, so we can use the Vorbis layout if we push it, but this may be
> >something we may want to review.

Vorbis layout can't easily change before Vorbis II, where a more
expansive system will indeed be required.  But I'd like to avoid going
down the path of trying to make everybody happy by supporting every
available variant of each feature. 

Monty
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