[Vorbis-dev] Mapping = 1 Ambisonic Vorbis flag

[Richard Lee]

>> That's cos we might want more bits from yus guys later if / when we have more than 16 channels ( more than 3rd order Ambi)

> Why would you want additional bits?

> B-format support is going to need a channel map even for second order because you may want to do mixed order-representations.

As it so happens, mixed orders up to 3rd are implicitly specified by the number of channels.

This is described Martin Leese's "File Format for B-Format" page at http://www.ambisonia.com/Members/mleese

There is no need for a channel map for the forseable future.

When we start distributing 4th order stuff (which requires >16 speakers) we can get around this by having missing channels simply silent.  Vorbis compression is very efficient at coding silence.  Thanks to Gregory Maxwell for this excellent suggestion on the sursound forum.

Be aware though that Martin's "official file format for downloadable B-Format files" has errors and ambiguities.

In particular, there is talk of "attenuation of W" which causes much confusion to newcomers to Ambisonics and old hands alike.

I intend to start an "Official Vorbis Ambisonic specification" which is meant to be used in conjunction with the official Vorbis "Mapping = 1" specification when if appears.  This will specify the signal in the various channels and also some parameters eg coupling modes.

I'll correct some serious errors in most public Ambi specifications and bring them into line with real life.  This would allow practical microphones to be designed.

At the same time, we'll retain a simplified spec to facilitate coding eg ITU 5.1 and various 7.1 modes efficiently to Ambisonics with good results.

So we probably won't need extra bits.  But we'd still like to have Bit1 as the Ambisonic flag rather than Mapping =1 if that's OK with yus guys
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> The second proposal, was from Sampo Syreen, who suggested to add the full OggPCM (option 2) channel mapping, ...  because we would also add support for surround formats >5.1.

This can stay in Ogg.  It is a VERY bad idea to have with Vorbis Ambi (Mapping = 1)  They are fundamentally VERY different in their approach.

Ambisonics defines the soundfield; WHAT SHOULD BE HEARD.  How you achieve this is completely separate and depends on your speakers, the layout etc.  This allows it to be played on 4.0 5.1 .. 7.1 ... zillion.1 speakers and in all cases extracts the best possible sound from the original recording and the speakers and the room.  It is this 'spatial coding' that gives its sonic advantage and allows 3 channel ambi to outperform 5.1  Having more channels does not only improve the sound in the direction of the extra speakers but improves the sound from ALL directions.

5.1 7.1 ...  zillion.1 are one channel / one speaker formats.  You will never keep up with this sensibly.  Crude mixing 7.1 to 5.1 or vice versa usually gives bad results.  In fact the best way to do this is to code these into Ambisonics and then play this via an Ambisonic decoder matched to your speakers and room layout.

5.1  7.1  ...  zillion.1 belong in the one channel / one speaker zone (Mapping = 0)  You will need ever increasing amounts of obfuscating metadata to describe it so please keep it in Ogg.

The best thing about this is that Native Vorbis can ignore it all for (Mapping = 1).  Lets keep it that way.
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What's required to do a full Vorbis Mapping = 1 (Ambisonic) spec?

We'll have

a)	Channel Definitions 
b)	Coupling Modes.  Gregory Maxwell is working on a Lossless Coupling mode optimised for Ambisonics.

Can present decoders decode the full set of Stereo Coupling modes described on the web page?  eg Dual, Lossless Stereo, the Phase Stereo mode ...

Can we specify minimum and recommended Lossless Coupling modes?  
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