[Vorbis-dev] Mapping = 1 Ambisonic Vorbis flag

[Gregory Maxwell]

On Sun, Sep 7, 2008 at 2:07 AM, Richard Lee <ricardo at justnet.com.au> wrote:
> Where can I find the Header file or whatever which specifies the "Mapping" flag.
>
> In feb - apr 2007, there was a lot of discussion about Ambisonics and Monty kindly stated that
>
> Mapping = 1     ; Denotes and Ambisonic file as opposed to = 0 which is 1 speaker/ 1 channel
>
> Has this been written explicitly into the standard?

I don't believe so, but it was always the intention.  It wouldn't be
in the standard because there is no specification on what to do inside
the mapping.

[snip]
> The reason why I'm searching for the actual physical definition of the flag is the unwashed Ambisonic Community would like this to be just a bit (bit 1) rather than a single value (Mapping = 1)
>
> 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)

Hm?

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.  A commons
usage would be second-order horizontal only (this is what would
probably be used for 5.1->b-format conversions), or things like
third-order horizontal plus first order height (which is more of a fit
for speaker arrays which can reasonably be constructed in a
residential setting).  I wouldn't use the mapping to indicate H/V
orders.

Or were you referring to the lack of a canonical selection of
orthogonal harmonics for higher orders?  I thought that had been
resolved. If not, I think it probably should be rather than expecting
Vorbis to handle it.