[Vorbis-dev] Ambisonics in Ogg Vorbis

[Sebastian Olter]

2007/4/26, Paul Martin <pm at nowster.zetnet.co.uk>:
>
> If the W channel is stored in the AMB file at a reduced level, and
> I'm decoding from an AMB file
>
[...]
> OR, am I to assume that the 3dB reduction in the W channel is a
> prerequisite the Furse-Malham decodes already take into account


Actually, Furse-Malham equations describe a second-order Ambisonic encoder.
(The scalability of Ambisonics makes it possible to use a subset of them
(first three) for 1st order, planar encoding.) There is a -3 dB coefficient
in the first of them

W = 1/sqrt(2) Σ a(n) = c Σ a(n),

in the other words we obtain the W signal by dividing the sum of all sound
sources by sqrt(2). Anyway, the exact solution (velocity decode) for a
rectangle rig should not assume any coefficient

a(n) = W/c + X/cos φ(n) + Y/sin φ(n),

[φ(n) is the angle of each speaker's position, measured anticlockwise from
forward direction], so
as long as you divide W by the coefficient used for encoding, you still have
velocity decode.
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