[Vorbis-dev] Ambisonics in Ogg Vorbis

[Paul Martin]

On Thu, Apr 26, 2007 at 01:17:00PM +0200, Sebastian Olter wrote:
> 2007/4/26, Paul Martin <pm at nowster.zetnet.co.uk>:

> >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).

"Sum of all sound sources" = the output of a (perfect)
omnidirectional microphone.

To turn that into the W signal, you need to drop its level by 3dB
(ie. divide by sqrt(2)). [FX: penny dropping]

Thank you very much for your help. As others have said, the available
documentation is either opaque or uses conflicting notation.

-- 
Paul Martin <pm at zetnet.net> (work)
  <pm at nowster.zetnet.co.uk> (home)