[Vorbis-dev] Ambisonics in Ogg Vorbis

[Paul Martin]

On Thu, Apr 26, 2007 at 08:48:27AM +1000, Richard Lee wrote:
> Paul Martin wrote :
> 
> >> In "SHELF FILTERS ... ", I use WXYZ in its strict Ambisonic
> >> sense as defined in
> 
>  http://www.york.ac.uk/inst/mustech/3d_audio/secondor.html
> 
> >You're dancing round the question. The specification of an AMB
> >file says that the W channel will be stored at a level of -3dB.
> 
> http://www.ambisonia.com/Members/etienne/Members/mleese/file-format-for-b-format
> 
> > "The W channel is attenuated by -3 dB (1/sqrt(2)) for all orders. 
> > That is to say, a source at 45 degrees azimuth (zero elevation)
> > wouldproduce equal gains in W, X, and Y."
> 
> > This means that my decode *is* the "Energy" one.
> 
> You've just described an ENCODING issue.
> 
> Your DECODER
> 
> > 0.3536,0.3536,0.3536,0.3536,\
> > 0.1768,0.1768,-0.1768,-0.1768,\
> > 0.1768,- 0.1768,-0.1768,0.1768,\
> 
> as Sebastian Olter points out is a "Cardioid" decoder.  Its OK but
> an "Energy" decoder is better and an "Energy" decoder with Shelf
> filters is better still.

Please see my other email.

If the W channel is stored in the AMB file at a reduced level, and
I'm decoding from an AMB file, do you expect me to ignore the effect
of that reduced level of the W channel when decoding?

OR, am I to assume that the 3dB reduction in the W channel is a
prerequisite the Furse-Malham decodes already take into account and I
shouldn't be correcting for it?

I am asking for clarity here. If I'm making mistakes with this,
others will too.

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