[LAU] ambisonics: fons ambdec and muse.demon.co.uk ambidec

fons at kokkinizita.net fons at kokkinizita.net
Thu Jan 6 23:33:53 UTC 2011

On Thu, Jan 06, 2011 at 11:14:46PM +0100, sh0099 at gmx.de wrote:

> I was wandering if i can use
> "First and Second Order Ambisonic Decoding Equations"
> from this page:
> http://www.muse.demon.co.uk/ref/speakers.html
> for setups in ambdec by fons?
> http://www.kokkinizita.net/linuxaudio/

You can use them, but why should you ?

Most of the speaker layouts on that page are available as ambdec
presets, and those will in general produce better results. 

Some terminology:

There are three major AMB decoder modes:

* 'max rV', this is called 'reproducing spherical harmonics' on the
  demon page,
* 'max rE', not available on the demon page,
* 'in phase', called 'controlled opposites' on the demon page.

In general, the best decoding is obtained by using 'max rV' at low
frequencies (< 500 Hz or so), and 'max rE' for higher frequencies.
This is what most of the config files supplied with ambdec will do.
As said, the demon page does not have any 'max rE' decoders.

> my problem is that the speaker position is of demons ambidec is in
> positions in the room (x,y,z) and not as dist,azim,elev as in fons
> ambdec

This is just conversion of cartesian to spherical coordinates.
If you don't know how to do that, you probably should not be
meddling with ambisonic decoder coefficients :-)

In case you want to go on:

Given  x,y,z:

dist = sqrt (x * x + y * y + z * z)
azim = atan2 (y, x)
t = sqrt (x * x + y * y)
elev = atan2 (z, t)

Azim and elev are in radians, to convert to degrees multiply by
180/pi. Note that for all layouts on the demon page, dist will
be 1 for all speakers. For ambdec you should use the real distance.



There are three of them, and Alleline.

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