[CM] ambix ambisonics command line

[Anders Vinjar]

Hi Juan!

    J> Does anyone know of a command line application that would process
    J> multichannel (interleaved) soundfiles into Ambisonics b-format or ambix?.

If all you want is the MS-type matrixing it should be straight forward
in an "a2b.scm" script for snd, or sox or ffmpeg or others, You can
combine the 4 A-format channels FLU,FRU,BLU,BRD using
MS-like-techniques:

        W = FLU+FRD+BLD+BRU
        X = FLU+FRD-BLD-BRU
        Y = FLU-FRD+BLD-BRU
        Z = FLU-FRD-BLD+BRU

But leaving out all the required tunings for A2B conversion from any
given FAO mic: eq, gain, phase - you wont get any meaningful B-format
output - rather a "quite-nice-space-like-feeling-surround-spread mic"
(wow, my new brand!  :-) .

For B-format you'll need a specific B-format encoder to a specific type
of mic, tuned to its geometry and specs of capsules, and further using
precise calibration-data fitted to your personal mic (*).

Fons Adriansens 'tetraproc' includes a CLI 'tetrafile', doing the A-B
conversion for Core Sounds Tetramic, and similar for their 2nd-order
Octomic.  I think he has some example *.tetra config-files for some
other brands.  He also offers to do the necessary measurements and
calibrations for your TetraMic to plug into his conversion tools.

Not sure what the other brands - Sennheiser, Röde etc. - offers for
CLI-versions of their tools.

(*) The dependence on both a specific decoder for a mic-brand, and
further individually calibrated per mic, is perhaps sub-realised?
Strange, given Ambisonics 100% reliance on extremely exact phase and
gain info to represent the recorded space.

One example is Nevaton who provides their VR-mic - otherwise very nicely
made with great sound (no-noise, full-range - and very expensive) -
without neither a fitting A2B decoder or necessary calibration data for
each mic, relying on a one-time measurement of one mic done by Angelo
Farino.  They even have suggested using Sennheisers or Röde's standard
A2B-encoder...

-anders