[LAU] Ambisonics microphone for Linux

Fons Adriaensen fons at linuxaudio.org
Thu Jun 25 15:49:35 CEST 2020

On Thu, Jun 25, 2020 at 02:59:36PM +0200, Michael Jarosch wrote:

> Still, there's one point that makes me prefer an all-in-one recording
> solution Zoom is offering: How is gain calibration working with an
> ambisonics mic like the Tetramic and f.e. a recording setup I have with
> a Presonus Digimax FS and a RME HDSP 9652?  

If you refer to setting the preamp/ADC gains, yes that is a problem.
The practical solution is to use a device that has calibrated gains 
instead of just analog gain pots.

But you don't need a sound level calibrator. Assuming the mic was
measured and a matching A/B matrix was computed from these measurements,
all you need to do is calibrate the preamps. What you need for that
is something that generates a constant mic level and balanced signal.
It could be as simple as a passive attenuator driven from a balanced
line output.

Such measurements and matrix calculations are done for each and every
Tetramic or Octomic (I do the calculations), and they involve a lot
more than just gain matching. 
> The Zoom does make it easy, here. Guess, they calibrate the input gain
> for each mic as the final step before packaging it for shipping.

Just the gain isn't enough, unless they use capsules manufactured to
very strict tolerances. That is the case for the Ambeo for example -
the Ambeo A/B processor just uses a fixed A/B matrix and filters,
and there is no individual calibration. I measured a few of them
and they were indeed remarkably similar. But it comes at a price.



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