< nothing below implies that an Ambisonics microphone is the best solution for all sound capture cases! >

On 10/21/20 6:37 AM, Augustine Leudar wrote:
Hi Steve,
An interesting proposal. You'll excuse perhaps my misunderstanding here,
but my understanding was that ambisonics A format consists of a omni plus
x,y,z to give 4 signals that are then converted to b format.

First order "A format" is just the four signals coming from the four capsules. They are matrixed and filtered to generate the B format signals, which are the equivalent of an omni and three figure of eight at right angles to each other. Ideally the B format polar patterns are frequency independent. In reality this is not the case (for any capsule), but a properly calibrated Ambisonics microphone is very good in that respect, with polar patterns consistent up to very high frequencies (as Fons points out elsewhere in the thread).

I built my
own ambisonics mic and decoder once a long time ago so perhaps things have
changed, I think I put Left + & - right = x Front + & - Back = y Top + & -
bottom = z All summed * 0.257 = w into my decoder.

Hmmm, just curious, did you also add high frequency non-coincidence compensation filters?

A simple add/substract matrix (maybe that is not all you did? - you also say later in the thread you eq'ed the capsules) will work fine for frequencies where the array can be considered small relative to the wavelength in air. The effects of the non-coincidence of the capsules starts to show up at 3Khz or so in my case, exactly where depends on the radius of the array, and needs to be compensated somehow. This is usually done by filtering the B format signals.

(see attached graph of unfiltered and filtered response at 0 degrees incidence for my TinySpHEAR mics - these are very old plots but useful anyway as they show the difference those filters make)

Best,
-- Fernando


. Anyway I digress -
this basically will give you a sounds position in 3D space , or at least on
the surface of a sphere proximity not being so well recreated. So if you
had say, 8 ambisonics mics pointed in 8 different directions ,in order to
record that small 8th of  a sphere (or in this case cube) in the direction
they are pointed in (which is what the ORTF does)  - how would this work if
the decoder if a format gives spherical coordinates rather than an 8th of a
sphere/cube (hope this makes sense), and with the capsules pointing in all
directions... ?
...
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