[SpHEAR-devel] latest musings (PCB, new Octathingy, calibration and more)

[Rob Hamilton]

I for one welcome our calibrating robot overlord...

... and, this is awesome!

Sent from my iPhone (in Mexico City)

> On Jun 23, 2018, at 3:43 PM, Fernando Lopez-Lezcano <nando at ccrma.Stanford.EDU> wrote:
> 
> Hi all,
> 
> I have been meaning to send an update about the project for a loooong time... Many many changes (not all of them in current git yet). I'll try to summarize...
> 
> I have a new design for the Zapnspark phantom power printed circuit board. I decided to make a double sided board as I was going to manufacture a few (double sided would be much harder with our small mill), and then went overboard and tried to make it as small as possible without using surface mounted components. I found some smaller form factor capacitors, which helped shrink it.
> 
> Using those, the four capsule microphone is much smaller, and the Octathingy benefits as well (smaller but not quite as much). There is a control variable that lets you switch between the two PCB sizes, and the assemblies mostly scale (but work is not finished yet - for example I need new windshield and shock mount 3d models).
> 
> Speaking of the Octathingy. I have been working on the calibration side a lot. Looking at many graphs made it more apparent that the initial design, which is a straight scale up from the four capsule design, was "clever" but not the best possible due to the resonance from the cavity formed by the capsules (but then Rode, SPS, Core Sound and others of course already knew about that - hmmm, but not Senheisser? :-).
> 
> So I went ahead with a newer "flower power" or "trumpets of doom" design with individual conical capsule holders and the smallest practical spherical core they could plug into. A measurement of a test 3D print (all dummy capsules except for one) showed better performance at high frequencies. Just yesterday I finished building the first full prototype, now I have to measure it (see pictures, all three prototypes and the comparison of the array between the last two).
> 
> Measuring... oh so not easy. I have to work on compensating the low end of the measurement frequency range as that is currently (I think) the weakest link in the calibration data. Juan Sierra here at CCRMA helped me understand Eric's paper on the matter, and it looks like I was doing the inverse filter and first reflection trimming in the wrong order (to begin with!). I have to see which type of window is best (right now using a full blackman centered on the impulse)...
> 
> The current Octathingy calibration includes equalizing the 8 individual capsules, deriving an 8x8 matrix of A to B filters for low and mid frequencies, and finally creating 8 individual filters for the B format components at high frequencies, all collapsable into a single 8x8 matrix of filters.
> 
> More about measuring...
> 
> I (well, CCRMA) bought a small robotic arm. And customized it to be a better measurement rig. The inverse kinematics are now working correctly and the arm is now pointing the microphone in the right directions. The arm is a bit short (longer would have been much more expensive), but with the current dimensions, and depending on how it orients the microphone, I can get vertical coverage up to around +/-40 degrees of elevation (and complete horizontal coverage of course), or +60-20 or so. We'll see what's best, or if both can be combined.
> 
> The arm is controlled from a SuperCollider program which does the inverse kinematics and will also will play back the sine sweeps and record the output of the capsules. Hopefully easier and more repeatable at the end of the road (which is still long). I really need better data...
> 
> Comments welcome...
> -- Fernando
> <octa_family_1.jpg>
> <octa_1.jpg>
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