Let's Design and Build a (mostly) Digital Theremin!

"... looking at the experts I don't seen any limitation with the conventional sensing, so I'll go with that."  - pitts8rh

Besides the crying baby syndrome, another issue with conventional farther-is-louder volume sense is sorta right there in the name.  The player's body has an increasingly confounding influence on the size (and so linearity) of the field the farther out the player's hand is - so the nearer the hand to the antenna, the more sure the player can be of the obtaining the expected result.  Farther-is-louder works against this by giving the player really good control over silence, and less good control the louder one plays.  Nearer-is-louder works with this, and much of the body's remaining influence can be expanded away by switching a secondary sensitivity (knee).  This obviously isn't a deal-breaker, as just about everyone plays farther-is-louder for historic reasons (if no other) but I believe it is a less than ideal situation from a body influence standpoint.  Similarly, for the pitch side closer-is-higher makes a lot of sense, because it places the harder to discern low pitches out in the more uncertain far field.

"... but really in the end neither of us should be giving advice about playing technique along with our theremins, except to "look to the experts to see how they do it". 

But there are no experts who play a digital Theremin?  All past and current expert players learned on an analog, where a rod makes both practical and engineering sense, where sensitivity is fixed and there is an iron trade-off between it and linearity, where farther is always louder - their techniques are entirely and necessarily based on these factors.  Most of their techniques can be accommodated, the remaining may require a pitch rod, and they will likely be satisfied with that - but who then should / will advocate for newly enabled alternatives that perhaps make more sense, at least to some?

One thing we haven't discussed this time around in the family feud ;-) is variable pitch field sensitivity.  To me this is much more of a game-changer than plates or rods in terms of influencing and producing new playing techniques.  Even Moog Inc. seems to have something similar (though more indirect) in the Claravox Modern mode.  I'm a little surprised there is no tradition-based push-back against it, but maybe Thereminists in general aren't complete sticklers? Most of the time I guess they're in the unenviable position of having to take whatever they can get.

I wish the same Vincent!  I would very much like to do a video conference with you when you get the kit up and working, just to show you around the thing.  Can you speak English at all or do you rely on a translator for TW and such?  My French is from watching movies with subtitles, so effectively none.  It's weird how we Americans rely so heavily on Europeans (and the rest of the world really) to know passable English - and it's weird how most seem to know English, and are often much better at it than me!

Yes, I speak English every day for my job. Most of the time with non-native English speakers (European, Chinese, Indian people), so, if you pardon a few mistake, I am Ok for a video conference when we can. 

BTW, Thanks for the confirmation of the positionning of coil + AFE boxes.

Remote Control

No matter how you generate / acquire / filter / process / etc. Theremin fields, the closer your hand is to the antenna the higher the quality of the information / signal.  On the D-Lev, which - unlike analog Theremins - has a highly linear near field, this "sweet spot" goes from right at the antenna to maybe a foot away.  Beyond that the capacitance of the player's body starts to intrude and confound the response.  The field can still be made highly linear farther out (I can make it so the open / closed hand gives the same response with my hand retracted almost entirely to my body) but the calibration of it becomes more and more dependent on the exact position of the body.  This effect is quite obvious, so I tend to set up the fields so that my volume hand is around 8" away, and my pitch hand is maybe 12" or so away nominally.  Analog Theremins generally don't have ways to position and shape the fields, so you sometimes see players playing much farther away to reach low notes, which is impressive looking, but it must be less comfortable for them, because any null error will become significantly magnified.

On the volume side, the traditional farther-is-louder configuration has an additional downside that was just brought up over on the Claravox thread but I hadn't thought of (because I never play that way): pitch hand interaction.  If there is audible volume change taking place in the far field, then moving the pitch hand around will also modulate the volume.  Non-traditional nearer-is-louder tends to swamp any pitch hand interaction.  One more brick in the wall in which the traditional volume sense may be problematic.

In conclusion, critical responses (higher pitch, higher volume) are best assigned to a linear near field, with less critical (lower pitch, lower volume) relegated to a necessarily less certain far field.  In a perfect world.

DPLL Sim

Back from the family vacation, this year it was in Connecticut, which has a very New England vibe (a good thing IMO).  Not very tan (besides my normally somewhat darkish skin that invariably gets me extra unwanted TSA attention at the airport) nor all that rested, but back nonetheless. 

Have been working here and there on a DPLL sim in LTSpice: [Link].  Here it is doing its thing:

It uses the "modulator" proprietary component in LTSpice (A1) as the NCO, as well as the proprietary XOR component (A3).  They both operate at 0-1V so some level shifting is necessary here and there to match the 3.3V supply of the AFE.  The digital phase error integration probably doesn't reflect reality, I've got it set to simulate quickly rather than with high fidelity.  And the hand capacitance is a total fudge to get the swing at the antenna and C divider in the ballpark.  But the rest is pretty real.

It's interesting that you can rather dramatically increase the coil DCR and not incur a huge penalty in voltage swing, i.e. it's not as directly proportional as you might think from the equations.  Makes me wonder if I should be sensing phase there rather than on the high impedance side of things, but the voltage there is so huge and the wave so pure, it's hard not to.  The easiest and most straightforward approach would be my 8 transistor oscillator (which uses low impedance phase sense) feeding a precision processor interrupt or similar.

MOAPA (Mother Of All Plate Antennas)

Rob Schwimmer said that playing the Theremin is like "having sex with a ghost."  Designing them places you in a near constant state of ghost busting, with no one to call.  I've been wondering if fully enclosing the coil and AFE in a Faraday cage - which would serve as the plate antenna too - might somehow reduce unwanted environmental interaction?

I'm going to put my 8 transistor oscillator in an egg roll tin with a 4mH inductor and see if it can make contact with the other side.  Our upstairs seems chock full of electrical spooks.

Hi,

AFE+coil literally inside the antenna... how does it work after all?

I have been thinking about a housing that would make possible to use plates or rods and that would be right and left handed convertible. 
(I'm right handed, not for personnal use but could be usefull... )
I'll post one or two sketches soon...

This is the housing that I am thinking about for a convertible D-Lev.

"AFE+coil literally inside the antenna... how does it work after all?"  - Mr_Dham

I need to try it.  It's kind of an inside-out very loosely coupled ferrite core.  Problems are keeping the low impedances (drive end of the coil, AFE, ribbon cable) sufficiently away from the tin skin, and scraping off paint to gain conductivity to the top.  I'm loathe to cut holes and destroy it until I've had the chance to think it through.

IIRC the magnetic field in a coil is concentrated at the windings - in the coil form walls rather than at the center of the coil form - so with a sufficiently large diameter coil one could probably safely put the AFE inside the coil itself, and cut a hole in the bottom of the tin directly below for electrical access.

"I have been thinking about a housing that would make possible to use plates or rods and that would be right and left handed convertible.  (I'm right handed, not for personnal use but could be usefull... )"

Interesting!  I've long thought that would be a good goal, but the ergonomics of even a non-ambidextrous D-Lev are rather daunting from a construction standpoint.  If you have plates for both axes it might be a little simpler to swap them via the system encoder, for a lefty in a pinch that is.  The axis swap is there mainly because it was a trivial (and fun!) thing to include.

My current lab unit is such that my left and right upper arms are at my body, and my forearms are parallel with the floor, and angled out a bit from each other.  The plates are angled out a bit the same to address my hands perpendicularly, with the volume plate more horizontal to the floor, the pitch plate more vertical.  I think it's best to take a very comfortable posture and go from there in terms of playing the thing and ergonomics.  I have to believe that most Theremins are designed to fit in a crude or fancy box first, and to be playable a distant second.

[EDIT] Vincent, I don't mean to quash any of your excellent ideas!  Not that you need my permission to do anything, please ignore me and experiment!  I very much look forward to your observations here.

A Slice Of Life

I'm trying to print the tiny tuner faceplates in white PETG but it's really fighting me.  Keep getting discolored areas on the first layer, which I think is over extrusion.  Also getting really weird messed up holes that don't make any sense as they are simply 4 wall towers all the way through the full thickness of the part.  The problem holes are always in the same place - even if I shrink the part 50% in the slicer! - which makes me think it's some sort of geometry issue between OpenSCAD and the Cura slicer.  So I switched over to the Prusa slicer and the ratty hole problem is solved, but now the holes are too small!  Had to fix that with some global wall compensation, which didn't make me super happy, but whatever.  You also lose all direct control over first layer flow in Prusa, which is idiotic and has kept me away from using it.  When it comes to 3D printing there's always something going wrong - too much art, not enough science.  It's a creepy thing to have to rely on.

Above: full size Cura print on right, 50% Cura print at top left, 50% Prusa print center left, 50% Prusa print with "XY Size Compensation" = -0.15 at bottom left.

[EDIT] Vincent, I don't mean to quash any of your excellent ideas!  Not that you need my permission to do anything, please ignore me and experiment!  I very much look forward to your observations here.

No worries, I like the discussion, experimentation and challenging ideas.

PV setting... I didn't catch it initialy, nice !
It prevents from swaping AFE boxes which is a good thing. Still remains to adapt to hand's movement (vertical VS horisontal) but it probably simplify and somewhat change my design at the end...

My point is to adapt the "central unit + 2 satelites" of the D-LEV so as I can put it on my camera stand (not to heavy), mount or unmount it, move it from home to gig or rehearsal and I sure need it to be comfortable to play. I'll push my relfexion forward and experiement.