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

Posted: 7/20/2021 4:53:51 PM

From: Northern NJ, USA

Joined: 2/17/2012

"1. Is it possible to offer kits with both? Then let people play and compare."  - ContraDude

For all my talk here, I don't want to discourage anyone from experimenting with the kit however they want, but I guess I'm anticipating a certain amount of trouble with rods, which could give the D-Lev a bit of an undeserved black eye in some scenarios, and could generate support headaches.  Folks think "Theremin" and they think rods, but I want them to keep an open mind about antenna geometry, and go into this with some knowledge of the engineering behind it all.  I intend to include a couple of cheeser plates, like aluminum foil on cardboard, because it needs to be playable out of the box - this is me subtly introducing the idea of plates in their minds, but also they're much easier to include than rods.  For your kit I'll include both for you to experiment with.

"2. I like the rubber ducky antennas  With a digital theremin, does the length of the antenna play a role in performance?"

The intrinsic capacitance goes up with surface area, and a longer antenna (for a given diameter) will have more surface area.  But a longer antenna has less mutual capacitance with the hand in the mid and near fields.  The main reason analog Theremins have long rod antennas is to somewhat linearize the field geometrically.  But when linearization can be implemented mathematically, the question then is: what is the ideal antenna geometry?  From an engineering perspective, a capacitor is formed by two plates, and while the hand isn't exactly plate shaped, a rod definitely isn't.  From a player perspective, a plate presents a softer and easier to hit target, though some who are highly adapted to rods might not appreciate nor welcome that.

"All I ask is for all players to be advised that you may encounter some differences in the feel of plate pitch antennas, and for new players to be careful what antenna system you choose to learn on.  The time investment that you make in learning to play any musical instrument makes your initial choices extremely important.  Sometimes the best way to decide how or what to play is to look at the players that you would like to emulate and see what they do and what equipment they use.  BTW - the volume antenna is much less of transition issue whether you choose a plate or loop."  - pitt8rh

Ah, very well put!  Honestly, I'm not bringing plates up (yet again) to poke anyone in the eye, it's just that I find myself talking about this subject a fair amount during show and tell.  Perhaps I'm too adamant about it in the first place, but as the number of units running around in the world increases, the potential headaches for me multiply, so I'm doing everything I humanly can to nip any possible issues in the bud.  Making Theremins in any quantity is sorta nuts in the first place, teetering as they are on a knife edge of physics, in a polluted environment of AC mains fields, fluctuating grounds, and mammoth AM broadcasters.

Next up: The traditional volume field is backwards!  (I keed.)

[EDIT] Yesterday's very special guest was none other than Rob Schwimmer!:

From left to right: Rob, me.

Rob was on his way from NYC to PA and dropped by for a couple of hours.  It was fairly mesmerizing to watch him play the Theremin and my wife's piano, he's quite fantastic on both!  And a really nice person.  I'm working to get Rob a D-Lev, not sure at this point what form that might take.

Posted: 7/20/2021 5:20:52 PM

From: Occitanie

Joined: 3/4/2012

To think that I was worried about putting 1.8Kg on a mic stand ;-)

Edit: was refering to:  "That's Roger's white P3 on the left, the "Bollamin" kit in the center, a laptop to show the librarian and power the units, and my skanky lab unit on the right."

Posted: 7/20/2021 5:58:40 PM

From: Basking Ridge, New Jersey, USA

Joined: 12/12/2020

Thanks, Dewster. Despite my total neophyte engineering mind, I actually understood most of what you were stating! The explanation regarding antenna length was especially fascinating and helped to explain things fine performers have discussed but only from a performance, not engineering perspective. That cleared up a lot!

Regarding your mention of volume field being backwards, I suspect you have a very fine explanation, from an engineering perspective, but it only makes sense to me, musically, if using the theremin to emulate percussion instruments. It also, arguably, makes sense from an articulation perspective, particularly when trying to play staccato (a difficult thing to do well on a Theremin). When volume is “backwards” I strongly suspect a good staccato is much easier. As a bassoonist, however, this becomes counterintuitive as no one plays a better staccato than the bassoon - indeed, composers rightfully exploit this characteristic. The reason for the spectacular bassoon staccato has to do with tone production and articulation technique. The tongue is placed directly on the tip of the reed, which prohibits the sound from being produced. Lifting the tongue initiates the sound and returning it to the reed stops it immediately. As a result, the tongue touches the reed twice - at the start and end of each note, thereby producing a very clean staccato. This is something  that is taught very early on. This is similar to the action of the hand with the traditional volume theremin except that the start and end of each not is not as distinct; also, the technique requires a rather large movement and it’s more challenging to control the hand.

For most everything else, I think entirely in terms of my well-honed orchestral musician mind 😀 when a conductor raises his/her hands and/or uses large gestures, volume (dynamics) increases.

Well, enough rambling for now (I’m just pondering theremin antenna technique).

Posted: 7/20/2021 8:03:12 PM

From: Northern NJ, USA

Joined: 2/17/2012

"The explanation regarding antenna length was especially fascinating and helped to explain things fine performers have discussed but only from a performance, not engineering perspective. That cleared up a lot!"  - ContraDude

A person can learn a lot about capacitance (and inductance) when designing Theremins.  Measuring the frequencies of an LC oscillator with your body in playing position and your hand at various playing positions (or a body & hand simulator, such as ILYA has done) and working your way back through the LC resonance equation to capacitance is quite illuminating, and gives you data you can stick in a spreadsheet and play around with.  This also reveals intrinsic C of the antenna (when the playing hand is located back at the body).  But what really opened my eyes was doing Finite Element Analysis on a virtual body & hand & Theremin antenna.  FEA breaks the total C down into mutual and intrinsic, and there you see that the intrinsic isn't a fixed value, but actually decreases as the hand approaches the antenna.  Which is somewhat unfortunate, as it lowers overall sensitivity to the movement of the hand.  Conceptually, what happens is more and more of the intrinsic field lines between the antenna and free space (including your body) get redirected to your hand as it approaches the antenna, converting a larger and larger portion of the intrinsic to mutual.  Lucky for us, the mutual increases faster than the intrinsic decreases, so the net effect is increasing C with decreasing hand distance.  Also lucky for us, the conductor closest to the antenna dominates here.

But any method (direct measurement of a real body, a simulation, FEA) will show that a square or round plate with an area a little larger than the open hand has the highest sensitivity to hand movement over typical Theremin playing distances.  It's not profoundly different than, say, the Etherwave rod antenna, but in this biz I would argue that you should at least try to take whatever you can get to reject interference, particularly if you are playing on stage.

Between the square and the round plate I would pick the square plate because it has a better off axis response both side-to-side and up-and-down, so you can get away with being a little sloppier when playing it.

"Regarding your mention of volume field being backwards..."

I was kind of kidding! :-)

"... I suspect you have a very fine explanation, from an engineering perspective..."

No, not really, more of a player perspective.

"... but it only makes sense to me, musically, if using the theremin to emulate percussion instruments. It also, arguably, makes sense from an articulation perspective, particularly when trying to play staccato (a difficult thing to do well on a Theremin). When volume is “backwards” I strongly suspect a good staccato is much easier."

Yes, I believe this to be true.  The muscles that drive the hand towards the antenna are likely stronger and more articulate than those that retract it, and there is the gravity assist at work too.

"For most everything else, I think entirely in terms of my well-honed orchestral musician mind 😀 a when a conductor raises his/her hands and/or uses large gestures, volume (dynamics) increases."

I know what you mean, but (just my 2 cents, anecdata dead ahead) the traditional volume sense has always felt completely alien to me, to the point where it was one of the major factors that turned me off to the EW we owned.  I just couldn't get into it.  But I think the piano damper pedal is backwards too!

When Rob was playing the D-Lev yesterday (with traditional volume sense) I noticed that we had to mute it whenever we did an ACAL and unmute it afterward, otherwise it made a bunch of noise during the ACAL timeout, which was one more thing to deal with.  Thereminists have to be extra vigilant between numbers to keep a hand on or near the loop, and often need to drape a grounded audio cord over the volume antenna when they walk away from it to shut the thing up.  Reverse volume sense has none of these issues, my lab D-Lev is on 24/7 and I never need to mute it.

Another kit buyer observed me playing with reverse volume sense and liked it.  I think that person maybe plays with their hand beneath the loop to get the same effect?

But everyone's different, it takes all sorts to make the world go 'round, and the D-Lev (as well as some analog Theremins) can do either.  If traditional volume sense is driving a person away from learning the Theremin in the first place, then I don't see a lot of harm at that point in reversing it.

Next up: Pitch correction is a good thing! (I keed!)

Posted: 7/22/2021 3:22:56 PM

From: Northern NJ, USA

Joined: 2/17/2012

Spare The Rod

Yesterday I went through the field calibration procedures for the Bollamin, and couldn't quite get the pitch field flat right at the antenna.  Though it's really hard to tell because it's so easy to be off-axis in the near field with a rod.  It's just slightly non-linear - certainly way better than any analog - but it's not my lab unit where the response is ruler flat and you know it because a near field plate isn't particularly difficult to play.

Nothing personal, but even if a player is highly acclimated to them, I just don't get the attraction of a pitch rod on a digital instrument.  The closer my pitch hand gets to the rod the more effort and precision I have to put into keeping it aligned horizontally, so it's a total PITA to play the near field where I have to watch it like a hawk - and I'd much rather be watching the tuner.  If a person is one of those who closes their eyes when they play, then a near field rod must be extra difficult.  I see now why so many Thereminists' eyes are crazy glued to the pitch rod.  There's no up-side that I can see and I believe I've heard all the arguments.

And traditional volume sense requires so much in the way of tedious baby sitting to keep it from squawking, it's no wonder some sort of mute is absolutely mandatory on analogs.

But, to each his/her own.

Posted: 7/22/2021 3:33:24 PM

From: Northern NJ, USA

Joined: 2/17/2012

Yesterday our very own ContraDude - Jeff La Marca - dropped by to demo Roger's P3 and borrow the Bollamin for a while:

It's always a pleasure visiting with Jeff!  He has a very keen eye and a strong musical background, and he established the D-Lev Theremin Groupies group on Facebook [link].

Posted: 7/22/2021 6:59:13 PM

From: Occitanie

Joined: 3/4/2012

Wish I was not so far from Northern NJ...

The two AFE + Coils box are designed to be two satelites as close as possible to antennas (behind the plates ?), aren't they ?

I am more and more thinking about having a central unit (screen, encoder, tuner, connectors) and two stands that would include antennas, one with plates the other with rods. Both would have specific receptacle for the AFE+coil boxes.
An idea to be explored...

Posted: 7/22/2021 6:59:57 PM

From: Minnesota USA

Joined: 11/27/2015

"There's no up-side that I can see and I believe I've heard all the arguments.  And traditional volume sense requires so much in the way of tedious baby sitting to keep it from squawking, it's no wonder some sort of mute is absolutely mandatory on analogs." - Dewster

I think you've listened to the arguments but it seems that you haven't heard them.  From an engineering standpoint where linearity is quantified with a ruler I too would come to the conclusion that plates are superior, simply because the near field and far fields (not using antenna terminology here) are more similar.  If you play along a line to and from the antenna then this is probably a very good setup, but I think you have to accept that not everyone plays the same way, and it can be difficult to understand a technique that you don't use.  And hand/fingering movements are far from identical at the near and far positions.

Whether conscious of it or not, players using rod antennas are at times playing off the cylindrical pitch fields intentionally at higher pitches.  It's one way to reduce the sensitivity of pitch to position by intercepting the pitches at a glancing angle off the cylindrical fields.  I'm always playing to the right of the antenna or sometimes directly at it depending on the jumps.  If you put a pitch plate on the theremin (and I have from time to time) I just move to the right of the plate so I can still be in the cylindrical pitch fields. But with the field distortion from cylindrical to flat with a plate antenna I would rather just have the predictable cylindrical shape surrounding a rod.

About aiming at the antenna?  My eyes are shut nearly all of the time when I'm playing, including when I make my recordings. The hand-to-antenna position is sensed, not seen.  I wouldn't be surprised if players that look like they are fixated on aiming at the antenna are just zoned out in another world where the eyes really aren't doing much.

And the reverse volume issue is perhaps another case of going against convention because it seemed to make more sense in the beginning.  I thought the more common volume sensing that feels normal now felt weird at first too, but  then I started thinking about how "up" generally means louder in the world of music, and it clicked almost immediately.  Is one better than the other?  I have no idea, but looking at the experts I don't seen any limitation with the conventional sensing, so I'll go with that.

To each his own, I agree.  Eric, you're a brilliant engineer, and I think you've designed an outstanding theremin.  But I don't think you're going to convince the bulk of skilled players that they are doing it wrong.   We've had many offline discussions of plate antennas versus rods, volume sensing, pitch spans, etc.  to the point it's almost a friendly version of a Hatfield-McCoy feud, 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".

Posted: 7/22/2021 8:34:28 PM

From: Occitanie

Joined: 3/4/2012

Theremin being mainly a self taught instrument (player density is too low), it is almost a second nature to challenge all things and habits that we have learnt, isn't it ?

Is there any risk trying plates after all ? That's what I find interesting in Eric's proposal: at least try it and make your own opinion.

No great advise to give. I am not an advanced player, enough level to have fun while playing, that's it...
Most propbably I'll get back to rods as with my other theremins but at least I'll have learnt something new.

Posted: 7/23/2021 2:07:16 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Wish I was not so far from Northern NJ..."  - Mr_Dham

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!

"The two AFE + Coils box are designed to be two satelites as close as possible to antennas (behind the plates ?), aren't they ?"

Yes, that generally works best.  You want to keep the coils away from each other and as near to their antennas as possible, but all within reason.  I've seen the interconnecting AFE ribbon cable pick up interference if it is draped over the tuner ribbon cable, so you have to watch out for that sort of thing.

"I am more and more thinking about having a central unit (screen, encoder, tuner, connectors) and two stands that would include antennas, one with plates the other with rods. Both would have specific receptacle for the AFE+coil boxes.
An idea to be explored... "

The Bollamin physical configuration isn't the most ideal, as the AFE & coil boxes are placed fairly close to each other, and close to the controller and tuner too - but it was a fun build!  For more serious use I would move the AFE & coil boxes farther to the left and right.  The most optimal configuration I've been able to come up with is my lab unit, where the coils are mounted directly behind U shaped plates, and this is probably what I will play from now on.  It is super comfortable, super linear, and rejects interference better than any other configuration I've tried.  But it's pretty strange looking, and fairly bulky to move from place to place (though so are cellos and tubas and pianos I guess).  I'm going to build a paper mache version soon, which should be less fragile, look a little better, and make transport easier.