Idea for Theremin Staccato Pedal

In terms of capacitive sensing, I think I'd try a "conducted away losses" synchronous AM approach.  Have a 2 layer PWB, drive the bottom layer with a low impedance periodic signal.  Amplify, band pass, and synchronously detect the signal at the top layer.  Human appendages near the top layer will conduct away some of the signal, lowering the detected amplitude.  A third layer added to the very bottom could be ground, but if the drive is low impedance enough this might not be necessary.  Insulate everything to eliminate ESD issues.

Hello Gordon,

Wow, you got the whole gang to come out on this one. (-:

Interesting subject you bring up, more attack on the volume control. We all have our own approach which may stimulate the creativity of some young mind still out there to develop something that all Thereminist might find useful. All of my volume control designs have been stand-alone and can be used with any theremin model, especially pitch only versions.

I have explored three different mediums, they include RF, active infrared, FSR and maybe ultra-sonic. Each has its own advantage over the other. My current model uses standard RF capacitive sensing so I can get a 24” shading window or it can be switched into a more abrupt smooth volume quieting, problem is you are stuck with abrupt until you switch it back.

If I interpret your description properly you want normal shading and an abrupt quieting attack always available?

I would design around an add-on sensor mounted on the box of the EWS. Switch it in or out. Normal shaded volume control would still be there with normal play.

A key component to all my volume controls has been the Vactrol “VTL5C1”. It will pass a line level voltage and is LED controlled so a lower analog control voltage works from any of the volume control methods. Also switching off the Vactrol is a clean click-free method of muting that is distortion free.

Edit: If I further my research on this topic it will be here  Reversible Volume Attack

(Why do bullies have the need to sit in the back of the classroom and shoot spit wads at the chalkboard of someone else trying to outline their own ideas... when the bully himself has never demonstrated anything that works?)

Christopher

Goodness me, what a lot of interesting responses. I am kind of excited about this idea - maybe it's contagious. :-)

Ilya. I think that proximity sensing in general has as much advantage as disadvantage. If I didn't, I wouldn't play the theremin! But I am open to all possibilities.

There will be tactile feedback here - to keep a good rhythm it is necessary to tap something physical - I have some toy electronic drumsticks that sense motion - they are almost impossible to play without tapping them against a surface. But do I think there is an advantage to sensing the foot approaching the pedal, and leaving afterwards.

Specifically, tiny variations in foot movement give variety and life to the sound - it is not automated and machine-perfect.

Think of it as a variation on envelope shaping - ADSR. Or rather, SR-AD.

Sustain - the foot is off the pedal. Release - the foot approaches the pedal. Attack - the foot leaves the pedal. Decay - moving the foot quickly up, it bounces or shudders a little at the top of its trajectory when it stops moving.

I should add that this is not a critical requirement. A switch with some envelope shaping control would be acceptable, but if I can avoid losing this subtle refinement I would prefer not to.

Dewster - I do rather like the burlesque and stag videos on archive.org - they're mild enough to be youTube friendly and they're public domain. :-)

I agree that it could be problematic for the precision thereminist. But I don't really know. I did note that Martin Taubman had pretty good intonation with his electronde. My own - non-precision - experience with treadles (wah, etc) is that they are not problematic for me - I can hold a steady pitch and operate a treadle quite easily - it's a question of putting the leg bones directly over the hinge, so any movement is restricted to the foot. Obviously with the staccato pedal the shortest movement would be the best, both for speed and balance. It would be interesting to hear from a precision thereminist about this, and to let one try it out for themselves. 

Thierry - same comments as for Ilya. Perhaps his idea of a pressure sensor would be preferable to a simple key? (I assume a pressure sensor would sense the difference between a gentle and a hard tap.)

Christopher - "you want normal shading and an abrupt quieting attack always available" Yes, exactly. And both at once. The snippetymen sample in particular illustrates the effect of very brief, abrupt silences in the middle of shaded notes. With the talking machine it adds plosive sounds (i.e. like T, B or D) to its vowel sounds.

I'm not sure about infrared. I have an Alesis AirFX which uses infrared sensors on three axes - X, Y & Z. It works but it's not brilliant when I compare it to the capacitive sensors of my theremin. It is also affected, it seems to me, by ambient light conditions. I guess an optical pedal would also be dependant on the lightness or darkness of the sole of my shoe?

I don't know much about ultrasonic - might it be affected by external sound sources? My first TV remote was ultrasonic - I could mute the TV by rattling a bunch of keys or by coughing loudly. 

"In terms of capacitive sensing, I think I'd try a "conducted away losses" synchronous AM approach.  Have a 2 layer PWB, drive the bottom layer with a low impedance periodic signal.  Amplify, band pass, and synchronously detect the signal at the top layer." - Dewster

 >> Engineer Technical level ;-)

Yeah - that was my favoured approach (TX/RX - as in the bottom layer being the TX, the top being the RX).. But I worry about the variation in capacitance between the TX and RX layers.. This design, if done capacitively, is going to be extremely critical.. One has 20mm in which to compress the entire 80db volume field..

If one was to have this field perfectly log (sonically linear) then we are talking about 4db/mm - a drift in the coupling through FR4 through temperature could easily amount to 12db, so the level at "foot down" could be -68db rather than -80db, and 0db move (maximum volume) move 3mm closer to the pedal - IMO the real problem is at the "floor side" (one wont notice a movement of the maximum volume point by 3mm, but you would notice a change of "silence" from -80db to -68db..

The above is compounded by the fact that "linearity" is quite unlikely - any linearizing will probably carry with it increased thermal drift which couldnt be tolerated with such tiny distances..

So I switched to "driven shield" (a conventional front-end of some king, either fixed or variable frequency, where the signal on the antenna/sensor plate is buffered with a high-Z input buffer that drives the shield) this scheme removes all "Floor / Ground" loading, and because the potential on the shield is exactly the same as the potential on the sensor, changes to the coupling between shield and sensor (due to temperature or whatever) shouldnt influence the sensor... Or at least not nearly as much as the TX/RX scheme. <<

But, to be honest, I dont think capacitive sensing is right for this application - Thierry's idea of using a strain gauge is a good one, and Force Sensing Resistors could be an even easier (and more robust) method.. A "Stomp plate" with one or more of these, and simple circuitry, and force of 100g to 10kg can be applied - any portion of this span could be selected anywhere in this range and amplified / offset to taste with a simple DC circuit containing an opamp or two....

Going extreme and using 4 of these FSR's, one should still be able to build all the electronics (including VCA) for under £70.. And its something a reasonably competent hobbyist could design and build.

Fred.

FSR Links: TekScan (data)

Quantum Tunneling Compound is another substance that can be used for force sensing - its extremely low cost (pills are small pieces of this compound that cost about 25p each, you would probably need at least 4 to balance the pedal plate) - But they are a real pig to work with.

Quantum Tunnelling Composite!

Fred, it may be a pig to work with, but it is perfect marketing copy for tech blogs. My inner nerd just had palpitations reading it. A quantum tunnelling expression pedal. Much cool. So wow.

And what a wonderful marriage of old and new technology to use it with a theremin. 

It looks like a very simple, very elegant solution from the data sheet. What is the problem that makes it hard to work with?

 Also, would it be complicated to make it switchable between it being normally muted and normally max volume, so that I can have it the way round I think it should be, and the rest of the world can do it their way. ;-)

Hello Gordon,

If my last conclusion was close to your thinking then I have a second question. In my stand-alone volume control I use pulse-width-modulation to control the Vactrol volume response, this is different than most. 

What if the volume loop responded like normal with excellent shading and anytime a rapid hand movement is detected “near the loop” it pops the sound on at full volume briefly like hitting a piano key. This could be setup to pop on abruptly with rapid hand movement in both directions or it can be the single normal direction. Rapid hand movement is the key.

This would be using basic logic to compare the hand speed at the loop. Using the Arduino comes to mind so the hobbyist of today can explore this volume method. My expertise is with C+ and the Pic by Microchips but the cross over is probably feasible for my old brain.

Stand alone volume loop controlling an AM Radio output.  Listen

Darn me, should be finishing my Altermen build.

Christopher

Hi Gordon,

QTC is a pig to work with because of the mechanical requirements.. You cannot solder to it, you cannot use metalized glue to bond contacts to it (the original data sheet advises this - newer data doesnt seem so adamant, so perhaps something has changed) , I doubted this advice, and wasted loads of pills trying to glue wires to them with silver loaded epoxy, or trying to glue one side to a substrate.. Also, the stuff I played with seemed quite theremin like - as in, troublesome - reactive to temperature and seems reactive to glues I tried (both conductive and non-conductive)

The pills are a tiny piece of rubbery composite that needs to have contact to both sides, or two contacts along its length.. The best operation is conduction across the pill (largest contact area) but this is the most difficult to achieve mechanically.

I experimented a lot with them, and only got then to work well when a mechanical engineer designed an assembly with gold contacts and plastic frame so the pills could be dropped in and were held secure by the assembly (I "designed" a joystick on this basis - but in fact it wasnt me who designed it really - it was the mechanical engineer who did.. I simply specified what was needed and discussed my problems, he designed the plastic framework and the actuation mechanism, I did a gold plated PCB the pills and assembly sat on..

It may just be that the technology was new and the compound poorly manufactured - it was back in the early days - we got some samples of the first available pills, and whether due to the pills or to the gross cost-cutting that the company (PML Flightlink, now defunct) applied to products, only one batch of 10 joystics were made, and they proved unreliable and never got to market.. It was (rightly I think) felt that out high-end inductive sticks (costing £50+ each) could be impacted if we produced a load of junk low-end products.. so instead the company started producing a load of crap inductive joysticks costing £50+... ;-)

A PCB with gold finger contacts the pills sit on, and a sheet of silicone rubber the same thickness as the pills, with cut-outs alligned to the contact positions, so the pills can be placed in these and held in position and an actuator surface (either conductive or not, depending on topology) placed on top to provide a force sensor 'sandwitch' should do the job.. and could even give directional data similar to a joystick..

The electronics is no problem, it could be "switchable" it could have simultaneous inverse functions, whatever.. You have resistance change from n pills, depending on where these pills are placed and how you "process" the data (sum them, find the difference between them, whatever) you could achieve whatever you want from the sensor pad - it can give X,Y and Z, or just Z, or !Z, or Z AND !Z, or whatever combinations - and likewise with voltages etc - these can be scaled and offset to taste if any force above the minimum required is applied to any sensor.

Oh - this is an issue for things like joysticks, but I dont think will be a problem for a pedal... This material is HIGHLY temperature sensitive.. I discovered this early in experiments, and included a dummy compressed pill in my joystick to provide a compensation reference..

Anything requiring mechanical fabrication of any kind (even cutting little squares out of silicon sheet) gives me the heebyjeebies - so for me, particularly for small quantity production, paying £20 for 4 FSRs I can just solder to a circuit, versus paying £2 for some QTC pills and spending a day making an inferior assembly, well - its a no brainer...

If thinking abiut production of say 10+, then it may be worth looking at having the required components for a "sandwitch" sensor made.

Also, the pills are easy to get, but the other stuff (cable, sheet,paint) I have tried to get within the last 10 years isnt easy to get, and certainly isnt cheap!

ps - these pills are tiny! - You really dont want to touch the contact area, and whatever contact material you use must not oxidise.. Aluminium and copper are good for a few weeks..depending on the environment.. so gold is probably best/required, particularly for signal level stuff.

There is no mechanical 'wiping' action at the contact points ( I strongly suspect the conductor -> QTC interface is the major weak link in an otherwise beautiful concept)  - I have not used any QTC for long enough to know their likely life-span under the conditions of a sensor sandwitch - There were only the joystick tests and I wasnt involved in that project anymore at that time - I know there were problems, but dont really know what they were - all I know is they were mechanical / contact related.

These have been about for nearly 30 years, in that time few have taken this technology - which wouldnt be the case if it was as good as it looks!

( When they first came out, with a LOT of publicity ( in the UK anyway) I was expecting them to replace contacts in switches and be a real winner, bringing a whole new spectrum of design possibility - If I could have, I would have invested every penny I had in the enterprise - the cost is so low, and they look so good, that IF I had been asked to predict their use in 2014 I would have guessed that at least 50% of new switches, including "white goods" power switches, would be using these or something similar.. Why use metal contacts with bounce and arcing when this far superior technology was available at lower cost ? ) 

But - I have never seen anything in production that uses tunneling pills, I know some UK Synth manufacturers were looking at them in the late '80s (I was trekking across manufacturers at the time looking for work) for things like polyphonic keyboards with poly aftertouch - nothing I have seen came of this.. The only applications I have seen use prefab assemblies similar to the FSR assemblies, at a price higher than conventional FSR's.

QTC MAY be ideal for this application - But I advise caution.

Oh, you could still call it a "Quantum Tunneling Expression Pedal" regardless of what technology you used - because quantum tunneling has been happening continuously in everything since the big bang! ;-)

Fred.

BTW, QTC has interesting properties with regard to capacitance.. The shape required fo assist tunnelling also increases "capacitive surface area" and a client of mine is using this for fabrication of capacitice sensor surfaces (I suggested the idea to him many years ago, and am named on his patent application) .. It only works if the compound is in galvanic contact with the plate, and not just floating in some dielectric paint, so the process is much more difficult - But the capacitive area can be massively increased in this way for short range sensors... Just as having a rough (course sand-blasted perhaps) theremin antennas would effectively increase their surface area.

The "compound" is just a load of conductive molicules with special mechanical "Quantum" properties (jaggedy pointed "extrusions" of exactly the right sort of dimensions) in a non-conductive "binder".

Christopher - I do like taking ideas and turning them on their head, so for that reason I like your suggestion. But I already use rapid hand movements near the volume loop to give a soft tremolo effect. (Besides, an articulation regulator is known as a kill switch because it kills the sound, not because it causes brief bursts of sudden, loudspeaker bursting, heart-attack inducing loudness. If I ever want to weaponise my theremin I think that is the way to go! :-)

Fred - so we would need a mechanical engineer, preferably one with experience of QTC, to design a harness for the pills - assuming they have a decent lifespan. And the fact that they are mostly a toy for hobbyists and not adopted by industry suggests they are not a good approach. The QTC cable looks like it would overcome much of the problem but is made of unobtanium. Er, ok, coolness factor aside, maybe that option is best left on the back burner.

---

So the consensus is that capacitive sensing is not the way to go. OK, I'm coming round to the idea of some sort of pressure sensitive pad or key. Here's a question that occurs to me. Velocity sensitive keyboards - how do they work? Can that technology be adapted to our purpose?

"Fred - so we would need a mechanical engineer, preferably one with experience of QTC, to design a harness for the pills - assuming they have a decent lifespan. And the fact that they are mostly a toy for hobbyists and not adopted by industry suggests they are not a good approach. The QTC cable looks like it would overcome much of the problem but is made of unobtanium. Er, ok, coolness factor aside, maybe that option is best left on the back burner." - Gordon

Yeah - but dont throw the baby out with the bath water! ;-)

For all practical purposes, the Force Sensing resistors do the same job - and they are easily connected, robust and reliable.. But a bit more money per part.

One could use a small FSR if you had the right mechanical construction..

Re velocity sensitive keyboards..  Usually there is a change-over pair of contacts, the centre 'wiper' actuated by keyboard movement - when a key is pushed, it breaks the upper contact, and near the bottom it makes the other contact.. The travel time is measured, and this is velocity.

The (IMO) best keyboard ever made (that I have seen) was one of the Sequential synths (T8?) which had an optical proximity sensor per key - this gave not only velocity but full positional data on a per-key basis (it was remembering this keyboard that caused me to suggest optical).

Most keyboards with polyphonic aftertouch use FSR's I think -

Oh, there is one sensor technology we havent mentioned yet (I think its tha last one ;-) which I was reminded of when blabbing about joysticks - Thats inductive sensing.. Inductive sensing could be usable - the inductive joysticks had an oscillator driving a coil connected to the stick, and 4 reciever coils on a circular PCB below it.. The RX coils were synchronously rectified and the amplitude from each was proportional to the displacement (distance) from the TX/Stick coil.. Distances variation was less than 1cm and resolution / stability was good... Replace the FSR with a coil, and the actuator with a coil, have a (much) more complex circuit, and you could get a pedal which is more position than force sensitive, acting over probably any span from 1cm to 5cm.

Alternatively - How about a metal locator pedal? A single coil in the 'pedal' and some iron on the sole of your shoe.... This could be nearly as quirky and troublesome as a capacitive sensor! ;-)

;-)

Fred. 

OK. That looks good too.

I've been thinking about design strategies, and starting with the simplest solution would seem a sensible approach - moving on to more complicated approaches only if necessary. So is this the simplest approach, or would that be a switch that is sensitive to very small pressures (and also is very thin to keep it as close to the floor as possible, and robust enough to stand a lifetime of being stamped on.) 

I would definitely want some control over the duration of the attack and release either from pressure/velocity sensitivity or a couple of pots. Range would be between the harsh clicks of my handheld kill-switch (sample: Articulator) and the soft bloops of the snappy Kees volume loop (sample: Boundary Logic.) Longer attacks/releases would be superfluous.

And, of course, with reversible logic. :-)