Invisible pitch-only theremin for performance art?

I had another mad idea of a theoretical nature.

Typically a pitch-only theremin has two electrodes - an active one; a metal rod or plate ( known as the pitch antenna) attached to a theremin circuit and a passive one - the player's body.

Can this arrangement be reversed, or to put it another way; is there a reason why the player's body could not be the active electrode, and a conveniently located earthed metal rod (for instance) be the passive electrode?

Sure, but it involves inserting a pitch-only theremin up your bum. OUCH!

(That's a joke, son. You had to know *someone* was going to say it.)

[edit]
G.C. writes, "In fact I think you might be the only person thinking it."

I can't say I would agree with that. I'm just the first person who had the opportunity to SAY it. Some Brits are well known for having a rather bawdy sense of humor.

Thank you for the compliment and the history lesson.

Oddly enough, no I didn't. In fact I think you might be the only person thinking it.

From now on I shall always think of you as a Hieronymus Bosch for the electronic age. :-)



[edit] I'm sure I don't know what you mean, Jeff. Brits doing ribald humour? Surely not.

http://www.youtube.com/watch?v=unmkX15AeN8

[/edit]

[i]"is there a reason why the player's body could not be the active electrode, and a conveniently located earthed metal rod (for instance) be the passive electrode?" - GordonC[/i]

The answer is yes and no ! ;-)

The sensed capacitance completes a circuit.. When the Theremin is operating, it has some form of connection to ground OTHER than via its antenna.. Ideally, this 'ground' connecion is a galvanic (electrical, solid wire) to the "ground" (usually 0V) on the Theremins circuit board.. When there is no galvanic connection, there should be a STRONG capacitive connection beween the circuits 0V and "ground" (this usually occurs through power supply components)

The signal on the antenna is referenced to the circuit boards 0V, so it is capacitance between this 0V and the antenna which is of greatest importance..

If the 0V and "ground" are effectively the same, them any "grounded" object approaching the antenna will be "seen" by the antenna. The body of the player has reasonable (due to comparatively large 'plate' area they represent) coupling to ground.. so everything (usually)works.

Now..

If you are carrying the Theremin.. The only (unless one is trailing a ground wire - and even this has its problems, apart from the obvious ones ;-) the 0V of the Theremin can only couple to ground via your body and its capacitive coupling to ground..

As you approach a grounded object, your bodys TOTAL capacitive coupling to ground will increase.. Hence the 0V coupling to ground will increase.. But this will not be 'seen' as much of a change (if any) to the sensed capacitance..

It is only the difference between the capacitive coupling of the antenna vs the capacitive coupling of 0V* which will be seen.. If both antenna and 0V are only coupled to ground through the same common 'plate' (your body) it wont work.. So it wont work!

*not exactly true.. but this is longwinded enough already! ;-)

There is a way which would work..

one will need a directional capacitive sensor - it would need to be constructed into something like a torch, ideally with a metal body in which the 'theremin' electronics was constructed - this metal body being connected to the circuitry's 0V. The 'bulb' side of the 'torch' would consist of a piece of circuit board or other metal insulated from the body - and would connect to the circuit's antenna input.

With this arrangement, one could aim the 'torch' at any grounded object, and the distance / capacitance between 'torch' and 'object' would determine the pitch.

The way this would work is that the capacitive and/or galvanic connection from the metal body to the hand holding it, and the capacitance of the person to ground, would form a return path for the signal from the antenna via any other grounded object in its sensing field..

Cool - could one player operate two simultaneously, one with each hand?

[i]"Cool - could one player operate two simultaneously, one with each hand?" - GordonC [/i]

yes.

These should be quite easy to construct.. but to get a reasonable sensing range, you want as large an antenna plate as possible.. the old fashoned metal torches with a large front lens would probably be ideal.. you want 5cm diameter or more antenna disc, and at least 1cm perimeter arround this disk to seperate it from any other metal.. I would make the sensor antenna on a piece of PCB - but you could use plastic and put a foil disk on this.

I might hold you to that one day but at the moment the new toy budget is in stasis. The real budget was just in an unexpected collision with a car. :-( Fortunately only the budget was hurt. And we got a newer car. :-)

Plus, I just got a new toy (http://www.abstractdata.biz/synthex/index.html) to explore (I'm going to connect it the same way as the ring-mod) that I think will give me plenty to figure out and will extend my sound palette quite a bit (hopefully into good places - I don't know yet - the power supply may be delayed by the postal strike) and additionally; I'm wondering about getting a little device to let me transfer the audio volume from one part of the effects chain to another (please tell me that's cheap and easy - just an envelope follower and a VCA) - in a while.

Gordon,

it might be worth looking at a different sensing mechanism fot the "theremin torch" - I usually abhor "theremins" based on sensing other than capacitance - but for this application, I think ultrasonic distance measuring would probably work better.

The capacitive "torch" could have a problem with frequency stability - the grip applied will affect the ground coupling, and therefore the pitch - also, if being used while walking / dancing / cavorting, the bodys capacitive coupling to ground will change.. you may get less coupling when standing on one leg, and more coupling if kneeling..

Optical sensing is probably not a good solution, as it will depend on the sensed object being reflective, and on the reflective angle being usable..

Ultrasonics would, I think, be the best option - particularly if the sensed objects are large and acoustically reflective.