Odd question: Pitch antenna design ...

"Not if they are behind you and if they moved closer they become the closest grounding object, seems clear to me. Basic response means just that, not every influence possible." - Amey

Hmmm.. Someone who is behind you is mostly "shielded" by your "capacitive shadow".. Unless they are a ghost (I have no idea about ghosts or their capacitive properties if they exist !) in order to to become the "closest grounding object" they would need to pass you on one side or the other, therebye no longer being "shielded", and therefore affecting the capacitance and theremin pitch long before they become the closest object.

I really dont understand why such an obvious fact which everyone who has ever played a theremin can demonstrate simply by switching the bloody thing on, is even being discussed or questioned! - It utterly astounds me!

I, for one, am not going to waste another word on this matter.

Fred.

.... For one, I didn't write what is highlighted in purple though... Please keep the stories and authors straight ;)

Sewer, yes! That is kind of what I was referring to! Good find! But the preset vibrato is kind of creepy!

".... For one, I didn't write what is highlighted in purple though... Please keep the stories and authors straight ;)" - Amey

Real sorry about that, Amey,  I have changed the offensive error - Fred.

"Amey; You might go to here to see what you are suggesting, sort of. For the antenna design." - Dana

This theremin is interesting - There appear to be two horizontal pitch antennas, and I suspect it operates by detecting the differential capacitance between these antennas. This design would reduce the problem of interference from the volume hand (and because distance from pitch antennas to hand will not vary greatly - the pitch being determined by differential capacitance, it should have a lot more linear relationship, and be a lot more imune to changes in background capacitance).

Technical mumbo-jumbo:

The horrible vibrato (and general tone of the instrument) leads me to believe that voltage controlled topology is being used - as in, the pitch antennas / ciircuitry are producing a voltage which drives a VCO, and a seperate LFO is also modulating the VCO to produce the vibrato. This topology is well suited to deriving the difference between capacitance sensed on two antennas.

             VIEWED SIDEWAYS

Quick diagram showing how I think this theremins antenna arrangement works: It can be seen that the vectors for 3 hand positions shown relate geometrically  - in the mid antenna position, capacitance seen by both antennas is equal (distance D3 = D4), at the top position,the capacitance on the top antenna is greater due to D1 being shorter than D2, at the bottom position the capacitance seen by the bottom antenna will be greater because D5 is shorter than D6..

The relationship between capacitances seen by each antenna will vary in a  consistant geometric way as the hand is moved on the Z axis. By converting each antennas capacitance to a voltage, and taking these voltages to a differential amplifier, an output voltage relating to hand position on the Z axis can be obtained.

The usual hand actions such as extending the fingers etc used for precision playing will not work with this antenna arrangement - forward - backward motion will have little effect as this design works on the Z axis only - other finger actions may be possible for precision playing, provided they act on the Z plane - for example, raising a thumb will add more capacitance to the top antenna.

I should just add that my diagram is extremely simplistic - The arm will significantly alter the capacitances seen by each antenna, acting, I believe, to further linearize the response. The body (and movement thereof) will have a far lesser effect on pitch than with normal theremins, assuming it is not 'tilted' on the Z plane - moving the top of the body towards the antennas would raise the pitch, leaning backwards (tilting the top half of the body away from the antennas) would lower the pitch - All assuming that my hypothesis about this theremins operation is correct.

 Fred.

Amey: (re Benedict Popescu vid) ".....But the preset vibrato is kind of creepy!"

The video Dana suggested is hard to judge because Popescu is not playing. He's "finger sync-ing".

The wobbly, extremely annoying, constant vibrato you hear is a semitone wide (which technically makes it a "trill") and, as you pointed out, it is generated electronically. Popescu is a Romanian born busker who entertains on the theremin (that contraption he built himself) and the musical saw. There are many videos of him on YT playing in the streets of various cities around Europe.

".....please, please not a wide vibrato...not to be mistaken for a trill." Clara Rockmore

"There is that math again that no one but an engineer would mention." -RS Theremin

Oh come now, surely we can discuss frequency and capacitance in the "Theremin Construction" section of TW.

"This theremin is interesting - There appear to be two horizontal pitch antennas, and I suspect it operates by detecting the differential capacitance between these antennas." -FredM

If the Theremin didn't exist, and I were building the first one, I might take this physical approach.  Left hand between two differential antennas for pitch, right hand for volume with closer=louder.  Guitarists would probably have an easier time of it anyway.  It certainly appears more theatrical, and more intuitive to the observer as to what the player is doing. 

I only listened once (couldn't bear a second pass) the vibrato and "finger-syncing" are pretty grating.

RS said: “When you reach into the pitch field you break a theremin out of the Null point, what determines the maximum distance this edge of the Null point can be moved away from the pitch antenna?

dewster ignored it.

Fred said: “Oscillator stability. In theory, if one had infinite stability, you could set the null point to any distance - one may only get 0.0001Hz difference frequency over a distance of 10m over some part of the "field" but it is practical electronics which is the actual limiting factor.”

Oscillator stability?   (Edit: IMHO the simplest answer would be the influence from the closes grounded object)

I was hoping for a break down of influencing variables and mathematical analysis which would predict the distance using modeling with the components used..

OK, enough of that nonsense, stability?....

Fred your graphic pulling rubber bands is nicely done!

IMHO the dual parallel antenna setup is 180 degrees out of phase with one another as can easily be done with an antenna fed off of each oscillator. The upper antenna raises the frequency like normal on approach and the lower pushes the frequency down. With the proper spacing between them I believe he may get improved linearity in this pitch field as he does not know there is a better method.  (-‘   If he would have type "theremin linearity" in Yahoo he would have found it at the top of the page.

Christopher

"RS said: “When you reach into the pitch field you break a theremin out of the Null point, what determines the maximum distance this edge of the Null point can be moved away from the pitch antenna?

dewster ignored it."  - RS Theremin

dewster was afraid he might get jumped on again if he had to use a little grade school algebra in his explanation.

I agree with FredM, the limit is set by the electronics, more specifically stability and thermal noise.  If you could stabilize it sufficiently, and if you could tolerate a (possibly huge) lag in the response, you could average the noise down to the point where you could probably detect someone picking their nose across the street.

Christopher "RS Theremin" wrote: "when I first participated at TW they made fun of the way I worded sentences, as time passed they made fun of my original theories and now they make fun of my logic."

I just wanted to know if that was the same "they" that killed Kennedy, faked the moon landing back in the 60's, and planted the dynamite in the twin towers to make it look like the buildings got knocked down by airplanes.

Just askin'