# Magnetic field ?

Posted: 12/27/2010 2:12:17 PM

Joined: 12/27/2010

Hello,
I am a French student so forgive me for my English...
I study how the theremin work. I have understood that it is composed of two oscillators (the first fixed the second variable).
Then when the player approach his hand near the antenna, he changes the capacity of a condenser linked to the variable oscillator.
However i would like to know how to explain this phenomenon.
Do you think there is a magnetic field, generated by the antenna ? Is it a wave ?.
In a nutshell i am seeking for the physic principle of pitch variation (before electronic approach).
Posted: 12/27/2010 3:48:29 PM

From: Colmar, France

Joined: 12/31/2007

It's a pure electronic approach. The body of the player can be considered as more or less virtual ground (earth) due to its conductive mass. The hand of the player forms a simple variable capacitor with the pitch antenna (2 insulated conductors + variable space = variable capacitor). Its variable capacitance (up to 1.5pF) is added to the internal fixed capacitance of the variable pitch oscillators' tank circuit and will thus lower its frequency while the fixed pitch oscillator remains at constant frequency. A heterodyne mixer (nonlinear/exponential addition = multiplication) builds the sum and the difference of both frequencies. The difference falls into the audible range and that's it.
No magnetism, no waves, no myth, just a weak electrostatic field between antenna and hand (and other grounded environment) which form a variable impedance.

C'est tout et c'est simple comme bonjour!
Posted: 12/27/2010 4:18:27 PM

Joined: 12/27/2010

Merci beaucoup Thierry pour cette réponse, (je me permet de parler Français, sur la plateforme française sa ne répondait plus).
J'ai ratissé large pour le phénomène...
Donc c'est simplement un champ électrostatique entre la main et l'antenne:
*De quel ordre de grandeur?
*Comment le mesurer ?
Mais je ne comprend pas comment ce champ électrique est généré. Le joueur est au potentiel 0 et on créer un potentiel sur l'antenne c'est ça?

If translation is needed it will be a pleasure to speak english ... as you see...
Posted: 12/27/2010 7:31:28 PM

Joined: 8/1/2008

Il n'est pas mon intention d'être ni indiscret ni impoli. Vous dites que vous êtes un étudiant français, mais votre message en langue française est bourré de fautes.

C'est donc bien curieux!

Quelle est votre langue maternelle?
Posted: 12/28/2010 3:32:06 AM

Joined: 12/27/2010

Toutes mes excuses pour les fautes c'est vrai que c'est pas correct.
Cependant j'admire le coté vraiment constructif de la réponse. Si l'orthographe et la bonne syntaxe vous passionne vous pourriez m'aider à corriger mes fautes ?

Merci beaucoup Thierry pour cette réponse, (je me permets de parler Français, sur la plateforme française ça ne répondait plus).
J'ai ratissé large pour le phénomène...
Donc c'est simplement un champ électrostatique entre la main et l'antenne:
*De quel ordre de grandeur?
*Comment le mesurer ?
Mais je ne comprends pas comment ce champ électrique est généré. Le joueur est au potentiel 0 et on crée un potentiel sur l'antenne c'est ça?

Ou sont les fautes restantes svp ?
Posted: 12/28/2010 3:44:19 AM

From: Colmar, France

Joined: 12/31/2007

[off topic]Il est peut-être canadien... ;-p [/off topic]

The electrostatic field exists between a voltage source and ground. The voltage source is the variable pitch oscillator. Depending on the theremin model its amplitude at the pitch antenna may be 4Vpp at 460kHz (SC/EPE/Jaycar/Enkelaar theremins) or at up to 800kHz (Theremax), 24Vpp at around 280kHz (Moog Etherwave Standard, Plus, Pro), 60Vpp at 170kHz (RCA AR1264).

The amplitude is only of secondary interest since we are only interested in the capacitive frequency deviation on the pitch side which is not depending on the amplitude. Theremin designers choose the amplitude to not to be letal on one side and not to weak in order to have stable oscillation and to be not too sensitive to external interference at the other side.
Posted: 12/28/2010 4:58:18 AM

From: A Coruña, Spain

Joined: 9/26/2010

Just out of pure curiosity, but I wonder, in case you know off the top of your head... what is the amplitude/frequency of the field in the case of B3 theremins?

And is this the parameter that makes two theremins incompatible with each other? i.e. is it true that two theremins using the same frequency cannot be operated near each other, or is that issue much more complex than that?
Posted: 12/28/2010 6:25:30 AM

Joined: 8/1/2008

darkwhite wrote:

Si l'orthographe et la bonne syntaxe vous passionne vous pourriez m'aider à corriger mes fautes ?

Si l'orthographe et la bonne syntaxe vous passionnent vous pourriez m'aider à corriger mes fautes ?

Never mind, darkwhite. French is impossible! Writing and speaking it properly is a lot like playing the theremin, it demands a high degree of precision. In English you can get away with anything (sort of like playing the kazoo).

AlKhwarizmi, I don't understand theremins at all, in spite of years of explanations from experts. For example, I can turn on one of my RCA's and play it by gesturing around the antennas of an Etherwave Pro placed next to it - AND THE E'PRO IS TURNED OFF!

As we say in English, gofigga!

Posted: 12/28/2010 6:40:22 AM

From: A Coruña, Spain

Joined: 9/26/2010

coalport: Wow, that's pretty amazing!

Does it sound like the RCA or like the EPro when you play it that way? I guess it sounds like the RCA since it's the one that is actually turned on, but who knows... with all these strange phenomena, I could believe anything.

I wonder if the same effect could be achieved by playing around a pair of antennas without an actual theremin. It would be funny for magic tricks/jokes if one hides the RCA :)
Posted: 12/28/2010 7:11:43 AM

From: Colmar, France

Joined: 12/31/2007

What makes the operation of multiple theremins one besides another so difficult, are in fact interferences. But the pitch field is only one source of troubles and the less important one.

a) Pitch field interference: Imagine an Etherwave Standard operating at 285kHz besides an Etherwave Pro operating at 281kHz. Both theremins will output the difference of about 4kHz which is added to the output signal.

b) Pitch field damping: This is the effect which coalport described above. The pitch antenna of a theremin stays connected to its variable pitch oscillator's tank circuit and forms an impedance even when the theremin is switched off. Depending on the oscillator's frequency of the "active" theremin, the "passive" circuit behaves capacitive if excited above its self-resonant frequency and inductive below. This virtual capacitance or inductance present in an active pitch field will naturally detune the active pitch oscillator. No mystic.

c) Volume circuit interference: The strongest and most often occurring problem. The volume circuit of most theremins is not sensitive to frequency changes, but since it works as a field damping detector, it behaves like an AM radio which catches almost all of the RF dirt which is around in the air and modulates it directly through the VCA onto the audio signal. During a theremin festival last spring in Germany I could demonstrate the importance of this effect to the participants. We had set up 6 theremins in a rather small room and my Etherwave Standard produced a very ugly and strange noise, emitted by the volume loop of Lydia K's tVox tour played by Thorwald Jorgensen. I moved now my Etherwave half-way behind Thorwald, so that he was exactly in line between the volume loop of the tVox tour and mine. Since Thorwald is somewhat taller and weights some pounds more than me, he absorbed perfectly the interaction between both volume loops and there was no more interference.