Nasty radio-sounding noises at Zero Beat?

Posted: 6/25/2008 12:58:54 AM
EricK

From: USA

Joined: 12/8/2007

Heres the post:
Posted: Fri Nov 30, 2007 12:08 am Post subject: Leon Theremin's Hidden tones...

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Friends,

I want to share with you a discovery that I made, well I don't necessarily claim that its my discovery because im sure that im not the only one that has discovered it.

I have an Etherwave Standard Theremin.

I read that Theremin was working on a way to get 2 pitches simeltaneously with his device, the second pitch coming from the left hand VOlume Antenna.

I don't think that he ever truly developed it.


My discovery

I was running my theremin through an Electro Harmonix Bass Balls pedal. For those who don't know its a twin dynamic filter for Bass Guitar. When turned on, especially on the overdrive setting where its mostly used, It truly boost the crap out of the theremins initial signal.

Heres how you find the sound:


WHat you do is you let it warm up as much as its going to and tune it in the traditional way of having your body standing outside of the field of capacitance.

There is a realm of sound hidden in the most extreme border regions just before you will hear the first click of the pitch oscilator. Thats where you want your right hand to be, right in the immediate outskirts of ALMOST activating the pitch Osc. Outside of this there is a silence, but as you approach this area it satarts to make a fuzzy sound.

Granted your goal is to put some headphones on with the volume boosted about as high as you can get it being extremely careful not to activate the oscilator as it will be painful to the ears. You want to boost the signal as much as possible with an amp or something.

Then what you do is you move your hand away from the volume antenna and listen close. You don't want to activate the pitch oscilator. You shoudl hear a cracking or some feedback in the form of what sounds like a waveshape coming from the volume antenna.

Experiment with this and let me know if it is doing that on your theremin and if its not then what Ill do is I will get out my theremin and Ill do it and Ill give even more precise directions, as this is something that I just pulled from my memory.


This is the sounds that I believe Mr Theremin would have eventually developed, of someone hasn't already. I think its having your left hand controlling the volume and the pitch of one wave and having your right hand control the pitch.


Let me know what you find out.

Eric

HERES THE RESPONSE:

Welcome to Theremin World, EricK.

The volume loop does indeed produce a tone. That is how theremins work - the pitch rod is effectively part of a pitch-only theremin. The volume loop is part of what is effectively a pitch-only theremin too - except that the pitch from the volume loop theremin is converted to amplitude in the circuitry connecting it to the pitch rod theremin.

It truly boost the crap out of the theremins initial signal

...and when there is too strong a signal in the circuit it feeds back into the theremin and causes "ghost tones." The easiest to detect is when feedback causes the tone of the pitch rod theremin to bleed through when the volume is muted. I do not have a pedal that powers up the signal as you describe, so I can't reproduce your experiment, but I imagine a similar effect is happening.

The area of the pitch field that you describe is in the zero beat zone, when the pitch rod theremin is silent or at least sub-sonic, but before the zone where the etherwave's auto-mute circuitry kicks in. (Without this, the theremin would growl at you if you walked away from it.)

It should be easy enough to tap into the volume circuitry and make it's tone audible, but I wonder about it's usefulness - a pitch that is locked to the volume sounds like a darned nuisance to me.
Posted: 6/25/2008 7:38:42 PM
DiggyDog

From: Jax, FL

Joined: 2/14/2005

I have gotten a similar sound when my Etherwave is running through my ART SGX-2000 effects processor.

It mainly happens with some of the distortion effects.
Posted: 6/25/2008 8:15:50 PM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

Hi EricK -
Lots of interesting stuff here to ponder..

[i]"but before the zone where the etherwave's auto-mute circuitry kicks in. (Without this, the theremin would growl at you if you walked away from it.)"[/i]

The schematics I have for the EW does not show a "auto-mute" (unless I am missing it and it is implemented in some obscure way) - I have ordered an EW, so should have a better idea when I have looked at actual signals on a 'scope..

My understanding was that the reason there is no 'growl' was due to the square-law capacitance / distance relationship.. When one has tuned the Theremin for zero-beat at minimum capacitance (player furthest from antenna) there is no significant reduction of cappacitance possible, so therefore no significant change in frequency.. (assuming zero-beat = 0Hz, then a increase of the pitch oscillator of 10Hz would still be inaudible)..

If there is a 'muting' function, I feel a little P'd off - I have spent ages on getting my oscillators super-stable, so that there is no growl when exceeding the zero-beat distance!

On the whole issue of a duo-phonic Theremin, I have explored this a bit, but shelved it for now.. I came to the conclusion that the Theremin is hard enough to play, without adding another voice which one must control! Having a 1V/octave output allows seperate VCO's etc to track the Theremin pitch at any interval. My thoughts on duophony were to have 2 pitch antennas wired to give a differential capacitance and a total capacitance - voice 1 would track the total capacitance (distance from antennas to player) and voice 2 would be tuned to voice 1 +/- the difference capacitance.. this would mean that with the players hand in the centre of the pitch antennas, both voices would be the same pitch - but changing position reletive to the antennas would 'detune' voice 2. My directional antennas would probably be needed to implement this.
Posted: 6/25/2008 10:12:50 PM
EricK

From: USA

Joined: 12/8/2007

Duophonic Theremin

Well its been so long since I read any articles or watched the Theremin documantary. TO the best of my reccollection, the first pitch was from the regular pitch antenna and the second came from the volume antenna. THe modulation of pitch of the Volume antenna occured from a side to side motion of the hand just as you woudl do with the pitch antenna, except the vertical direction of the hand still modulated the volume as well.
The radio squelches I observed were definately controllable, but since they didn't occur while the pitch antenna was producing a sound then I can't tell where they came from of if they are not directly related to the pitch oscillator.

Frankly I don't know anything about the nature of the circuitry, im just an ameteur synthesist and Moog afficianado. I thought I might have been on to something when i boosted the signal but Eureka, I can't build anything lolol.

Difficulty of duophony

Okay so I read in the Moog Freq Box manual about using the expression pedal to modulate the VCO to create a "Pedal Theremin" Well frankly this is extremely difficult but it does allow for some great portamento effects via the Freq's VCO. Now the Freq Box also has an envelope Follower circuit in it, which I think is a Thereminist's best friend because it allows the Volume antenna to produce a voltage. (Env Followers transform amplitude of the input signal to a variable voltage) If you don't have an Etherwave Pro or a Theremin with Pitch and Volume CV outs, then an Envelope follower will allow for modulation via the volume antenna.
WHat I was trying to do was to use the Volume antenna to modulate the mix amount of the Freq Box, fading it to an equal volume level of the theremin to create a dual VCO sound, and then use the expression pedal to modulate the Frequency of the Freq Box, thus creating a duophonic theremin.

This technique is hard!

So to make a long winded post short, duophonic theremins would be insane to learn to master, but I don't think impossible.

Sorry if thats a hijacked thread.

Respectfully,
Eric
Posted: 6/25/2008 10:14:21 PM
EricK

From: USA

Joined: 12/8/2007

OH, Fred,
I don't know if I 100 percent agree with what the guy posted about my hidden tones, but I really don't know enough about the instruments function to say one way or the other.

Eric
Posted: 6/25/2008 10:19:11 PM
EricK

From: USA

Joined: 12/8/2007

Differential and total capacitance....like a ring modulator?

I wonder though if that would always play at the same harmonic interval.

Weren't you the guy who wanted people to post Ideas and suggestions for theremin construction. Send me a PM if so because I have something that I especially want.

Eric
Posted: 6/26/2008 1:20:33 AM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]"Weren't you the guy who wanted people to post Ideas and suggestions for theremin construction. Send me a PM if so because I have something that I especially want."[/i]

Yep - that was me.. Boy, did I get more than I could handle ! LOL! ;)

Email me - see my picture.. email address is there in a way that the robots cant find! - I hope your idea aint too radical - I have made so many variants on my original ideas since posting on TW, that I have spent months redesigning... I am now desperately trying to get the system together before Lippstadt (September) - I wont even look at doing any specials until after that.. unless, of course, it is truly revolutionary -- Or there is enough cash up-front to lift me a few inches above the brown sticky stuff I am sinking in! :)


Posted: 6/26/2008 1:46:02 AM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]"Differential and total capacitance....like a ring modulator?"[/i]

Not exactly - I have thought of several approaches.. 1) Have 2 HF oscillators, one for each pitch antenna, both running at different mean frequencies.. Process each antennas frequency to produce a voltage (no hetrodyning).. Get the average value of these voltages, and get the difference of these voltages.. Drive a VCO/VCF from the mean, Drive 2nd VCO/VCF from the mean, but add a portion of the difference (difference can be +ve or -ve).

2.) Have a single HF oscillator driving both antennas, but have a series resistance / inductance / (capacitance?) to give an AMPLITUDE difference between the antennas when either is loaded with more capacitance (by the player) than the other. Do conventional hetrodyning of the oscillator to produce audio, and a pitch-voltage converter driven from this audio.. Each antenna would need a buffer + amplitude discriminator connected to it (this could be quite simple, as there should be no phase difference, and a differential amplifier followed by synchronous rectificaton and filtering should give a difference voltage out)
.. The difference output voltage would need to be inversely multiplied (in analogue) by the pitch voltage - it would then be ready for summing with the pitch voltage, and able to drive a VCO/VCF which tracks the 'main' (true Theremin) signal, but is 'bent' by the hand position reletive to the 2 antennas..

3.) I have one other scheme - but I aint telling anyone what it is! ;)

None of the above is simple - which is why its on the shelf for now..

Posted: 6/26/2008 2:42:58 AM
FredM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]"I don't know if I 100 percent agree with what the guy posted about my hidden tones, but I really don't know enough about the instruments function to say one way or the other."[/i]

It all makes sense - except for the 'mute' function.. To get a frequency dependent mute signal would require a filter and voltage follower, and the resulting DC would need to be fed to the VCA.. I dont see any of these implemented in the EW..

Also, such a 'mute' would not be able to descriminate between "closer to the antenna than zero-point" or "Further from the antenna than zero-point", so the oscillator would need to be stable anyway, as drift in either direction would cause the 'mute' function to be turned off..

So.. There aint a mute - which means that the VCA is wide open if the volume antenna isnt sensing some-thing/hand.. And in this state, any audio frequencies being produced by any frequency interactions within the theremin will be passed unnatenuated to the output..

And what a lot of potentisal candidates there are! Volume oscillator + any harmonics thereof.. Pitch + reference oscillator + any harmonics thereof.. .. Any signals which are spaced apart by an audio frequency could, either through the mixer, or through circuit non-linearities, produce audio which gets to the VCA.

I think the most likely is Volume oscillator fundamental or harmonics mixing with pitch / reference oscillator harmonics.. As the pitch / reference oscillators usually run at lower frequency than the Volume oscillator, I strongly suspect that pitch / reference oscillator harmonics are the primary cause.
Posted: 6/26/2008 2:49:27 AM
EricK

From: USA

Joined: 12/8/2007

I emailed you.

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