"Tanks" For Nothing!

Posted: 8/28/2013 12:02:29 AM

From: Colmar, France

Joined: 12/31/2007

I didn't talk about the series coil's SRF. That's another chapter. I have seen in practice that playing with that (the Epro's high register tone spacing is adjusted by a small variable 1-6pF capacitor in parallel to the first of the four linearization coils in the pitch arm) has nothing mysterious. The virtual SRF of this "additional" parallel tank circuit is still ways higher than the VPO's frequency, so that this tank behaves as a tunable/variable and slightly frequency depending inductance. Varying this SRF means simply varying the overall series inductance and thus the Ls/Lp ratio and at the same time the frequency gap between the oscillator's parallel tank circuit and the antenna's (overall) series tank circuit. 

Posted: 8/28/2013 1:56:22 AM

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

Thank you for your (IMO) insightful thoughts Thierry -

Yes - I think you are right.. Particularly with regard to player style vs linearity.

And if you are right, I suspect this would explain why different instruments are percieved as more / less linear by different players - as in, there is no such thing as a linear theremin 'box' - there can only be a linear theremin if one includes the most important component when evaluating the theremin - and this component is the player..

Reminds me of the discussion re bonding with the theremin, perhaps what I said there, almost in jest, about the player actually being one with the electronics, and the theremin ony coming "into being" when they are "one", could have some truth - perhaps..

And perhaps my attempts to quantify linearity, and my efforts involving tubes coated in carbon, and artificial hands with distance measuring sensors, so as to get absolute quantifiable data on performance would have been a complete waste of time even if they had worked!

LOL ;-) ... Such a fine line between hysterical laughter and tears!


Posted: 8/28/2013 6:48:47 AM

From: Colmar, France

Joined: 12/31/2007

My "problem" with all that linearization stuff was and is still that everything can well be described in a mathematical way but the results are not always obvious. I have spent days and nights, weeks and months, setting up and solving complex fourth and sixth order polynoms which allows me to "prove" everything I am telling. But all these mathematics which give global solutions in form of new functions and parametric equations made me often loose the view on these particularly small zones which are important for understanding the pitch field geometry. 

That led me finally to a different approach: In the meantime I use all that theory only to optimize the match between my thinking model and reality, and to see tendencies. The remaining part of research and understanding is better done by feeling and sensing which gives a more precise and usable experience  than analyzing very small parts of an infinitely long parametric curve.

Finally, that's why I finished up recommending every theremin designer not only to analyze theoretically as many instruments and circuits as possible, but to put his/her hands on them to get "the feeling".

Posted: 8/28/2013 3:09:17 PM

From: Northern NJ, USA

Joined: 2/17/2012

Thanks for your thoughts Thierry, I do very much understand where you're coming from.  And not to denigrate your skills at all, I believe there is a fundamental difference between repairing / adjusting Theremins and designing / constructing them in the first place.  With the former you are trying to squeeze the best performance out of an existing design.  With the latter you are trying to find the most optimal solution from a bewildering array of options. 

Many of these options have been implemented poorly, others obscured by various means, and the vast majority have not been implemented or perhaps even thought of yet.  Trying to read the mind of the designer when looking at whatever is known / revealed of his/her designs is impossible, though we of course still attempt to do so.  I could be that Theremin / Moog / etc. just needed a way to stimulate the EQ / antenna tank and picked whatever oscillator topology that seemed to work best, and we are reading too much into that choice.  It could also be that there is some minor linearizing effect associated with these topologies (that I haven't seen any evidence of) but we don't know if the topologies were explicitly chosen for that purpose or even with that in mind.

I have to use Excel, Inca, Spice, verilog, and toy setups on my bench to make sense of all this and to not go insane in the process.  If simulation doesn't match reality then the model obviously needs work.  When simulation does match reality I have some modicum of proof that I understand what I am doing and can progress from there.  I don't know how else to approach this exercise in musical instrument electrical engineering and actually get anything done.  If / when I get some prototypes in the field I will certainly heed the input of players, but I'm not at that point yet.

But it does seem to me that the parallel tank, for whatever minor virtues it may possess, is simply not worth all the very real trouble it brings with it.

[EDIT] In retrospect, the name of this thread should probably be "Tanks" For (almost) Nothing! because the tank may of course be a useful component in the general capacitive sensitive circuitry of a Theremin.  But I wanted to emphasize that an LC oscillator in the traditional sense isn't necessary (nor perhaps even desirable) for stimulation / phase detection of the (IMO) primary resonant structure in a Theremin, which is the EQ coil / antenna capacitance series LC tank. 

I'm nobody, but my advice to the budding analog or digital Theremin designer is this: Don't start your investigations by studying LC sine wave oscillators in books with an eye toward tacking on the EQ coil and antenna later, to do so is to take your eye off the ball.  Resonance of the EQ coil and antenna is of primary importance, the stimulation of it doesn't necessarily have to be sinusoidal, and phase detection can be other than 0 degrees.  Keep your mind open to things that haven't been tried (in Theremins) yet.  Bring all of your knowledge to the table and you just might budge this otherwise fairly stagnant field of study.

Posted: 8/28/2013 4:26:20 PM

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 believe there is a fundamental difference between repairing / adjusting Theremins and constructing them.  With the former you are trying to squeeze the best performance out of an existing design.  With the latter you are trying to find the most optimal solution from a bewildering array of options. " - Dewster

Dewster, I understand your perspective in the above, but I think you may be missing an important aspect - this is that IF it is possible to squeeze better linearity from an existing design, then some aspect of that design IS enabling better linearity.. If there was no linearization function occurring as a result of the topology, then one could not improve it.

If we look at the Lev theremins, it can be seen that the antenna EQ components represent a substantial cost - When Lev "honed" the RCA for production, cost was a major issue - they were damn expensive.. too expensive to realize Lev's proclaimed dream of them becoming a "common" or "affordable" instrument for the masses.. If he could have cut the cost and removed the EQ components (or removed the tank components and used direct-to-antenna series LC oscillator), I have no doubt that he would have done this.

Technically, there was no reason that Lev was forced to opt for large EQ coils if these were not required for some real purpose - Series LC oscillators ("capless") could have been built as, or more efficiently with then available components - and Lev certainly knew this and had played with this type of topology on many other things he built..

I strongly suspect that we (theremin designers / developers) are often going over ground that Lev trod many years ago - and that, IF we stick to components and/or topologies which were available in the 20's to perhaps the 50's, we will end up implementing some form of linearization using a series antenna LC with seperate oscillator having its own LC (be this parallel or series - the Lev oscillator actually uses a form of coupled series LC as its tank)

BUT - the above only applies IF we stick to those components and topologies - IMO, as soon as we break away from those constraints and use semiconductors, everything changes - we have more computational power, either digital or analogue - we can implement stable analogue real-time maths functions such as squaring and exponentiation, or with fast modern digital components can compute these and other functions with high enough resolution, and enough speed..

Lev never had those - oh, perhaps squaring and exponentiation could have been performed with available analogue of the time, but the cost of doing so would have been astronomical - one look at the VCA using tube heater current and one can see that we are far better placed today to implement functions such as analogue multiplication at high speed.

IMO, there was a "wave" which engulfed electronic musical instruments, but almost entirely bypassed the theremin - this was analogue computing (or computing in general).. Perhaps in this regard Bob Moog was right when he declared that the theremin was not a "synthesiser" - The synthesiser started as an analogue computer - and it is, effectively a computer - whether using analogue computation or using digital computation - every synthesiser, be it analogue, FM, AWM, modeling or whatever, is a computer.. (it could be argued that the theremin is also a 'computer' or 'synthesiser' because it produces waveforms which are multiplied - I have argued this before, LOL - but in this case I am arguing 'about' deliberate computation functions at the sensor side ;-)

And correction of performance matters - correcting the response from a sensor or "antenna" - is the sort of task computational circuits can chew up with ease.. And I strongly suspect that if Lev had been here today and was designing the RCA for minimum cost, there wouldnt be big coils (or any antenna-side equalization)..... But, alas, I also suspect that even if he had wanted to build it the way our old RCA's are built, he wouldnt have been able to do so - because I think that something has gone so badly wrong with the education we recieve today compared to what Lev recieved in his day, that had he been educated today, he would not have been able to design a modern equivalent of the RCA using its original topology.


Posted: 8/28/2013 5:33:37 PM

From: Colmar, France

Joined: 12/31/2007

OK... seen dewsters latest statements he appears to me rather not interested in what I told and thus I'll stop wasting my time by sharing my knowledge and experience with him, especially since he makes a difference between repairing/adjusting and designing Theremins. In my eyes the latter can not be done without the first, since the first leads to studying and understanding of the different construction principles, and to see and feel the strong and the weak points of each design. Thus it seems to me to be an absolutely required prerequisite before one even starts considering his own theremin development.

Dewster is not the first and also for sure not the last self-proclaimed theremin builder who will perhaps come out one day with a design which he considers as revolutionary and has everything to fascinate engineers. I have seen some of these people presenting their prototypes to our renowned (and/or professional) theremin players and I have seen or can imagine their reactions:

Lydia Kavina (always smiling and positive): "Oh, very nice, especially the housing! But can you play a simple melody on it, too?"

Carolina Eyck (more German and more direct): "Hmmm... long pause hmmm... I'm not sure if this is an improvement compared to my EPro. You should perhaps ask an expert about how to improve it."

Gordon Charlton (connects first his active subwoofer): "It's a bit thin in the sub-bass register, isn't it? Just wait, I'll connect my modified Etherwave and show you what is a true wall of noise!"

Wilco Botermans (fiddling around with cables and effect devices): "Where is the FireWire plug for my MaxMSP interface?"

Thorwald Jorgensen (always trying to avoid conflicts): "I'd need perhaps more time to get used to it." thinks: "Actually it does neither feel nor sound like a theremin for me."

Randy George (not from this world): "Well... it's somewhat earthy... makes a levitation while meditating for a short while ...it will perhaps sound more ethereal when I put it onto Croatian weed and play it barefoot"

Pamelia Kurstin (giggling): "Do you know what? Let's have a beer and a cigarette and talk about something else!"

Amethyste Spardel (admiring everything new): "Wow! What a beautiful instrument. I will buy it just now, paint it with pink varnish, and then immediately record a video covering the latest 10 top-charts from Enya!" 

Peter Pringle (very direct): "Sorry, I have no time, I'm just recording a new video on the Hoffman Theremin!" returns to his computer and posts the twentieth pamphlet on LevNet about people who dare wasting his precious time while they don't understand what makes a good theremin.

Posted: 8/28/2013 8:18:48 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

This thread screams for my input about things that are obvious once you know the answer but out of respect I will remain quiet.

Thierry I never took you as one with a sense of humor but the above is really funny.

You're French, you were being funny weren't you?


Edit: The reason I delete so many of my posts is because I want to prevent future theremin explorers from having to weed through the many posts to nowhere.

Posted: 8/28/2013 8:31:33 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Ha! Excellent. :-)

i think I came out of that roasting rather well. (Thinks: Maybe if I route the signal through my Nuclear Destruction pedal and play it with a chainsaw...)

Posted: 8/29/2013 2:18:46 AM

From: In between the Pitch and Volume hand ~ New England

Joined: 12/17/2010

That cannot be me you are talking about cause I don't buy everything I see cause I still don't have an etherwave standard to my grand collection of 3 theremins. I don't play ALL of Enya songs on the theremin cause at least I do know which ones wouldn't be suited for the instrument. 

...and I friggin' hate pink!

Posted: 8/29/2013 7:23:27 AM
randy george

From: Los Angeles, California

Joined: 2/5/2006

Thierry, you pegged me incorrectly. :) I can only guess you've formed your perception of me from a peek through a small window of my lifetime. ... so, to remove any misunderstanding, here:

Randy George (sensitive player with absurdly methodical scientific approach): "hmmm, that lowest 2.5 octaves is horribly non-linear, the tone is wretched, the volume antenna doesn't offer enough dynamic range while still allowing a quick enough response, and the tuning fluctuates way too much for me to be able to maintain precision with pitch because of the microscopic movements I employ in my technique... I'll send the rest of the detail in an email... oh never mind that, the only way this is going to work is if we met in person..."

don't worry, I take no offense to anything. If you were accurately describing me, I would be honored to accept the roasting... if inaccurate, then I must chime-in to set the record straight. I have been to Croatia, but I've never smoked a thing in my life. the last time I did a meditation was some time in 2009, when I discovered at that I did not benefit very much from it. i don't use words like 'earthy' to describe anything. I do love the Earth, our planet, but who doesn't...

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