Linearity - players' point of view

Posted: 9/3/2013 4:03:07 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 . ...................................

Joined: 12/7/2007

Sorry - this isnt a "players" perspective really - its technical, and could be regarded as at least a little OFF TOPIC.. But perhaps players would still be interested in reading / commenting on the idea of directional antennas...

"And if used to reduce the bulk antenna capacitance, a directional antenna could dramatically increase raw (%F/pF) sensitivity.  You might be able to make a Theremin you could play with your hand a meter or more distant." - Dewster

Yes - with a fully implemented active directional antenna, you do get a dramatic improvement in raw sensitivity.

However, I have found that in order to drive the shield and simultaneously keep radiated signals from this shield to acceptable (legal) levels, one needs a secondary shield - It gets quite involved and quite expensive.. One needs to generate and control high voltage high frequency signals, and keep the phase of these under tight control, and these drive into a highly capacitive load.. There may be a simpler way to implement a fully active shielded antenna, but I have not yet found it.

So I cut back the design to reduce the complexity - ending up with a front end which was slightly less sensitive than a conventional 'open' antenna. One does not, I think, need sensing beyond about 70cm for theremins. The directional antenna does nothing to improve or reduce linearity - except when one plays at the edge of the field, where any left/right movement is converted to pitch change and is really bothersome - this is only a possible problem close to the antenna where the focus is narrowest.

It is possible to use 'low grade' shielding with conventional theremin topology, but this does (due to greatly increased antenna capacitance seen by the antenna) make conventional (antenna L) equalization a lot more difficult to achieve - so my experiments mainly went on the direct-to-voltage topology, where linearization was performed by analogue computation.

I do not see directional antennas, on their own, as a fix for linearization - perhaps the opposite.. I only really mentioned them in the context of sensing the 'background' capacitance without the players capacitance being "in field" (re Randy's question).. To be honest, I dont think this approach will solve anything.

But within the context of direct-to-voltage or AFE for a digital theremin, where one does post-antenna processing of the signal to 'bend' the response for linearity, I think directional antennas have huge advantage - the antenna response (whatever it is) will be much more predictable (there is far less influence from the environmental / background capacitances, so one has a nearly constant 'baseline) and one is starting with a nearly constant profile to which one can apply whatever post-antenna / front end linearization scheme one chooses..

Even the crudest passive (ground shielded) directional antenna, which adds almost nothing to cost, can work well when connected to a suitable front end - but does not work at all with conventional theremins employing series LC antenna equalization.. This kind of antenna has a massive fixed "background" capacitance which effectively replaces any fixed tank capacitance, but provides shielding.. For example, replacing the 180pF capacitor on the simple SC theremin, or better, replacing the capacitance required for a "tank" in a series LC oscillator - a big advantage of series LC being the higher antenna voltage.

One problem with forming capacitance between the grounded shield and the 'sensor' antenna is the problem of thermal stability - this is another reason for going the voltage route - one can sense the temperature and easily provide a correction voltage to compensate for this error.

I think Dewsters Capless FE, or some oscillator with a series LC tank connected directly to the antenna sensor , is probably the best form of topology for passive shielded directional antennas, but this topology must be followed by some form of computational linearization, and really also needs a thermal correction signal.. so its unlikely that one could directly take a variable oscillator signal from the Front end for heterodyning, or at least not unless its frequency (or the Reference frequency) was modified in a controlled manner to facilitate linearization / correction.. (This is not too difficult to do though, if one has a CV input to one of the oscillators driven from a voltage computed to correct the curve) .. But IMO, its better to just generate a linear voltage from the Front End, and use this voltage to drive voice modules (which can be complete heterodyning theremins) - this gives one the ability to scale the number of octaves and perform register switching and even have multiple voices operating simultaneously.

I have provided one thereminist with a early prototype of an uncompensated 'one stick' "theremin" (derivitive of my H1 without EQ coil [wrong.. looking at my notes I see this particular unit I did have EQ coil] ) using a passive directional antenna driving a conventional direct-to-antenna heterodyning front end, to one leading  'experimental' thereminist.. However the silence from this player makes me suspect its (as I predicted) not particularly useful, even for non-classical playing.. Active equalization will be needed to get a response better than what is available from a simple Jaycar / SC theremin.

IMO, another huge advantage for directional antennas is that other musicians can move about close to you (one less excuse for thereminists, LOL ;-), and provided they stay out of focus, they will not influence  pitch - This was my first motivation for exploring the idea. I had a "working" prototype back in my earliest days when I was messing with Digital / PSoC, but at that time had no idea about the required range - my antennas had a 5V signal on them - easy to "shield" - but utterly useless for any real theremin application.. Moving up to the required real theremin voltages and ranges, I gave up on directional antennas for a while, and concentrated on matters which I only became aware of after exposure to real instruments and competent players.


Posted: 9/4/2013 10:36:56 AM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Fred wrote:

"However the silence from this player makes me suspect its (as I predicted) not particularly useful, even for non-classical playing"

No, it's not that. When I received it I was not in the best of health and it was inconvenient for that and other reasons, so set it aside until the time was right.

As the school summer holiday has now finished I am looking forward to some quality R&R and hope to be able to resume my project using your directional antenna quite soon. 



Posted: 9/4/2013 11:27:43 AM

From: Scotland

Joined: 9/27/2012

On the subject of Linearity.

'Playing the theremin on pitch is indeed difficult and would still be difficult even with a perfectly linear field, due to having no feedback other than the sound itself. But I think good linearity definitely helps. If I know that if I open my hand from position 1 to position 4 at any point in the range I'm going to get a 4th, playing is slightly easier than if I have to adapt to the fact that the notes are closer to each other in the higher octaves. Mind you, I do adapt and it comes quite naturally with practice...'  AlKhwarizmi (Page 1)

When I started learning the fiddle, I never looked at it and thought, 'Oh no, there isn't a linearity up the fingerboard so that's going to make it SO difficult to play...I might as well give up now'. I just got on with the job in hand and learned how to play the thing. No easy way out, just practice practice practice and after a while, it's fine, really it is.
Have you ever wondered why fiddlers like playing one fiddle? That's because not all fiddles are created the same. There is a small variation in length, different neck widths and different set ups: all of these things can conspire to making one fiddle very different to play from another.

I can never imagine a fiddle with as big an octave range having constant linearity-the neck would be very long and I don't think there would be anyone with an arm long enough to play it. Or else the spacing between the notes would have to be very small. Whatever, the fact remains that such a fiddle is not going to happen and it's foibles will remain.

However, there is such an analogy with the theremin as I see it: what use would a constant linearity 6 octave range be in the distance of an arm's length? Note spacing so small as to be almost useless?

A narrower range would probably be usable but then the wider octave range would not be available without some kind of range switch.

All which leads me to think that what we have gives us the best of both worlds and as AlKhwarizmi puts it 'I do adapt and it comes quite naturally with practice...'



PS-I realize I am probably the only person who thinks this.

Posted: 9/4/2013 12:19:48 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 . ...................................

Joined: 12/7/2007

Hi Gordon,

You have a really peculiar antenna, a one-off - I abandoned the approach because I could see it wasnt any use for precision playing - also, it is not a "hard" directional antenna - the shielded side is inductively coupled to ground and has some 'wrong side' sensitivity* ... The only reason I let this out of my lab was because you intend to do processing of the signal and may be able to compensate for linearity wierdness. I hope you dont misunderstand my last comments as any kind of "prod" LOL ;-) .. I hope the unit serves your purpose, but the performance of the unit doesnt affect me in any way - for me it (the antenna) was some junk I will never use, so if you do find it useful thats great! - but, you can always unplug the antenna and use a conventional stick.


*I was attempting to produce some linearization and increase in amplitude using a series antenna inductor, and simultaniously reducing the effective shield capacitance by tuning this.. I think linearity and immunity / range may be a little better than direct-antenna front end, but its too fiddly and imperfect to even think about using for production theremins.

Posted: 9/4/2013 3:21:41 PM

From: Northern NJ, USA

Joined: 2/17/2012

And the 2013 TW award for most ominous avatar in the developer category goes to...

(drumroll, opens envelope)


*wild applause*

[EDIT] Sorry Fred, not trying to minimize your current plight.  Here's hoping only the best for you in your newly adjusted living arrangements!

Posted: 9/4/2013 7:46:44 PM
randy george

From: Los Angeles, California

Joined: 2/5/2006

"My Etherwave Pro has a very linear pitch response right up to the antenna, and also I've tweaked the bass interval response to get linear response in the bass range too.  but the catch is, that this linearity works only for me because of my body size, and playing method."  - randy george

"I think this is an argument against the inclusion of fixed linearizing compensation circuitry in the EQ network, and perhaps also against the aggressive application of any linearization method." -dewster

When I'm talking about the linearity only working for my body size/playing method, I'm specifically speaking about the precision linear interaction that I require. What is non-linearity to me, could seem to be very acceptable linearity to someone else (i think this because many people do not measure their linearity with software/hardware tools, but only with their ears)  If anyone stepped up to my instrument, I guarantee they would have a much more linear experience than an EWS or some other instrument. (as long as they generally played without large body movements / only moving their arm/hand/fingers).  I hope that one day, someone can open up the Etherwave Pro and explain why it works the way that it does.  until then we have to piece together findings.

There are four significant things I 'did' to the instrument to obtain the linear interaction that I currently experience with the instrument.    

1) changed the tuning of the instrument to something beyond what was recommended in the original spec.  Original specification from Moog was that the Fixed Pitch Oscillator should be 282 KHz +/- 100Hz.    I adjusted it to 282.9 KHz. The playable range increased as a result... meaning, I had access to higher notes near the antenna... Also as a result I noticed more consistent linearity between the mid range and the high range.

2) On the front panel of the Epro, there is the small recessed pot labeled Pitch Response. I have meticulously studied the effects this parameter has on bass range linearity. This dial affects two things at once: the null zero-beat area width and bass range interval spacing.   if it is turned CCW, the zero beat area increases in size, while the spacing of bass range intervals decreases in size.   If the dial is turned CW, the zero beat area decreases in size, while the bass range intervals increase.  I adjusted this pot while carefully noting its associate voltage. it is a very sensitive dial, the full range covering 0 to -12V.  the setting of -8.99V ended up giving me the most satisfactory results.

3) over the past 4 years I had been experimenting with the orientation of my body with respect to the theremin, and I've settled on a position finally last year.  I noticed that the horizontal and depth distances of the instrument had effects on the linear interaction also.... because of the changes that are made to the 'capacitance changing functions' (having to do with the non-linear distribution of surface area of my human arm as it extends/retracts, the effect depicted nicely by Fred's illustrations)... also vertical adjustments to the height of the instrument (by raising/lowering it on it's tripod base) affected linearity to a small degree because they would slightly affect the vertical location of the pitch antenna that my hand was approaching. lastly because I sit when I play I can choose where my right leg sits on the ground... slightly further into the field would expand bass intervals slightly. 

4) The final thing was something that I stumbled upon almost by accident. I'll describe how the solution came to be in another post (because it's a long story), but basically I've found a way to increase my body's capacitance.  I attach my body to the electrical system earth ground and trim the additional capacitance with a linear 10K potentiometer.   How I know this actually works...  I turn the dial and the Epro's null (zero beat) shifts in location towards and away. And the bass range linearity is affected because it means the changes in capacitance introduced by my arm have slightly greater effect,  as if I was a larger person... however the slenderness in my fingers I think counters the effect.  i have no way to accurately measure the effect.


I don't think any thing that I have done takes away from the what is going on with linearization in the theremin circuitry, and I have/had basically determined one thing... Bob put enough magical stuff into this instrument for it to be calibrated to the preferences of some 'greedy' player ;) in the future. But he left no manual on how to actually achieve this.   The factory Epro manual is basically a joke. A hot rodding guide, such as the one included with the EWS, never existed for the E-Pro because it seems Moog Music (not Bob) chose to guard the secrets in the event they would create a new flagship theremin.  Only after many years of many emails and calls to engineers from Moog Music did I obtain even the small amount of info that I currently have.  A schematic has never surfaced.

So what is left for a player to do...   Nothing... [or]  Call Thierry Frenkel ... [or]  If you are 'insane' like me,  you do everything you can to get what you want (even though I am very lacking in technical understanding) 

at this point, I think it is very important that the fixed linearization compensation circuitry still be included in theremin designs,  but it seems to me (after all the exploration I've done on my own) that it is only part of a larger equation.  I would love to see some instrument in the future that could address all issues in one easy to adjust package, but I'm not very hopeful that will ever happen... especially if all the variables have not been acknowledged and agreed upon by everyone.  i guess the first step will always be sorting out the facts from the theory.

Posted: 9/5/2013 2:56:29 AM

From: Northern NJ, USA

Joined: 2/17/2012

"I would love to see some instrument in the future that could address all issues in one easy to adjust package, but I'm not very hopeful that will ever happen... especially if all the variables have not been acknowledged and agreed upon by everyone.  i guess the first step will always be sorting out the facts from the theory."  - randy george

As Fred interestingly observed, the Theremin was pretty much bypassed by the last wave (or two) of synthesis technology, so it's likely the developers one runs across will tend to be either stuck in the past or obsessed with attempting to do something radically new.  It's a small enough field that a single person could make a fundamental change (which makes it quite attractive from a development point of view) but IMO that change requires someone who has the time and inclination to come up to speed on the older technology and the subtleties of how it operates, and who also knows enough about modern technology to cast the Theremin into something of a new mold - i.e. one has to know how to play the game before one goes off breaking the rules.  And in any musical instrument / technology intersection there is the question of whether the technician doing the implementing is enough of a musician (or vice versa) to get the major elements in place and substantially right.

I believe as you do that the first move is to sort out the fundamentals, because only then can one throw out the old chaff with any confidence and begin to work in the new wheat.  While it would be nice if all the developers could come to a consensus on what is actually going on in every Theremin in terms of linearity, timbre, etc. I don't think a broad consensus will happen soon.  I don't believe consensus is absolutely necessary for things to move forward, but the lack of it does tend to hamper and confuse things.

Posted: 9/5/2013 5:42:47 AM
randy george

From: Los Angeles, California

Joined: 2/5/2006

well said, dewster.  I'm glad this forum exists [Thx Jason] because with it we know there exists a small handful of people who are interested enough to engage in discussion to further development.

When I can get my overhead video rig set up,(currently I'm waiting for some new video equipment) I will make a birds eye view video demonstrating the linear interaction I am getting from my EPro.

Fred, I am very interested in the directional antenna solution you have devised. Though, I'd like to re-read and digest what you have written before I comment further.  I think the topic deserves its own thread.

Posted: 9/5/2013 3:16:23 PM

From: Northern NJ, USA

Joined: 2/17/2012

"I'm glad this forum exists [Thx Jason] because with it we know there exists a small handful of people who are interested enough to engage in discussion to further development."  - randy george

I'm also glad the TW fora exist and for the same reasons - double thanks to Jason!

Posted: 9/5/2013 4:19:27 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 . ...................................

Joined: 12/7/2007

"And the 2013 TW award ..."

LOL ;-) Thank you, Dewster..

I know the formal action when recieving an award is to give a speech.. But I will spare you that! ;-)

I also with to add my thanks to Jason for TW and particularly the way he manages it - Just recently I read about "Nomo-Phobia" - a new phobia inflicting people whos lives are "contained" in their mobile phone - and the fear of loss of this appliance. I deliberately dont own one (well, I have a simple pay as you go one for emergency, but rarely use it) - But I have certainly suffered from NoTW- Phobia! Thanks also to other TW staff. 


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