Let's Design and Build a (mostly) Digital Theremin!

" For a digital Theremin one is only looking for good delta F, the absolute F is fairly unimportant. " - Dewster

"run the variable oscillator at ~1MHz, divide the output by 4 to get ~250kHz, and run that into the mixer. " - Dewster

Hi Dewster,

First, Sorry about imposing analogue thinking onto the issue - You are of course absilutely right - the issue of oscillators drifting with respect to each other is not applicable to what you are doing.

In exploring register switching, I did mess about with VFO's running at HF (>800k) because this would have been an easier route than multiplying LF (<500k) oscillators using PLL. As you know, the E-Pro uses logic (square wave) division - and by using XOR mixing one gets a triangle wave which, with a bit of filtering, becomes sine-like. I do not know what audio processing or wave-shaping the E-Pro does, but from what I have heard and deduced, it is similar to the processing / shaping done on the EW.

My method of wave-shaping when using square-wave (logic level) inputs is done at the mixer - my design (which, probably foolishly, I published here) takes the VFO and Ref signals and provides difference frequency outputs with triangle, ramp and square outputs - One does not need any shaping on the audio side, all you need is filtering - Using well established subtractive synthesis you have a source of all the available harmonics from these waveforms - wheras from a triangle or square wave you only get odd harmonics, and must distort the audio to generate even harmonics (which are essential for string quality in the tone).

Generating registers by division is extremely simple - but alas, in my expierience, the results are far inferior to starting with LF and multiplying it up. I think one of the reasons may be related to oscillator coupling - the bass response (in terms of linearity and stability) seemed to be best when I had the oscillators running at 330kHz which were divided down (/2) to 165kHz before being mixed.. This was the 'bass'setting and could happily produce notes playable down to 32Hz with no problems of jitter or 'motor-boating'. Going to /4 allowed playing down to 16Hz.

Going to 660kHz or above and adding an extra /2 did not give me the stable results I got from lower frequency oscillators.. This does not mean that it wont work - In theory it should work fine, and someone else may manage to get it to work..

It was dissapointing for me though - I wanted to eliminate PLL multiplication - I need frequencies at least 8x the "natural" in order to generate an 'audio' frequency 8x that which is heard - I use this for fast update on the frequency to voltage converter .. So the ideal oscillator frequencies need to be minimum 800kHz (using mixed signal / square waves into the mixer, one needs good filters to roll off the unwanted HF - so having the divided frequencies above 100kHz is the minimum I am happy with).

My latest exploration have been into analogue multiplication (as in, take the square wave into a filter tuned to the wanted nth odd harmonic, the BW is sharp enough to reduce level of other harmonics, but wide enough to pass the frequency variation on the required harmonic.. simple to square this up..)

Fred.

No need to apologize Fred, I'm the one who forgot about the absolute frequency needs of the analog world.  Back to this:

"For me, the biggest problem is knowing where to draw the line at trying to improve performance.. Does one glue a thermal sensor to the antenna and compensate for this"  - FredM

I ran across this in the past and again today (while looking for capacitance guard banding):

http://www.stools.com.cn/html/About.asp?ID=39

Love that URL! Shades of "Waiting For Guffman" ("stool capital of the world...").

At the end (7.2) they show the things that can change the dielectric constant of air:

Change in dielectric constant of air


This change can be enough to affect the accuracy of precision detectors, but it can be easily compensated by building a reference capacitor in air with materials similar to the sense capacitor’s and using a balanced bridge detector.

That solution doesn't seem to apply to Theremins very well with the expensive / large coils and such (unless incorporated into the fixed oscillator).

They also mention ppm changes in material dimensions:

Insulators
Insulators have a much wider range of thermal expansion than metals. Glass, quartz, and diamond expand between 0.5 and 3.3 ppm/ deg C Plastics vary from polyimide’s 20 ppm/deg C through epoxy’s 45-65 ppm to Teflon’s 100 ppm/deg C..

Nature abhors a Theremin (and stability in general)!

[EDIT] More thermal expansion at http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html, wood is maybe a better candidate than PVC pipe for coil formers.  They have similar relative permittivity (http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html) but PVC being hollow might give slightly less self-capacitance (probably not much of an issue for tightly wound single layer coils).

"but it can be easily compensated by building a reference capacitor in air with materials similar to the sense capacitor’s and using a balanced bridge detector"

If only the above was true, my work with capacitive sensors would be a lot easier! - The trouble with constructing a "reference capacitor" is that, unless it is subject to EXACTLY the same conditions as the actual sensing head, it can (and often does) make matters worse.. For example, humidity - this can form droplets - if the reference plates do not have the same airflow over them, the droplet formation / evaporation will be different between the sensing plates and the reference plates...

But far worse is the problem of airbourne pollutants - dirt settles on surfaces and between plates.. one can clean the sensor head - but unless you devise a simple way to clean the reference plates, they accumulate error over time.

In fact - Capacitive sensing should really be left to do what it can just about manage - detecting gross changes like button pushing or non-critical balanced applications like touch screens.. Using this intrinsically unstable and environmentally sensitive method for anything requiring any precision or repeatability is folly!

Which is the only reason nutters like us are drawn to it! Lets see - what is the most critical sense we probably have - well, probably pitch. So lets take the most unstable medium (air's dielectric) and employ the most difficult electronic sensing method (capacitance) and use these to create an instrument which changes pitch (to which we are most sensitive)..

LOL ;-).. Lev was a genius! He pulled off the greatest technical joke perhaps ever! He even got Lennin into it (or perhaps Lennin was in on the joke.. "stupid Americans will buy into this - they'l buy into anything! - And you can spy on their factories while you're out there..")

Sorry - No disrepect meant.. I do not believe the above .. I think Lev was as (or more) facinated by the challenges as we are.. From a technical perspective the challenge is  (or can become) all consuming - One is fighting nature, and trying to "out-think" it.

And I will probably be obsessing about some theremin related matter right up to the moment I draw my last breath..

To be honest, the whole subject of flux in a vacuum is WAY out of my depth - I was "forced" to try to grapple with it when researching so-called "free energy" collectors which (alledgedly) sucked "vacuum energy". We "know" that there is a "quantum froth" - "matter" and "antimatter" coming into and out of existance - we "know" that the sum total of the universe is zero - that what we have is due to imbalance...

But there are things we cannot test anymore in the old "Edison" or "hilbilly" manner - We need mega-money machines like linear accelerators and the like - and even these are "toys" when it comes to addressing the big questions in physics.

So sure - we can "test" different materials in a theremins pitch field - we find that a broomstick in the field raises the pitch - then what? We can hypothesise as to why we get this result.. IF we are methodical, we can follow up with other tests to determine the likely accuracy or error of our hypotheses.

It gets tiresome - but some hypotheses have stood the test of time and become theories.. This does not mean that they must be correct, but it does mean that they have never been shown to be incorrect, and that there is nothing related to the theory which cannot be explained by the theory and/or other supporting "validated" theories.

And sadly, it is extremely rare to find a "flaw" in the established theories of physics - Oh, flaws will exist - but mostly they are insignificant anomalies which only require minor modification to the theory. Breaking a fundamental theory is what every true scientist would love to do, as it opens up whole new perspectives (and also brings fame and all the other perks ;-).

And the theories related to the mechanism by which theremins work are in this group - and everything one ever observes with regard to what happens when any object is placed in the sensing field can with (astronomical but not infinite) certainty be ascribed to capacitive effects alone, and can be verified (both practically and mathematically) to be soley due to capacitance... Now, it MAY be that "capacitance" is a myth - that we have entirely misunderstood what is really going on - that capacitance is an "analogy" just like thinking about oscillating springs as representitive of oscillators is an analogy, and in fact there is some as yet unknown science lurking in them fields..

All I know for sure is that, if I stick a broom stick into the pitch field, and the pitch goes up, it is likely that the capacitance "seen" at the antenna has increased..

I can speculate that (A) there is moisture in the stick giving it conductivity, that the stick is coupling to ground (capacitively - directly due to its proximity to ground, and / or by coupling to me who is holding it) and to the antenna.... Or (B) The dielectric constant of the stick is increasing capacitive coupling between the antenna and grounded object/s.

But thats all you can do - speculate.. Unless one was to spend a week with high-end instrumentation to determine as many possible factors as you can, you wont be able to verify the mechanism/s.

But whenever one does factor in enough data, you will find that capacitance explains everything...And theremins do work in a vacuum - Capacitive sensors have been used in space, and more significantly, experiments have been performed with capacitive plates related to exploring the Casimir and other effects.. Also, if theremins didnt work in a vacuum, vacuum capacitors wouldnt work.. And they most certainly do! (but are too expensive to use in theremins, sadly! ;-)

There are no "flux density changes heard in the audio signal ".

Sorry - I wish there was ! - Life would be so much more fun ;-)

Fred.

One of the funniest things I've read lately (from an engineering standpoint so everyone else please look away) was someone stating capacitance was undefined if there was no charge.  One wag said that he went to his lab and shorted out some capacitors and none of them disappeared!

[EDIT] Ah, here he is, John Woodgate, lab wag after my own heart:

>So are you saying there doesn't need to be any potential difference to
>arise between the turns for there to be an inter-turn capacitance?

Indeed. An uncharged capacitor doesn't cease to exist.

>I think that's a metaphysical question that more properly belongs to
>the realm of philosophy, John. :-)

I checked before making such a controversial statement. I short-
circuited some of the capacitors in my stock, and they didn't disappear.

>Ah, but shorted them at what frequency?
>;->

About one every 20 seconds, I think.

David Knight states in his Self-res.pdf: "The clue lies in the metamaterial permittivity factor εrh ; which is effectively defined as: 'that factor which makes the self-capacitance come out correctly'."  All impressive mathematics aside, engineering is often a quite seat-of-the-pants kind of thing.

RS (talking to myself at this point) have you read about the 1880's Michelson-Morley experiments that disproved the existence of the aether?  IMO one of the weirdest things about our universe is that literal nothingness has the ability to propagate light and other electromagnetic radiation with no visible means of support.  Loads of super smart people thought otherwise as the concept is exceedingly counter intuitive, but here we are staring it in the face every day we use a cell phone (not that you, or I for that matter, own one - and not that there's anything intrinsically wrong with that).  The universe is most likely stranger than anyone can imagine (but at the same time it's not a scientifically conceptual free-for-all).

" I can't get into this theory talk, I need to take another pill." - RS

LOL, ;-)

To me, speculative "theory talk" and thought experiments etc are fun and "enlightening" SOMETIMES.

I worked as a physicist for 3 years in a large team (about 30 chemists, biologists, medical doctors, technicians etc) and theoretical exchanges and "brain storming" occurred daily - we were creating a new "branch" of science, it became established as Biophysical Chemistry - It deals with chemistry at the physics level - the physical forces acting an molicules - how monolayers (layers one molicule thick) of phospholipids behave, how to produce them and use them, how individual protein molicules behave when hit by high energy photons and the like.. to name a few of the kins of questions the experiments were seeking information on.

Wild hypotheses were postulated for some bizarre results, and there was a willingness to entertain the possibility that some fundamental theory was flawed.. But if we had gone off at a tangent and ditched all the prior theories every time some anomaly popped up, we would have got nowhere -

We used established science first - and when some observation didnt fit, we checked the experiment. In every case we found some fault with the experimental methods or equipment or maths - Never any fault with the theories! - As a result we were free to explore the new science without being bogged down with trying to verify or refute the "old science" - we never needed to look at fundamental theories - they worked!

And this is what theories are good at - they are tools.. It almost doesnt matter whether they are "right" or "wrong" - they are at least usable models until something better comes along.

In terms of actually understanding the "truth" about the universe, we need the theories to be right - And this is the area where "this theory talk" has an entertaining place.. But from a perspective of using the theories as a tool and a common "language" to assist technical communication, it doesnt actually matter if the theory is "right" or "wrong" - just so long as it works, ind is "right enough" that no errors can be found in it,  it explains everything relating to it, and there is no part of it which is missing or requiring some magical suppliment to make it work.

And understanding the "truth" about the universe is impossible anyway - we can sharpen our physical  tools and theories for working with the physical model, but that is all - these tools are useless if we dont live in the physical model - just as the "spiritual tools" or flawed hypotheses or "theories" are useless in the "physical" model.

Fred.

"So you saw that brief post, if my 3" x 3" oscillator board was some how suspended in a reasonable vacuum it may eliminate most thermal drift?"  - RS Theremin

If the vacuum includes the antenna too then it would get rid of the changes in antenna capacitance due to the changes in the dielectric constant (or as the kids these days call it, relative permittivity) of air due to pressure, temperature, and humidity.  The oscillator itself would still drift due to local heating effects (including thermal expansion of the antenna itself).

A Theremin for use only in outer space would be a really, really tiny market!

"A Theremin for use only in outer space would be a really, really tiny market!" - Dewster

LOL ;-)..

You need to expand your thinking a bit, Dewster!  .. You are forgetting the huge potential market to the intergalactic nomads.

Why stop at "world theriminisation" Or "World Thereminization" as you Americans spell it.. ?  ;-) Lets go for "Interplanetary Theriminisation" or "Intergalactic Theriminisation" - or why not embrace the universe and all parallel "versions" thereof - If one did this, and sold only one theremin per universe, one would still have infinite demand! (ignoring the potential difficulty of having an infinite number of theremin developers competing for this infinite market...)

I am thinking this market may be enough to justify the effort of writing my book .. But getting it translated could be a problem - you seen the absurd price being asked for a babel-fish these days?! - they make the E-Pro price seem cheap! - and its even harder to find a healthy Babel fish than it is to find an original Lev theremin or a sentient antenna!

(the dangers of inserting an unhealthy babel fish in your ear are profound - believe me, ive been there, and done that! ;-)

Fred.

Oh - back to the question - Circuits "housed" in a vacuum containment will probably drift more .. No heat will escape through convection, so component temperatures will only obtain equilibrium via tracks etc - this could introduce problems.

Ovens - Passive or active? A "passive" oven is not really an "oven".. With my "ovens" I set a temperature above the maximum temperature any component will ever reach as a result of the highest ambient temperature combined with the maximum current / dissipation through it.

Have all the components in a good thermally conductive medium, have some heater (resistors or whatever) and the temperature controller.. Encase this in thermally non-conductive material..

Power up starts the heater/s - they drive hard quickly heating the assembly to say 55C, then slowly continue heating until the target temperature is reached (say 60C). One wants the thermal leakage to be slightly greater than the maximum component dissipation, so that one drives the heaters at low power for the minimum that is possible whilst maintaining control.. you need a damped analogue control - you certainly dont want to hear vibrato because the TC of the control loop is too fast! .. A non-linear response is required for best control.

The best "oven" I have built (for expensive lab equipment) used a peltier device for both cooling and heating - the temperature is set for normal ambient (say 25C) and the board is heated or cooled to keep this temperature constant.. If the temperature setting is carefully selected, this scheme draws less current than any other.

Speaking of tiny markets, I'm wondering about perhaps tiering my digital Theremin into one or more products aimed at different segments.  (I loathe tiering in general, it seems to be overused lately as a way to extract the most money from markets rather than a way to satisfy the most users.)  But for every flush, experienced power user out there itching to enlarge their personal harem, there are probably 10 or 20 curious cash strapped noobs looking to test the waters. The EW Standard is $389, and the Burnes B3 Deluxe is $249 - both on Amazon.  (There's also the Burnes B3 for $199 but it looks fairly toy-like and cramped in that tiny inverted project case.)

I'm thinking an entry level unit wouldn't have to do too much other than make some basic sounds and have the standard single dimensional volume antenna.  It could drift a bit with temperature so I could use standard ferrite coils, maybe with negative tempco caps.  If at all possible (sufficient logic left over) I'd install nearfield/farfield linearity correction, a multi-LED tuning indicator, and pitch preview.

I'm of course most interested in creating a flagship model with as many bells and whistles as I can possibly cram in there, but I'm finding this pie-in-the-sky product something of a constantly moving development target (better is the enemy of good, etc.).  An entry level model which necessarily has fewer features but that is still a fully functioning Theremin (however you define this) would help me test the market, as well as my abilities to supply it, and give me something to compare future hopefully superior products to.  My main fear is whatever perceived weaknesses the stripped down model might possess will be interpreted as general weaknesses of the digital approach.  A corollary fear is that by not putting my absolute best foot forward to begin with I'll be alienating future professional, more discerning / demanding buyers. 

Does anyone have any thoughts on this?  What is a reasonable target price & feature list for an entry level Theremin that is somewhat spare, yet good enough to keep one interested and inspire one to take the next step?  (I would of course be seeking the input of excellent players to hone the final product.) 

My own feeling is a fairly linear seeming pitch field with wide note spacing and comfortable volume field are at or near the top of the list, followed by tone that has some variability and isn't harsh, selling for around $250-$300.