Theremin Lip-Sync Scandal

"That's probably true, but more because there's never been a market for it than because the technology isn't there. Plenty of other types of standalone volume controls exist and are on the market." - Jo

I think there is some truth in this..

To make a quality theremin volume control, you would need to charge a lot more than the price of most pitch-only theremins.. And I think the biggest market would probably be owners of these..

And there are some big technical issues - the primary issue is that, as such a unit is likely to be used with some unknown pitch-only theremin, what frequency does one use? you cannot - you need to make your units frequency user adjustable so they can retune it if it interferes with their theremin...

... And making it user tunable is not just a simple twiddle of one control - If one wants your volume unit to have a nice volume profile (log) you need to tune both its equalization and its operating frequency.

My conclusion was that it was more potential trouble than it was worth, and that the quality of most pitch-only theremins simply was not worth having a good volume control for - better to just add a pitch section and sell a theremin for 50% more.

On the subject of volume - I dont know if anyone else does this - but I am using the reference oscillator as a fixed frequency volume oscillator - This overcomes all the tuning problems.. I have trimmable VFO, a tunable pitch antenna circuit, and a tunable volume antenna circuit, and a rock-stable fixed frequency reference oscillator which feeds both the mixer and the volume circuit.. One less variable oscillator, and no possibility of volume - pitch heterodyning.

"so I know just enough from each world to have plenty of ideas, and not enough to get them anywhere usually." - Jo

Dont let that worry you! - IMO, the big problem these days is that individuals focus is too 'tight' - Innovation, IMO, is best served by those with enough 'broad-spectrum' knowledge and interest to be able to pull ideas from many fields - Yes, it does pose problems in actually realising the ideas - But, if motivated, you can focus in on specific areas when needed - and you can usually find experts to help on matters where gaining the required competence is beyond your ability.

I think the world would be a better place if we had more "drop-out's".. And these "drop-outs" retained their self-esteem and recognised that they may have more potential for "greatness" than those who have been "responsible" !

Fred.

 "This doesn't come as standard? Why on earth not?" - From GordonC' post

The reason is simple I think - Because it wasnt on Lev's original! LOL - And also because an awful lot of theremins over the decades have been collosal rip-offs, highly priced with every possible cost-cutting measure applied.

And I think it is, sadly, understandable - Volume of sales are small and unpredictable - When I fitted the complex routings on my 16 theremins for the Royal festival hall, this addition cost me about £3 per theremin - these additions were never used - that £48 was out of my pocket - I never made a penny on the bloody things!

It is really difficult to know what to include, and what to leave out. I personally cannot imagine providing a theremin without good routing options - but I come from a modular synth background - If I was to include what I really want, there would be a full patchbay on the back of the theremin and a panel with so many knobs it would be the envy of every synth freak!

Something in the middle - that way, everybody will be unhappy!  ;-)

Fred.

 "This doesn't come as standard? Why on earth not?" - From GordonC' post

GordonC's post shows not only that there is perhaps a need for this additional functionality, but also that these things and much more can be realized - on demand.

But as far as I am concerned (I created Gordon's extension, adding 2 VCAs and some volume CV processing among others), I don't see this demand. It's now two years that Gordon is "on Tour" with that and I had only one request for a similar extension in all that time...

On the subject of volume - I dont know if anyone else does this - but I am using the reference oscillator as a fixed frequency volume oscillator - This overcomes all the tuning problems.. I have trimmable VFO, a tunable pitch antenna circuit, and a tunable volume antenna circuit, and a rock-stable fixed frequency reference oscillator which feeds both the mixer and the volume circuit.. One less variable oscillator, and no possibility of volume - pitch heterodyning. - FredM

So you use a single reference oscillator for both the pitch and volume sections?

To be fair, the same people who see my effects loop as obvious now did not say "why do you not have an effects loop" in the five years before I had it fitted.

This is not unusual with innovation - a great many things are obvious only with the benefit of hindsight.

Add to this the observation that the classical theremin community is has been very slow to adopt the use of effects amongst thereminists - prior to Peter's use of the Talking Machine it was pretty much limited to reverb. And a looper for the solo performer.

And for the experimental thereminist the path laid down by Moog guides one towards the use of CV and moogerfoogers.

So perhaps my effects loop has been "the cure for which there is no known disease".

And maybe this will change if it transpires that Peter has opened a door to a wider usage of effects - certainly putting distortion in the effects loop of my etherwave standard is the best way to condition the etherwave signal before feeding it through a Talking Machine - my experience is that it is preferable to just using the regular audio out.

Almost the same principle was used by Anthony Henk who created the theremin on which Lydia played in the mid-nineties and the one which you can see in the Midsomer murders recording with Celia Sheen.

There is one XTAL reference oscillator which runs at 150kHz and whose signal is directly mixed with the signal from the variable pitch oscillator. At the same time the 150kHz reference goes through a doubler stage (if transformer + 2 diodes) and excites the volume resonant circuit which is tuned to 300kHz which makes that the signal is attenuated the more hand capacitance is seen by the volume loop. Another rectifier stage converts this into a volume CV which is fed into the VCA.

Fred said: "On the subject of volume - I dont know if anyone else does this - but I am using the reference oscillator as a fixed frequency volume oscillator - This overcomes all the tuning problems.. I have trimmable VFO, a tunable pitch antenna circuit, and a tunable volume antenna circuit, and a rock-stable fixed frequency reference oscillator which feeds both the mixer and the volume circuit.. One less variable oscillator, and no possibility of volume - pitch heterodyning."

Here we have a situation my understanding of word usage confuses me, you just described the Jaycar Theremin?

"When I fitted the complex routings on my 16 theremins"

Also what do you mean or do with theremin routing?

Christopher

Fred, now I'm gonna be figuring out just how your setup works in my head for a while.  The VFO frequency should be different enough from the other oscillators to avoid interference still, right?  This should give you a pretty low difference frequency on the volume side, and with a fairly wide sweep?  Is there a fixed or adjustable bandpass, and about what would the bandwidth for it be?  And how would this arrangement apply, just for the sake of argument, if one were to use your volume control on a different theremin?

Not going to say too much yet 'bout what I am doing - I may yet get back to making these instruments!.. Oh hell - what does it matter!?

The key is the "Antenna tuning" - Unlike conventional designs, I tune my pitch and reference oscillators once - (actually, I dont tune the reference - its crystal derived - so I only tune the pitch oscillator - set and forget)

The antenna resonances are electronically adjusted using technology which is older than the theremin - Saturable core reactors or sometimes called "magnetic amplification". The pitch oscillator is tuned while the pitch antenna is tuned for optimum linearity at the reference frequency. Adjustment to the pitch tuning is then only made by adjusting the antenna circuit using DC (control voltage) which is either manually altered (user tuning knob) or altered by DC from a DAC driven by a processor, so the player can put their hand at the null position, press a button, and the instrument auto-tunes.

Now - the volume circuit does not have a variable frequency oscillator - it drives the reference frequency into its antenna circuit (which is tuned by a DC CV as described for pitch) and the resonance of this circuit changes in response to hand position, affecting the amplitude and phase of the signal seen on the volume antenna, and affecting the current flow through the antenna circuit. These parameters are "monitored" by an analogue circuit which produces the volume CV (a controlled portion of this CV is fed back to the saturable reactor which tunes the volume antenna circuit, and this gives controlled linearization to the volume - this way one can adjust the response from snappy to gentle - could be put on a user accessible control).

Both the maximum volume position and the minimum volume position can be adjusted, either manually or automated.

There is no interference, because signal radiated from the volume antenna is at the reference frequency - any unintended mixing with the pitch oscillator produces the same audio frequency as the intended mixing of reference and VFO.. !On my original, I multiplied the reference frequency *2 using a PLL - 'unintended' Mixing of the *2 reference with the VFO can give slight 2nd harmonic addition, but this is no problem.

My last prototype before I was "grounded" used a small PSoC MCU which was clocked from a crystal (24MHz) and the reference oscillator was derived from this as were multiples to provide a *2 reference for the volume circuit, and *16 frequency for pitch-voltage, and other divisors selected when the register was switched - A PLL multiplied the pitch oscillator and this was also fed to the PSoC, giving, effectively, a single chip "Epro-like" mixer.

There is quite a lot of fairly fancy stuff required to get the above scheme to work well - in particular, the saturable reactors are not "off the shelf" components - to get a good volume response from a fixed frequency oscillation scheme, great care is needed in the design of the magnetic components - and then there is a real problem getting these made reliably (particularly if you get them made in China!)

But thats How I do (or did) it!

Fred.

"And how would this arrangement apply, just for the sake of argument, if one were to use your volume control on a different theremin?" - Jo

Not going to work with most theremins - they are "tuned" by adjusting the reference oscillator. My scheme only works if the reference oscillator is rock stable and not adjustable.

But thats How I do (or did) it!

Fred.

I've gotta say, very neat, but nothing I want to get into.

Adjustment to the pitch tuning is then only made by adjusting the antenna circuit using DC (control voltage) which is either manually altered (user tuning knob) or altered by DC from a DAC driven by a processor, so the player can put their hand at the null position, press a button, and the instrument auto-tunes.

Is that a knob and a digital tuner, or one or the other depending on the individual instrument?  Does this setup account for users with different hand geometry and people who want to tune to different intervals?  I've found that tuning to null is the first step to get you in the ballpark, then I have to fine tune to get a nice perfect fourth from my position one to position four in every playable register.  Also some people tune so the zero-beat range is very far behind them.