Possible EW Modification board - *continuation*

This is a continuation from here (http://www.thereminworld.com/forum.asp?cmd=p&T=4127&F=1&p=3)

Have had a chance to sleep on my proposal.. I think it was a bit "off the cuff" and could do with some detail:

1.) I doubt that it would be as simple as one schematic leading to one board design leading to production.. I think there may be a fair bit of testing and modification to get it perfect, and probably a few PCB modifications before the final PCB is produced..

What I am saying is that it will require reasonable commitment - as all serious projects do.

2.) I would like to see this development kept 'open' .. I would like to have discussion between the project 'facillitator' and myself mainly conducted on-line in public view - probably here on TW with links to schematic and board layouts etc..

3.) I would like it to be possible for those following / reading the "thread" to build and experiment with us if they are so inclined and able to do so.. And for people to have access to the final layout and schematic, and to be free to make the modification for their own use.

4.) I think that for the majority of people, the option of buying a ready made board from us will be the best and cheapest option - but for others, all information required should be available.

5.) I think there will be many people who would prefer the modification to be fitted by a competent engineer, so Ideally, the person dealing with selling / distributing these modifications, aould also be competent to fit them and perhaps do a general service on the EW at the same time.

I feel that I'm perhaps a complete idiot. That's why I didn't dive too deeply into this discussion. But I want to utter some thoughts in order to understand what's happening:

The question was if one could get a bowed string-like sound out of the Etherwave Standard. Yes one can. I just don't understand why the discussion turns only around the waveform which is not the most important part of the effect, the rest being the playing technique: As Clara Rockmore pointed out, you have to play the notes and what is happening between the notes. So if you want to emulate a bowed string instrument, you have to play as on a bowed string instrument, that means that with your right hand you have to decide about appropriate vibrato and matching transitions between the tones: Would a Cello player put another finger on the same string, with or without moving the hand's position? Would he go from one string to another? Your left hand: Would he constantly move the bow or slow down or change the direction during the transition? A question which you have to to answer with your both hands in the air. It's not mainly because of the RCA's waveform that Clara played so beautifully, but because she thought and played on the theremin as on a violin or a cello. That's the secret. A convenient basic setting on the Etherwave standard to round up the tone would be the waveform knob at 11 o'clock and the brightness knob at 3 o'clock...

Concerning the previous technical discussion: See a theremin as a Swiss watch or a gearbox. You cannot simply modify only one gearwheel... Example: If you decouple the oscillators of the Etherwave by inserting buffer stages, the linearity in the lower octaves will be much worse, the octave distance increasing exponentially. Cheap integrated mixers as the NE602 or the MC1496 require small signal amplitudes at their inputs while the Etherwave's oscillators produce >16Vpp, a level needed to overcome the unlinearity of the mixer diode and to give the headroom for the wave shaping by "controlled distortion"... etc...

So: Never touch a running system... If you like the Etherwave, buy it and play it. If you don't like it, don't buy and play it, look for something different or design something yourself!

This reply is in two parts.

FredM:
I think perhaps at this point you are beginning to discuss the finer points of turd polish. I personally feel that your energies would be better directed at bringing your own theremin design to completion. We less technical folks who want a better option than the toy theremins that are available on the market would love you for it. Thank you greatly for all your help so far, but I think that wasting time on the Etherwave is beneath you. :)

Thierry:
Your post I'm going to say has infuriated me. I wonder if you actually took time to consider what you wrote here. A little background on myself, I've played violin since age 9, at age 12 I took up the viola which I still play, I'm now in my mid-30s. I fully understand playing technique and the method of vibrato is not unknown to me. My original question to ThereminWorld at large was "ok, I have an Etherwave, what is the next step up because I feel I'm ready for a professional instrument?" I feel like I've walked into a violin shop with a chinese made plywood starter violin and asked to see the semi-pro selection only to have the shop owner berate me and tell me that there is no reason my shitty $300 starter instrument can't sound exactly like a Stradivarius and since it doesn't it's my playing that is at fault.

The Etherwave was a great starter, I've enjoyed learning on it. But now I'd like an instrument that sounds better. Your suggestion to buy a better one is great, where are those exactly? Build your own was another idea, I'm fully willing to pay someone to build one, as I mentioned, any thoughts on which design is best? Design your own you say, I'll get right on that as soon as I go get an electrical engineering degree.

Frankly, if this is the response a musician gets from simply asking, "where does one find high quality theremins," then no wonder the musicians I know don't take the theremin as a serious instrument.


Once again, sorry FredM, none of this is directed at you, you've really gone above and beyond with this thought experiment, but yes, work on your own design. Sounds like it'll be a much needed quality injection to the theremin world.

[i]"So: Never touch a running system... If you like the Etherwave, buy it and play it. If you don't like it, don't buy and play it, look for something different or design something yourself!" -Thierry [/i]

Thierry, the last thing I want is any kind of contest with you - I have no real interest in developing this board either for the fun of it, or for money.. And, as I have stated many times, I do not see myself as an EW expert – All I am doing here is trying to help solve a problem.

Your statement quoted above, as I see it, is completely at odds with what you actually do! Are you saying that if people don’t like the volume response of the E-Pro, they should “look for something different or design something themselves” ? I don’t think so! ;-) and you don’t follow your advice “Never touch a running system”..

[i] “See a theremin as a Swiss watch or a gearbox. You cannot simply modify only one gearwheel...” [/i]
Oh, Please! I could not modify a gearwheel, but a skilled watchmaker could.. and would also know what other wheels need changing. A skilled watchmaker would not be the best person to select to change a component value in an EW – An analogue Electronics engineer would probably be your best bet for this task!

[i]“Example: If you decouple the oscillators of the Etherwave by inserting buffer stages, the linearity in the lower octaves will be much worse, the octave distance increasing exponentially.”[/i]

I am aware of this (I agree that a watchmaker may not be..) , and have simple ways of overcoming this – in particular, the oscillators can be coupled in a controlled manner – This has NO impact on the methods I propose to solve the problems I have identified.

[i]” Cheap integrated mixers as the NE602 or the MC1496 require small signal amplitudes at their inputs while the Etherwave's oscillators produce >16Vpp, a level needed to overcome the unlinearity of the mixer diode and to give the headroom for the wave shaping by "controlled distortion"... etc... “[/i]

On the EW, there is an attenuator (R23/R24) to reduce the signal levels to cater for the LM13700.. Matching signal levels and impedances is the every day task of Analogue designers and engineers – Why make it any big deal? There is no magic or mystery about it – most hobbyists understand the principle.

You don’t want to be involved in this project, that’s fine.. That is all you needed to say –
I would love to have constructive input from you, but constructive advice is the offering of possible solutions, not portraying problems which are simple to overcome, as insurmountable obstacles.

Fred Mundell
Fundamental Designs Ltd.
Electronics Consultant.
<- See Profile Image for Email.
Designer of Theremins and other alternative electronic music controllers and instruments.

Ooooops! It seems that I misunderstood or misinterpreted the initial problem. The question which arrived here was "How can I make the Etherwave Standard sound like a bowed string instrument?" and my answer to this special question was that modifying the circuit would not implicitly be the best solution in my opinion. One can naturally imagine thousands of other reasons which would perhaps justify the suggested or still other modifications, but this seemed me not to be the question.

Compared to the Etherwave Pro which has definitively a "bug" which can be cured, the Etherwave Standard is in my eyes rather something which works coherently on a certain limited level as does a good bicycle. One might imagine adding a motor to this bicycle in order to make it more powerful, which would create the need for more powerful brakes, a more stable frame etc.
Someone might invest the time in doing such "deep" and individual modifications but this risks to be not a "mass market" solution, as are VW beetles with a Ferrari motor.

I didn't portray simple problems as insurmountable obstacles but I tried to show with my weak English vocabulary that if you start with a small modification in one circuit stage, you risk to have to do lots of following modifications and end up with a complete redesign, so that the final result will have almost nothing in common with the original Etherwave. That's why I suggested rather designing something from scratch.

In ever case I didn't want to be offensive, I think I misunderstood the problem and gave answers to a question which was not asked. Please excuse me for any emotions which I have unwillingly caused!

I'm always ready to give constructive input and to share my Etherwave knowledge if there is a concrete and differentiated problem. But doing a complete redesign without having a clearly described design goal is out of my league.

[i]"FredM:
I think perhaps at this point you are beginning to discuss the finer points of turd polish. I personally feel that your energies would be better directed at bringing your own theremin design to completion. We less technical folks who want a better option than the toy theremins that are available on the market would love you for it. Thank you greatly for all your help so far, but I think that wasting time on the Etherwave is beneath you. :)" - Carricre [/i]

Thanks Carricre -
And yes, getting a better class of Theremin is what I will do, and my top priority..

This has not been wasted effort however - Every bit of learning and discovery helps to advance the art, particularly if done on a public forum where it can inspire others or be the seed for ideas.

"I think perhaps at this point you are beginning to discuss the finer points of turd polish" -

I think perhaps not - I think the biggest "turd" is right at the start of the signal path, and that everything thereafter only acts to "polish" it.. Feeding clean, non-interactive signals into the simulation, I found that the action of the brightness and waveform controls was good (much to my surprise - I had thought that the VCA / Distortion stage was highly likely to be the source of the 'turd').. And I therefore think that the EW [b] MIGHT [/b] become a pleasant sounding instrument Even over the complete tone (brightness and waveform) pallette.

The difficulty is that there is no way to implement modification to achieve this, on the existing board - The board should, ideally, be redesigned - but the other way is a "piggy back" board to replace the turd with a decent sound production stage.

There are a lot of EW owners - And a lot of these will not be able to afford to buy an expensive new Theremin when I bring mine to market..

And there is [b]probably[/b] nothing wrong with the EW as a starting / intermediate Theremin - Except (in my opinion) its horrible sound.

To me, the sad thing about Theremin technology, is the way it has stagnated.. The RCA was quite an advanced piece of technology in its day.. but todays theremins, compared to the advanced state of other electronic equipment, are mostly crude and "hobby level"..

The present situation will not continue - Even without my Theremins, High accuracy capacitive sensing is developing rapidly for other applications - IC's dedicated to capacitive range sensing are starting to appear, and some of these are getting close to the resolution and linearity required to better anything the Theremin designers have developed to date.. The days of having expensive non-linear bad sounding, unreliable Theremins is coming to an end soon - with or without me!

[i]"I'm always ready to give constructive input and to share my Etherwave knowledge if there is a concrete and differentiated problem. But doing a complete redesign without having a clearly described design goal is out of my league." - Thierry [/i]

Lets set a few design goals:

[b]1)[/b] Confirm the source of distortions which appear on the pre-heterodyne waveforms.. Ideas on these are:
[b]1a)[/b] Distortions caused by oscillator synchronisation via C2+C6 oscillator coupling.
[b]1b)[/b] Distortion caused by frequency / phase modulation due to changes in capacitance of D4 which results when D4 is reverse biased.

[b]Hypothesis:[/b]
FM/PM occurs due to capacitance change across D4, and this FM/PM causes distortion which produces harmonic sidebands both above and below the fundamental.. Isolating the change in capacitance (by buffering the waveforms) would eliminate this distortion.

[b]2)[/b] Depending on the above (true/false/to be verified) either buffer the waveforms, and/or use a mixer such as a LM633 to replace the diode mixer.

[b]3)[/b]The above would not remove the possibility of coupling the oscillators for purpose of [u]controlled [/u] synchronisation.. a capacitor (or trimming capacitor) of perhaps 3pF to 7pF or a network consisting of a couple of capacitors in series and a shunting resistor to ground in the centre of these (T network) connected between Q1C and Q3C could achieve this.. if it was needed for synchronisation and linearization.

[b]3a)[/b] If the primary source of distortion is caused by synchronisation of the oscillators, and if synchronisation is required for linearity, Then this may not be simple to solve and the validity of the project is in question.

Getting a clear answer to questions 1, 1a, 1b would require a spectrum analyzer which I don't have. So all I can do is observing the oscillators' waveforms with the oscilloscope before and after unsoldering C2 and C6, but I'm not sure if one may be able to see a difference...

Concerning the variable junction capacitance of D4: I'm not sure if this variation from 0.9 to 4.5pF will make an effect, seen that this is in series with a 15pF capacitor an then parallel to a 3300pF capacitor. Naturally, a theremin works with even slightest changes in capacitance, but mostly on the other side of the linearization coil... One could try to reduce this effect by connecting the anode of D4 not to the sum point of C2 and C6 but D4 only in parallel to R24 so that R23 would do supplementary decoupling, but this would clamp the upper DC limit at +0.6 instead of +0.2V...

...and I'm not convinced that the actual waveshaping section is working fine... I find it rather unpleasant that the waveform is frequency and amplitude dependent... but that's another problem.

Thanks for your input, Thierry..

[i]"Concerning the variable junction capacitance of D4: I'm not sure if this variation from 0.9 to 4.5pF will make an effect, seen that this is in series with a 15pF capacitor an then parallel to a 3300pF capacitor." - Thierry [/i]

Nor am I.. It will have some effect, and if this effect does cause what I hypothesize, I think it may be more significant than one would expect.. But I am not sure.

The practical problems of doing live testing to evaluate this are not trivial -

I will set up a simulation test bed, and derive the theoretical results, only if these indicate that the idea is potentially valid would I bother to persue it further.


[i]" One could try to reduce this effect by connecting the anode of D4 not to the sum point of C2 and C6 but D4 only in parallel to R24 so that R23 would do supplementary decoupling, but this would clamp the upper DC limit at +0.6 instead of +0.2V... ." - Thierry [/i]

Yes, this could be a simpler way - but I think one would require a bit more complexity - The diode across R24 would place C23 directly across it without any resistance to form a functional filter - it would also cause attenuation of the HF signals pre-heterodyning.. I dont think it will work.

But the basic idea could be sound.. Perhaps having a 10k resistor in series with each capacitor (C2 and C6) and placing an additional 10k resistor in parallel with D4, and leaving the circuit otherwise unchanged, would reduce the effect substantially.. It would also reduce oscillator coupling - so one may need to implement a seperate coupling path for controlled synchronisation.. An additional pair of capacitors wired like C2+C6, but their common point taken to ground via a preset (20k set at mid position giving 10k to ground, which could be adjusted for whatever coupling one wanted).


[i] "...and I'm not convinced that the actual waveshaping section is working fine... I find it rather unpleasant that the waveform is frequency and amplitude dependent... but that's another problem." - Thierry [/i]

Nor am I .. Simulations cannot be fully trusted..
But, if there are problems on the input signal, then one cannot be sure where the biggest 'turd' is, or even what is a 'turd' and what is toilet paper.. If the VCA is also a turd, we can solve that by turning it into a good VCA and using some other way to implement harmonic variation.. but one thing at a time ;-)

Back to the issue..

I do not have the time or inclination to take this project on my own.

So I am ending my involvement now..

If someone is interested in taking this project on (doing ALL the hands-on stuff as described earlier) Please indicate this, and I will take on the design / theoretical / development side..

If no one is interested in doing that, then there is little point in continuing hypothetical discussions here - I think that, without breadboarding and experimenting to confirm or smash ideas, nothing more of value can be said.. Likewise, if no one is going to take this up, there is no point in me spending hours running simulations which, whilst possibly enlightening, will end up being wasted time on my part.