Pitch Drifting .. .. ..

Posted: 7/18/2011 11:32:11 AM

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

Joined: 12/17/2010

I recently noticed that when I sustain a given note while increasing the volume hand, there is a slight pitch drift happening with my Wavefront.
At the moment, it is not too bad but I assume that with time it will get progressively worse? I asked Gene Segal if I should detach the circuit board and send to him rather than sending the whole cabinet - Here is the reply I received from Gene on that subject:

"Are you positioned slightly towards the volume antenna when you play? Try adjusting your body position to see if there is a difference."

Well, I am pretty much standing in the middle of the cabinet slightly toward the pitch antenna. What if the position I found myself being comfortable implied being closer to the volume antenna, I would have to find another position so that wouldn't happen? Really?

"Also, this may occur from slight moisture in the wood cabinet, which will improve with time."

Does moisture really matter that much in this case? I've had this Theremin since March, if that is the reason how long does it take to "settle"?

"I doubt this is a circuit board issue, and if there is slight pitch-volume coupling, there is no reason to believe that it will get worse in time."

But I noticed it and it does bother me. This theremin was NOT chunk change... I feel that perhaps he's trying to avoid me sending in the board to him to take a look at it...

I'd be appreciative of any comments or suggestion here... THank you.
Posted: 7/18/2011 12:22:20 PM
Jeff S

From: N.E. Ohio

Joined: 2/14/2005

Is the drift consistant? Does it always drift only up or only down? This would be indicative of a coupling issue.

If not, it may be that your body position is changing when you lift your volume hand. Also remember that your pitch range is very compressed, so any instability in your stance will have a greater effect than other people's.

To determine which, place your hand firmly on the top of the theremin cabinet to hold a note rock steady. Then, lift your volume hand and listen to find out if it is in fact the theremin or you.

Normally, pitch drift due to temperature or humidity occurs over a matter of a few to several minutes, not within a single movement of the arm.
Posted: 7/18/2011 12:44:43 PM

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

Joined: 12/17/2010

"To determine which, place your hand firmly on the top of the theremin cabinet to hold a note rock steady. Then, lift your volume hand and listen to find out if it is in fact the theremin or you."

Jeff, I did that igher pitch and lower pitch - both changed...

Right now, with added vibrato, the variation is masked, but I am afraid one day the problem will get worse.

I do live in NH and we have pretty humid days here... I'll have to test on other less humid days if that happens (when will that ever be? lol)
I guess I could load up the theremin's inside cabinet with silica gel packs and see if that problem is caused by humidity :)
Posted: 7/18/2011 7:06:02 PM

From: Redmond, WA

Joined: 9/1/2007

Hmmm...I don't remember experiencing this problem with the Wavefront when I had it...but I did notice something similar with an RCA theremin. Is this similar to what's going on (maybe more pronounced, though):

http://www.youtube.com/watch?v=UAkIscBOQXA (http://www.youtube.com/watch?v=UAkIscBOQXA)

Also, I noticed this happening at home once, but only with my Etherwave Standard (Not the Wavefront). I was running an experiment with a portable jump-starter as a battery pack. My resolution was to tie the audio cable to my waist (touching my skin) in order to "ground"(?) myself to the theremin.

I'd suggest setting up in different parts of the house to see if there is interference, and also try wrapping the audio cable around a limb or your waist (touching the skin). You should notice an initial increase in pitch that will cause you to need to retune, but see if the slight drift is eliminated as a result.

If so, you may have grounding issues on your outlets. how old is your residence? Could it be that the power grid is not fully/properly grounded?
Posted: 7/18/2011 7:28:35 PM

From: Canada

Joined: 8/1/2008

Amey, the problem you describe is not uncommon.

The two electrostatic forcefields - volume and pitch - overlap to a certain degree. It sounds to me like you are playing your volume antenna too close to your body and therefore your movements are slightly disturbing the pitch antenna forcefield. Try moving just a couple of inches closer to the pitch antenna side of your instrument, and try to play the far side of the loop rather than the narrow part where it plugs into the cabinet. This will require that you extend your volume hand a little further out and away from your body but it should cure the problem.
Posted: 7/19/2011 6:20:54 AM

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

Joined: 12/17/2010

thank you everyone for your replies... I'll do more position testing and location to see if it does go away.

Dan - i viewed the link you gave - it is kinda like that, but yes - a lot less pronounced :)

Thank you Coalport for the suggestion - I'll try different position and see how it goes :)

I'll report my findings later... :)

Have a great day everyone!
Posted: 7/19/2011 11:39:41 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

Hi Amey,

You may find that this is an issue to do with your body's capacitive coupling to ground..

If your body's coupling to ground is low, then the extra coupling between your volume hand / arm to the ground (earthed) components in the theremin (or even the effective "ground" coupling to the volume antenna) will influence the total capacitance sensed by the pitch antenna..

The problem with the above is that, as your local coupling to ground changes (due to changing your position, the position of the instrument, humidity etc) the percentage of extra coupling to ground via the volume hand/arm will change.. so it will seem that the instrument is drifting.

If the above is the cause, the best way to fix / reduce the problem is to increase your body's direct-to-ground coupling.. You can test if this is the cause by physically connecting yourself to ground - a wire onto a ground point (ideally directly to the theremin ground) to a piece of metal foil or other good conductor tucked into your right sock would do the trick..

But beware! Grounding yourself can be a hazard.. if you touch anything 'live' you could get a fatal conduction path.. which is why I said right sock.. connecting a grounded conductor to your left leg and touching something live with your right hand is the most dangerous conduction path!

If the above solves the problem, you can implement a permanent fix by having a grounded conductive mat under you (which increases your body's 'direct' capacitive coupling to ground) when you play (something which I think greatly improves theremin stability - provided ones body stays in a constant position reletive to the mat, and/or one is aware of its contribution to the capacitive coupling and pitch).. The other way is to get a grounding strap (http://uk.farnell.com/desco/09060/grounding-wrist-strap/dp/1568936) (these are available for electrostatic work) and connect this to ground via a 1n - 10nF safety isolating capacitor (http://uk.farnell.com/vishay-bc-components/bfc233820102/capacitor-class-x2-1nf-310vac/dp/1215444).

One other way to increase the body's capacitive coupling to ground is by having a pile of screened wires running under your playing area! .. It is quite astounding the difference even one grounded screened cable can make.. If you are a tidy person who keeps all the cables away behind the theremin and out-of-sight, wou will be reducing yout ground coupling!

At HO2010 I ended up having ground "antennas" placed inside the plinths on which the instruments stood, to increase the ground coupling to players and make it more predictable.

Hope this helps,

Ooops.. Have just seen that Carport has already suggested this..
Posted: 7/20/2011 3:29:49 AM

From: Colmar, France

Joined: 12/31/2007

I have sometimes the same issue and it's also a body-to-ground problem in my 300 years old half-timbered house with wooden floors, especially when the weather is hot and dry and when I'm wearing shoes with rubber soles.

So I take off my shoes and play barefoot, standing on the (insulated) audio cable. This indirect capacitive grounding is enough to stabilize everything.
Posted: 7/20/2011 6:09:09 AM

From: Canada

Joined: 8/1/2008

In all the years I have been playing the theremin this is the first time time I have ever heard that a thereminist can be improperly grounded. No one has ever mentioned it before (not that I recall) and I have never experienced this phenomenon. Any problems I have ever had of the sort described here I have resolved with slight changes to my playing position or adjustments to the instrument itself.

Of course, as I said elsewhere, I usually play barefoot.

I have noticed, particularly with RCA theremins, that if you are holding a long note with a slow decrescendo, the movement of the volume hand for the extended fade can change the note that your pitch hand is playing. As you gradually move your volume hand closer to the loop, your note will sharpen.

If you hear this happening, and you are unable to play your loop any further from your body than you already are, then simply compensate by moving your pitch hand away from the rod as you move your volume hand toward the loop. Your ear is the key to everything. We are, after all, THEREMINISTS.
Posted: 7/20/2011 11:55:36 AM

From: Redmond, WA

Joined: 9/1/2007

"In all the years I have been playing the theremin this is the first time time I have ever heard that a thereminist can be improperly grounded."

You may be quite right. Personally, I didn't mean to suggest that the thereminist may not be properly "grounded"...or maybe I did (I'm not specifically familiar with all of the implications of the word). Perhaps you, or someone, could fill me in on exactly what "grounded" is and why theremins must be "grounded".

The phenomenon that I experienced with my Etherwave Standard was (seemingly) caused by the theremin being plugged into a battery pack and not a wall outlet (never happened when using wall outlet). The theremin seemed "lazy", if you will, but when I wrapped the audio cable around my waist, it "woke up".

So, what exactly is happening here? Is this actually a "grounding" issue with either the theremin or the thereminist? I got the idea to wrap the cord around my waist from the Gakken Premium's wrist strap...is this a similar phenomenon? Could it be that the theremin only needs to be "linked" to that which is controlling it in order to get proper responsiveness, and the floor is able to do this when using a proper "grounded" power outlet? The theremin/thereminist "circuit" simply needs to be completed for full performance, correct?

I'm curious as to whether one could play a theremin while falling through the sky? Barring wind resistance causing control issues (as well as fear of death, as lack of wind resistance would moot a parachute's effectiveness), would the theremin (plugged into a battery pack) perform better with the audio cable "linked" to the player's skin? If so, how much better?

If I'm in a field with my theremin on a battery pack and I have this issue, would a metal spike from the theremin into the dirt help fix this problem as effectively as a cable around the waist?

Anyone who knows, please let me know if I am way off on this...I very well could be. :)

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