multiple theremins......conflicts?

so my band has a gig tomorrow and we've enlisted a second theremin player. (which reminds me of a joke: what do you call 2 thereminists in a room?... a convention)
anyway, at practice last night due to space constraints in my studio we were about 6 feet apart. I didn't think our fields were overlapping, but apparently they were, because if joey held a note on his, while mine was muted, I could control his output with my pitch antenna! and vice versa. and if we both played simultaneously there was a weird doubling effect taking place. I guess they were heterodyning off each other. Any one else experienced this?
And just how far DO 2 theremins need to be apart from each other to not experience this?
btw,they are both etherwave standards, mine a big briar patched thru an airfx, his a moog patched thru a moogerfooger ring mod.
any advice, comments welcomed
thanks
Sean

At Hands Off 2007 theremin symposium (25 conventions = 1 symposium by your figures, by the way) Lydia Kavina, Barbara Buchholz, Carolina Eyck and Charlie Draper played as a quartet with two Etherwave Pros and two tVox Tours, each at the corners of a square ten metres or so across, by my recollection. Apparently this was satisfactory to their standards.

When we arranged 17 theremins of various types around the audience about about with 3 metres separating each instrument and with care taken not to have two of the same model next to each other it was close enough for rock and roll, as they say. (Joke: How do you get 17 thereminists to play in tune with each other? Shoot 16 of them.)

When I played two etherwave standards simultaneously they interfered with each other in complicated and unpredictable ways. At times the fields shrank to nothing, at other times the volume of one seemed to control the pitch of the other and so on. (Mind you, having a different echo on each did make it hard to tell exactly what was going on.)

You could try playing inside faraday cages. Or put an earthed (grounded) kitchen foil screen between you. That might work. (Or not. I'm not knowledgeable about electronics.) Or change the base frequencies of one of the etherwaves, if you have the necessary skills. My etherwave and Kees Enkelaar theremins operate on unrelated frequencies and do not interfere at all, even when I invert the kees and place it on top of the etherwave with each one's rod through the other one's loop. Of course they do mute one another in that position, but as far as I know they don't actually interfere with each other. Even if it looks like they are doing just that.

BTW I'm still exploring my airfx. Do have any favourite effects on it? At the moment I have it after a delay, on Strobe Gate (30).

Faraday cages for thereminists ... hmm, sounds like a business plan in the making for some enterprising young enthusiast :) (Us "old" ones are too set in our ways to take on such a project :) )

Don

[i]"You could try playing inside faraday cages. Or put an earthed (grounded) kitchen foil screen between you. That might work."[/i]

A grounded foil screen will work, but will add a huge increase in background capacitance, reducing sensitivity and probably requiring re-tuning of the instruments..

[i]"(Us "old" ones are too set in our ways to take on such a project :) )"[/i]

LOL! - Perhaps some of us "old ones" are still foolish enough to embark on such a venture! ;)


Sorry - What follows is probably only of interest to those with good [b]TECHNICAL[/b] / [b] ELECTRONICS[/b] knowledge..

A way which WILL** work (the basis of my directional antennas) is to connect the input of a high frequency DC amplifier to the antenna, and connect the output of this amplifier to the screen - the screen then tracks the waveform on the antenna, and is maintained at the potential seen by the antenna.. Because there is no potential difference between the antenna and the screen, there is no capacitive loading caused by the screen.. any 'outside' signals or capacitances hitting the low impedence screen are blocked. If multiple Theremins employ this scheme, with their independent screens blocking each others fields, one should be able to entirely block interactions.

[i]*This method is similar to the guard ring employed in sample/hold circuits, where the output of the amplifier is taken to a 'ring' on the PCB which goes 'round the holding capacitor.. the principle is identical - if the amplifier is exactly unity gain, then the voltage on the input cannot leak away, as its leakage path is obstructed by a low impedence 'barrier' maintained at the same voltage as the input voltage.[/i]

Practical problems..
Voltages on antennas can be high (easily 50V P-P) and impedence high, so the amplifier connected to the antenna needs to be quite special - it must not load the antenna, it must track the voltage on the antenna closely, and it must be able to output the voltage range on the antenna.

**I have not played with screening theremins from each other in this way, but have achieved directional antenna screening using this method. It is far easier to implement this when using low voltage antenna circuits (inductorless - RC type oscillators or 'digital' capacitance sensing circuits) but much more difficult if one is interfacing to conventional LC theremin circuits.. On my original PSoC designs, I was able to shape the sensing field using this method.. But the range was too short.. The complexity and expense of applying this scheme to higher voltage 'conventional' Theremin circuits has caused me to put this development in the 'do later' box.

By amplifying the antenna signal and "redistributing" it in the ether I think you may risk to increase interference problems...

One theremin may be shielded but the second will suffer since the output impedance and thus the ERP of the shield will be still stronger

I hope that I'm wrong

[i]"One theremin may be shielded but the second will suffer since the output impedance and thus the ERP of the shield will be still stronger"[/i]

The above would be true IF only one Theremin was shielded.. The low Z output from the shielded screen would radiate a strong signal which would be 'picked up' more readily by the other Theremin..


This is why I stated [b]"If multiple Theremins employ this scheme, with their independent screens blocking each others fields, one should be able to entirely block interactions."[/b]
.. for this system to work, EVERY theremin would need its own shield carrying a low z 'copy' of its antenna signal.. These shields would need to block the fields of each other shield.

But please understand - I have NOT tried this, and implementing this scheme IS NOT SIMPLE! .. Each Theremin will require a high frequency DC Coupled unity gain amplifier with high z low capacitance input which connects to the antenna, and a low z output for connection to the screen. The amplifier will need to track the waveform on the antenna as closely as possible, with minimum phase difference, and this buffered signal must appear on the screen regardless of what capacitance is seen by this screen or what other signals are imposed on it.. For example, if two screens are placed next to each other, both following the signals from their antennas, the signals on the screens must be strong enough so that no mixing occurs due to capacitive coupling between the screens.. This means that the output z driving the screens must be extremely low.. ie a lot of power.

** I have assumed here that we are talking about pitch antenna interaction - it may be that volume antennas also require shielding .. Also, although frequency difference between volume and pitch oscillators in large, it may be that the low-z signals from the screen/s could swamp the volume circuits unless these antennas are individually screened.

Also - connection of the antenna to the amplifier is a problem.. you cannot use a long wire! ;) So really the amplifier/s need/s to be built into each Theremin, and low z output connectors provided for connection to the screen/s.

I hope to have this as an option on my Theremins - primarily for implementation of directional antennas.. this may be enough to greatly reduce or overcome Theremin interaction.

I didn't manage to express my thoughts clearly in this foreign language.

The more theremins you shield the more ERP is in the air, risking to break through the shields.

[i]"The more theremins you shield the more ERP is in the air, risking to break through the shields."[/i]

True..
With low z "shields" acting as a (comparatively) high power "transmitters" there may well be complications.. As I stated, I have not tried this - and have only done 'proof of concept' using a single screen to counteract unwanted capacitance originating outside the 'focus' area of the antenna. Also, I did notice that there was some reduction of sensing range when the shield was connected - which leads to the issue of ERP and the effect of radiating a strong 'copy' signal.

There is also probably going to be a problem with regard to legislation.. ERP from a Theremin 'sensing' antenna is extremely low, but ERP from a screen could be quite high - high enough to cause it to fall into the "radio transmitter" classification.

I do not think it likely that signals will 'break through' the shields (although possible) but there may well be other problems - there usually are with Theremins! ;)

wouldnt 2 theremin's shred ina band? someone must do this! it would shred!

Take two different theremin models, set them up with 5 meters distance, connect each one to mains through a RF filter block.