Greatly extended pitch antenna range

Posted: 11/1/2017 12:44:09 PM
Gazmono

From: City of London, England

Joined: 11/1/2017

Hi all!

I want to know if it is do-able to greatly extend the distance sensitivity of a theremin. Years ago I made an art-work which is five and a half feet tall, made of wood and perspex and already has a theremin built into it. I have always wanted to make it more interactive; when approached (by one person at a time) lights and audio volume increase in a linear fashion. Ideally I would like pitch and volume to start increasing five to eight feet away. Apologies if this info is already out there but I have spent half a day trying to find posts on this site but have only found cryptic references to experiments that people were doing years ago by RStheremin and Torceador claiming this sort of distance , but also other posts saying that there is a maximum of two feet realisticly achievable. I could use ultrasonic detectors, but they are too directional and all the circuits I've seen give a stepped output rather than a linear response. I had given up on the idea of using the theremin and was looking at using an x-box Kinect, but that involves programming which I hate and I would dearly love to keep everything analogue and use the magic of the ether, which is kind of what the art-work is about. The theremin is used for sci-fi sound effects (with a digital delay) rather than for music. Any ideas would be greatly appreciated, thanks. p.s. I do have a fair bit of experience with electronics.

Posted: 11/1/2017 5:29:01 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Hi Gazmono, 

If you read technical books or papers where they mathematically solve capacitance for various geometries, you'll see that they often express the capacitance abstractly using ratios of area and distance.  For instance, the overly simplistic formula for two plates is:

  C = epsilon * A / d

So if you want to "scale up" a Theremin to work over larger distances, you need to scale up the area of things that interact: make the antenna physically larger, and use a body instead of a hand.

There will be increased intrinsic capactiance, where the antenna "sees" the rest of the universe as a plate, so the LC resonance will be lower.

Another thing that helps a lot is high voltage swing at the antenna.  If the inductor is high Q at the operating frequency, and if the drive doesn't introduce appreciable phase error (even tiny phase errors in high Q resonant circuits can cause significant amplitude reduction) then you can easily get hundreds of volts at the antenna with minimal voltage drive.  This swamps external noise and gives you a good signal.

You need stability in order to detect small changes in frequency, for high Q and temperature stability I would suggest using air-core inductors.  Digital approaches can also increase stability.

Finally, there can be a lot of environmental noise (mains hum, RF) picked up by the antenna, which can be filtered to some degree, preferably with a digital CIC comb / low-pass filter.  Filtering gives you a better signal to work with and eliminates mains intermodulation ("motor-boating").  You could low-pass just about everything I suppose if you don't need to play music on it, many digital Theremin implementations (not mine) have quite low cutoff points (<10Hz).  Maybe a multi-pole analog or digital filter set to 1Hz or thereabouts.

Posted: 11/2/2017 7:18:01 PM
rupertchappelle

Joined: 5/8/2017

First off, traditional volume mapping has the volume decreasing as you approach the antenna.

A volume antenna and circuit are not useful in my opinion.

If my memory serves me correctly, increasing the mass or area of the antenna should increase the range. You can try this with aluminum foil attached to the pitch antenna. I would suggest doing this rather than mucking up the internal adjustments as you may want to play the thing once again.

Now, the greater the range of the pitch antenna, the less the effect of pitch change and the greater the noise.

Which is why I stopped entertaining the idea of having dancers around a theremin.

It just don' t work, even if it seems a good idea.

If you do this with IR range sensors and program responses with a computer connected to a sound generator or video generator, you may have more success for what you may be thinking of.

Now for some good news, when I had my Etherwave in the basement and left it on for a while, occasionally with would growl and moan as a I approached it from  about 16 feet away. That would be good news for someone who is not a musician.

Posted: 11/3/2017 9:58:41 AM
xtheremin8

From: züriCH

Joined: 3/15/2014

very nervous system?: very nervous system is not the hot shit anymore but still a good thing to sense motion and translate it into a musical context. 

Posted: 11/3/2017 6:20:15 PM
rupertchappelle

Joined: 5/8/2017

Theremin Bollards:

https://www.google.com/search?q=theremin+bollards&oq=theremin+bollards&aqs=chrome.0.0l2.3234j0j4&sourceid=chrome&ie=UTF-8

And what you really want - 24 foot range enough???

205 Environmentally Sealed Pitch Theremin Module

http://harrisoninstruments.com/205/205.html

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