MIDI interface for just about any Theremin

Posted: 1/24/2011 12:48:59 AM

From: USA

Joined: 8/25/2009

So after enjoying my 15 minutes of internet fame with the laser harp project - I get asked regularly if it's possible to build a MIDI interface for a Theremin.

After completing the tube Theremin I thought it was about time I had a go at doing just that...

http://www.stephenhobley.com/blog/2011/01/24/pitch-and-volume-tracking-midi-interface-for-the-theremin/ (http://www.stephenhobley.com/blog/2011/01/24/pitch-and-volume-tracking-midi-interface-for-the-theremin/)

Inspired by an old Casio keyboard, Peter Pringle's videos, and few other things I just finished the first prototype - it seems to function rather well with all three of my Theremins.
Posted: 1/24/2011 7:22:01 AM

From: A Coruña, Spain

Joined: 9/26/2010

Very interesting! In case you finally sell these as a kit, how complex would it be to ensemble? Would it be suitable for people with no knowledge of electronics or specific tools?
Posted: 1/24/2011 8:18:04 AM

From: USA

Joined: 8/25/2009

The hardware is not that complex, as it's the software that does most of the work. You would have to be comfortable using a soldering iron to assemble it. If you could build a Theremax, then you'll have no problem with this.

Maybe if I were to offer some boards and pre-programmed microprocessors that would simplify things a bit.

One improvement I want to add is mapping the detune information to pitch bend. In theory it should be possible to capture the vibrato information from the player. This could be mapped out using the full 14 bits over a small 1-2 semitone range, thuse reducing the aliasing you sometimes get with pitch bend.

It would require a "well made" host synth.
Posted: 1/25/2011 5:46:23 AM

From: USA

Joined: 8/25/2009

I made a slight change to the firmware last night so that pitchbend information is used to track the microtonal changes in pitch in between semitones.

The result was pretty amazing (and I don't say that often). Now the synth responds just like the Theremin tone, and tracks it almost 100% over 5-6 octaves.

A lot depends upone the synth patch too - you need to use glide/portamento to eliminate note attack. I'm thinking of writing a dedicated software synth just for the Theremin.

I need to figure out a way to film it so that you can see the device in action and what I do to play it...
Posted: 1/25/2011 6:19:57 AM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005

Wouldn't OSC be a better choice than MIDI for an instrument that doesn't really have a note-on event?
Posted: 1/25/2011 7:10:15 AM

From: A Coruña, Spain

Joined: 9/26/2010

I don't even have a soldering iron, although I must confess that this kind of things makes me want to get and learn to use one.
Posted: 1/25/2011 7:13:07 AM

From: USA

Joined: 8/25/2009

GordonC - Yes it would.

At the moment I don't own anything that responds to OSC messages.

I would probably use something like SynthMaker to create a new synth - and I don't know if that supports OSC.

It's certainly worth considering for the future.
Posted: 1/25/2011 7:57:53 AM

From: Canada

Joined: 8/1/2008

The late Bob Moog solved this problem in a very interesting and creative way when he built the Ethervox MIDI theremin with software designer Rudi Linhardt.

The E'Vox will operate in five "modes", one of which is PITCH SMART MODE which operates as follows (as quoted from the manual).

"As the left hand pulls away from the volume antenna, the amplitude of the theremin volume control signal increases and is converted into a value that is displayed on the performance screen. At the instant that the left hand's movement is reversed, a MIDI Note On message is sent with a Note On velocity value equal to the last value displayed. The Note On number is determined by the position of the right hand in relation to the pitch antenna."

One of the problems of attempting to play a MIDI keyboard or module using a gestural trigger is that notes produced by pitchbend have quite a different sound from what is produced by the keyboard (or keyboard triggered module) itself. For example, a cello program C2 pitchbent up three octaves is totally different from the sound of the same program at C5, although they are both the same pitch.

Bob Moog told me once that the way around this would be to create a "lattice" of interlaced faders. Each note would be triggered separately in a gestural sweep but the effect would be seamless because as each new note swelled in volume, the preceding note would fade in precise sync with it so that no audible artifacts would be produced. No individual note would have to be pitchbent up or down any more than a halftone.

Sadly, this technology was never explored and we have since lost our dear, wonderful theremin mentor, Bob Moog.

One of the things that distinguishes the RCA theremin (and vacuum tube theremins in general) from their transistorized counterparts is that the timbre morphs slightly from register to register giving the instrument a very "organic" and often uncannily human sound. This has to do with the heating and cooling of filaments as the hand moves within the playing arc.

Interlaced faders, as proposed by Bob Moog, would do the same thing for a MIDI interface. On a keyboard sampler where you have a separate sample for every minor third, the sound would shift in sync with pitch hand gestures.

I don't know a great deal about these things and it is quite possible that these days there are easier roads to Rome.

Posted: 1/25/2011 8:06:51 AM

From: USA

Joined: 8/25/2009

"create a "lattice" of interlaced faders"

If I understand you correctly - then this is what I achieved last night - and it amazed me how well it works. Each note essentially blends into the next note with the pitchbend function set to just a 1 semitone range.

This small range is enough to "blend" the subtle pitch differences from one note to the next. Useful for synths that do not possess a CV input.

I've not been able to find an Ethervox manual - so any info on this Theremin is greatly appreciated.

BTW - Totally agree on the vacuum tube sound - I don't own an RCA, but the Keppinger sounds very voice-like in the higher registers. Going to have to build some synth patches tonight.

Posted: 1/25/2011 10:03:17 AM

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

Coalport kindly scanned the entire E-Vox manual for me, and a full set of high resolution photos of the innards..

Alas, the hard-drive which I loaded these onto died a few days later - I was not able to recover the files, so the drive has been sent for recovery.. I did upload the photos to Theremin / General group at Element 14 (http://www.element-14.com/community/groups/theremin-general-resources?view=documents) but the manual never made it there before the HDD died.

As soon as I recover my files, I will post the manual to element 14.

Regarding theremin sound (tube vs solid state) the 'morphing' effect occurs with all analogue theremins - it is a function of the dynamic change in the period of the waveform.. it would not occur on any type of sampling technology or any design where waveforms have fixed period for each cycle.. so I do not believe this is the reason for the 'tube sound'

With regard to the 'fader' idea - I can see this working only if the 'active' voices are always at exactly the same frequency and phase, and mixed such that the sum amplitude is constant.. with this, one could do frequency shifting over a large range.. but I do not see much point.. One would need to specifically design the MIDI instrument to perform this, and its function would be quite limited (and limiting) for any other MIDI application, I think... Also, one would lose the waveshape 'morphing' by using this scheme... If one wanted to design a MIDI theremin 'expander' I think there are better and easier ways to do this.

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