The Open Theremin is an Open source theremin piggy backing onto the Arduino motherboard.
It is programmable via the USB connector on the Arduino and can be powered by the PC/USB connection or an external DC supply.
As explained in the Open Theremin setup instruction guide, the theremin has to be manually calibrated to adjust for specific differences in the user antennae to those suggested.
This is done via a two part calibration process: firstly, the variable capacitor for the tank circuit of the pitch antenna is adjusted so that an audible frequency of 600Hz is achieved. Then after a long depression of a small button, the variable capacitor in the Volume circuit is again adjusted to an output of 600Hz.
Once this has been done, the theremin is auto calibrated for zero beat and maximum distance away from the volume antenna by another shorter depression of the button (3 audio beeps over about 2-3 seconds) signal that this calibration is taking place.
The theremin is now ready to play.
(NB In my experience the theremin is best switched on for around an hour for manual calibration and at least 30 minutes before auto calibration and playing.)
Observations on playing the Open Theremin.
Initially finding the correct playing distance from the instrument was kind of tricky for me since I was used to adjusting the pitch/volume fields manually with potentiometers-relying on a 2-3 second window to get in the ‘playing position’ was strange but after a while it works.
The instrument gives an octave range of just over 5 octaves and this seems to be fixed within the distance chosen at the auto calibrate, ie you can have 5 octaves in an arm’s length to the antenna or 1.5m from the antenna. Irrespective, you will have the full 5 octaves.
My only other theremin doesn’t have such an octave range and so initially playing this instrument felt a wee bit strange.
That said however, now it feels fairly easy to play (not-with-standing the usual difficulties of playing the theremin!) with what feels like a fairly constant linearity (see graph in the photos area).
Up until a few days ago I was playing through a small battery/mains powered Vox amp but have bitten the bullet and bought a Behringer K450FX (I believe that our Thierry has one of these). This amp has an exceptionally neutral response with a good power output and with the Open Theremin played through it two things are immediately revealed: the extended base response of the theremin and at high frequencies, a noise superimposed on the ‘music’ signal. This can be heard (I think) on the audio file ‘Scan UpDown’. Also, under quiescent conditions (volume hand away from the volume antenna at zero beat) a similar but quieter noise can be detected.
It has to be noted that with a lesser amp I had not been aware of these ‘dirt’ signals over the main ‘music’ signal.
After several manual calibrations at several different times I wonder if success in the setup of the instrument is pivotal at this point? It has to be said that it is quite tricky and from experience, how well one cap is adjusted will affect how well the other one can be adjusted: if the first one is maladjusted then adjusting the second one will either be difficult or when adjusted will not have a significant effect on the respective antenna. I have noticed that if the two caps are out of adjustment the octave range will be less and the volume rise/fall will not be smooth but will have the potential of break-up.
Certainly, I have noticed that with a lesser octave range, the noise ‘dirt’ at the high frequencies is less, due I think to the cut-off of octave range before these frequencies are reached.
I have noticed also on the Open Theremin site that a reference has been made to the effect that the program code has been altered in an updated version of the program to reduce noise.
Playing the instrument poses no problem as far as I can see and there is no detectable lag-so far it seems to respond quickly to any movements that I can throw at it.
The sound the instrument produces is determined by a waveform table which can be altered, re-saved and then substituted in the program for the new waveform. I have tried this to varying degrees of success although the waveform which is standard with the program is perfectly acceptable. Two others are available on the Open Theremin site to try.
Also, the CV out has not yet been programmed-to be fair, I have been in contact with Urs Gaudenz, the designer about this and he has said that since the project is a open source, he has left this un programmed so that it may be programmed to suit individual needs and to that end, he has said that he is happy to programme the output if I suggest a voltage range per octave, which I have not done yet.
Sound files are on Soundcloud at https://soundcloud.com/roy-min/sets/test-sounds-for-the-open
‘Scan UpDown’ is a sweep of the octave range up and down (there is a glitch towards the end which is due to my laptop hard drive not the theremin!)
‘NGL1 and 2’ is part of Neil Gow’s Lament to His Second wife-as always, all bum notes and dodgy glissandos are the fault of the player not the instrument!