Urs also made a very clear presentation of this MIDI feature on his website: http://www.gaudi.ch/OpenTheremin/index.php?option=com_content&view=article&id=200&Itemid=121, many thanks !
### Open Source Theremin based on the Arduino Platform
Open.Theremin is an arduino shield to build the legendary music instrument invented by Leon Theremin back in 1920. The theremin is played with two antennas, one to control the pitch and one for volume. The electronic shield with two ports to connect those antennas comprises two heterodyne oscillators to measure the distance of the hand to the antenna when playing the instrument. The resulting signal is fed into the arduino. After linearization and filtering the arduino generates the instruments sound that is then played through a high quality digital analog audio converter on the board. The characteristics of the sound can be determined by a wave table on the arduino.
For more info on the open source project and on availability of ready made shield see:
3. In "Application.cpp", take care of selecting MIDI mode that correponds to your cituation (put "//" in front off inadequate line - MIDI through serial is selected by default here):
//Serial.begin(31250); // Baudrate for real midi. Use din connection https://www.arduino.cc/en/Tutorial/Midi or HIDUINO https://github.com/ddiakopoulos/hiduino
The MIDI open theremin generates NOTE ON/OFF messages and Continuous Controler changes (MIDI CC) depending on settings and hands positions next to antennas.
When left hand moves away from VOLUME ANTENNA (LOOP) and volume crosses a settable threshold (Volume trigger), a NOTE ON is generated. VELOCITY depends on how fast left hand is moving. Right hand's position next to PITCH ANTENNA (ROD) determines the starting NOTE NUMBER.
When right hand moves next to PITCH ANTENNA (ROD), PITCH BEND messages are generated (if activated) to reach exact pitch as long as pitch bend range will do. Beyond, a new NOTE ON followed by a NOTE OFF for the previous note are generated if legato mode is activated. Pitch bend range can be configured (1, 2, 7, 12 or 24 semitones) to align with synth's maximum capabilities.
The picture at https://github.com/MrDham/OpenTheremin_V3_with_MIDI/blob/master/MIDI%20Open%20Theremin%20V3%20HMI.bmp gives an example of possible HMI: on "Value" pot, red lines have 4 positions, grey lines have 5 positions and yellow lines have 8 positions. On "Parameter" pot you see coloured lines indicating which colour to follow for the "Value" pot.
The Quick Guide at https://github.com/MrDham/OpenTheremin_V3_with_MIDI/blob/master/Quick%20guide%20open%20theremin%20midi.bmp works well with this HMI. Print it on a portrait A4 sheet of paper, plastify it and take it with you theremin everywhere you go...
Manipulation of "Rod antenna MIDI CC" and "Loop antenna MIDI CC" is not error proof. MIDI newbies should be advised to change their value in MUTE mode.
Audio processing from antennas to output jack, including volume and pitch pots, LEDs and button functions, is exactly the same as in open theremin V3. You can play the Audio and the MIDI side by side.
When legato mode is activated, if you trigger a note (with volume loop) and go in one direction (with pitch antenna) a new note will be triggered at the limit of pitch bend range.
Legato mode is used as a workaround for a limitation of MIDI (max 24 semitones pitch bend). Maybe some synth can perform pitch bend on more that 2 octaves but none of mine does...
### With Legato Mode = ON and Pitch Bend Mode = OFF, the notes generated don't seem to be in a given scale. For example I can't play in C major if I select Pitch Bend Range = 2.
Effectively according to the sequence described above (Fade-in / Picth Variation / Fade-out), the first note played influences the next notes played: they will always be distant of N * Pitch Bend Range. For example, with Pitch Bend Range = 2, if you start with a C and move the right hand, you can only play C, D, E, F#, G#, A#. This is as designed.
If you want to play in a given scale you need to set Legato Mode = ON, Pitch Bend Mode = OFF, Pitch Bend Range = 1 and to use a synth with a "force to scale" capacity (Some plugins can do that, for example http://www.thepiz.org/plugins/?p=pizmidi midiForceToKey). Then select the expected scale and play. Out of scale notes will be replaced by the closest one in the scale.
"#define PLAYER_ACCURACY 0.2" -> Pitch accuracy of player. Tolerance on note center for changing notes when playing legato. From 0 (very accurate players) to 0.5 (may generate note toggling).
