Control Voltage / Gate
Control Voltage and Gate signals (abbreviation CV/gate) are analog methods of controlling synthesizers, filters, drum machines and other similar equipment with external signals. The OpenTheremin V4 generates two control voltage signals form the pitch and volume antennas. Additionally a gate signal can be generated to control effects or envelopes. While the OpenTheremin has an integrated tone generator (Audio output) these signals can be used to expand your setup and enrich your thremin sound by control any equipment with CV and gate inputs.
Coralie Ehinger has developed a unique way of combining the Theremin instrument with modular synthesizers while preserving the classical tone of the instrument. See here for here videos.
Control Voltage
The control voltage signals (CV) of the OpenTheremin V4 are accessible via the 3.5mm jack marked CV. The jack provides both the pitch and the volume CV signal through the two channels. To access the two signals best use a stereo to 2x mono splitter cable.
Splitter cable 'stereo to 2x mono'.
The pitch signal is on the red (right) plug/socket, the volume signal on the black/white (left) plug/socket.
Be careful to get a true 'stereo to 2x mono' cable as most splitter cables on the market are '1x stereo to 2x stereo' instead.
OpenTheremin connected to a Waldorf 2-pole filter through CV.
By default the OpenTheremin V4 is set to output logarithmic control voltage with 1V/Octave. This is a common standard as used by synthesizers such as Moog and Roland. The characteristic can be changed to linear control voltage with 819Hz/V as used by Korg or Yamaha by changing a software flag. Just change CV_LOG=1 to CV_LOG=0 "build.h" file and re-upload.
For the volume most synthesizers "exponentiate" the volume control voltage themselves, thus OpenTheremin send the "raw" volume signal to CV.
Gate
The gate signal can be tapped by a splitter cable via the OpenTheremin Audio output. The black/white (left) channel is the mono audio signal, the red (right) channel is the gate output. The gate triggers at a fixed level of the volume antenna. To activate the gate set the trigger levels by change the following lines in the code and re-upload:
In the file "build.h":
// Set the trigger levels for the Gate signal (0 to 127, 128=off)
#define GATE_ON 20 // That's the level which will drive the Gate high when volume increases from lower
#define GATE_OFF 16 // That's the level which will drive the Gate low when volume decreases from higher
The code for the control voltage and gate was contributed by "Theremingenieur" Thierry Frenkel. For the gate he developed a special impedance sense algorithm to prevent shorts when plugging in mono jacks. Kudos and a big thank you to Thierry.