We introduce the basics of a Control Voltage / Gate system to let different modular synths interact with each other.

Invented by Robert Moog and Herbert Deutsch in the early 1960s, the Control Voltage / Gate is an analog system based on electrical voltage, through which it is possible to control synthesizers and drum machines.

There are three types of controls:
1. Control Voltage: it’s an electrical signal with which it is possible to modify various parameters, such as the pitch of an oscillator, the envelope of a sound or the cutoff frequency variation of a filter.
It can be unipolar (it only has a positive modulation between a voltage of zero Volt and one of +N Volt) or bipolar (it has a modulation both positive and negative, which varies between -N and + N Volt).

These voltages are established by the individual manufacturer.
For its Eurorack system, Doepfer uses a bipolar voltage between -2.5 V and +2.5 V for LFO modules, and a unipolar voltage between 0 V and 8 V for ADSR modules.
Volca Modular instead works with voltages between -3.3 V and +3.3 V while Bastl Kastle uses only voltages between 0 and +5 V.

Also for note instructions there are two construction standards, called Volt / Oct and Hertz / Volt.
In the first one, invented by Moog and now used by most manufacturers, for every Volt increased in voltage corresponds an octave increased in pitch.
In the second one, used by the old Korg and Yamaha synths, at the doubling of the voltage corresponds an increase of one octave of pitch.

Volt/Ottava1 V2 V3 V4 V5 V
Hertz/Volt1 V2 V4 V8 V16 V

They can be CV generators, in addition to keyboards and sequencers, also LFO, Sample and Hold, ADSR and Envelope Following modules.

2. Gate: it’s a signal that can assume only two values, zero Volt and different from zero Volt (voltage always dependent on the manufacturer).
A Gate command is commonly associated with the Note ON signal and it’s maintained for the entire duration of the note.
When a key on a keyboard is being pressed the Gate rises to + N V and when the key is released it drops to 0 V.

Doepfer uses a Gate / Trigger voltage of +5 V while Volca Modular uses +3.3 V. The old Roland systems instead used voltages of +10 V.

3. Trigger: it’s a signal that takes only two values (zero and different from zero) but has an almost instantaneous duration in time.
It can be used to reset an LFO to the beginning of its cycle or to restart an ADSR while the Gate signal is still active.

In this example, pressing a key on the keyboard will send both a CV voltage, relative to the pitch of the note, to the oscillator and a Gate voltage to the ADSR module, which will advance as programmed to the Attack, Decay and Sustain stages.
A Trigger command sent by us at the right time will restart the ADSR module, which will re-open the Attack, Decay and Sustain stages.
When we release the key, bringing the Gate voltage back to 0 V, the ADSR module will start the last Release stage.

After having dealt with the theoretical bases related to a CV system, we will be able to move on to practice and have different semi-modular synths interact with each other.

In this video a Korg Volca Modular is controlled by an Arturia Microbrute and a Bastl Kastle.
Since those three synths work at different CV / Gate voltages, it won’t be possible to connect directly one another.
In order to be able to control the intonation of the Volca Modular and its Gate via an external sources, Korg has implemented a module that can scale external input voltages to +/- 3.3V.

In our example we will connect the left channel / tip of a Y cable to the Microbrute Gate output and its right / ring channel to the Pitch output. The stereo end of the Y cable will then be inserted into the CV-Input module of the Volca, from which we will take the CV / Gate voltages properly scaled.

Regarding the integration of Volca with Kastle, we can simply connect each input to each output with the supplied Patching Pin Cable, with the foresight to maintain a Commond Ground signal, which can act as a Zero Volt reference between the two machines.
In this case we can simply use the sync cable supplied with Volca and connect the Volca’s Sync Out to the Kastle’s audio output.

Kastle’s Oscillator Output will be connected to the audio input of Volca’s second Low Pass Gate, to use Kastle as a second voice, while Kastle’s Triangle LFO output will be linked to Volca’s reverb amount modulation.

The audio sum of the two Volca LPGs will then be sent to the Audio Input of the Microbrute, so we can filter the signal, which will then be sent to a Monotron Delay, to add some effect to our final sound.

After laying the basics to understand a CV system, in the next post we’ll see how to use a MIDI / CV interface to control our modular system with a hardware sequencer.