In a previous article about Voice Model in Subtractive Synthesis, a model for constructing voice in subtractive synthesis was examined in detail. It’s time to move on to the concepts of Control Voltage and Trigger/Gate signals, and also to understand the principles of control in a modular or any classic synthesizer.
Control is relevant for both subtractive and any other type of synthesis. Control devices or control modules are also necessary and many of them are basic, along with the devices that make up the voice model.
There are two types of control signals: Trigger/Gate and Control Voltage (CV). These are two fundamentally different types of signals and it is important to understand their difference.
The vast majority of devices in a modular synthesizer have control inputs and their parameters can be dynamically changed by external voltage. Even the control devices themselves can also be controlled using an external signal or, as it is called correctly, a control voltage (CV).
For digital or virtual-analogue synthesizers control voltages do not exist as such, but control is performed in a similar way, and the connection is called a patch and, unlike analogue and modular synths, it is supplied to the controlled parameter “virtually” through the menu settings. Patching possibilities in digital synthesizers are strictly limited by the number of patches, which cannot be said about modular ones, where the hands are completely untied for any experiments, including those not provided by the manufacturer…
In simple terms, the control voltage is needed in order to «twist the knobs» of the synthesizer without you. We are have only two hands, but there can be an incredible amount of parameters for control. And from this abundance, you can create a whole automated ensemble obeying strict (or not very) rules, synchronized with the rhythm, or completely abstract and chaotic. Tying the voice model with control is called automation, and controlling a parameter is called modulation.
The control voltage for the Eurorack format usually varies from 0 to 10 volts (sometimes up to 12), but can also take negative values. And for vintage analog synthesizers or 5U format modules (Synthesizers.com, Moog Modular, STG Soundlabs) can reach up to 15 volts. There is no particular rigor for analog devices and it is impossible that the Eurorack module can be break with a control voltage of 15 volts. But even with such flexibility, it’s still better to understand what is happening.
For clarity, consider an example
We need to automate the filter so that it continuously opens and closes smoothly. This means that we will control its main parameter (cutoff frequency) with a low frequency sine wave. We feed the voltage from the low-frequency oscillator module (LFO) to the control input with a smoothly varying sinusoid voltage. That is, we control the filtering according to the sine law – we open and close the filter cyclically. In simple: The higher the voltage, the stronger the “handle” of the filter is “turned” and there is no need to twist it, the control voltage does this for you. And when you turn the handle of the device manually, then you also change the voltage in the control circuit of the device. If the control voltage is positive, then the “knob rotates” in the direction of increasing the parameter value, if negative, in the direction of decreasing (see the video below).
What is important to understand: the control voltage changes the value of the parameters of the device in the same way as if you turned the knobs yourself. It can be used to control the pitch, cutoff of the filter, gain level, or any other parameter. And sometimes it’s even possible to go beyond the limits of turning the knob with the help of a control voltage and get extreme operating modes of the device and new sound capabilities.
Triggers and Gates
A special case of control voltage are trigger signals and gates. These are pulse signals with a positive voltage of 3-7 volts. This voltage value is different for different manufacturers, usually equal to 5 volts, and for old synthesizers all 12 volts may be required.
Triggers and Gates determine the rhythm. They are used to indicate a keystroke or the moment you hit the drum. At its core, triggers and gates are one and the same, with one small difference.
When you press the synthesizer key, you can release it right away or hold it down and extend the prolong of the note sound – this is a Gate-signal. It has a duration of opening. Commonly used to control the rhythm of melodic sequences.
When you hit a drum or percussion instrument, the working surface of the percussion instrument is briefly touched – this is a Trigger. The trigger has a fixed opening duration (usually about 5-10 milliseconds or more) and is used to create rhythmic patterns and sequences for triggering percussion modules or to determine the clock signal for sync the devices.
The main control devices in classical synthesis are:
- Envelope Generator (EG, ADSR, AHDSR) is usually used to control the dynamics of a note, activated by a gate signal;
- Low Frequency Oscillator (LFO) generates waves of lower frequencies than a standard sound oscillator and is used to control or modulate;
- Sequencer (SEQ) is used to create sequences of control voltages and their rhythmic characteristics;
- The MIDI converter to the control voltage interface (MIDI – CV / GATE) is used to translate the digital control signal received from the MIDI controller, computer or groove box into control voltage signals and trigger/gate signals. It can also broadcast synchronization signals, equipment reset, start-stop and others signals.
The sequencer and MIDI interface combine two control methods at once – control voltage (CV) and Gate / Trigger signals. Each of the listed control devices is worthy of separate attention and will be described in detail in the corresponding article.
In this article, the concepts of voltage control in modular and analogue sinthesis, concepts of modulation, and automation were discussed. The main types of control signals – CV and Trigger / Gate are considered and a brief overview of the main control modules is carried out.