Motivation
There is a lot of jargon around concepts in modular synthesis. This guide introduces some of the basic terms and concepts. Hopefully this will help you decode marketing material from manufacturers and the content of forum posts. We will focus on Eurorack here as other modular systems are different.
Types of signals
Control Voltage (CV)
Control voltage (CV) is half of what ties a modular system together, the other half being audio signals. CV is how a module signals to other modules what they should do. Most of the time, a CV input on a module is associated with one of its knobs, thus the CV can be used to adjust that knob, thus one module can adjust settings on another. Some modules output CV signals, others receive CV signals, and many will do both.
CV can be set to a steady level or vary over time and they can range from the negative to positive voltages. In Eurorack modular systems, CV signals are very flexible, interchangeable, and experimenting with patching CVs into various inputs is half the fun.
- unipolar vs bipolar
- gate vs trigger
- 1v/8
Audio
Audio signals are electrically identical to CV. Audio signals, however, are usually intended to eventually be heard. Most of the time audio signals vary their amplitudes more than 20Hz, in the range of human hearing. CV signals, if they vary at all, will do so at a rate lower than 20Hz, below the range of human hearing.
Types of modules
Voltage Controlled Oscillator (VCO)
These are a basic building block of a modular system and one of many types of module that generate audio signals. They are called oscillators as they essentially wiggle a voltage between positive and negative ranges at a given rate. The rate, which affects the pitch of the audio, is adjustable by a knob or via a CV signal, hence “voltage controlled”.
There are VCO’s that output basic waveforms (sine, sawtooth, square waves etc.) or a blend of basic the waveforms. These audio waveforms can be routed to other modules to be shaped, mangled, etc. into more exciting sounds.
Some VCO modules will have more than one oscillator. For example, a module may feature two oscillators and blend the waveforms together before they are output. Other modules may have the two oscillators feed their output into the pitch control of the other, these are sometimes called “complex oscillators”.
Voltage Controlled Filter (VCF)
Filters are one of my favorite types of modules. Audio signals can carry a range of frequencies, each of which contributes to the overall character or timbre of the sound. VCFs shape the sound by reducing or emphasizing some of the frequencies.
The cut-off frequency control sets a threshold frequency. In a low-pass filter (LPF), the frequencies below that cut-off threshold are kept in the signal or “passed”, while frequencies above the cut-off are rejected or reduced. In a high-pass filter (HPF), the situation is reversed: frequencies below the cut-off are rejected while those above the cut-off are passed. A band-pass filter (BPF) passes frequencies near the cut-off.
The cut-off frequency can often be CV-controlled, hence “voltage controlled filter”. The “FM” input, in the context of filters, is a CV control that also adjusts the cut-off frequency.
Another common control is “resonance” or “Q”, which adjusts how much of the frequencies right at the cut-off threshold are emphasized. Turning resonance to a high setting can induce “self-oscillation” in some filters. A high resonance setting is one of the ingredients of the classic “acid” sound.
The “poles” on a filter determines the sharpness of the filter. The more poles are in the filter, the more quickly frequencies are reduced the farther they are from the cut-off. One, two, or four poles are common configurations.
Voltage Controlled Amplifier (VCA)
Voltage controlled amplifiers take an input signal and output a copy of the signal with the amplitude changed, usually in the range between zero and the original signal’s amplitude. The output signal’s amplitude can be controlled by a CV signal. A VCA’s input will have an accepted range of voltage usually from 0v to +5V. Thus, a +5V at the level input will set the VCA to output a copy of the input at 100% amplitude.
VCAs can be designed for use with CV signals or audio signals. Those suited for use with CV signals feature a “linear response curve”, meaning a 50% level setting outputs the original level at 50% amplitude. A VCA designed for audio use has an exponential response curve to account for how human hearing perceives loudness.
The usefulness and utility of these modules is reflected in how often forum posts repeat the adage “you can never have too many VCAs”.
Low-frequency Oscillator (LFO)
Low-frequency oscillators are one of the basic CV sources. Similar to VCOs, they generate waveforms, but at much lower frequencies. These signals can be fed to any number of modules.
Envelope Generator
Envelope generators are a special type of function generator.
- AR vs ADSR
- unipolar
Function Generator
The name of these modules is inherited from the lab equipment used in early days of synthesis.
Attenuator / attenuverter
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Mixer
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Low-pass Gate
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Sequencer
Sequencers are my favorite modules. Their purpose is to provide a way to generate
Random / noise
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Quantizer
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Logic
- analog vs. digital logic
Utility
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Sample & Hold
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Multiples
- passive vs. buffered
- aka “mults”
Clocks
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Audio effects
- delays, reverb, chorus
Samplers
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Output
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