Modular synthesizer

The modular synthesizer is a type of synthesizer consisting of separate specialized modules. Modules output voltages. The modules are not hardwired together but are connected together, usually with patch cords, to create a patch. The voltages from the modules may function as audio signals, control voltages, or logic conditions.

Development
The first modular synthesizer design was presented by Harald Bode at the 1960 AES convention, also attended by Bob Moog. This was the first patchable modular system with control voltage capability. The earliest commercial modular synthesizers were developed, in parallel, by R.A. Moog Co., and Buchla in 1963. ARP (in 1970), Serge (1974), and EMS (1969) versions were soon to follow. In 1976, the Japanese company Roland came out with the Roland System 100. Also in the early 1970s, there were at least two mail-order electronics kit vendors Paia Electronics, and Aries, marketing different lines of simple DIY modular synthesizer systems. The Aries 300 Music System was modeled on the circuits produced by Bernie Hutchins and published as Electronotes. In the UK in the 1980s the Digisound 80 modular synthesizer, designed primarily, by Charles Blakey was sold as a kit by the company Digisound Ltd. Many of the early modules appeared in the early to mid-1980s as construction articles in two British electronics magazines - Electronics Today International (ETI) and Electronics & Music Maker (E&MM). These days hardware offerings range from individual modules through complete systems in cases to kits for hobbyist DIY constructors. Many manufacturers augment their range with products based on recent re-designs of classic modules, the original patents having lapsed. Many hobbyist designers also make available bare PCB boards and front panels for sale to other hobbyists.

Form Factors
Many early synthesizer modules had modules with height in integer inches: 11" (e.g., Roland 700), 10" (e.g., Wavemakers), 9" (e.g., Aries), 8" (e.g., ARP 2500), 7" (e.g., Polyfusion, Buchla, Serge), 6" (e.g., Emu) and width in 1/4" inch multiples. More recently it has become more popular to follow the standard 19" Rack unit system: 6U (Wiard), 5U (8.75" e.g., Moog, Modcan), 4U (e.g., Serge). Two rack 3U unit systems in particular are notable: the Frac Rack system (e.g., Paia) and the similar Eurorack system (e.g., Doepfer). Further minor variations exist where European or Japanese manufacturers round a U measurement up or down to some closer convenient metric equivalent; for example the common 5U modules are exactly 8.75" (222.25mm), but non-American manufacturers may prefer 220mm or 230mm.

Electrical
Differences are with plugs that match 1/4-inch or 6.3mm jacks, 3.5mm jacks, and banana jacks, with main DC power supply (typically ±15 V, but ranging from ±18 V to ±12 V for different manufacturers or systems), with trigger or gate voltages (Moog S-trigger or positive gate), with typical audio signal levels (often ±5 V with ±5 V headroom), and with control voltages of volts/octave (typically 1 V/octave, but in some cases 1.2 V/octave.) Most analog modular systems use a volts/octave system, sometimes termed linear voltage control; some (such as Korg MS-20, ETI 4600) use a volts/hertz system with excellent temperature stability but less flexible control.

Semi-modular synthesizers
A modular synthesizer has a case or frame into which arbitrary modules can be plugged; modules are usually connected together using patch cords and a system may include modules from different sources, as long as it fits the form factors of the case and uses the same electrical specifications.

A semi-modular synthesizer on the other hand is a collection of modules from a single manufacturer that makes a cohesive product. Modules may not be swapped out and often a typical configuration has been pre-wired. However, the manufacturer provides mechanisms to allow the user to connect modules in different orders.

Matrix Systems
Matrix systems use pin matrixes or other crosspoint switches rather than patch cords. Historic examples with pin matrixes include the EMS Synthi 100, EMS VCS-3, ETI International 4600, Maplin 5600. The ARP 2500 used a matrix switch.

Patch Override Systems
The different modules of a semi-modular synthesizer are wired together into a typical configuration, but can be re-wired by the user using patch cords. Some examples are the ARP 2600, Anyware Instruments Semtex, Cwejman S1, EML101, Evenfall Minimodular, Future Retro XS, Korg MS-10, MS-20, MS-50, PS-3100, PS-3200 and PS-3300, Mungo Enterprises State Zero, and Roland System-100.

Electronically Reconfigurable Systems
Reconfigurable systems allow certain signals to be routed through modules in different orders. Examples include the Oberheim Matrix and Rhodes Chroma, and Moog Voyager.

Hybrid modular synthesizers
Hybrid synthesizers use hardware and software combination. Include the fully self-contained Arturia Origin, the Clavia Nord Modular and Clavia Nord Modular G2 which need an external computer to edit patches.

Modules
There are three principle functions in a modular synthesizer, signal, control and logic/timing. The function is not determined by the module, rather by how it is used. For example, an oscillator may function as a signal, a control when the output controls a parameter of another module, and logic when providing a logic function, such as a clock, trigger or gate. Separate specialized modules are connected by patch cords or (rarely) through a matrix, to create a patch. Inputs and outputs are an electric voltage.

There exist many different types and sub-types of modules – even modules with the same basic function will have different inputs, outputs and controls on various models. There are some standards which manufacturers followed for their range of synthesizers, such as 1V/oct; however, connecting synthesizers from different manufacturers may require cables with different kinds of jacks.

Some standard modules found on almost any modular synth are: SOURCEs - characterized by an output, but no signal input; it may have control inputs:
 * VCO – Voltage Controlled Oscillator, a continuous voltage source, which will output a signal whose frequency is a function of the settings. In its basic form these maybe simple waveforms (most usually a square wave or a sawtooth wave, but also includes pulse, triangle and sine waves), however these can be dynamically changed through such controls as sync, frequency modulation, and self-modulation.
 * Noise source - A source that outputs a random voltage. Common types of noise offered by modular synthesizers include white, pink, and low frequency noise.
 * LFO - A Low Frequency Oscillator may or may not be voltage-controlled. It may be operated with a period anywhere from a fortieth of a second to several minutes. It is generally used as a control voltage for another module. For example, modulating a VCO will produce frequency modulation, and may create vibrato, while modulating a VCA will produce amplitude modulation, and may create tremolo, depending on the control frequency. The rectangular wave can be used as a logic / timing / trigger function.

PROCESSORs - characterized by a signal input and an output; it may have control inputs.
 * EG - is a transient voltage source. A trigger in the presence of a gate, applied to an Envelope Generator produces a single, shaped voltage. Often configured as ADSR (Attack, Decay, Sustain, Release) it provides a transient voltage that rises and falls. It can be triggered by a keyboard or by another module in the system that produces a rapidly rising trigger in the presence of a gate. Usually it controls the amplitude of a VCA or the center frequency of a VCF, but the patchable structure of the synthesizer makes it possible to use the envelope generator to modulate other parameters such as the frequency or pulse width of the VCO. Simpler EGs (AD or AR) or more complex (DADSR—Delay, Attack, Decay, Sustain, Release) are sometimes available.
 * VCF - Voltage Controlled Filter, which attenuates frequencies below (high-pass), above (low-pass) or both below and above (band-pass) a certain frequency. VCFs can also be configured to provide band-reject (notch), whereby the high and low frequencies remain while the middle frequencies are removed. Most VCFs have variable resonance, sometimes voltage-controlled.
 * VCA - Voltage Controlled Amplifier, is usually a unity-gain amplifier which varies the amplitude of a signal in response to an applied control voltage. The response curve may be linear or exponential. Also called a two-quadrant multiplier.
 * RM - Ring modulator - Two audio inputs are utilized to create sum and difference frequencies while suppressing the original signals. Also called a four-quadrant multiplier or balanced modulator.
 * Mixer - a module that adds voltages.
 * Slew limiter - is a very-low frequency lowpass filter which slows down changes in voltage. This can be used to create glide or portamento between notes.
 * S&H - Sample and hold, is usually used as a control-voltage processor. Depending upon the design, usually an ascending edge (trigger), captures the value of the voltage at the input, and outputs this voltage until the trigger input reads another voltage and repeats the process.
 * Sequencer, is a compound module which produces a sequence of voltages, usually set by adjusting values on front panel knobs. Basic sequencers will be stepped by a trigger being applied to the trigger input. More complex designs may have the sequencer step forwards or backwards, or only run for a limited number of stages.