Modular synthesizer


Modular synthesizers are electronic musical instruments composed of separate synthesizer modules that represent different functions. The modules can be connected together by the user to create a patch. The outputs from the modules may include audio signals, analog control voltages, or digital signals for logic or timing conditions. Typical modules are voltage-controlled oscillators, voltage-controlled filters, voltage-controlled amplifiers and envelope generators. A modular synth is made up of individual modules which are combined into an instrument. A semi-modular synth allows patching within a fixed set of circuits and fixed user interface.

History

German engineer Harald Bode developed a modular sound processing system in the late 1950s, and this and his paper on using semiconductors in sound processing greatly influenced future development. The first prototype of the Moog synthesizer was introduced at the 1964 Audio Engineering Society convention, and the first Buchla Modular Electronic Music System was released in 1965.
The Moog and Buchla were built of separate modules which created and shaped sounds, such as envelopes, noise generators, filters, and sequencers, connected by patch cords, the arrangement of which are called a "patch", specifying a particular sound. At this time, Moog developed the 1v/octave standard for representing musical pitch.
Early modular synthesizers were large and very expensive, mostly sold to university music departments, composers of advertising jingles and to a few wealthy musicians. After the success of Wendy Carlos’ 1968 album Switched-On Bach, there was a surge in modular synth sales, but by 1970 when ARP Instruments introduced their ARP 2500 modular system, sales were already in decline. After 1970, the synthesizer market predominantly shifted to semi-modular and fully integrated synthesizers, such as the semi-modular EMS VCS3, ARP 2600, and the integrated Moog Minimoog.
ARP largely abandoned modulars after 1971, but Moog continued to produce their System 35 and 55 until 1981,. Several new companies entered the market, though, in the 1970s. E-mu introduced the E-mu Modular System in 1973-4. Roland released the semi-modular Roland System 100 in 1975, followed by the modular System 700 in 1976 and the System 100m in 1979. During the same period, Korg released their PS range of semi-modular polyphonic instruments and their MS range of semi-modular monophonic keyboards.
Modular synthesizers have also been distributed in DIY kit form since 1972, when the Serge synthesizer was developed by Serge Tcherepnin, Rich Gold and Randy Cohen at CalArts. Also in 1972, PAiA Electronics released their first of several DIY modular systems, the 2720 system. Electronic magazines published modular synth construction articles, too, for example the Elektor Formant, and Practical Electronics PE Synthesizer. Bernard Hutchins published the Electronotes newsletter which, during its publication history, published hundreds of synthesizer circuits that were incorporated into commercial designs.
Moog and Buchla had different approaches to synthesiser function. Moog’s instruments were based on subtractive synthesis, with oscillators producing fixed basic waveforms then shaping and mellowing them with a voltage-controlled filter, controlled by a keyboard. Buchla used waveshaping and frequency modulation to create new dynamic waveforms, typically controlled by touch-pads.
By the late 1970s, highly-integrated and often polyphonic keyboard synthesizers, samplers, sound modules, and other MIDI-connected gear dominated the synth market and modular synthesizers had fallen out of favor compared to cheaper, smaller digital and software synthesizers. However, there continued to be a community who chose the physically patched approach, the flexibility and the sound of traditional modular systems.
Since the late 1990s, there has been a resurgence in the popularity of analog synthesizers aided by physical standardization practices, an increase in available retro gear and interest, vastly decreased production costs and increased electronic reliability and stability, the rediscovered ability of modules to control things other than sound, and a generally heightened education through the development of virtual synthesis systems such as VCV Rack, MAX/MSP, Pd and Reaktor etc.

Types of module

The basic modular functions are: signal, control, logic and timing. Typically, inputs and outputs are an electric voltage.
The difference between a synthesizer module and a stand-alone effects unit is that an effects unit will have connections for input and output of the audio signal and knobs or switches for users to control various parameters of the device while a synthesizer module may have connections for input and output, but will also have connections so that the device's parameters can be further controlled by other modules
There exist many different types of modules. Modules with the same basic functions may have different inputs, outputs and controls, depending on their degree of complexity. Some examples include the voltage-controlled oscillator, which may have options for sync, linear or exponential frequency modulation, and variable waveshape; the voltage-controlled filter that may have both resonance and bandwidth controls; and the envelope generator which may provide outputs at each stage of the process. Examples of more complex modules include the frequency shifter, sequencer, and vocoder.
Modular synthesizers may be bulky and expensive. There are some standards that manufacturers follow for their range of physical synthesizers, such as control voltages, and gate and trigger thresholds providing general compatibility; however, connecting synthesizers from different manufacturers may require cables with different kinds of plugs.
Image:Doepfer A-100.jpg|thumb|A Doepfer A-100
German engineer Dieter Doepfer began his interest in modular synth in the late 70s building an Elektor Formant then designing some extra modules for it. In the 1990s he believed modular synthesizers could still be useful for creating unique sounds, and created a new, smaller modular system, the Doepfer A-100. This led to a new standard for modular systems, Eurorack; as of 2025 several hundred companies, including Moog and Roland, were developing Eurorack modules.. There are also larger 4U and 5U systems available.

Typical modules

Modules can usually be categorized as either sources or processors. Standard modules found in a modular synthesizer are:
Sources - characterized by an output, but no signal input; it may have control inputs:
  • VCO – Voltage-controlled oscillator, outputs a signal whose frequency is a function of input control voltage and settings. In its basic form, these may be simple waveforms, however these can be dynamically changed through such controls as sync, frequency modulation, and self-modulation.
  • Noise source - 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 a frequency modulation perceived as vibrato, while modulating a VCA will produce amplitude modulation perceived as tremolo, depending on the control frequency. A rectangular wave output from an LFO can be used as a logic output for timing or trigger functions on other modules.
  • EG - An envelope generator is a transient voltage source. A trigger control signal applied to an envelope generator produces a single, shaped voltage. Often configured as ADSR it provides a control voltage that rises and falls. Usually it controls the amplitude of a VCA or the cutoff 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 or more complex are sometimes available.
  • Sequencer or analog sequencer, is a family of compound module types that may be a source or a processor. As a source, depending upon the configuration, it may produce a sequence of voltages, usually set by adjusting values on front panel knobs. The sequencer may also output a trigger, or gate, at each step. Sequencers are stepped by a trigger being applied to the trigger input. Designs may allow for stepping forwards or backward, oscillating patterns, random order, or only using a limited number of steps. An example of an analog sequencer and controller with this level of complexity is the Doepfer A-154, A-155 combination.
Processors - characterized by a signal input and an output and may have control inputs:
  • VCF - Voltage-controlled filter, which attenuates frequencies below, above or both below and above a certain frequency. Most VCFs have variable resonance, sometimes voltage-controlled.
  • VCA - Voltage-controlled amplifier, 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.
  • LPG - Low-pass gate, is a compound module, similar to a VCA combined with a VCF. The circuit uses a resistive opto-isolator to respond to the control voltage, which also filters the sound, allowing more high-frequency information through at higher amplification.
  • RM - Ring modulator - Two audio inputs are utilized to create sum and difference frequencies while suppressing the original signals. Also called an analog multiplier or frequency mixer.
  • Mixer - A module that adds voltages.
  • Multiple - Fan out a voltage output to multiple inputs.
  • Slew limiter - A sub-audio low-pass filter. When used in a control voltage path to an oscillator, produces a portamento effect.
  • S&H - Sample and hold, is usually used as a control-voltage processor. Depending upon the design, usually an ascending edge, 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,, as a processor, may have a signal input into each step,, which is output, when stepped to. An example of this type of sequencer is the Doepfer A-155.
  • Custom control inputs - It is possible to connect any kind of voltage to a modular synthesizer as long as it remains in the usable voltage range of the instrument, usually -15V to +15V.