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Modular synthesizers are synthesizers composed of separate modules for 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.
The first modular synthesizer was developed by German engineer Harald Bode in the late 1950s. The 1960s saw the introduction of the Moog synthesizer and the Buchla Modular Electronic Music System, created around the same period. The Moog was composed of separate modules which created and shaped sounds, such as envelopes, noise generators, filters, and sequencers, connected by patch cords.
By the 1990s, modular synthesizers had fallen out of favor compared to cheaper, smaller digital and software synthesizers. German engineer Dieter Doepfer 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 2017, over 100 companies, including Moog and Roland, were developing Eurorack modules.
Types of modules
The basic modular functions are: signal, control, logic/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 the musician to control various parameters of the device (for example, the rate of a chorus pedal) 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 (for example, to connect a low-frequency oscillator module to a delay module to get the chorus effect.)
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 (VCO), which may have options for sync (hard or soft), linear or exponential frequency modulation, and variable waveshape; the Voltage Controlled Filter (VCF) that may have both resonance and bandwidth controls; and the Envelope Follower which may provide outputs at each stage of the process. Examples of more complex modules include the frequency shifter, sequencer, and vocoder.
There are some standards which manufacturers followed for their range of physical synthesizers, such as 1V/octave control voltages, and gate / trigger thresholds providing general compatibility; however, connecting synthesizers from different manufacturers may require cables with different kinds of plugs.
In the past, modular synthesizers were often bulky and expensive. Due to the continuously variable nature of knobs and sliders, reproducing an exact patch can be difficult or next to impossible. In the late 1970s, modular synthesizers started to be largely supplanted in pop music by highly integrated keyboard synthesizers, racks of MIDI-connected gear, and samplers. 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,[when?] there has been a resurgence in the popularity of analog synthesizers aided by physical standardization practices, an increase in 'retro' gear and interest, 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.
Modules can usually be categorized as either sources or processors 
Some standard modules found on almost any modular synthesizer 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 may be 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.
- EG - An envelope generator 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 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 (AD or AR) or more complex (DADSR—Delay, Attack, Decay, Sustain, Release) are sometimes available.
- Sequencer, also sometimes called an Analog Step Sequencer, is a family of compound module types which may be a source or a processor, see also below. 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, and/or gate, at each step (stage). Sequencers are stepped by a trigger being applied to the trigger input. Designs may allow for stepping forwards or backwards, oscillating patterns, random order, or only using a limited number of stages. 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; it may have control inputs:
- 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 attenuated. 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.
- LPG - Low pass gate, is a compound module, similar to a VCA and a VCF, except that the circuit uses a resistive opto-isolator (vactrol) to respond to the control voltage, which also filters the sound as it amplifies, allowing more high frequency information through at higher amplifications.
- 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 usually a sub-audio lowpass filter. When used in a control voltage path to an oscillator, this can be used to create glide or portamento between frequencies.
- 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, (see also above), as a processor, may have a signal input into each step, (location or stage), 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.
Modern manufacturers of modular hardware synthesizers (alphabetical)
Hardware offerings range from 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; often both the original and subsequent reworked designs are available free on the internet, the original patents having lapsed. Many hobbyist designers also make available bare PCB boards and front panels for sale to other hobbyists.
- Buchla Electronic Musical Instruments (formerly Buchla & Associates)
- Doepfer Musikelektronik (A-100)
- Moog Music (formerly Big Briar, formerly Moog)
- Synthesis Technology
- PAiA Electronics
- Analogue Systems
- Sound Transform Systems
- Studio Electronics
- Synton Fenix
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), 3U (Eurorack).
Two 3U unit standards in particular are notable: Frac Rack (e.g., Paia), which uses the entire 3U for the front panel, and Eurorack (e.g., Doepfer) which has a 2mm horizontal lip that the front panels are seated between. 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.
Other differences are in the plugs used, which can match 1/4-inch or 6.3mm jacks, 3.5mm jacks, banana jacks, or breadboard patch leads, in the main power supply, which is most often ±12 V or ±15 V, but can range from 2.5±2.5 V to 0±18 V for different manufacturers or systems, in the 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 system in which the frequency is exponentially related to the pitch (such as 1 volt/octave or 1.2 volts/octave), sometimes called "linear" because the human ear perceives frequencies in a logarithmic fashion, with each octave having the same perceptual size; some synthesizers (such as Korg MS-20, ETI 4600) use a volts/hertz system, where the frequency (but not the perceived pitch) is linear in the voltage.
Modular software synthesizers (alphabetical)
There are also software synthesizers for personal computers which are organized as interconnectable modules. Many of these are virtual analog synthesizers, where the modules simulate hardware functionality. Some of them are also virtual modular systems, which simulate real historical modular synthesizers.
- Arturia Modular V
- CreamwareAudio Modular III
- Moog Model 15
- Pure Data
- VCV Rack
- Wren for Windows (open-source)
Computers have grown so powerful and inexpensive that software programs can realistically model the signals, sounds, and patchability of modulars very well. While potentially lacking the physical presence of desirable analog sound generation, real voltage manipulation, knobs, sliders, cables, and LEDs, software modular synthesizers offer the infinite variations and visual patching at a more affordable price and in a compact form factor.
A modular synthesizer has a case or frame into which arbitrary modules can be fitted; 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, an instrument. Modules may not be swapped out and usually a typical configuration has been pre-wired. The “modules” are typically not separable and may physically be parts of a contiguous circuit board. However, the manufacturer provides mechanisms to allow the user to connect modules in different orders and often to connect external components or modules (chosen and supplied by the user) between those of the instrument.
Matrix systems use pin matrices or other crosspoint switches rather than patch cords. The ARP 2500 was the first synthesizer to used a fixed switch matrix. The pin matrix was made popular in the EMS VCS-3 and its descendants like the EMS Synthi 100. Other systems include the ETI 4600, and the Maplin 5600.
In digital times the clean logical layout of these matrices has inspired a number of manufacturers like Arturia to include digitally programmable matrices in their analog or virtual analog synthesizers. Many fully digital synthesizers, like the Alesis Ion, make use of the logic and nomenclature of a "modulation matrix", even when the graphical layout of a hardware matrix is completely absent.
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 Semtex, Cwejman S1, EML101, Evenfall Minimodular, Future Retro XS, Korg MS-10 / MS-20 / MS-50 / PS-3100 / PS-3200 / PS-3300, Mungo State Zero, Roland System 100 and Moog Mother-32 .
Electronically Reconfigurable Systems
Hybrid modular synthesizers
Hybrid synthesizers use hardware and software combination. In alphabetical order:
- Arturia Origin by Arturia (fully self-contained)
- Clavia Nord Modular and Clavia Nord Modular G2 (these need an external computer to edit patches)
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- Mechanical specifications