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The modular synthesizer is a type of synthesizer consisting of separate specialized modules. The modules are not hardwired together but are connected with patch cords, to create a patch. There are both audio and control signals, which are interchangeable.
Types of modules 
Modular synthesizers fall into two broad categories, analog, and digital. The analog modular synthesizer is a type of synthesizer consisting of separate specialized modules connected by wires (patch cords) to create a so-called patch. There are three basic kinds of modules: source, processor and logic. The basic modular functions are as: signal, control, logic/timing. 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 may create vibrato while modulating a VCA may create tremolo. The pulse wave can be used as a timing / trigger function.
- 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 output of a VCA or 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.
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 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 combines multiple signals.
- 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.
- 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.
Modular synthesizers can be bulky and expensive. 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 continues to be a loyal following of musicians who prefer the physically patched approach, the flexibility and the sound of traditional modulars. Since the late 1990s,[when?] there has been a resurgence in the popularity of analog synthesizers spurred on 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 MAX/MSP and Pd.
Historic manufacturers of modular synthesizer hardware 
The earliest commercial modular synthesizers were developed, in parallel, by R.A. Moog Co., and Buchla in 1963. The synthesizer both broadened the spectrum, and greatly eased the creation of electronic music, which before was made via tape splicing, use of primitive electronic oscillators, and earlier electronic or electromechanical instruments such as the theremin and the Ondes Martenot. 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 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). Joseph A. Paradiso's Massive Modular Synth is among the world's largest home-designed and built synthesizers.
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.
- Analogue Solutions (Concussor)
- Analogue Systems (RS Integrator)
- Blacet Research
- Buchla (200e)
- Club of the knobs
- Cyndustries (Cynthia)
- Doepfer Musikelektronik (A-100)
- Elby (Panther Series)
- Fénix (Fénix)
- Future Sound Systems
- Kilpatrick Audio
- Macbeth Studio Systems
- Synton (Synton)
- Mattson Mini Modular
- Oakley Sound Systems
- PAiA Electronics
- Pittsburgh Modular
- Plan B
- Sound Transform Systems
- Synthesis Technology (MOTM)
- Synthetic Music Systems
- Synthetic Sound Labs
- The Harvestman
- Tiptop Audio
- Wiard Synthesizer Company
Technical specifications 
Form Factors 
Many early synthesizer modules had modules with height in integer inches: 11" (e.g., Roland 100), 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.
Other 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.
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.
- Alsa Modular Synthesiser http://alsamodular.sourceforge.net/
- ARP 2600 by ARP Instruments, Inc
- Bidule By plogue
- CreamwareAudio Modular III
- Jeskola Buzz
- Kyma by Symbolic Sound Corporation
- Moog Modular V by Bob Moog
- Pure Data
- SynC Modular
- Tassman by Applied Acoustics Systems
- Timewarp 2600 by Way Out Ware
- Thor Polysonic Synthesizer (included with the current version of Propellerhead Reason)
- Zebra2 by Uh-e 
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.
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, 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. In alphabetical order:
- Arturia Origin by Arturia (fully self-contained)
- Clavia Nord Modular and Clavia Nord Modular G2 by Clavia (these need an external computer to edit patches)
- Modular Synthesizer for AudioCubes (Audiocubes)
See also 
- Synthlearn's full list of modular synthesizers by manufacturer
- 120 years of Electronic Music has information on classic modular synths.
- Vintage Synth Explorer Information on vintage synth gear, including Moog and other modular synths.
- Modular Analog Synthesizers Return! Article about new modular systems.
- Current listing of analog synthesizer manufacturers
- Eurorack Modular Synthesizer Manufacturers A guide to manufacturers of Eurorack modular synth modules.
- Using the Moog modular synthesizer
- Sound samples from classic synthesizers like the Moog Modular
- Retrosound - Nord Modular
- Muff's Modules & More
- ModularSynth.net Analog Modular Synthesizer discussion forum
- Wavemakers: kb electronic music instrumentation History of Wavemakers modular synthesizers 1970-1980
- ETI 4600 owners site
- DIY Hardware and Software Discussion forum at Electro-music.com
- Synth-DIY Discussion forum
- Ken Stone's Do-It-Yourself Page
- Music From Outer Space All the information you need to build your own modular.
- Music From Outer Space Synth diy-page
- SDIY wiki a wiki about Synth-DIY including modular synthesizers