A synth which is constructed from discrete functional modules (such as VCOs, VCAs, envelope generators, mixers, etc.), which the performer can purchase and install individually as needed to construct any desired configuration. The first practical synthesizers as we know them today, beginning with the work of Donald Buchla, Bob Moog and Herb Deutsch in the early 1960s, were modular in design construction. In a typical modular synth, all inputs and output within each module are accessible via jacks on the front panel, and are routed by the use of patch cords; there is no pre-defined routing. (If no patch cords are plugged in, the modular makes no sound.)
Modular synthesizers are composed from functional modules, usually each purchased separately by the user, and installed in a cabinet or frame which provides structural support, physical enclosure and protection from damage, and room to mount power supplies and other support components as needed. Generally, a module consists of a front panel, which is fastened to cabinet mounting rails at the top and bottom, and circuit boards and components which are exposed behind the panel and are attached to and supported by the panel. Power is supplied to the module by either having it plug into a backplane in the cabinet, or, more commonly, connecting a cable from a power supply to a connector on one of the module's circuit boards. A given combination of physical dimensions, mounting hole patterns, power supply voltages, and means of connecting to the power supply is known as a format. It is common today for a format to be supported by multiple manufacturers; this was much less common with the first generation of modular synths in the 1970s. There are no generally accepted standards for how the modules in a modular synth should be arranged; layout of modules is highly personal and most performers have their own preferences. It is not uncommon for someone who purchases a used modular to completely disassemble it and rearrange the modules.
Modular synths are considered by many to be the ultimate in complex-sound creativity, as the number of possible signal routings in any non-trivial configuration is nearly infinite, and the system is fully open to interconnection with external devices at any point in the signal chain. However, this flexibility and lack of pre-wired connections makes this type of synth more difficult to learn and to play. Also, the modular method of construction is relatively space- and cost-inefficient, meaning that a large configuration will require a substantial amount of time, space, and money to set up. (Among other things, this means that as a practical matter most modular synths are monophonic, as a polyphonic setup requires multiple copies of everything.) All current modular synths employ analog signal routing and most of the module circuitry is analog, although there are some digital processing modules available. Modular synths were originally popular from about 1968 to 1980. They disappeared from the market in the 1980s, but made a significant comeback in the late 1990s and have continued to gain adherents since, with over fifty manufacturers currently active in the market.