A device which generates control signals telling a synth what notes to play and when to play them. The term is used to apply to three different entities. In order of invention:
(1) In the context of modular synthesizers, a sequencer generates a series of control voltages and gate signals, usually intended to cause the synth to play a repeating series of notes. (In this application, the control voltage is routed to a voltage controlled oscillator to control its frequency, and the gate signal is routed to an envelope generator to control the sounding and dynamics of the notes.) The typical sequencer in a modular synth is capable of 8 or 16 steps, whose outputs are all tied to a common output bus. Each step has a knob which determines the control voltage and the duration of the gate signal that it generates. A counter determines which step is in control of the output bus at a given time; the counter is advanced by an applied clock signal, which determines the tempo of the played notes. This type of sequencer first appeared in the 1960s; they were expensive and temperamental beasts, and not very flexible. The technology has improved, but the basic design remains the same. The use of this type of sequencer remains mostly confined to use with modular synths.
(2) Sequencers with digital memory first appeared in the 1970s (the canonical example was the EMS Synti-AKS with its microprocessor-controlled sequencer built into the case lid), and these quickly evolved to be able to store longer and more complex patterns as well as additional performance information.In the 1980s, synth manufacturers began including sequencers in some of their synth designs, and equipping them with MIDI outputs so that one sequencer could easily control multiple devices. Since they were so often used to control drum machines, models started to appear which incorporated both the drum machine and the sequencer in one device. Today, stand-alone digital sequencers are far less common then sequencers built into drum machines or workstation keyboards.
(3) MIDI interfaces started to appear on computers almost as soon as the initial standard was adopted in 1983. In 1985, the Voyetra company introduced the first sequencing software for the IBM PC, launching a third category of sequencer: those implemented purely in software running on a personal computer. From this point software sequencers evolved far beyond simple pattern-remembering. The first innovation was to record and playback MIDI data as if it were audio being recorded by a tape machine. Graphical display and editing capabilities soon appeared, using the well-known piano roll notation display paradigm. Soon, additional abilities such as SMPTE synchronizing with tape machines, hard disk recording and mixing, and ability to load patches and parameter settings into sequencers, were added. As of 2014, this use of the term "sequencer" is becoming obsolete, as software sequencer functions have largely been incorporated into DAW software pacxkages.