A music sequencer is an application or a device designed to play back musical notation. The original kind of sequencer is now known as a step sequencer to distinguish it from the modern kind, which records a musician playing notes.
NOTE FROM ELECTROSPECTIVE: It is widely accepted that Raymond Scott invented the first electronic sequencer, the first model as early as 1946.
From the Raymond Scott site http://RaymondScott.com/1946.htm
“Standing 6 feet high and covering 30 feet of wall space, the sequencer consisted of hundreds of switches controlling stepping relays, timing solenoids, tone circuits and 16 individual oscillators. If you walked behind the wall during the operation the music produced would be all but drowned out by the cacophonous klickety-klack of the relays as they switched positions.” -Herb Deutsch, author & Hofstra University music professor, recalling a visit to Raymond Scott’s self-design & built studio.”
The first sequencers were primitive devices that played rigid patterns of notes using a grid of (usually) 16 buttons, or steps, each step being 1/16th of a measure. These patterns of notes are then chained together to form longer compositions. Step sequencers are monophonic by nature, although some are multitimbral, meaning they can control several different instruments but only play one note on each of those instruments. Step sequencers are mostly used in drum machines and groove-boxes.
With the advent of MIDI and in particular the Atari ST, programmers were able to write software which could record and play back the notes played by a musician. These sequencers didn’t play mechanical sounding notes of exactly equal length, but rather recorded and played back expressive performances by real musicians. These were typically used to control external synthesizers, especially rackmounted sound modules as it was no longer necessary for each synthesizer to have its own keyboard.
As the technology matured, sequencers gained more features, including the ability to record multitrack audio, and the ability to control virtual instruments known as plug-ins. This enabled musicians to replace their synthesizers with software equivalents that ran inside the sequencer software.
The terms “music sequencer” and “digital audio workstation” are often used interchangeably, as modern sequencers combine both sets of features.
Although the term “sequencer” is today used primarily for software, workstation keyboards include their own proprietary built-in MIDI sequencers. Drum machines and some older synthesizers have their own step sequencer built in. There are still also standalone hardware MIDI sequencers, although the market demand for those has diminished greatly due to the greater feature set of their software counterparts.
The Fairlight CMI’s built-in sequencer, known as Page R, was the basis of Ultimate Soundtracker and hence all subsequent trackers. These combine step sequencing with sample playback in a single application.