Atari ST

The Atari ST is a home/personal computer that was commercially popular from 1985 to the early 1990s.

Operating system: Atari OS

Thanks to its built-in MIDI ports it enjoyed success as a music sequencer and controller of musical instruments among amateurs and professionals alike, being used in concert by bands such as Tangerine Dream and 808 State.

There was plenty of MIDI-related software for use professionally in music studios, or by amateur enthusiasts. The popular Windows/Macintosh applications Cubase and Logic Pro originated on the Atari ST.

Sources:
http://en.wikipedia.org/wiki/Atari_ST

Related:
Computer History
Operating Systems

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