Musical Instrument Digital Interface

Musical Instrument Digital Interface (MIDI) is a hardware specification and protocol standardized in 1983 to communicate note and effect information between synthesizers, computers, music keyboards, controllers, and other electronic music devices. It is basically a high-speed serial connection with separate connections for MIDI in, MIDI out and MIDI through (to allow devices to be daisy-chained).

Description
The basic unit of information is a "note on/off" event which includes a note number (pitch) and key velocity (loudness). There are many other message types for events such as pitch bend, patch changes and synthesizer-specific events for loading new patches.

There is a file format for expressing MIDI data which is like a dump of data sent over a MIDI port. The MIME type "audio/midi" isn't actually registered so it should be "audio/x-midi".

History
Basic MIDI support was added through a QuickTime Musical Instruments extension for QuickTime 2.0. A Macintosh MIDI Manager had also been in development for QuickTime 2.1, but was never released for classic Mac OS. MIDI support was updated in QuickTime 2.5, which was released on July 22, 1996.

Support for MIDI in Mac OS X (now macOS), iPadOS, and iOS 4.2 or later is managed through the Core MIDI framework.