MIDI is an acronym for Music Instrument Digital Interface. MIDI is a shared standard adopted by the music synthesis industry as it undertook the transition from analog to digital hardware. The purpose of MIDI is to capture performer actions using control devices and to transmit these actions to sound-processing devices. Control devices include keyboards, breath controllers, modulation wheels, sliders, foot pedals and — after the introduction of the MIDI file format — computers. Sound-processing devices include synthesizers and digital mixers.

The original 1981 MIDI specification mandated only features which enabled digital devices to communicate with one another in real time. These features included:

Central to the information protocol is the notion of a channel. Workhorse MIDI messages (such as “Note On”, “Note Off”, “Program Change”, and “Control Change”) all embed a channel number ranging from 1 to 16. MIDI channels are best explained by stating that a message issued on a channel would affect all of the notes playing on that channel but none of the notes playing on other channels. Thus you can have any number of notes playing on a given channel (so long as the synthesizer can accommodate them), but all of these notes must share the same program/timbre. Consistently, the action of a pitch-wheel control will cause all the notes currently playing in the channel to shift upwards or downwards in pitch — every note shifting by the same pitch-interval.

The next evolution of MIDI was the 1988 Standard MIDI File format. This format basically wraps MIDI messages with timing information. It also introduced for meta-messages which contain “information about the MIDI sequence … that are not to be sent over MIDI ports”. Such messages include “Set Tempo” and “End of Track”.

The MIDI file format made it possible to record, edit, and play back live performances on MIDI instruments. It also kick-started the MIDI sequencer industry. Interfaces came onto the market which allowed one computer to send MIDI messages out through multiple ports, each with its own block of 16 independent channels. Each MIDI port was identified in software by a device name.

A third standard, “General MIDI”, was introduced in 1991. General MIDI gives specific mappings between MIDI programs instrumental sounds, and also between MIDI key numbers and percussion sounds.

© Charles Ames Page created: 2013-09-20 Last updated: 2015-04-14