32. Musical Instrument Digital Interface

Electronic Music Interactive so far has only considered sound, its representation and its synthesis. The discussion now begins to examine how these sounds are triggered, or in other words, turned on and off. The discussion will also survey how their relative dynamics are controlled, and look at many other aspects of music that are vital in shaping an expressive musical work.

In the homes and studios of professional and amateur musicians, and at major computer music installations around the world, MIDI-based music performance and composition systems are used to create music. MIDI, which stands for Musical Instrument Digital Interface, is a protocol that allows messages to be sent as digital data from one musical instrument to other musical instruments connected in a MIDI system.

A MIDI system is made up of sound-producing synthesis modules capable of receiving and understanding messages transmitted in MIDI protocol, and a keyboard controller or computer able to transmit MIDI messages to the sound modules in the system.

Diagram 32-1: A MIDI system.

These transmitted MIDI messages are musical in nature and might consist of instructions such as "play note middle C and play it softly", "now have the loudness of note C gradually intensify for two beats", "note C gently fade in volume with subtle vibrato added", and finally, "turn off note C."

A sound-producing module receiving messages like these then responds by playing the note or notes, and by adding the nuances according to the instructions in the messages. MIDI messages can be played from a keyboard controller in real-time to control a sound-producing module, or the directions can be recorded by a computer program, fully edited and refined, and then forwarded to the modules in the MIDI system. The MIDI system is conceptually very similar to a player piano where instructions come from the placement of small punched holes on a paper roll, with the holes controlling which notes will play and when they will play.