MIDI (Musical Instrument Digital Interface) is a standardized protocol for exchanging control or performance information, but not the actual audio signals, between suitably equipped electronic musical instruments and computers. That is, it is a set of instructions that provides a highly compact means of storing music with, for example, the ability to contain an entire song within just a few hundred MIDI messages, in contrast to conventional audio data, which requires sampling thousands of times per second.
A second advantage of MIDI is that it allows easy modification of individual notes, including their pitch, volume and duration, without having to rerecord a song or other piece. A third advantage is that it makes it simple to change any or all of the instruments in a piece.
MIDI was initially developed in the early 1980s in response to the rapid growth in often-incompatible electronic music equipment and was eventually adopted by every major manufacturer. This, plus its several advantages over recordings, has helped it survive despite its age and despite all of the other advances that have occurred in electronic music over the subsequent years.
The MIDI protocol allows a device to send data, referred to as messages, for up to sixteen different channels, or voices, simultaneously over a single cable. The most basic message is the Note-On, which includes the channel, the note pressed on a MIDI keyboard (for example, the number 60 representing middle C), the note's velocity (i.e., how hard the key has been pressed), and a timestamp. A corresponding Note-Off message will turn that note off when the key is released.
Some of the various other data that can be transmitted by the MIDI protocol are attack, decay, sustain, release, panning, aftertouch, pitch bend, and modulation.
General MIDI (GM), created later in the same decade, is a set of 128 standard instruments and other sounds, called patches, that were not specified in the original MIDI standard. They are ordered into groups according to the type of instrument or other sound, and include, for example, 1 for acoustic grand piano, 25 for nylon acoustic guitar, 41 for violin, 57 for trumpet, 115 for steel drums and 127 for applause.
All wavetable synthesizers and sound cards use this set of sounds to assure compatibility among the manufacturers. Files saved in GM format will play the correct instruments regardless of the manufacturer of the GM-capable MIDI device upon which it is replayed, such as a sound card in a computer or a high quality keyboard. The only difference in the final listening experience is the quality of the sampled instruments that the MIDI playback devices provide.
MIDI 2 was introduced in 2020 after well over a decade of work by both industry groups and individual manufacturers and was made possible by the tremendous advances in electronic hardware and software that occurred since the release of MIDI 1. It is a backwards-compatible extension of MIDI 1 that features a bidirectional communication ability. This allows instruments and controllers to exchange data in both directions, enabling real-time feedback and interaction between devices, in contrast to the previous single direction information flow, generally from the controller to the instrument.
Among its other features are greatly increased resolution and expressiveness, made possible by a 32-bit resolution, as contrasted with the previous seven-bit resolution and 128 steps, and an upgrade in velocity from seven-bit to 16-bit messages. This provides musicians with finer control over parameters such as velocity, pitch, and timing, thus making possible more realistic and more dynamic performances.