Introduction to the MIDI Standard
MIDI is an acronym that stands for Musical Instrument Digital Interface. It’s a way to connect devices that make and control sound — such as synthesizers, samplers, and computers — so that they can communicate with each other, using MIDI messages. This lets one keyboard trigger sounds on another synthesizer, and it makes it possible to record music in a form that allows for easy note editing, flexible orchestration, and song arrangement. Virtual instruments — computer programs that simulate hardware synthesizers and samplers — also communicate with computer sequencing software running on the same computer using MIDI messages.
A Brief History
MIDI evolved as a standard to enable communication between the more compact and affordable synthesizers that were available in the early 1980s, after the era of large, expensive modular analog synthesizers. MIDI was meant to allow someone to control multiple synthesizers from a single keyboard, so as to generate, for example, the massive layered sounds popular in some ’80s pop music. Formerly, such connections between instruments were not standardized, so incompatibilities were common. The MIDI standard was completed in 1983 by a consortium of musical equipment manufacturers (including Korg, Oberheim, Roland, Sequential Circuits, and Yamaha). Products featuring the standard, such as the popular Yamaha DX7, were on the market soon after.
Before long, sequencing software for personal computers could take advantage of the MIDI communications protocol to let users record, store, and edit music, as well as manage large collections of synthesizer sounds.
Communication by Message
The most important thing to understand about MIDI is that it is based on the idea of message-passing between devices (pieces of equipment or software). Imagine a common situation: you have a keyboard synthesizer and would like to record a sequence using the sounds that are in that synthesizer. You connect the computer and synthesizer so that they can communicate using the MIDI protocol, and start recording. What happens?
When you play your recorded sequence, the computer sends MIDI messages back to the synthesizer, which interprets them and creates audio in response. Because the music handled by the computer is in the form of encoded messages, rather than acoustic waveforms, it’s possible to change the sound of a track from a piano to a guitar after having recorded the track. That would not be possible if you were recording the sound that the synthesizer makes.
To address this problem, the MIDI standard includes the General MIDI (or GM) specification. The most important part of this is a standard assignment of instrument types to patch numbers. For example, in a General MIDI compatible sequence, a violin sound will always be patch number 41. The violins on two different keyboards will not sound exactly the same, but at least they will sound like violins.
A similar problem affects drum kit patches: the assignment of individual drum sounds to keys on the keyboard is not guaranteed to be compatible between different synthesizers. General MIDI specifies a map of typical drum sounds to keys. It also declares that channel 10 is the drum channel, so that a sequence can depend on finding drum sounds there.
To enhance compatibility between different MIDI sequencing and music notation programs, even those running on different operating systems, the MIDI standard defines a specification for the Standard MIDI File. This type of file (usually having the file extension “.mid”) represents multi-track sequences, complete with patch selections, notes, pitch bend, and other controls. A wide variety of programs can read and write SMF files. The format is especially useful in conjunction with the GM patch set, to enhance portability between different systems.
