Work in progress!

MIDI (Musical Instrument Digital Interface) is a technology enabling synthesizers and other electronic musical instruments to be connected to each other, or to a computer. This is useful for a variety of reasons, some of the most popular ones being:

• Playing several synthesizers using just one keyboard (avoiding the need for a Rick Wakeman style rack of keyboards)
• Recording a musical performance on a computer
• Playing back music recorded on a computer

How it works

MIDI simply allows information about which notes a musician is playing to be sent down a bit of wire. Any professional quality electronic keyboard will have a socket on the back labelled "MIDI Out" which sends out a message every time the player hits key. (The message will contain information about the note, which 'voice' should be used to play it (e.g. was that the piano, the bass guitar or the drum kit?) and the speed at which the key was hit, ususally used to determine how loud the note should be.) Likewise, any decent synthesizer will have a socket labelled "MIDI In", and if you wire a cable from the MIDI Out on the keyboard to the MIDI In on the synthesizer, the synthesizer should make the appropriate noises for any notes played on the keyboard.

This picture is complicated a little by the fact that a lot of electronic keyboards have a built-in synthesizer. (Many people think of a keyboard when someone says a synthesizer - in fact synthesizers without keyboards (often called 'synth modules') are very common, and keyboards without synthesizers (often called 'controller keyboards') are also widely used.) The slightly confusing issue here is that there are two ways to get sound out of such a synthesizer: you can either just play notes on its own keyboard, or you can plug the MIDI Out from a different keyboard into its MIDI In and play it that way.

Recording with MIDI

Many computers have MIDI In and Out sockets, but they tend to be used slightly differently. Although the MIDI In could be used to play the computer's sound card from a keyboard, and in theory the computer's keyboard be used as a piano keyboard to send messages through the MIDI Out, there is a much more appealing use of the MIDI ports of a computer: music recording.

You can buy programs which will store any messages that come in through the MIDI In port. These can then be played back through the MIDI Out again and again. (The program simply has to note down the timing with which the messages arrive, and make sure they go out with the same timing.) So the performance has been recorded and can be played as often as you like. This is a reasonably fun trick, but there are two refinements that make this a whole lot more useful: multi-track recording and layering.

In traditional studio-based recording, rather than recording an entire piece in one go, it is possible to record parts of it individually and then mix them together. In extreme cases this could consist of recording, say, drums, vocals and each guitar part seperately. For groups of musicians with considerable amounts of 'creative friction' this has the inestimable advantage that you can record an entire album without any of the musicians ever having to be in the same room at the same time. Purists will note that this tends to destroy a certain amount of spontenaity, but it has less drastic applications: even if the piece is recorded as a single play through, recording each instrument on its own 'track' and mixing the results together later on in the production process will allow the balance of sound to be adjusted, and for effects to be applied selectively to individual players.

MIDI recording software typically allows a similar level of flexibility - it is possible to record two performances, one after the other, and then play the results out simultaneously. If applied without care this will of course lead to cacophony, but used judiciously it can enable a single player to build up a piece with several parts by playing one at a time.

The second big bonus that MIDI recording offers is the opportunity to fix your mistakes. Because MIDI recording software is essentially recording the time, pitch, volume and duration of each note played, it is possible to make the recording software able to change any of these facets of any note. So if you played a performance that was perfect apart from one tiny detail, you don't need to go back and play it again (or settle for an otherwise less good take), you can just fix the error leaving everything else intact. So you can tweak down the volume of the note you inadvertently hit a little hard, or adjust the entry you played a little late to be on time.

This can be taken to extremes - many MIDI recording programs have a feature to take an entire performance and correct any timing it considers to be a bit sloppy. (This feature is called 'quantization'.) The problem with this though is that it can make the music sound a little regimented, and will also destroy any intentional deviations from the 'correct' rhythm; a great deal of musical expression is achieved through taking slight liberties with the timing, a subtlety lost on such an 'enhancement' facility. And in fact such facilities are often of limited use in any case - if your playing is bad enough to need it, it's probably bad enough that the software will fail to guess what you were trying to play and end up making it worse.