MIDI, short for Musical Instrument Digital Interface, is the standard electronic language "spoken" between electronic instruments and the computerized devices which control them during performances. Developed in the early 1980s, this technology allows a keyboardist to kick off a drum synthesizer with one key or a computer to store a sequence of composed notes as a MIDI file, for example. The keyboard, drum synthesizer and computer all recognize the same set of binary code instructions.
Before the development of the MIDI system, professional keyboardists would often need to set up towering banks of synthesizers, pianos, organs and other electronics in order to perform live. They would go from instrument to instrument in order to produce the necessary sounds. With the introduction of MIDI, these same musicians could connect all of the peripheral keyboards together with 5-pin DIN cables and control them all through one master keyboard. A synthesizer set for background strings, for example, could "teach" another keyboard how to generate that sound through a MIDI connection.
This technology is not restricted to musical synthesizers, however. It is not unusual to find other stage equipment, such as lighting banks, under the control of MIDI-compatible computers. Each light may be assigned a specific channel and turned on or off according to a master program. These programs may also control effects pedals for guitarists or pre-recorded sequences to supplement the sound onstage.
MIDI files do not actually record the sound of the keyboard instrument, but rather record instructions on how to recreate that sound elsewhere. For instance, a keyboardist might play Beethoven's Moonlight Sonata on a MIDI-compatible synthesizer connected to a computer. The file would change each note into a series of 1s and 0s, similar to binary code language. The coding incorporates other aspects of the performance besides notes, including dynamics, note-bending and changes in key pressure.
If someone wanted to play that recorded version of the Moonlight Sonata on a different computer, the MIDI file would play exactly what the original keyboardist played on the original instrument. The sound reproduction qualities of the computer itself may present a problem. The computer's sound generation card might render a very weak-sounding version of the MIDI file, with some unpleasant electronic noises. Modern computers with advanced sound cards have eliminated many of these reproduction problems, but many people still associate MIDI files with a less-than-spectacular performance.
Because these files are relatively small and easy to produce, they have become very popular for use on websites, video game programs and MIDI-compatible cellular phones. The ring tones on many cellphones are actually MIDI files which reproduce the original tunes using the phone's own sound card.