MIDI (Musical Instrument Digital Interface)

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MIDI, which stands for Musical Instrument Digital Interface, is a connectivity standard for transmitting digital instrument data.  It is mainly utilised by computers, electronic keyboards and synthesisers.  In addition, it is supported by other instruments like beat boxes, electronic drums, and even digital stringed instruments, like guitars and violins.

MIDI is a protocol designed for recording and playing back music on digital synthesisers, allowed for by many different types of personal computer sound cards.  It was originally designed to control one keyboard by using another, but  was quickly adapted for the personal computer.  At its most basic, MIDI sends data about how music is created.  The command set contains note-ons, note-offs, key velocity, pitch bend, and many other methods of controlling a synthesiser.  The sound waves created are already saved in a wavetable in the receiving instrument or sound card.

With a program that offers this interface, you can produce music using a standard keyboard, or some other input device.  You can play your MIDI creation with the same program (or with another program), as long as you have a sound card to function as a music synthesizer.  The MIDI program may have a graphical user interface that looks like a sound studio control room.  Many sound cards come as a package with MIDI software.

MIDI data contains different kinds of information.  When a single key on a synthesiser is pressed, it sends the note played, the velocity (or how firm the note is pressed), and how long the note is held.  If several notes are played at once, the MIDI data is sent for all the notes simultaneously.  Other pieces information that may be transmitted over a MIDI connection include the instrument ID, sustain pedal timings, and controller data, like pitch bend and vibrato.

When a synthesiser is connected to a computer through a MIDI connection, the notes played can be recorded by Digital Audio Workstation (DAW) software in MIDI format.  The MIDI information can be played back by transmitting the recorded MIDI notes to the keyboard, which outputs them as audio samples, such as piano or strings.  Most DAW software supports MIDI editing, which allows you to adjust the timing and velocity of individual notes, change their pitch, and add / delete notes.  MIDI information is often displayed in a digital format, with lines representing each note played.  Many programs can translate MIDI information to a musical score.

A MIDI recording contains both instrument data and the notes played.  The actual sound is played back using samples from real instruments.  By changing the output instrument, a MIDI track for piano can be played back with a guitar sound, or vice versa.

Before, MIDI connections utilized MIDI cables connected to a 5-pin MIDI port on each device.  Nowadays, most of the MIDI devices have standard computer interface linking mechanisms, like USB or Thunderbolt ports.  These modern interfaces offer more bandwidth than traditional MIDI ports, which in turn allows more tracks with more data to be sent at the same time.

The MIDI protocol utilises eight-bit serial transmission with one start bit and one stop bit.  It has a 31.25 Kbs data rate, and is asynchronous.  Connection is made through a five-pin DIN plug, whereby three pins are in use at any given time.

General MIDI

The MIDI standard outlines 128 General MIDI (GM) instruments, which already exist on most computers as software instruments.  The sound of each GM instrument may vary between different computers and keyboards because the samples used for the instruments may be different, but the GM instrument ID is consistent across devices.  For example, GM instrument #1 is assigned to an acoustic piano, whereas #20 is assigned to a church organ, and #61 is assigned to a French horn.  Modern synthesisers include hundreds—or even thousands—of other instruments that can be chosen, most of which offer more genuine sound than the original, standard GM options.