MIDI information is transmitted as messages, a string of 8-bit bytes. There are a number of different types of MIDI messages in various lengths. All messages start with a status byte, and some messages have data bytes following the status byte.

The status byte tells the receiving device what type of message is being transmitted (note on, note off, etc.) and which channel the message is intended for. MIDI supports 16 channels, and each channel can represent a different instrument (piano, flute, guitar, etc.). The bits of the status byte are defined as follows:

ý D7: always 1

ý D6-D4: message type (3 bits = 8 message types)

ý D0-D3: channel number (4 bits = 16 channel numbers)

Following the status byte are usually data bytes, although some messages consist of only the status byte. For instance, the message that tells the instrument to turn on a note is followed by two data bytes. The first byte is the note number (0-128), and the second byte is the velocity, which usually translates into volume. Data bytes always have the most significant bit (D7) set to 0.

MIDI provides a number of messages in addition to note on/note off. These include messages to change the patch (which instrument is synthesized when a note is played), messages to turn all notes off, and so on.

MIDI also provides for messages that are exclusive to a specific instrument, such as a keyboard that has features not supported by the generic MIDI commands. These features can be exercised with the special MIDI commands.

Some MIDI devices can send the contents of their internal memories to a computer via the MIDI port. This permits the user to retain important data and restore the state of the instrument if the internal battery fails or if someone programs something they shouldnýt.

One problem with MIDI is a lack of standardization. Selecting patch 7 on one synthesizer may select an electric guitar sound, while on another synthesizer the same patch may produce trumpet sounds. General MIDI was introduced as a means of standardizing at least some of these so that a particular note would sound the same on all synthesizers. The quality of the reproduction, of course, depends on the quality of the synthesizer wavetable.

MIDI is a powerful interface for controlling electronic music, and it is not surprising that most electronic instruments support it.