MIDI

Designed in 1982/3 by music instrument manufacturers
A standardized and efficient means of conveying music performance information as electronic data
Information transmitted via MIDI messages

MIDI Controllers: convert performance gesture to MIDI output messages
MIDI Sequencers: allow for the capture, storage, editing, combination, and replay of MIDI messages
MIDI Synthesizers: receive input MIDI messages and respond with user-specified sounds

5-pin DIN connector (only three pins used)
MIDI input, output, and thru connections
MIDI input uses an opto-isolated circuit to avoid ground loop
unidirectional serial bit stream at 31250 bits per second
10 bits transmitted per byte (start bit, 8 data bits, and one stop bit in 320 microseconds)

A schematic of a MIDI (IN and OUT) interface (from http://www.borg.com/~jglatt/).

Message Type	Status Byte (Hex)	Message Name	Data Bytes	Data Byte 1	Data Byte 2
Channel	0x8c	Note Off	2	Note Number	Note Velocity
	0x9c	Note On	2	Note Number	Note Velocity
	0xAc	Aftertouch	2	Note Number	Key Pressure Value
	0xBc	Control Change	2	Controller Number	Control Value
	0xCc	Program Change	1	Program Number
	0xDc	Channel Pressure	1	Pressure Value (greatest of all depressed keys)
	0xEc	Pitch Bend	2	Value LSB	Value MSB
System Exclusive	0xF0	System Exclusive	n	Vendor ID	anything
System Common	0xF1	Time Code Qtr Frame	1	Time Code Value
	0xF2	Song Position	2	Position LSB	Position MSB
	0xF3	Song Select	1	Song Number
	0xF4	Undefined	0
	0xF5	Undefined	0
	0xF6	Tune Request	0
	0xF7	End of Sysex	0
System Real Time	0xF8	Timing Tick	0
	0xF9	Undefined	0
	0xFA	Start Song	0
	0xFB	Continue Song	0
	0xFC	Stop Song	0
	0xFD	Undefined	0
	0xFE	Active Sensing	0
	0xFF	System Reset	0

Channel Voice Messages:
- musical performance data
- Note On, Note Off, Aftertouch, Control Change, Program Change, Channel Pressure, Pitchbend
Channel Mode Messages:
- affect the way a receiving instrument will respond to Channel Voice messages
- MIDI controller numbers 120 - 127
- controller #120: all sound off
- controller #121: reset all controllers
- controller #122: enable/disable Local Control
- controller #123: all notes off
- controller #124-127: Omni Mode on/off, Mono Mode, or Poly Mode

System Exclusive:
- can be used to send more general or currently undefined data between devices
- manufacturers of MIDI equipment may define their own formats
- manufacturers are granted unique identification (ID) numbers which are included as part of the message
- manufacturers are required to publish the details of their System Exclusive data formats
- certain System Exclusive ID numbers are reserved for special protocols (MIDI Sample Dump Standard, MIDI Show Control, MIDI Machine Control)
System Common:
- intended for all receivers in the system
- quarter frame messages are sent periodically to keep track of the running SMPTE time
- it takes 8 quarter frame messages to specify the current SMPTE time
System Realtime:
- used to synchronize all of the MIDI clock-based equipment within a system, such as sequencers and drum machines
- a timing clock message is sent 24 times per quarter note
- to help ensure accurate timing, these one-byte messages are given priority over other messages
- may occur anywhere in the data stream (a Realtime message may appear between the status byte and data byte of some other MIDI message)

General MIDI is a specification for synthesizers that respond to MIDI messages. It was first created in 1991.
General MIDI devices produce a specified set of instrument sounds for the 128 program change numbers.
MIDI channel 10 is reserved for percussion instruments, with an agreed upon set of sounds selected by MIDI key number.