The musical Instrument Digital
Interface (MIDI) specifies the standard
hardware and data format that allow
devices to communicate with one
another. Developed by researchers in
Japan, MIDI Specification 1.0 made its
debut in 1983. Soon afterwards, Roland
introduced the MPU-401 (MIDI
Processor Unit model 401), which gave
PC users the power of MIDI. By providing
an affordable way to interconnect
musical devices, MIDI opened the
door to the music world for computer
users.
Communications
Intercommunication between
devices is achieved through messages.
Each message consists of a status byte
and a stream of data bytes sent serially
over an asynchronous communications
line. Status bytes have their most significant
bit (MSB) set, while data bytes
have their MSB reset. In MIDI communications,
only messages are transmitted,
not the actual sounds. Messages tell
a device to turn a note on, turn a note
off, change the pitch, and so forth. Fig.
1 shows how MIDI messages are structured.
Four bits of the channel status byte
are used to decode the channel for a
total of 16 channels that may be individually
accessed. A MIDI device may
be assigned to one or more channels.
Fig. 2 shows the meaning of each bit in
the channel and system status bytes,
The input and output of a MIDI
device are labeled MIDI IN and MIDI
OUT, respectively. Some devices have
a MIDI THRU port which replicates
the MIDI IN signal for output and
allows multiple devices to be chained
together.
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