MIDI (Musical Instrument
Digital Interface) is the standard format used
for communication of digital messages to & from electronic keyboards, MIDI
sequencers, sound modules, computers, and many other MIDI devices.
Sound Module
A sound module generates musical notes in
response to digital messages received through a MIDI cable. The source of
the messages can be from a MIDI instrument such as a keyboard, or from a
sequencer with pre-recorded MIDI data. The sound module may have hundreds of different instrument sounds
(called voices) to choose from and can play 16 selected voices
simultaneously (one on each of the 16 MIDI channels).
MIDI Sequencer
A MIDI sequencer, (usually a computer with MIDI software) , is used to
record, edit, and play back MIDI messages. While playing a part on the
keyboard the sound module plays the selected voice and at the same
time the sequencer records the MIDI messages. Recorded parts can be
played back while an additional
part is being recorded. A musical
arrangement can be created by recording one part at a time, each in a
different voice and on a different MIDI channel.
MIDI messages
-
Program change # : Tells
the sound module which voice to use for note messages received on a
given channel.
-
Note # :
Designates a particular note
-
Note on : Tells the sound
module to play the designated note.
-
Velocity: Tells how hard
the note was struck on the keyboard.
-
Note off : Tells when a
designated note is released.
-
Controller messages affect all
concurrent notes on the same channel as the message. examples are: volume,
modulation and pitch bend.
Applications
The block diagram below shows an example of
a MIDI system.
Most MIDI devices have connections for
three MIDI cables ( in, out, & thru). Messages originating from a
device are transmitted at MIDI OUT. Messages received at MIDI IN are repeated (re-transmitted) at MIDI THRU. This arrangement allows multiple
devices to be connected together in daisy chain fashion. In the diagram
above the translator converts the two formats (MIDI & USB) in both
directions. Messages are looped back in the
computer, so the keyboard receives at MIDI IN the same thing that it
transmits plus whatever is transmitted by the computer, and
repeats all at MIDI THRU to the sound module.
The diagram below shows a multi track
recorder connected to the MIDI chain. The recorder receives MTC (Midi
Time Code) messages transmitted form the computer when the sequencer is
in play mode. This allows the recorder to stay in sync with the
sequencer so that parts played one at a time from the sequencer and
recorded on separate tracks of the recorder will be in time with each
other. This is the MIDI system currently in use at TexasTrax.
Recording parts on the computer first,
instead of directly to the recorder, allows convenient editing in the
MIDI sequencing program. Editing may include correcting
wrong notes, adjusting timing of notes (too early / too late),
adjusting velocity of notes (too hard / too soft), and adding
controller messages. After all parts of an arrangement are recorded and
edited they can be played from the computer and recorded as a stereo mix
on two tracks of the recorder, or soloed one part at a time from
the computer and recorded on separate tracks of the recorder. The later
method allows more flexibility on final mix down which is especially
desirable after vocal tracks have been added on the recorder.
Note: Modern MIDI computer programs allow
vocal parts to be recorded directly to the computer's hard drive, and
mixed with the MIDI tracks.
For those who would like to acquire more
extensive knowledge of MIDI, a good place to start is The MIDI Manual
by David Miles Huber, published by SAMS a division of Prentice Hall
Computer Publishing.
