96
Viscount
Physis Organs
Reference Manual
14.5 WHAT MIDI IS
The MIDI interface (Musical Instrument Digital Interface) allows instruments of different makes and kinds
to communicate with each other, using this very specific protocol of codes. This allows the creation of
systems of MIDI instruments, offering much greater versatility and control than is possible with single
instruments. To make this communication possible, all MIDI instruments have two or three 5-pin DIN
connectors called:
-
MIDI IN:
The connector through which the instrument receives the MIDI data transmitted by other
units.
-
MIDI OUT:
The connector through which the instrument sends the MIDI data it has generated to
other units.
-
MIDI THRU:
This connector, not always provided on all instruments, is used for connecting several
units in series, since it transmits the MIDI data exactly as they are received by the respective MIDI IN
port.
Most instruments equipped with MIDI interface transmit MIDI messages which specify, for example, which
note has been played and with what dynamic, by means of the MIDI OUT connector. If this connector is
connected to the MIDI IN connector of another MIDI instrument, such as an expander, the second instrument
will respond exactly to the notes played on the transmitter instrument.
The same type of information transfer is used for recording MIDI sequences. A computer or a sequencer
can be used to record the MIDI data generated by the transmitter instrument. If these recorded data are sent
back to the instrument, it automatically repeats the recorded performance.
MIDI is able to transmit a multitude of digital data by means of just one cable, and thus just one connector.
this is thanks to the MIDI channels. There are 16 MIDI channels, and in a similar way as for radio
communications in which two stations can only communicate if they are tuned to the same frequency (or
channel), two MIDI instruments connected together are only able to communicate if the transmitter instrument
channel is the same as the receiver instrument channel.
MIDI messages subdivide into channel messages and system messages. The following is a short description
of these messages:
CHANNEL MESSAGES
NOTE ON
This message is transmitted when a note is depressed on the keyboard. Each Note On message contains the
following information:
Note On
: when a key has been struck;
Note Number
: the key which has been pressed, and therefore the relative note played;
Velocity
: note dynamic (i.e. the force applied when the key was struck).
Note messages are expressed as a number from 0 to 127, with middle C represented by number 60.
NOTE OFF
This message is transmitted when a key struck previously is released.
When it is received, the sound of the note relating to the key is switched off. Each Note On message contains
the following information:
Note Off
: a key has been released;
Note Number
: which key has been released;
Velocity
: dynamic (i.e. how fast the note was released).
N.B.:
A Note On message with Velocity=0 is considered equivalent to a Note Off message.
