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 velocity (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: velocity (i.e. how fast the note was released).
Program Change
This message is used to select the programs or sounds of the receiver instrument.
There is also a specific standard called General MIDI which describes which sound should be recalled for each
Program Change received. This association is usually described by means of a table included in the user manual
of the instrument which adopts the standard.
This message contains the following information:
Program Change: voice or program change;
Program Change Number: the number of the program or voice to be activated;
Control Change
These are control messages (often associated to trimmers or pedals) used to add expression to the performance,
allowing you to set (and control in real time if necessary) voice parameters such as volume (CC n.7) or the position
of the swell pedals (CC n.11), etc.
This message contains the following information:
Control Change: a controller has been adjusted
Controller Number: which controller has been adjusted
Controller Position: the position of the controller
SYSTEM MESSAGES
System Exclusive
These messages can only be interpreted by an instrument made by the same producer as the transmitter device
(in some cases only by the same model). They mainly relate to the instrument’s sound generation and
programming parameters. The organ uses these messages to control all the internal parameters and for switching
the voices on and off.
Real Time
The Real Time messages also include the Active Sensing code, sent to keep the dialogue between two MIDI
instruments alive. When the receiver instrument does not receive any MIDI data or the Active Sensing code in a
time interval of about 300 milliseconds, it considers the MIDI connection to have been deactivated, so it switches
off any notes still active. Remember that the transmission and reception of this message is optional, so not all
instruments are equipped to handle it.
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