Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
7
When you have your system properly connected and configured, you’re ready to go.
Now you can “arm” the sequencer for recording, press a key on the controller, and record that
note in the sequencer. Most likely pressing that key doesn’t even make a sound, since we
haven’t yet told the sound where to go. Your controller may look like a piano, but it’s really just
an input device sending a digital message to your computer in some agreed upon format. This is
the purpose of the MIDI transmission protocol. In order for the two devices to communicate, the
connection between them must be designed to transmit MIDI messages. For example, the cable
could be USB at the computer end and have dual 5-pin DIN connections at the keyboard end, as
shown in Figure 6.6. The message that is received by your MIDI sequencer is in a prescribed
MIDI format. In the sequencer, you can save this and any subsequent messages into a file. You
can also play the messages back and, depending on your settings, the notes upon playback can
sound like any instrument you choose from a wide variety of choices. It is the synthesizer that
turns the symbolic MIDI message into digital audio data and sends the data to the sound card to
be played.
Figure 6.6 5-pin DIN connection for MIDI
MIDI Data Compared to Digital Audio 6.1.3
Consider how MIDI data differs from digital audio as described in Chapter 5. You aren’t
recording something through a microphone. MIDI keyboards don’t necessarily make a sound
when you strike a key on a piano keyboard-like controller, and they don’t function as stand-alone
instruments. There’s no sampling and quantization going on at all. Instead, the controller is
engineered to know that when a certain key is played, a symbolic message should be sent
detailing the performance information.
In the case of a key press, the MIDI message would convey the occurrence of a Note On,
followed by the note pitch played (e.g., middle C)
and the velocity with which it was struck. The
MIDI message generated by this action is only
three bytes long. It’s essentially just three
numbers, as shown in Figure 6.7. The first
byte is a value between 144 and 159. The
second and third bytes are values between 0 and 127. The fact that the first value is between 144
and 159 is what makes it identifiable by the receiver as a Note On message, and the fact that it is
a Note On message is what identifies the next two bytes as the specific note and velocity. A
Note Off message is handled similarly, with the first byte identifying the message as Note Off
and the second and third giving note and velocity information (which can be used to control note
decay).
Aside: Many systems interpret a Note On
message with velocity 0 as “note off” and use
this as an alternative to the Note Off
message.
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