2. Introduction to MIDI

The Musical Instrument Digital Interface (MIDI) is a protocol that allows electronic musical instruments, computers and other devices to communicate with one another. It was standardized in 1983 so that electronic musical instruments of the era could communicate with one another between manufacturers. It was a relatively simple protocol designed to communicate aspects of a typical musical performance: notation, pitch, velocity and more.

Forty-three years later every operating system still understands it, no drivers required.

MIDI controllers

A device that's capable of sending MIDI data to another MIDI-capable device is called a MIDI controller. A simple example to consider would be a simple, piano-style keyboard like this:

But they can vary widely in appearance and modes of interaction. Buttons are certainly common, but you might also find some that incorporate dials:

Others use more abstract and interesting modes of interaction, including mapping motion or breath to MIDI signals. For example, this controller uses three accelerometers to map hand gestures to MIDI messages:

It turns out they're not terribly difficult to build and you can find a lot of home-brewed MIDI controllers out in the wild. They can get much more elaborate in a hurry:

Some can be downright bananas:

Everything on the table today is a MIDI controller, and none of them know anything about music.

Anatomy of a MIDI message

When a MIDI controller "speaks" to another MIDI-capable device or computer they are sending and receiving MIDI messages with one another. The protocol underlying this communication is quite simple in practice but a little verbose when explained. Still, I'll try.

90status: note on, channel 1

3Cdata 1: note 60, middle C

7Fdata 2: velocity 127

Every MIDI message consists of 3 bytes of 8 bits (0–255)

Represented in binary, a message might look like this:

10010000 | 00111100 | 01111111

There are only 2 types of bytes: status and data

Every message consists of 1 status byte and 2 data bytes. A status byte will always begin with the number 1 and data bytes with the number 0.

1x0010000 | 0x0111100 | 0x1111111
^status     ^data1      ^data2

For data bytes that leaves 7 bits to express the data in that byte. That gives us an integer range of 0-127.

For status bytes, the next 3 bits after the first describe the type of status message while the remaining 4 bits describe the channel. To break down our binary representation:

1x001x0000

The channel messages. Each status value is for channel 1; add the channel number 0 to 15 for the rest.

Status	Name	Data 1	Data 2	What it is for
128	Note Off	note	release velocity	A key came up. Velocity is how fast; most devices send 0.
144	Note On	note	velocity	A key went down. Velocity 0 counts as Note Off. Buttons and pads send this.
160	Polyphonic aftertouch	note	pressure	Pressure on one held key. Rare.
176	Control Change	controller	value	A knob, slider, pedal or sensor moved. Most of what is not a button.
192	Program Change	program	none	Pick a sound or preset. Two bytes only.
208	Channel aftertouch	pressure	none	Pressure on the whole keyboard. Two bytes only.
224	Pitch Bend	low 7 bits	high 7 bits	The bend wheel, 14 bits of resolution.

System messages have no channel. They start at 240.

Status	Name	What it is for
240	SysEx	Any length, ends with 247. Whatever the manufacturer wants: Launchpad colours, firmware updates.
248	Clock	24 ticks per quarter note, so devices can play in time.
250	Start	Play from the top.
251	Continue	Play from where you stopped.
252	Stop	Stop.
254	Active Sensing	A heartbeat every 300 ms so a receiver knows the cable is still there.
255	Reset	Everything back to power-on state.

The whole thing fits on one page: the MIDI Association's Summary of MIDI 1.0 Messages. The MIDI 1.0 specification is the long version.

In Processing we seldom have to process these binary representations directly. Java's built-in javax.sound.midi hands us the three numbers split up. The helper tabs in this workshop turn them into stick.x, mf.pressed(3) or pad.justPressed(x, y). In p5.js the Web MIDI API gives us an array:

[144, 60, 127]

If you're working with existing musical hardware it's helpful to have this deeper understanding of how and why the messages are structured the way they are. It's helpful to know that receiving a 144 in your first byte means a note is being turned on in the first channel and that a 128 would indicate that a note is being turned off.

However, if we're building non-musical experiences and creating our own hardware these numbers can be repurposed to represent whatever you want!

Buttons send notes, everything else sends CCs

A button is a note on when pressed, a note off when released. A joystick axis, a slider, a knob or a light sensor is a control change, 0 to 127. The Explorer shows which numbers your device uses. Each device page lists them.

The escape hatch: SysEx

One message type can be any length. System Exclusive starts with F0, names a manufacturer, carries any number of 7-bit bytes, and ends with F7. The Launchpad's RGB colours and scrolling text are SysEx. Browsers ask a separate permission before a page may send it. Processing sends it with a SysexMessage and never asks. The Launchpad's text command is taken apart in the Launchpad Marquee notes.