tags: mus-407 midi

MIDI Message Format

A [midi] message consists of one or more [byte]s in the following format:

• a status byte followed by zero, one, or two data bytes

• status bytes begin with a 1, data bytes begin with a 0

• 1 byte = 8 bits

• 1 [bit] = smallest digital unit, corresponding to zero or one ("off" or "on")

• a UART chip (Universal Asynchronous Receiver/Transmitter) receives and processes binary signal into usable data

• MIDI messages sent at 31,250 bits/sec

  • exact division of 1 mHz to facilitate synchronization with early CPUs

Example

Scenario: pressing and releasing a key on a MIDI keyboard controller

How many MIDI messages are generated? Two.

• A note-on message when we press the key
• A note-off message when we release the key

How many bytes are generated? Six. Each message contains three bytes:

• A status byte specifying message type and channel
• A data byte specifying note number
• A data byte specifying note velocity

Binary vs. Decimal Data

Every MIDI message starts with exactly one status byte followed by an arbitrary amount of data bytes.

Example:

• A MIDI Note On message starts with a status byte indicating that it is a 'Note On' message, followed by additional metadata such as key number, velocity, etc.

Note-On Message, Deconstructed

10010010

Breaking up into sections, starting from left:
1 -> indicates status byte
001 -> message type, 8 possibilities (001 = note on)
0010 -> message channel, 16 possibilities: 0010: channel 3 (0-indexed)

00111100
Note number: 60 (decimal)

01100101
Note velocity: 101 (decimal)

Sources

• MUS 407 MIDI