• Each MIDI connections can communicate of 16 channels (0-15)
• MIDI devices can switch channels they send messages over.
• Devices listening for message can selectively listen on specific channels, or all channels at once.

• MIDI messages are composed of several bytes. Just how many depends on the type of messages
• The first byte is called the Status Byte and it has two parts, the message type and the channel information. See also https://midi.org/summary-of-midi-1-0-messages
  • Message info is the first four bits
  • Channel info is the four least significant bits
  • The top most bit is always set. This identifies the byte as the one containing the message type and channel info (ie, status)
• The subsequent bytes contain the message data.
  • The top bit is unset.

These messages will be 4 bytes. There is an extra header byte that comes first which contains the cable and command.

• Send message bytes using Hex notation, prefixed with 0x. For example, 00111100 (60) is sent as 0x3c.
• See also https://kb.iu.edu/d/afdl

The first byte is the status indicating message type (1001 = 9/Note On) and channel (0100 = 4/midi channel 5)

• Top bit is always set
• Message type: 9 = Note On
• Channel: 4 (Channel 5)
• This means the Note On message is in the range of 9..24 (eg, 9 + the channel 0 - 15)

In order to determine if the status byte is On or Off, we just need to look at the first 4 bits. We can mask out the last four bits with a bitwise & operator.

if ((statusByte & 0xF0) == 0x90) { // ON message }
if ((statusByte & 0xF0) == 0x80) { // OFF message }

1. 0xF0 in binary is 11110000.
2. The & operation is performed between these two numbers. For example, if the status byte is 10000011 (0x83 in hex, which is a Note Off message on channel 4): 10000011 (status) & 11110000 (0xF0) is 10000000 (0x80). This operation effectively “masks” the lower 4 bits (which represent the channel) and keeps only the upper 4 bits (which represent the message type). By comparing the result to 0x80 or 0x90 (10000000 or 10010000), this checks for note Off or On, respectively.

The next two bytes are note and velocity data:

• Top bits are unset indicating these are the message values
• 127 is therefore the maximum value

Other than sending a note off message (0x80), another common way is to send an On message for a given note with value of 0

The message structure is Status byte, LSB, MSB

1. Status byte: Always 0xE0 to 0xEF (224 to 239), where the lower 4 bits represent the MIDI channel (0-15).
2. LSB (Least Significant Byte): The lower 7 bits of the pitch bend value.
3. MSB (Most Significant Byte): The upper 7 bits of the pitch bend value.

The point here is that the pitch bend value is a 14-bit number, ranging from 0 to 16383, where 8192 represents no pitch bend. In other words you get higher resolution than just 0-127

See also https://www.instructables.com/Send-and-Receive-MIDI-with-Arduino/ for a good example.

https://ccrma.stanford.edu/~craig/articles/linuxmidi/

https://www.music.mcgill.ca/~gary/rtmidi/

• https://github.com/FortySevenEffects/arduino_midi_library
• https://github.com/arduino-libraries/MIDIUSB

• https://www.kieranreck.co.uk/MIDI-6N137-vs-6N138-vs-6N139/

Note the 220ohm resistor is optional and included as a safety precaution against connecting miswired cables and whatnot (eg, current flowing into the TX pin).

https://github.com/gbevin/SendMIDI Send MIDI from the command line.

https://github.com/surfacepatterns/midisnoop Receive and print MIDI messages. Use this in conjunction with sendMIDI to try things out. Uses Alsa or JACK midi drivers. See also Mixxx, Ardour, JACK and MIDI