Using MIDI with RtMidi

Although the MIDI protocol is quite old and has several drawbacks, it is still widely used and is appropriate for many applications. Read the MIDI section in the Computer Music Basics for a deeper introduction.

The development system used in this class relies on the RtMidi framework. This allows the inclusion of any ALSA MIDI device on Linux systems and hence any USB MIDI device. The RtMidi Tutorial gives a thorough introduction to the use of the library.

ALSA MIDI

The Advanced Linux Sound Architecture (ALSA) makes audio- and MIDI interfaces accessible for software. As an API it is part of the Linux kernel. Other frameworks, like JACK or Pulseaudio work on a higher level and rely on ALSA.

Finding your ALSA MIDI Devices

After connecting a MIDI device to an USB port, it should be available via ALSA. All ALSA MIDI devices can be listed with the following shell command:

$ amidi -l

The output of this request can look as follows:

Dir     Device        Name
IO      hw:1 ,0 ,0   nanoKONTROL MIDI 1
IO      hw:2 ,0 ,0   PCR-1 MIDI 1
I       hw:2 ,0 ,1   PCR-1 MIDI 2

In this case, two USB MIDI devices are connected. They can be addressed by their MIDI device ID (hw:0/1).

The MIDI Tester Example

The MIDI tester example can be used to print all incoming MIDI messages to the console. This can be helpful for reverse-engineering MIDI devices to figure out their controller numbers.

The MIDI Manager Class

The MIDI Manager class introduced in this test example is used as a template for following examples which use MIDI. For receiving messages, RtMidi offers a queued MIDI input and a user callback mode. In the latter case, each incoming message triggers a callback function. For the queued mode, as used here, incoming messages are collected until retrieved by an additional process.

The midiMessage struct is used to store incoming messages. It holds the three standard MIDI message bytes plus a Boolean for the processing state.

/// struct for holding a MIDI message
typedef struct  {
    int byte1             = -1;
    int byte2             = -1;
    double byte3          = -1;
    bool hasBeenProcessed = false;

}midiMessage;