MIDI (or Musical Instrument Digital Interface) is a method of representing musical concepts in a form suitable for use in computers. MIDI defines 16 different channels, along which messages are passed to instruments or synthesizers that understand the MIDI protocol; notes are played by sending appropriately crafted NoteOn messages that are followed by NoteOff messages. MIDI channels can be manipulated with special controller messages to alter the pitch of instruments, or their volume or timbre, and they can also tell the instrument or synthesizer what sound to play using Program Change and Bank Select messages.

Typically Program Change and Bank Select messages are collectively referred to by the singular term Patch Change.

Key features of Ardour MIDI handling

• MIDI, just like audio, exists in regions. MIDI regions behave like audio regions: they can be moved, trimmed, copied (cloned), or deleted. Ardour allows either editing MIDI (or audio) regions, or MIDI region content (the notes), but never both at the same time. The e key (by default) sets Internal Edit Mode, which allows the editing of MIDI data in a given region.
• All MIDI I/O is done either by ALSA or JACK (depending on which backend was chosen when starting Ardour), for sample accurate timing and maximal efficiency when communicating with external software synthesizers.
• Every MIDI track has its own JACK MIDI port for input; it may have an arbitrary combination of audio and MIDI outputs, depending on the signal processing in the track; the full flexibility of JACK connectivity is present for MIDI just as it is for audio.
• Full automation for MIDI tracks, integrated with the handling of all MIDI CC data for each track.
• Controllers (CC data) can be set to discrete or continuous modes; the latter will linearly interpolate between control points and send additional data.

Notable differences compared to other DAWs and sequencers

• Fader (volume) control currently operates on transmitted MIDI data, not by sending CC #7.
• All note/data editing is per-region. There are no cross-region operations at this time.
• By default, copying a MIDI region creates a deep link—both regions share the same data source, and edits to the contents of one will affect the other. Breaking this link is done by selecting MIDI > Unlink from other copies from the region context menu, after which the selected region(s) will have their own copies of only the data that they visually display on screen. The region will no longer be trimmable back to its original length after an Unlink operation, and the operation cannot be undone.