AU Instrument Plugins

An AU (Audio Unit) Instrument is an AU Plugin that can convert MIDI messages to audio. You can select an AU Instrument by clicking the instrument slot's down arrow. The AU Instrument plugins appear in the list's Plugins section.

AU instrument windows have a Channel selector (top right). This is not only the channel used for recording, as with the built-in instruments, it is also the channel on which MIDI data is sent to the plugin. Some plugins respond to all MIDI channels in the same way, in which case the channel setting does not matter. Sometimes you can set a plugin to Omni mode to achieve this. Other plugins, especially multitimbral plugins that can produce a different sound per channel, require that the Channel setting be the same as the channel used by the plugin.

AU Instruments respond to all the streams in the track's MIDI file. However, in most cases, it will be easier to use multiple tracks (each using its own instance of the AU Instrument) instead.

MIDI 2.0 / MPE

AU Instruments receive per-note pitch bend, per-note brightness, and poly aftertouch via MPE. Switching to MPE happens automatically when per-note pitch bend or per-note brightness is sent to the plugin. MPE will not be used if you've set the per-note pitch bend range to zero.
Plugins may have an MPE mode setting that needs to be set manually.

In macOS 12, AU plugins that support the MIDI 2.0 protocol will receive MIDI 2.0 instead of MPE.

MIDI output from the plugin is converted from MPE to MIDI 2.0. You can avoid this by setting the per-note pitch bend range to zero. In this case, MPE to MIDI 2.0 conversion will only occur if the plugin sends MPE configuration messages (RPN 6).