I/O plugins provide an way to do pre- and post-processing outside the normal session. Pre-plugins run before Ardour does any processing, post-plugins run after Ardour has done all processing. The rationale is to provide a relatively lightweight pre-/post-processing workflow as compared to using busses.
A common use case is wet recording where a number of plugins are applied directly to the physical input. The processed signal then can be routed to any number of tracks or busses in Ardour. This is a lot like doing some of the processing with a chain of guitar pedals, then feeding the signal to an Aux In port on a mixing console or an input port on a multi-effects digital pedalboard.
New I/O plugins can be added in the I/O Plugins dialog (Window > I/O Plugins).
Right-clicking opens the same menu for plugin selection available for mixer channel strips:
Double-clicking opens the Plugin Selector dialog.
Once a plugin has been selected and added, it shows in either Pre- or Post-process section depending on your choice.
I/O plugins have the same user interface for setting input and outputs that is also available in mixer channel strips. The button above the plugin name opens a drop-down menu for quickly choosing an input port. The button below opens the drop-down menu for choosing the output port.
Additionally, new tracks automatically connected to an I/O plugin can be easily created in the Recorder window by clicking the + button and then setting a new for that track.
