Ardour's Strict I/O mode enforces matching channel counts through the plugin chain; Flexible I/O allows width changes

Ardour offers two I/O modes for each track. The golden rule of signal flow is that ‘the number of outputs of one link of the process chain defines the number of inputs of the next, until the panner.’ Under Flexible I/O this rule runs free: a mono signal can become stereo after a stereo plugin, and the chain stays stereo from there on. Under Strict I/O, ‘plugins have the same number of inputs as they have outputs,’ so adding a plugin never changes the strip’s channel count. Strict I/O prevents accidental mono-to-stereo widening inside a track’s processing chain, which matters when you want to control stereo width deliberately. Flexible I/O is useful when layering plugins that expand the signal, such as stereo effects on a mono source. (MIDI synths are an exception: they add audio outputs even under Strict I/O.)

Examples

Strict I/O: a mono vocal track stays mono even with a stereo reverb inserted. Flexible I/O: a mono drum track becomes stereo after a stereo enhancer plugin, enabling independent L/R processing downstream.

Assessment

Explain what happens when a mono track in Strict I/O mode has a stereo plugin inserted. Then explain the same scenario in Flexible I/O mode and describe one practical reason to prefer each.