The Undo and Redo buttons at the top of the FabFilter Pro-L 2 interface enable you to
easily undo changes you made to the plug-in. With the A/B feature, you can quickly switch
between two different states of the plug-in.
The Undo button at the left will undo the last change. Every change to
the plug-in (such as dragging a knob or selecting a new preset) creates a new state in the
undo history. The Undo button steps back through the history to restore the previous states
of the plug-in.
The Redo button cancels the last undo command. It steps forward through
the history until you are back at the most recent plug-in state.
The A/B button switches from A to B and back. Before switching, the current state of
the plug-in is saved, so if you click this button twice, you are back at the first state. The
button highlights the currently selected state (A or B). In the example above, the A state is
The Copy button copies the active state to the inactive state. This marks the
current state of the plug-in and allows you to go back to it easily with the A/B button. After
clicking Copy, the button disables itself to show that both states are equal, so there is
nothing to copy anymore.
If the plug-in parameters are changed without using the plug-in interface, for example with
MIDI or automation, no new undo states are recorded.
The Undo and Redo buttons will disable themselves if there is nothing to undo or redo.