Gran FDN uses a fairly primitive style of artificial reverberation — an Feedback Delay Network — to create strange, reverberant spaces.
A feedback delay network uses delay lines, configured in a recursive feedback matrix, to produce rapid echo density. It’s a bit like a cross between a reverb and a multi-tap delay.
Instead of delay lines, Gran FDN uses granular modules in a 4×4 matrix. Each granular module feeds into itself and the other granular modules.
Wet and dry each have their own levels.
The “reverb” controls are all highly interactive. Size is a combination of grain size and position — think of it as pre-delay and reverb size rolled into one.
Texture controls the texture of the granular modules — as it is increased, the gain will decrease but the echoes will become less distinct.
Feedback-decay controls the decay of the reverb, but so many controls affect gain that it can oscillate in some settings — adjust carefully.
Diffusion controls the gain of the diffuser modules — at its lowest setting, the multi-tap nature of the algorithm becomes more apparent.
Pitch 1 and pitch 4 control the speed/pitch of the first (the shortest) and fourth (the longest) granular modules in the matrix respectively. Adjust one or the other for shimmer or sub-octave effects, or have them fight with one another, pitching up and down respectively.
The damping controls are two-fold: along with allowing you to control the tone of the reverb, they also act as anti-aliasing filters, to reduce some of the frequencies granular modules seem to produce when fed back on themselves.
The left footswitch controls a freeze, which circulates randomly between the granular modules. The middle stompswitch is its tap tempo (it also responds to MIDI clock).
The right stompswitch maxes the feedback-decay, producing oscillations (in some settings) and long decays (in other settings).