Diffusion is the amount of smear that is introduced as time progresses. It’s essentially a dense field of delays created by allpass filters connected in series. Because they are allpass filters the higher frequencies tend to get delayed, while lower frequencies don’t (this is due to the phase shift at the centre frequency).
This is what the manual says about diffusion.
0 to 1.0. The amount of diffusion. A value of 1.0
yields maximum diffusion or recirculation in the
allpass filter. A value of zero yields no diffusion.
This is what the manual says about the diffusion size
0 to 1.0. The size control scales the delays of all
of the allpass filters. A value of 1.0 gives the largest
size and a value of zero gives the smallest.
I hope this helps.
