Does anyone know how to slow down or speed up an echo, so that, for instance, I could put a snare drum file in the track and set it so that it repeats at a certain rate, then slows down or speeds up.

Sort of like dub bands, and The Orb do. I want to set the parameters though, so that it does it automatically rather than having to do it manually every time.

Hopefully that makes some sort of sense?