Originally introduced as a key component of analog synthesizers, envelope controlled filters came into prominence in the early 1970s with the release of the legendary Mutron III. Envelope filters are wah-wah type sliding filters that are triggered by an input voltage. Unlike standard wah pedals whose center frequency is controlled by moving a treadle, an envelope filter's center frequency is controlled by playing dynamics, allowing for an almost unlimited variety of effects based on expressive nuances in a player's performance.
AF-9 Auto Filter page at Maxon 's website...