UK company Mutronics has the Mutator, a stereo analog filter and envelope follower. The two, separate voltage controlled filters similar to those found in analog synthesizers are used to process external sound sources. Each filter can be controlled by either its own LFO (that's Low Frequency Oscillator) and/or its envelope follower. The envelope follower "follows" the dynamic contour of the input source signal and supplies a corresponding controlling voltage to the filter. You can also elect to use another sound source such as a drum sample, guitar or microphone as the envelope follower's input. In this way, one sound's dynamics is superimposed tonally over another sound. There is also gate mode where a specific threshold level can be set above which the envelope follower starts working.

There are four different LFO sweep waveforms and you may link the two filters for stereo panning effects. You can buy the Mutator with or without MIDI and you can retrofit the unit later on with MIDI when you want MIDI. With MIDI, additional control is possible "on top" of the local hardware controls. I.E. MIDI controller 1 or Modulation Wheel controls filter resonance on top of the front panel resonance knob setting. There is a LFO re-trigger function that restarts the LFO from a MIDI note for in-time, sequencer-controlled stereo filter effects. The MIDI pitch value controls the cut-off frequency while MIDI note on and MIDI note off information operates the envelope.