Some MIDI instruments, particularly those from Arturia, include an external input for processing audio signals through the synthesizer's filter, VCA, and effects modules. That's cool enough, but of course, what's even cooler is that is that you can then use MIDI to trigger filter and VCA envelopes, turn filter resonance up high and use a keyboard to "play" the filter frequencies as the audio goes through it, and moreā€”the only limit is the extent to which elements within the synthesizer can interact with the input signal.

Arturia's Mini V can also serve as a signal processor by feeding audio into the External Input. A volume control (highlighted in red) determines the level of the audio going through the synthesizer.