The Motor Synth MkII from Gamechanger Audio is, by some margin, the most original synthesizer design I've seen (and heard!) in some time. It's a desktop unit that generates audio waveforms via its eight variable-speed internal drone motors – that's "drone" as in UAS/UAV/quadcopter. As its eight brushless DC motors spin inside the MkII’s sturdy metal enclosure, they deliver two separate voices with three types of optically generated waveshapes each (sine, saw, and square), plus a fourth "electromagnetic inductive" waveshape that uses eight magnetic pickups instead of the optical sensors (similar to a tonewheel organ). A third digital voice is available (DCO), providing a total of three voices that can be mixed in five different octave ranges each.
There's analog filtering, Drive, and digital modulation, as you'd find on many synths, and a healthy breadth of up to seven envelope types for amplitude, filters, or modulation. The DCO uses a separate digital multimode filter that conveniently shares the same controls as the analog filter section with the press of a button on the well-designed front panel. You can feed external audio into the synth as a carrier signal to trigger the built-in sequencer, act as an envelope follower, or use the Motor Synth MkII as a vocoder (Gamechanger advises using a preamplified mic for the vocoder input).
Nerdy bits: The sine, saw, and square waves are generated by an infrared optocoupler pair that reads three distinct graphical patterns imprinted on reflective optical discs mounted to the drone motor shafts. These patterns, located on the bottom side of the discs, represent sine, saw, and square waveforms. Higher pitches are created as the motors and discs spin faster. The Motor Synth MkII has transparent windows on the front and back, allowing you to observe the eight motor discs in action as they respond to your playing and patch changes in real time – much like the hot rod with the transparent hood in the original Grease film! The MkII is capable of playing notes either in unison (four motors per note per voice) or playing up to four notes of paraphony (there is a single analog filter stage per voice).
The often shrill but harmonic sounds that the eight mechanical motors produce (not the synth output, mind you – I'm talking about the sound of the instrument’s mechanical workings in the room) are fascinating enough that I found myself frequently recording them with contact mics. It's like a tiny, reactive, spinning choir of birds.