Frequency modulation
The low price for a used unit (in the 1990s it fell to under $200) and partial analog design of the Poly-800 made it appealing for modification by hobbyists. There is a modification that adds two knobs to the VCF, increasing the filter's range and expressiveness, known as the Moog-Slayer filter modification; the FM-800 filter modification which adds a pseudo Frequency modulation synthesis (FM) control to the filter; a modification for external audio input; and some users have added a switch to control the filter slope, adjusting it from a 4 pole (24db/oct) to a 2 pole (12db/oct).