
"This modification to the Poly-800 series of synthesizers adds the ability to control Filter Cutoff with DCO1. This is commonly known as FM control of a filter. Although it's technically not Frequency Modulation, the results are capable of sounding very much like an FM synthesizer. A knob allows the effect to be turned from completely off, to far beyond the normal capabilities of a Poly-800. This modification very nicely compliments the Moog Slayer modification for a total of 3 knobs full of fun."
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FM800 filter mod