sentences:  Yamaha's later FS1r was the most advanced of their FM synthesizers, adding 8-operator synthesis and formant synthesis (hence FS), but it was a rack-mounted module and never received much attention, and so it was discontinued rapidly, with the result that units today are both hard to find and expensive.Some subsequent products by Yamaha included FM to certain extents.The CS6x, Motif, and Motif ES lines can perform FM using the PLG150DX expansion cards, which enable monotimbral voicing with up to 16 notes of polyphony per card.