via this auction
"The organ sound uses FM synthesis which is the same technology that Yamaha incorporated later into the DX7. It has 3 voices total. The other two voices are pure analog. One is a string synth similar to the Arp String synth, the other voice is an analog synth with adjustable cut-off, attack, decay, etc.. All three voices can be combined, you can control the volume of each voice, and they can be routed through the Leslie simulator, which is quite realistic. The SK15's are very similar to the SK 20's with the exception of the multi-pin Leslie connector on back of the SK20. Unlike other SK synthesizers, this SK15 lets you control the Leslie speeds with a foot control (included) via 1/4 inch jack on the back panel (a very useful modification). The leslie ramps up or down, gradually increasing or decreasing speed. It's a great effect to use on crescendos, or on choruses of songs... The organ section gives you the option, of percussion, sustain, and vibrato."
