Abbreviation for Smart FSK (see WFTD archive FSK). Smart FSK was adopted a few years after FSK and MIDI had been in common use. Professionals were all switching over to SMPTE based systems which were much more reliable and accurate than the old FSK based ones, but the cost of SMPTE readers and generators was extremely high at the time, and units that translated it to MIDI were yet another separate big expense. So a few companies, most notably at the time were TASCAM (MTS30) and JL Cooper (PPS1), adopted Smart FSK which added the ability to include location information to the FSK signal. These boxes, which translated between MIDI and SFSK and back again, were much less expensive than the SMPTE based ones and they still allowed users the luxury of starting from within a song and having their sequencer locate to the proper spot to begin playback. Also, because the system was continually able to “locate” itself the common problem of FSK devices getting one or two beats behind (due to drop outs and glitches) went away. Nowadays SFSK isn’t used much because SMPTE and MTC based systems are so inexpensive, as well as being even more versatile and reliable.