Abbreviation for Frequency Shift Keying. An audio tone (frequency) modulated by a square wave, which is used both for data transfer and also for sequencer and drum machine synchronization. FSK is the sound that you hear your fax or modem making as it establishes communication. In the early days of electronic music, before MIDI (yes, I was there), drum machines or sequencers were synchronized to each other or to a tape machine via this method. Back then the only information transmitted was a rate which was interpreted as tempo by the machines. There was no location information included so the song always had to be started from the very beginning in order to achieve proper sync. If there was any drop out or glitch along the way one had to go back and start at the very beginning of the song to reestablish sync. It was cumbersome and unreliable to say the least, and that is why formats such as SFSK, DTL, SMPTE, and MTC were later adopted.