Frequency Shift Keying(FSK) The use of {frequency modulation} to transmit digital data, i.e. two different {carrier} frequencies are used to represent zero and one. FSK was originally used to transmit {teleprinter} messages by radio ({RTTY}) but can be used for most other types of radio and land-line digital telegraphy. More than two frequencies can be used to increase transmission rates. (1997-07-14)