FSK

from The Free On-line Dictionary of Computing (8 July 2008)

Frequency Shift Keying
FSK

   <communications> (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)