Frequency Shift Keying (FSK) has until recent years been the most widely
used form of digital modulation, being simple both to generate and to detect, and
also being insensitive to amplitude fluctuations in the channel. FSK conveys the data
using distinct carrier frequencies to represent symbol states. An important property of
FSK is that the amplitude of the modulated wave is constant.
Consider the case of unfiltered binary FSK shown here. This waveform can
be viewed as two separate ASK symbol streams summed prior to transmission.