how fsk is used in telegraphy?
2 Frequency shift keying. If a digital signal is used to modulate a carrier in frequency, this is known as frequency shift keying (FSK). It is used in voice frequency telegraph systems (Chittleburgh et al., 1957) and for wireless telegraphy in the h.f. band (Hills and Evans, 1973). ... It may be termed non-coherent FSK.