DIGITAL METHODS
A form of modulation which uses a digital data signal to modulate a sine-wave carrier is called digital modulation. The three types mainly used are amplitude-shift keying (ASK), frequency-shift keying (FSK) and phase-shift-keying (PSK). They correspond approximately to AM, FM, and PM and are used especially in data communication systems.
ASK : On/off keying is the earliest modulation method. A continuous radio frequency wave (CW) is interrupted in a recognizable pattern (Morse code). To provide audibility the carrier is heterodyned with a beat frequency oscillator (BFO) in the receiver. The use of a modulated continuous wave (MCW) eliminates the need of a BFO but the bandwidth of the signal is increased. The problem with on/off ...
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