What is Digital Modulation ?

In digital modulation, an analog carrier signal is modulated by a digital bit stream. The aim of digital modulation is to transfer a digital bit stream over an analog passband channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.

These are the most fundamental digital modulation techniques:

·         In the case of PSK, a finite number of phases are used.
·         In the case of FSK, a finite number of frequencies are used.
·         In the case of ASK, a finite number of amplitudes are used.
·         In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.