<P> According to one definition of digital signal, the modulated signal is a digital signal . According to another definition, the modulation is a form of digital - to - analog conversion . Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission . </P> <P> The most fundamental digital modulation techniques are based on keying: </P> <Ul> <Li> PSK (phase - shift keying): a finite number of phases are used . </Li> <Li> FSK (frequency - shift keying): a finite number of frequencies are used . </Li> <Li> ASK (amplitude - shift keying): a finite number of amplitudes are used . </Li> <Li> QAM (quadrature amplitude modulation): a finite number of at least two phases and at least two amplitudes are used . </Li> </Ul> <Li> PSK (phase - shift keying): a finite number of phases are used . </Li>

A bit that can change over time is called a