When modulating analog signals, all analog signal values of the useful signal are modulated continuously. Since digital signals are not value-continuous, but jump from one characteristic state to another, this is referred to as keying rather than modulation. All keying methods are based on the three modulation methods mentioned above: Amplitude modulation, frequency modulation and phase modulation.
Amplitude shift keying(ASK): In amplitude shift keying, the carrier frequency is modulated with two different amplitudes. One amplitude corresponds to the logical "0", the other to the logical "1". The carrier frequency remains constant with this method. With on-off ke ying( OOK), the carrier signal is switched on at digital "1" and the carrier frequency is switched off at "0".
Frequency shift keying(FSK): With frequency shift keying, different frequencies are assigned to the two digital values. The digital value "0" is keyed with the carrier frequency "f1", the digital value "1" with the carrier frequency "f2". This means that depending on the state of the useful signal ("1" or "0"), a different frequency is transmitted. This type of modulation switches between two frequencies. The amplitude of the carrier frequencies is constant.
Phase shift keying(PSK): In phase shift keying, the two digital states "0" and "1" are assigned two phase positions of the carrier frequency, for example 0° for the signal with the state "0" and 180° for the signal with the state "1". During phase shift keying, the carrier frequency and amplitude are constant.
In the aforementioned shift keying methods, a carrier oscillation is modulated with only two characteristic states. This results in a relatively large bandwidth requirement and a relatively low data rate. In order to make better use of the bandwidth and increase the speed, various methods have been developed in which different phase states are used simultaneously and several retiming methods are combined. One example of four-phase shift keying is quadrature phase shift keying( QPSK), and one for the combination of amplitude shift keying and phase shift keying is quadrature amplitude modulation( QAM).
All modulation techniques discussed so far operate with one carrier frequency and are therefore also referred to as single-carrier modulation. In addition, there are modulation techniques such as Orthogonal Frequency Division Multiplex( OFDM) that operate with multiple carrier frequencies; these are called multicarrier techniques.