When modulating analog signals, all analog signal values of the useful signal are modulated continuously. Since digital signals are not continuous but jump from one characteristic state to another, they are referred to as shift 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): In frequency shift ke ying, 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): With phase shift keying, two phase positions of the carrier frequency are assigned to the two digital states "0" and "1", 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.
All modulation techniques discussed so far operate at 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 work with several carrier frequencies; these are called multicarrier techniques.