In analog PM, the phase of the AC signalwave, also called the carrier, varies in a continuous manner. Thus, thereare infinitely many possible carrier phase states. When the instantaneous data inputwaveform has positive polarity, the carrier phase shifts in one direction; when theinstantaneous data input waveform has negative polarity, the carrier phase shifts in theopposite direction. At every instant in time, the extent of carrier-phase shift(the phase angle) is directly proportional to the extent to which the signalamplitude is positive or negative.
In digital PM, the carrier phase shifts abruptly, rather than continuouslyback and forth. The number of possible carrier phase states is usually a power of2. If there are only two possible phase states, the mode is called biphasemodulation. In more complex modes, there can be four, eight, or more differentphase states. Each phase angle (that is, each shift from one phase state to another)represents a specific digital input data state.
Phase modulation is similar in practice to frequency modulation (FM). When theinstantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, theinstantaneous phase changes. But PM and FM are not exactly equivalent, especially inanalog applications. When an FM receiver is used to demodulate a PM signal, or whenan FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear;that is, phase and frequency do not vary in direct proportion.