Analog Signals are continuous time varying signals where as digital signals are discrete time signals.
- Amplitude modulation (AM), in which the height (i.e., the strength or intensity) of the signal carrier is varied to represent the data being added to the signal.
- Frequency modulation (FM), in which the frequency of the carrier waveform is varied to reflect the frequency of the data.
- Phase modulation (PM), in which the frequency of the carrier waveform is varied to reflect changes in the frequency of the data (similar but not the same as FM).
- Polarization modulation, in which the angle of rotation of an optical carrier signal is varied to reflect transmitted data.
- Pulse-code modulation in which an analog signal is sampled to derive a data stream that is used to modulate a digital carrier signal.