Modulation is the addition of information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform -- constant height (amplitude) and frequency. Information can be added to the carrier by varying its amplitude, frequency, phase, polarization (for optical signals), and even quantum-level phenomena like spin.
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Modulation is usually applied to electromagnetic signals -- radio, lasers/optics and computer networks. Modulation can even be applied to direct current (which can be treated as a degenerate carrier wave with amplitude 0 and frequency 0) mainly by turning it on and off (as in Morse code telegraphy), or applied to alternating current (as with power-line networking).
Common modulation methods include the following:
- 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.
A computer modem allows a computer to connect to another computer or to a data network over a regular analog phone line by using the data signal to modulate an analog audio tone. A modem at the far end demodulates the audio signal to recover the data stream. A cable modem uses network data to modulate the cable service carrier signal.
Sometimes a carrier signal can carry more than one modulating information stream. A process called multiplexing combines the streams onto a single carrier -- e.g., by encoding a fixed-duration segment of one, then of the next, for example, cycling through all channels before returning to the first, a process called time-division multiplexing (TDM).
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