Modulation is the process of converting data into radio waves by adding information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform -- constant height, or 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.
Modulation is usually applied to electromagnetic signals: radio waves, lasers/optics and computer networks. Modulation can even be applied to a direct current -- which can be treated as a degenerate carrier wave with a fixed amplitude and frequency of 0 Hz -- mainly by turning it on and off, as in Morse code telegraphy or a digital current loop interface. The special case of no carrier -- a response message indicating an attached device is no longer connected to a remote system -- is called baseband modulation.
Modulation can also be applied to a low-frequency alternating current -- 50-60 Hz -- as with powerline networking.
Types of modulation
There are many common modulation methods, including the following -- a very incomplete list:
- 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 phase of the carrier waveform is varied to reflect changes in the frequency of the data. In PM, the frequency is unchanged while the phase is changed relative to the base carrier frequency. It is similar to 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.
- Quadrature amplitude modulation (QAM), which uses two AM carriers to encode two or more bits in a single transmission.
Radio and television broadcasts and satellite radio typically use AM or FM. Most short-range two-way radios -- up to tens of miles -- use FM, while longer-range two-way radios -- up to hundreds or thousands of miles -- typically employ a mode known as single sideband (SSB).
More complex forms of modulation include phase-shift keying (PSK) and QAM. Modern Wi-Fi modulation uses a combination of PSK and QAM64 or QAM256 to encode multiple bits of information into each transmitted symbol.
Modulation and demodulation
Modulation is the process of encoding information in a transmitted signal, while demodulation is the process of extracting information from the transmitted signal. Many factors influence how faithfully the extracted information replicates the original input information. Electromagnetic interference can degrade signals and make the original signal impossible to extract. Demodulators typically include multiple stages of amplification and filtering in order to eliminate interference.
A device that performs both modulation and demodulation is called a modem -- a name created by combining the first letters of MOdulator and DEModulator.
A computer audio 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. 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 the channels before returning to the first -- a process called time-division multiplexing (TDM). Another form is frequency-division multiplexing (FDM), where multiple carriers of different frequencies are used on the same medium.
Why use modulation?
Multiple carriers of different frequencies can often be transmitted over a single media, with each carrier being modulated by an independent signal. For example, Wi-Fi uses individual channels to simultaneously transmit data to and from multiple clients.
In another form, wavelength-division multiplexing (WDM) modulates multiple laser wavelengths/frequencies on long-haul fiber links to increase the total available bandwidth.