modulation | communications | Britannica.com
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`Modulation
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`COMMUNICATIONS
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`The Editors of
`Encyclopædia
`Britannica
`
`MWRITTEN BY:
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`odulation, in electronics, technique for
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`impressing information (voice, music,
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`picture, or data) on a radio-frequency
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`carrier wave by varying one or more characteristics
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`of the wave in accordance with the intelligence
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`signal. There are various forms of modulation, each
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`designed to alter a particular characteristic of the
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`carrier wave. The most commonly altered
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`characteristics include amplitude, frequency, phase,
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`pulse sequence, and pulse duration.
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`AMPLITUDE MODULATION.
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`In amplitude modulation (AM), auditory or visual
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`information is impressed on a carrier wave by
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`varying the amplitude of the carrier to match the
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`fluctuations in the audio or video signal being
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`transmitted. AM is the oldest method of
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`broadcasting radio programs. Commercial AM
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`stations operate at frequencies spaced 10 kHz apart
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`between 535 and 1,605 kHz. Radio waves in this
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`frequency range are effectively reflected back to the
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`Earth’s surface by the ionosphere and can be
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`detected by receivers hundreds of miles away. In
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`multiplexing
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`carrier wave
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`telecommunication
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`data transmission
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`Thomas Alva Edison
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`amplitude modulation (AM)
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`information processing
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`frequency modulation
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`addition to its use in commercial radiobroadcasting, AM is employed in long-distance
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`shortwave radio broadcasts and in transmitting the video portion of television programs.
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`FREQUENCY MODULATION.
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`In frequency modulation (FM), unlike AM, the amplitude of the carrier is kept constant,
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`but its frequency is altered in accordance with variations in the audio signal being sent.
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`This form of modulation was developed by the American electrical engineer Edwin H.
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`Armstrong during the early 1930s in an effort to overcome interference and noise that
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`affect AM radio reception. FM is less susceptible than is AM to certain kinds of
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`interference, such as that caused by thunderstorms as well as random electrical currents
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`from machinery and other related sources. These noise-producing signals affect the
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`amplitude of a radio wave but not its frequency, and so an FM signal remains virtually
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`unchanged.
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`modulation | communications | Britannica.com
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`telecommunication:
`Modulation
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`FM is better adapted than is AM to the transmission of stereophonic sound,
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`audio signals for television programs, and long-distance telephone calls by
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`microwave radio relay. Commercial FM broadcasting stations are assigned
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`higher frequencies than are AM stations. The assigned frequencies, spaced
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`200 kHz apart, range from 88 to 108 MHz.
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`PHASE MODULATION.
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`The phase of a carrier wave is varied in response to
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`the vibrations of the sound source in phase
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`modulation (PM). This form of modulation is often
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`considered a variation of FM. The two processes are
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`closely related because phase cannot be changed
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`without also varying frequency, and vice versa. Also,
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`the rate at which the phase of a carrier changes is
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`directly proportional to the frequency of the audio
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`signal.
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`Like FM, PM minimizes various types of interference
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`to broadcast reception at frequencies below 30
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`MHz. The two techniques are commonly used
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`together. FM cannot be applied during the
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`amplification of a sound signal in broadcasting, and
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`so PM is used instead. PM is also utilized in some microwave radio relays and in certain
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`kinds of telegraphic and data-processing systems. Other important applications of PM
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`include communications between mobile radio units employed by the police and
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`military.
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`PULSE-CODED MODULATION.
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`In pulse-coded modulation (PCM), the intelligence signal converts the carrier into a series
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`of constant-amplitude pulses spaced in such a manner that the desired intelligence is
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`contained in coded form. Continuous signals, such as voice messages, television pictures,
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`and computer data, are commonly transformed into the Baudot Code or its variations,
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`which are composed of patterns of 5 or 7 “on” and “off” pulses. PCM minimizes
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`transmission losses and eliminates noise and interference problems because the
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`receiving unit need only detect and identify simple pulse patterns. Moreover, the pulses,
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`unlike continuous signals, can be regenerated electronically by repeater stations along
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`the transmission route with virtually no distortion.
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`PCM, invented by H.A. Reeves of the United States in 1939, is employed by many
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`communications companies and organizations, including Comsat and Intelsat, for
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`telegraph, telephone, and television transmission. The technique has proved especially
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`useful for the exchange of digital information between computer terminals.
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`PULSE-DURATION MODULATION.
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`Another kind of pulse modulation is pulse-duration modulation (PDM), in
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`which intelligence is represented by the length and order of regularly
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`modulation | communications | Britannica.com
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`recurring pulses. A familiar example of PDM is the International Morse Code,
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`used in ship-to-shore communications, amateur radio, and certain other
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`forms of radiotelegraphy.
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`PDM was devised by the American
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`physicist Raymond A. Heising in 1924. Besides its use
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`in telegraphic communications by means of
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`microwave radio relay systems, its chief application
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`is telemetering.
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`LEARN MORE in these related articles:
`
`telecommunication:
`
`Modulation
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`science and practice of
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`transmitting information
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`by electromagnetic
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`centres on the problems involved in transmitting large
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`picked up by a receiver tuned to this
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`channel. The process by which the
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`information is attached to the carrier
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`wave is modulation. Modulated carriers are isolated in their
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`separate slots or channels; if transmitters are geographically
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`Modulators and demodulators
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`A carrier wave is a radio-frequency wave that carries information. The information is attached to the carrier
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`wave by means of a modulation process that involves the variation of one of the carrier-frequency
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`characteristics, such as its amplitude, its frequency, or its duration. (All of these processes are discussed in
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`greater detail in the article telecommunication system.)
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`modulation | communications | Britannica.com
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`Assorted References
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`·
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`·
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`major reference (in telecommunication: Modulation)
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`radio (in radio technology: Modulators and demodulators) (in radio technology: Tuned circuits and the
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`superheterodyne principle)
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`telemetry (in telemetry: Transmission.)
`
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