Intelecommunications, acarrier wave,carrier signal, or justcarrier, is a periodicwaveform (usuallysinusoidal) that conveys information through a process calledmodulation. One or more of the wave's properties, such as amplitude or frequency, are modified by an information bearing signal, called themessage signal ormodulation signal. Thecarrier frequency is usually much higher than the message signalfrequency; this is because it is usually impractical to transmit signals with low frequencies over long distances (due to attenuation).
The purpose of the carrier is usually either to transmit the information through space as anelectromagnetic wave (as inradio communication), or to allow several carriers at different frequencies to share a common physical transmission medium byfrequency division multiplexing (as in acable television system).
The term originated in radio communication, where the carrier wave creates the waves which carry the information (modulation) through the air from the transmitter to the receiver. The term is also used for an unmodulatedemission in the absence of any modulating signal.[1]
Inmusic production, carrier signals can be controlled by a modulating signal to change the sound property of an audio recording and add a sense of depth and movement.[2]
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The termcarrier wave originated with radio. In aradio communication system, such as radio or television broadcasting, information is transmitted across space byradio waves. At the sending end, the information, in the form of a modulation signal, is applied to an electronic device called atransmitter. In the transmitter, anelectronic oscillator generates a sinusoidalalternating current ofradio frequency; this is the carrier wave. The information signal is used tomodulate the carrier wave, altering some aspects of the carrier, to impress the information on the wave. The alternating current is amplified and applied to the transmitter's antenna, radiating radio waves that carry the information to thereceiver's location. At the receiver, the radio waves strike the receiver's antenna, inducing a tiny oscillating current in it, which is applied to the receiver. In the receiver, the modulation signal is extracted from the modulated carrier wave, a process calleddemodulation.
Most radio systems in the 20th century usedfrequency modulation (FM) oramplitude modulation (AM) to add information to the carrier. The frequencyspectrum of a modulated AM or FM signal from a radio transmitter is shown above. It consists of a strong component(C) at the carrier frequency with the modulation contained in narrowsidebands(SB) above and below the carrier frequency. The frequency of a radio or television station is considered to be the carrier frequency. However the carrier itself is not useful in transmitting the information, so the energy in the carrier component is a waste of transmitter power. Therefore, in many modern modulation methods, the carrier is not transmitted. For example, insingle-sideband modulation (SSB), the carrier is suppressed (and in some forms of SSB, eliminated). The carrier must be reintroduced at the receiver by abeat frequency oscillator (BFO).
Carriers are also widely used to transmit multiple information channels through a single cable or othercommunication medium using the technique offrequency division multiplexing (FDM). For example, in acable television system, hundreds of television channels are distributed to consumers through a singlecoaxial cable, by modulating each television channel on a carrier wave of a different frequency, then sending all the carriers through the cable. At the receiver, the individual channels can be separated bybandpass filters usingtuned circuits so the television channel desired can be displayed. A similar technique calledwavelength division multiplexing is used to transmit multiple channels of data through anoptical fiber by modulating them on separate light carriers; light beams of different wavelengths.
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The information in a modulated radio signal is contained in thesidebands while the power in the carrier frequency component does not transmit information itself, so newer forms of radio communication (such asspread spectrum andultra-wideband), andOFDM which is widely used inWi-Fi networks,digital television, anddigital audio broadcasting (DAB) do not use a conventional sinusoidal carrier wave.
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Carrier leakage is interference caused bycrosstalk or a DC offset. It is present as an unmodulated sine wave within the signal's bandwidth, whose amplitude is independent of the signal's amplitude. Seefrequency mixers.
The dictionary definition ofcarrier wave at Wiktionary
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