Editing Carrier wave (section)

==Overview==
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The term ''carrier wave'' originated with radio. In a [[radio communication]] system, such as radio or television broadcasting, information is transmitted across space by [[radio wave]]s. At the sending end, the information, in the form of a modulation signal, is applied to an electronic device called a [[transmitter]]. In the transmitter, an [[electronic oscillator]] generates a sinusoidal [[alternating current]] of [[radio frequency]]; this is the carrier wave. The information signal is used to [[modulation|modulate]] 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 the [[radio receiver|receiver]]'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 called [[demodulation]].

Most radio systems in the 20th century used [[frequency modulation]] (FM) or [[amplitude modulation]] (AM) to add information to the carrier. The frequency [[spectrum]] 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 <math>f_C</math> with the modulation contained in narrow [[sideband]]s ''(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, in [[single-sideband modulation]] (SSB), the carrier is suppressed (and in some forms of SSB, eliminated). The carrier must be reintroduced at the receiver by a [[beat frequency oscillator]] (BFO).

Carriers are also widely used to transmit multiple information channels through a single cable or other [[communication channel|communication medium]] using the technique of [[frequency division multiplexing]] (FDM). For example, in a [[cable television]] system, hundreds of television channels are distributed to consumers through a single [[coaxial 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 by [[bandpass filter]]s using [[tuned circuit]]s so the television channel desired can be displayed. A similar technique called [[wavelength division multiplexing]] is used to transmit multiple channels of data through an [[optical fiber]] by modulating them on separate light carriers; light beams of different wavelengths.