Frequency Modulation

In communications, a type of modulation of a signal in which the frequency of the carrier is varied with changes in the value of the signal it is carrying. In broadcasting (such as radio broadcasting), a particular sound emanating from a radio station is converted into electrical energy, which is referred to as its signal. This signal wave is overlayed on top of a radio wave, called a carrier wave. In FM broadcasting, the frequency of the carrier wave conforms to the changes in the frequency of the signal (or sound) being transmitted. AM broadcasting, as we are all aware, is subject to static, which occurs at the top and bottom of a wave cycle. In contrast, frequency modulation keeps the amplitude of the carrier wave constant, thus the tops and bottoms of the signal wave can be eliminated, which is why FM radio generally remains static free.

In data communications, a similar process of modulation is used to overlay a signal representing computer information onto a carrier wave (which is transmitted over a telephone line, rather than through the air). Frequency modulation is a similar means of varying the frequency of the carrier signal in accordance with changes in the data signal.

See Modulation. See also Amplitude Modulation.

In halftone photography, the term frequency modulation is used to refer to a halftoning technique (also known as stochastic screening) in which the sizes of the halftone dots remain constant but theit distribution within an image is varied according to whether they correspond to shadows (lots of dots), middle tones (a moderate amount of dots), or highlights (few dots). See Halftone and Stochastic Screening.

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