In communications, a type of modulation of a signal in which the amplitude 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 AM broadcasting, the amplitude (or height) of the carrier wave conforms to the changes in amplitude 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). Amplitude modulation is a similar means of varying the amplitude of the carrier signal in accordance with changes in the data signal.
In halftone photography, the term amplitude modulation is used to refer to a halftoning technique (the conventional form of halftone screening) in which the sizes of the halftone dots are varied according to whether they correspond to shadows (large dots), middle tones (medium-sized dots), or highlights (small dots). An alternate means of halftone screening is known as stochastic screening, or FM screening. See Halftone and Stochastic Screening.