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Amplitude Modulation or AMThis system of modulation has been used since the beginning of radio and, of all systems, is most extensively used today. It is defined2 as "modulation in which the amplitude of a wave is the characteristic subject to variation." An amplitude-modulated wave is defined2 as follows: "An amplitude-modulated sinusoidal wave is one whose envelope contains a component similar to the wave form of the signal to be transmitted." Amplitude-modulated waves were discussed in Chapter 11. By the process of amplitude modulation, two sidebands are created, and each of these contains, independently of the other, all the information of the original modulating signal to be transmitted. These sidebands are created by simultaneously impressing the carrier wave and the modulating signal wave on a modulating circuit that distorts the waves and produces, because of the distortion, sum and difference frequencies. This has been summarized by Everitt who, in discussing amplitude modulation, has written,3 "Modulation is obtained whenever a component of the output wave is made proportional to the product of two input waves." If a radio-program signal, which covers an audio-frequency band of approximately 100 to 5000 cycles, is used to modulate a carrier-frequency wave of 1,000,000 cycles, the modulated wave, as transmitted by a standard amplitude-modulation broadcast station, will consist of three components: the carrier wave of 1,000,000 cycles; the upper sideband extending from 1,000,100 to 1,005,000 cycles; and the lower sideband extending from 999,900 to 995,000 cycles. As explained for carrier telephone systems, sometimes the carrier and one sideband are suppressed in radio systems, but this is not done (at present, 1949) in broadcasting. Amplitude-modulated radio transmitters are of two general types, those employing high-level modulation, and those employing low-level modulation. High-level modulation is defined2 as "modulation produced at a point in a system where the power level approximates that at the output of the system." Low-level modulation is defined2 as "modulation produced at a point in a system where the power level is low compared to the power level at the output of the system."
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Last Update: 2011-05-30