Text: A process of modulating or varying the amplitude of a radio frequency alternating current to correspond to any wave form of speech vibrations as in radio broadcasting. The system involves the introduction of additional power into the circuit of the radio frequency generator until the desired wave form variations are obtained. This is done particularly in the case of a vacuum tube transmitter, in which case the voltage supplied to the plate of the tube is altered (increased or decreased) to accomplish the purpose.

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Source: 190