It is modulated by a microphone (transmitter) in the telephone set according to the acoustic signal from the speaker. The result is a varying amplitude direct current, whose AC-component is the speech signal extracted at the central office for transmission to another subscriber.OverviewAmplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting messages with a. In amplitude modulation, the instantaneous of. In and, is the variation of a property of a according to an information-bearing signal, such as an which represents sound, or a which.