A simple form of amplitude modulation is the transmission of speech signals from the traditional analog telephone set using a common battery local loop. The direct current provided by the central office battery is a carrier with a frequency of 0 Hz, that is modulated by a microphone ("transmitter") in the telephone set according to the acoustic signal from the mouth of 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.