Editing Amplitude modulation (section)

=== Analog telephony ===
A simple form of amplitude modulation is the transmission of speech signals from a traditional analog telephone set using a common battery local loop.<ref>{{cite book |author=AT&T Bell Laboratories Staff |editor=R.J. Rey |title=Engineering and Operations in the Bell System|edition=2 |location=Murray Hill, NJ|year=1984|page=211|publisher=AT&T Bell Laboratories|isbn=0-932764-04-5}}</ref> The direct current provided by the central office battery is a carrier with a frequency of 0&nbsp;Hz. 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.