Editing Amplitude modulation (section)

{{short description|Electronic method of transmitting information with a carrier wave}}
{{Use dmy dates|date=October 2018}}

[[File:Amfm3-en-de.gif|thumb|right|250px|An audio signal (top) carried by a [[carrier signal]] using amplitude modulation (middle) and frequency modulation (bottom).|alt=Animation of audio, AM and FM modulated carriers.]]
{{Modulation techniques}}

'''Amplitude modulation''' ('''AM''') is a  [[signal modulation]] technique used in electronic communication, most commonly for transmitting messages with a [[radio wave]]. In amplitude modulation, the instantaneous [[amplitude]] of the wave is varied in proportion to that of the message signal, such as an [[audio signal]].<ref name=S>{{Cite book|page=85|title=Machines and Inventions|first=Robert H.|last=Smith|publisher=Time Life|year=1993|location=Alexandria, VA|isbn=0-8094-9704-2}}</ref> This technique contrasts with [[angle modulation]], in which either the [[frequency]] of the carrier wave is varied, as in [[frequency modulation]],<ref name=S/> or its [[Phase (waves)|phase]], as in [[phase modulation]].

AM was the earliest modulation method used for transmitting audio in radio broadcasting. It was developed during the first quarter of the 20th century beginning with [[Roberto Landell de Moura]] and [[Reginald Fessenden]]'s [[radiotelephone]] experiments in 1900.<ref>{{cite web|url=http://www.aminharadio.com/radio/files/Artigo-Revista-PCP-USA.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.aminharadio.com/radio/files/Artigo-Revista-PCP-USA.pdf |archive-date=2022-10-09 |url-status=live|title=Father Landell de Moura : Radio Broadcasting Pioneer : FABIO S. FLOSI : UNICAMP – University of Campinas, State of São Paulo|website=Aminharadio.com|access-date=15 July 2018}}</ref> This original form of AM is sometimes called '''double-sideband amplitude modulation''' ('''DSBAM'''), because the standard method produces sidebands on either side of the carrier frequency. [[Single-sideband modulation]] uses bandpass filters to eliminate one of the sidebands and possibly the carrier signal, which improves the ratio of message power to total transmission [[Power (physics)|power]], reduces power handling requirements of line repeaters, and permits better bandwidth utilization of the transmission medium.

AM remains in use in many forms of communication in addition to [[AM broadcasting]]: [[shortwave radio]], [[amateur radio]], [[two-way radio]]s, [[Airband|VHF aircraft radio]], [[citizens band radio]], and in computer [[modem]]s in the form of [[quadrature amplitude modulation]] (QAM).