Editing Amplitude modulation (section)

==History==
[[Image:Telefunken arc radiotelephone.jpg|thumb|One of the crude pre-vacuum tube AM transmitters, a Telefunken [[arc converter|arc transmitter]] from 1906. The carrier wave is generated by 6 electric arcs in the vertical tubes, connected to a [[tuned circuit]]. Modulation is done by the large carbon microphone ''(cone shape)'' in the antenna lead. ]]
[[Image:Meissner radiotelephone transmitter.jpg|thumb|One of the first [[vacuum tube]] AM radio transmitters, built by Meissner in 1913 with an early triode tube by Robert von Lieben. He used it in a historic {{convert|36|km|mi|abbr=on}} voice transmission from Berlin to Nauen, Germany. Compare its small size with the arc transmitter above. ]]

Amplitude modulation was used in experiments of multiplex telegraph and telephone transmission in the late 1800s.<ref name="Bray">{{cite book
 | last = Bray
 | first = John
 | title = Innovation and the Communications Revolution: From the Victorian Pioneers to Broadband Internet
 | publisher = Inst. of Electrical Engineers
 | year = 2002
 | pages = 59, 61–62
 | url = https://books.google.com/books?id=3h7R36Y0yFUC&pg=PA61
 | isbn = 0852962185}}</ref> However, the practical development of this technology is identified with the period between 1900 and 1920 of [[radiotelephone]] transmission, that is, the effort to send audio signals by radio waves. The first radio transmitters, called [[spark gap transmitter]]s, transmitted information by [[wireless telegraphy]], using pulses of the carrier wave to spell out text messages in [[Morse code]]. They could not transmit audio because the carrier consisted of strings of [[damped wave]]s, pulses of radio waves that declined to zero, and sounded like a buzz in receivers. In effect they were already amplitude modulated.

===Continuous waves===
The first AM transmission was made by Canadian-born American researcher [[Reginald Fessenden]]<ref name="t786">{{cite web | title=Reginald Fessenden (U.S. National Park Service) | website=NPS.gov Homepage (U.S. National Park Service) | date=1932-07-22 | url=https://www.nps.gov/people/reginaldfessenden.htm | access-date=2024-12-16}}</ref> on December 23, 1900<ref name="t779">{{cite web | last=Reel | first=Monte | title=Island Is Birthplace of Broadcast | website=Washington Post | date=2000-12-17 | url=https://www.washingtonpost.com/archive/local/2000/12/17/island-is-birthplace-of-broadcast/61d6fb5e-ecd2-4db4-b9ba-8b89e5c167b0/ | access-date=2024-12-16}}</ref> using a spark gap transmitter with a specially designed high frequency 10&nbsp;kHz [[induction coil|interrupter]],<ref name="AGARD_1992">{{cite report |author=Advisory Group for Research and Development (AGARD) |date=October 2, 1992 |title=ELF/VLF/LF Radio Propagation and Systems Aspects |url=https://apps.dtic.mil/sti/tr/pdf/ADA267991.pdf |publisher=North Atlantic Treaty Organization (NATO) |access-date=2024-12-16}}</ref> over a distance of {{convert|1|mi|km|spell=in}} at Cobb Island, Maryland, US. His first transmitted words were, "Hello. One, two, three, four. Is it snowing where you are, Mr. Thiessen?".<ref name="t779" /> Though his words were "perfectly intelligible", the spark created a loud and unpleasant noise.<ref name="AGARD_1992" />

Fessenden was a significant figure in the development of AM radio. He was one of the first researchers to realize, from experiments like the above, that the existing technology for producing radio waves, the spark transmitter, was not usable for amplitude modulation, and that a new kind of transmitter, one that produced [[sinusoidal]] ''[[continuous wave]]s'', was needed. This was a radical idea at the time, because experts believed the impulsive spark was necessary to produce radio frequency waves, and Fessenden was ridiculed. He invented and helped develop one of the first continuous wave transmitters – the [[Alexanderson alternator]], with which he made what is considered the first AM public entertainment broadcast on Christmas Eve, 1906. He also discovered the principle on which AM is based, [[heterodyning]], and invented one of the first [[detector (radio)|detector]]s able to [[rectifier|rectify]] and receive AM, the [[electrolytic detector]] or "liquid baretter", in 1902. Other radio detectors invented for wireless telegraphy, such as the [[Fleming valve]] (1904) and the [[crystal detector]] (1906) also proved able to rectify AM signals, so the technological hurdle was generating AM waves; receiving them was not a problem.

===Early technologies===
Early experiments in AM radio transmission, conducted by Fessenden, [[Valdemar Poulsen]], [[Ernst Ruhmer]], [[Quirino Majorana]], [[Charles Herrold]], and [[Lee de Forest]], were hampered by the lack of a technology for [[amplifier|amplification]]. The first practical continuous wave AM [[transmitter]]s were based on either the huge, expensive [[Alexanderson alternator]], developed 1906–1910, or versions of the [[Poulsen arc]] transmitter (arc converter), invented in 1903. The modifications necessary to transmit AM were clumsy and resulted in very low quality audio. Modulation was usually accomplished by a [[carbon microphone]] inserted directly in the antenna or ground wire; its varying resistance varied the current to the antenna. The limited power handling ability of the microphone severely limited the power of the first radiotelephones; many of the microphones were water-cooled.

===Vacuum tubes===
The 1912 discovery of the amplifying ability of the [[Audion tube]], invented in 1906 by [[Lee de Forest]], solved these problems. The vacuum tube [[feedback oscillator]], invented in 1912 by [[Edwin Armstrong]] and [[Alexander Meissner]], was a cheap source of [[continuous wave]]s and could be easily [[modulated]] to make an AM transmitter. Modulation did not have to be done at the output but could be applied to the signal before the final amplifier tube, so the microphone or other audio source didn't have to modulate a high-power radio signal. Wartime research greatly advanced the art of AM modulation, and after the war the availability of cheap tubes sparked a great increase in the number of radio stations experimenting with AM transmission of news or music. The vacuum tube was responsible for the rise of [[AM broadcasting]] around 1920, the first electronic [[mass communication]] medium. Amplitude modulation was virtually the only type used for [[radio broadcasting]] until [[FM broadcasting]] began after World War II.

At the same time as AM radio began, [[telephone companies]] such as [[AT&T Corporation|AT&T]] were developing the other large application for AM: sending multiple telephone calls through a single wire by modulating them on separate [[carrier signal|carrier]] frequencies, called ''[[frequency division multiplexing]]''.<ref name="Bray" />

===Single-sideband===
In 1915, [[John Renshaw Carson]] formulated the first mathematical description of amplitude modulation, showing that a signal and carrier frequency combined in a nonlinear device creates a sideband on both sides of the carrier frequency. Passing the modulated signal through another nonlinear device can extract the original baseband signal.<ref name="Bray" /> His analysis also showed that only one sideband was necessary to transmit the audio signal, and Carson patented [[single-sideband modulation]] (SSB) on 1 December 1915.<ref name="Bray" /> This advanced variant of amplitude modulation was adopted by AT&T for [[longwave]] transatlantic telephone service beginning 7 January 1927. After WW-II, it was developed for military aircraft communication.