Editing Amplitude modulation (section)

===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 [[modulation|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 company|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" />