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AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation (AM). AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is authorized in the medium wave band worldwide, and also in parts of the longwave and shortwave bands. Radio broadcasting was made possible by the invention of the amplifying vacuum tube, the Audion (triode), by Lee de Forest in 1906, which led to the development of inexpensive vacuum tube AM radio receivers and transmitters during World War I. Commercial AM broadcasting developed from amateur broadcasts around 1920, and was the only commercially important form of radio broadcasting until FM broadcasting began after World War II. This period is known as the "Golden Age of Radio". Today, AM competes with FM, as well as with various digital radio broadcasting services distributed from terrestrial and satellite transmitters. In many countries the higher levels of interference experienced with AM transmission have caused AM broadcasters to specialize in news, sports and talk radio, leaving transmission of music mainly to FM and digital broadcasters.
==History==
(詳細はamplitude modulation (AM) radio transmission was developed during the two decades from 1900 to 1920. Before this, the first radios transmitted information by wireless telegraphy, in which the radio signal did not carry audio (sound) but was switched on and off to create pulses that carried text messages in Morse code. This was used for private person-to-person communication and message traffic, such as telegrams.
The entrepreneurs who developed AM "radiotelephone" transmission did not anticipate broadcasting voice and music into people's homes. The term "broadcasting", borrowed from agriculture, was coined for this new activity (by either Frank Conrad or RCA historian George Clark〔) around 1920.〔 Prior to 1920 there was no concept of "broadcasting", or that radio listeners could be a mass market for entertainment.〔 Promoters saw the practical application for AM as similar to the existing communication technologies of wireless telegraphy, telephone, and telegraph: two-way person-to-person commercial voice service, a wireless version of the telephone.〔 Although there were a number of experimental broadcasts during this period, these were mostly to provide publicity for the inventor's products. True radio broadcasting didn't begin until around 1920, when it sprang up spontaneously among amateur stations. AM remained the dominant method of broadcasting for the next 30 years, a period called the "Golden Age of Radio", until FM broadcasting started to become widespread in the 1950s. AM remains a popular, profitable entertainment medium today and the dominant form of broadcasting in some countries such as Australia and Japan.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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