Globally, the adoption of stereo broadcasting was never great, and declined after 1990. With the continued migration of AM stations away from music to news, sports, and talk formats, receiver manufacturers saw little reason to adopt the more expensive stereo tuners, and thus radio stations have little incentive to upgrade to stereo transmission.
In the late 1970s, in an unsuccessful effort to stem the exodus of the music audience to FM, the US AM radio industry developed technology for broadcasting in . Stereo is the standard in the music , and had adopted a stereo standard early, in 1961. The technology was challenging because of the narrow 20 kHz of the AM channel, and the need for backward compatibility with non-stereo AM receivers. In 1975 the US requested proposals for AM stereo standards, and four competing standards were submitted: 's V-CPM (Variable angle Compatible Phase Multiplex), 's PMX, 's (Compatible Quadrature Amplitude Modulation), and Kahn- system. All except the Kahn-Hazeltine system used variations on the same idea: the mono (Left + Right) signal was transmitted in the as before, while the stereo (Left − Right) information was transmitted by .
Radio systems need a to some property of the energy produced to impress a signal on it, for example using or (which can be or ). Radio systems also need an antenna to convert into , and vice versa. An antenna can be used for both transmitting and receiving. The of in radios allow individual stations to be selected. The electromagnetic wave is intercepted by a tuned receiving . A receives its input from an and converts it into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc. Radio frequencies occupy the range from a 3 kHz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum.
brought home to nations the strategic importance of long-distance radio; in addition to its military uses in keeping contact with its fleets and overseas forces, a country that didn't have radio could be isolated by an enemy cutting its . In the US, before the war, the radio industry was fragmented by patent monopolies held by competing giant firms, so the best long-range radio technology was owned by two European firms: the British and the German . At its entry into the war in 1917, the US government temporarily took control of the entire US radio industry for the war effort, including the transatlantic wireless stations of these foreign firms. After the war, due to fear of foreign ownership of the US radio industry, there was an abortive effort to create a federal radio monopoly. Instead, in 1919 the US government brokered a patent cross-licensing and market-sharing agreement between the competing US corporate giants , , , and . Foreign firms were forbidden to own US radio stations, and US assets of Marconi and Telefunken were sold to a newly created firm, the , RCA. AT&T, Westinghouse, and GE would manufacture radio equipment, and RCA would be the marketing and transmitting arm. This "radio group" oligopoly controlled the US radio industry into the 1940s.
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