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Main article: History of radio, FM radio.
In the United States, FM radio stations broadcast at frequencies of 88–108 MHz. FM radio, and later stereo FM radio, were both developed in the United States primarily by Edwin Armstrong.
W1XOJ was the first FM radio station, granted a construction permit by the FCC in 1937. On January 5, 1940 FM radio was demonstrated to the FCC for the first time. FM radio was assigned the 42 to 50 MHz band of the spectrum in 1940.After World War IIWorld War II was the most extensive and costly armed conflict in the history of the world, involving the great majority of the world's nations, being fought simultaneously in several major theatres, and costing tens of millions of lives. The war was fough, the FCC moved FM to the frequencies between 88 and 106 MHz on June 27, 1945, making all prewar FM radios worthless. This action severely set back the public confidence in, and hence the development of, FM radio. On March 1March 1 is the 60th day of the year in the Gregorian calendar (61st in leap years). There are 305 days remaining. Events BC 29 BC Horace writes the ode Occidit Daci Cotisonis agmen''. AD First thousand years 286 Maximian proclaimed junior Roman emperor., 1945Events January January 5 The Soviet Union recognizes the new pro-Soviet government of Poland. January 7 British General Bernard Montgomery holds a press conference in which he claims credit for victory in the Battle of the Bulge. January 12 World War II: W47NV began operations in Nashville, TennesseeFor other cities named Nashville, see Nashville (disambiguation). Nashville is the capital city of Tennessee, a state of the United States of America. Nashville is located on the Cumberland River in Davidson County. Nicknamed "Music City, U. it is the hom becoming the first modern commercial FM radio station.
New technology was added to FM radio in the early 1960sCenturies: 19th century 20th century 21st century Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s Years: 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 Events and trends The 1960s was a turbulent decade of change around to allow FM stereo transmissions, where the frequency modulated radio signal is used to carry stereophonicStereophonic means having two channels of audio. This is almost always taken to mean one for each ear, left and right, but could also be interpreted as any two-channel audio system. See also monophonic and quadrophonic. sound, using the pilot-tone multiplex system.
This multiplexMultiplex is either a word derived from multi- + plex (fold) or a synthetic portmanteau combining the words multiple and complex and can be another word for many or (literally) manifold''. It has several meanings: # Multiplex (cinema), a contraction of "mes the left and right audio signal channels in a manner that is compatible with mono sound, using a sum-and-difference technique to produce a single "mono-compatible" signal, which has a baseband part that is equal to the sum of the left and right channels (L+R), and a higher-frequency part that is the difference of the left and right channels (L-R) amplitude modulated on a 38 kHz subcarrier. A 19 kHz pilot tone is then added, to allow receivers to detect the presence of a stereo-encoded signal.
This signal can then be passed through the FM modulation and demodulation process as if it was a monophonic signal, and the stereo signals extracted from the demodulated FM signal by reversing the multiplexing process.
Simple mono FM receivers will not extract the left and right signals, but simply reproduce the baseband part of the "mono-compatible" signal. (This relies on the fact that the subcarrier-modulated part of the mono-compatible signal is in a part of the audio spectrum that is inaudible to people, and the pilot tone is a low-intensity tone in a part of the audio spectrum that is inaudible to most people).
This backwards compatibility was important, as when the FM stereo system was introduced in the U.S. in the 1960s, mono FM transmissions had been in service since the 1940s, and there was a large installed base of mono receivers that needed to be able to receive stereo broadcasts without any modification.
Stereo receivers could automatically switch between "mono" and "stereo" modes based on the presence of the pilot tone. They were also equipped with a notch filter to remove the pilot tone. In poor signal conditions, stereo receivers could also fall back to mono mode, even on a stereo signal, allowing improved signal-to-noise performance in these conditions.
The stereo multiplexing system has been further extended to add an extra, even higher frequency, 57 kHz subcarrier, which is used to carry low-bandwidth digital Radio Data System information, allowing digitally controlled radios to provide extra features.
Visit http://transmitters.tripod.com/stereo.htm for an article that explains the concepts of FM Stereo, with SPICE analysis and waveforms at different points of the multiplexing process.
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