FM radio
FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound.
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.
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 II, 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 1, 1945 W47NV began operations in Nashville, Tennessee becoming the first modern commercial FM radio station.
New technology was added to FM radio in the early 1960s to allow FM stereo transmissions, where the frequency modulated radio signal is used to carry stereophonic sound, using the pilot-tone multiplex system.
This multiplexes 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 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.
See also: AM radio, History of radio.
History of FM radio in the US
FM stereo technology
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