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FM broadcasting in the United States

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FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound.

History of FM radio in the US

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 II, the FCC moved FM to the frequencies between 88 and 106 MHz on June 27, 1945. The change in frequency was to said to be for avoiding possible interference problems between stations in nearby cities and to make "room" for more FM radio channels. However, it has been alleged that the FCC was influenced by RCA chairman David Sarnoff, and that the change had the covert goal of disrupting the successful FM network that Edwin Armstrong had established on the old band. Radios built for the original FM radio band could be retrofit with converters, but many were just replaced. The greater expense was to the radio stations themselves that had to rebuild their stations for the new FM radio band. The move of the FM band, an organized campaign of misinformation by RCA (a company that competed with FM radio by focusing on AM radio and the emerging technology of television), and adverse rulings by the FCC, severely set back the development of FM radio. On March 1, 1945 W47NV began operations in Nashville, Tennessee becoming the first modern commercial FM radio station. However, FM radio did not recover from the setback until the upsurge in high fidelity equipment in the late 1950's.

During the 1970's, FM radio experienced a golden age of integrity programming, with disc jockeys playing what they wanted, including album cuts not designated as "singles" and lengthy prog rock tracks. The film "FM" (1978) portrays a fictional group of Los Angeles "maverick" FM disc jockeys reacting against the new formatting changes at their station.

FM stereo technology

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 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 noisy quality of stereo signals compared to mono signals can be attributed to the FCC's 1961 selection of the AM-subcarrier GE/Zenith standard over the high-fidelity FM-subcarrier Crosby system.

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.

FM radio channel assignments in the US

In the United States, frequency-modulated broadcasting stations operate in a frequency band extending from 87.8 MHz to 108.0 MHz, for a total of 20.2 MHz. It is divided into 100 channels, each 0.2 MHz wide, designated "channel 200" through "channel 300." In actual practice, no one (except the FCC) uses these channels numbers; the frequencies are used.

To receive a station, an FM receiver is tuned to the center frequency of the station's channel. The lowest channel, channel 200, extends from 87.8 MHz to 88.0 MHz; thus its center frequency is 87.9 MHz. Channel 201 has a center frequency of 88.1 MHz, and so on, up to channel 300, which extends from 107.8 to 108.0 MHz and has a center frequency of 107.9 MHz.

Because each channel is 0.2 MHz wide, the center frequencies of adjacent channels differ by 0.2 MHz. Because the lowest channel is centered on 87.9 MHz the tenths digit of the center frequency of any FM station in the United States is always an odd number. FM audio for television channel 6 is broadcast at a carrier frequency of 87.75 MHz, and many radios can tune down this low; a few low-power television stations licensed for channel 6 are operated solely for their right to use this frequency and broadcast only nominal video programming. For the same reason, assignment restrictions between TV stations on channel 6 and nearby FM stations are stringent: there are only two stations in the United States (KSFH and translator K200AA) licensed to operate on 87.9 MHz.

Originally, FM stations in a market were generally spaced four channels (800 kHz) apart. This spacing was developed in response to problems perceived on the original FM band, mostly due to deficiencies in receiver technology of the time. With modern equipment, this is widely understood to be unnecessary, and in many countries shorter spacings are used. Other spacing restrictions relate to mixing products with nearby television, air-traffic control, and two-way radio systems as well as other FM broadcast stations. The most significant such taboo restricts the allocation of stations 10.6 and 10.8 MHz apart, to protect against mixing products which will interfere with an FM receiver's standard 10.7 MHz intermediate frequency stage.

Commercial broadcasting is licensed only on channels 221 through 300, with 200 through 220 being reserved for noncommercial educational broadcasting. In some markets close to the Canadian or Mexican border, such as Detroit, Michigan and San Diego, California, commercial stations operating from those countries target U.S. audiences on "reserved band" channels, as neither Canada nor Mexico has such a reservation.

FM stations in the USA are now assigned based on a new table of separation distance values from currently licensed stations, based on station "class" (power output, antenna height, and geographical location). These new regulations have resulted in approximately double the number of possible stations, and increases in allowable power levels, over the original "bandplan" scheme described above. All powers are specified as Effective Radiated Power (ERP) which takes into account the multiplier effect of multiple antenna elements.

FM broadcast zones in the U.S.

The USA is divided into Zone I (roughly the northeastern quarter of the US mainland, excluding the far northern areas), Zone I-A (California south of 40 degress latitude, Virgin Islands, Puerto Rico), and Zone II (all other locations). The FM station classes are:

  • D – 10 watts, Alaska only
  • LP10 – 10 watts, 30 meters antenna height, new Low Power FM rules
  • LP100 – 100 watts, 30 meters, new Low Power FM rules
  • A – 100–6,000 watts, 100 meters, All Zones
  • B1 – 25,000 watts, 100 meters, Zones I and I-A
  • B – 50,000 watts, 150 meters, Zones I and I-A
  • C3 – 25,000 watts, 100 meters, Zone II
  • C2 – 50,000 watts, 150 meters, Zone II
  • C1 – 100,000 watts, 299 meters, Zone II
  • C0 – 100,000 watts, 450 meters, Zone II
  • C – 100,000 watts, 600 meters, Zone II
  • Note: The B classes and the C classes have different "protected" signal area regulations with different separation rules.

High power is useful in penetrating buildings, diffracting around hills, and refracting for some distance beyond the horizon. 100,000 watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther (e.g., 150 miles, 240 km) if there are no competing signals.

A few old "grandfathered" stations do not conform to these power rules. WBCT-FM (93.7) in Grand Rapids, Michigan, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. This huge power level does not usually help to increase range as much as you might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space. Nevertheless, when there were fewer FM stations competing, this station could be heard near Bloomington, Illinois, almost 300 miles (500 km) distant.

See also

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