FM broadcasting in the USA
|
|
FM broadcasting in the USA began in the 1930s. The use of FM radio been associated with higher sound quality music radio.
| Contents |
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 1950s.
During the 1970s, 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 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, both due to being forced off of another channel.
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 (NCE) broadcasting. In some "twin city" markets close to the Canadian or Mexican border, such as Detroit, Michigan and Windsor, Ontario, or San Diego, California and Tijuana, Baja California, commercial stations operating from those countries target U.S. audiences on "reserved band" channels, as neither Canada nor Mexico has such a reservation. Because of this necessary sharing, the FCC reserves a few other channels for such NCE stations.
FM stations in the USA are now assigned based on a table of separation distance values from currently licensed stations, based on station "class" (power output, antenna height, and geographical location). These regulations (see Docket 80-90) 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 magnifying effect (gain) of multiple antenna elements.
US_FM_broadcast_zones.png
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, U.S. Virgin Islands, Puerto Rico), and Zone II (all other locations). The highest-power stations are class C in zone II (not to be confused with ITU region II, the Americas), and class B in the others. There are no B stations in zone II, nor any C stations in the others. (See the list of broadcast station classes.) Canada is also divided in this manner, based on the most highly-populated regions.
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 one 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
- FM broadcasting
- FM band
- AM radio
- AM stereo
- List of broadcast station classes
- History of radio
- List of radio stations
- RDS (Radio Data System)
External links
- An Introduction to FM MPX (http://www.smoke.com.au/~ic/mpx.html)
- Some history of the FM multiplex system (http://www.airwaves.com/archive3/v9_76.html) (search down the page for "pilot-tone multiplex system")
- "Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country (http://www.salestores1.com/woreltab.html)".
- Stereo for Dummies (http://transmitters.tripod.com/stereo.htm) Many graphs that show waveforms at different points in the FM Multiplex process