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HISTORY See Also: History of radio AM was the dominant method of broadcasting during the first two thirds of the 20th Century and remains widely used into the 21st . The Central Intelligence Agency World Factbook lists approximately 16,265 AM stations worldwide . AM radio began with the first, experimental broadcast in in Pittsburgh , Pennsylvania ). Radio Programming boomed during the " Golden Age Of Radio " ( 1920s – 1950s ). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music. OPERATION AM radio technology is simpler than either FM radio or DAB . An AM receiver detects amplitude variations in the Radio Wave s at a particular frequency. It then amplifies changes in the signal Voltage to drive a Loudspeaker or Earphone s. The earliest Crystal Radio Receiver s used a crystal Diode detector with no amplification. FREQUENCIES AM radio is broadcast on several frequency bands:
The allocation of these bands is governed by the ITU 's Radio Regulations and, on the national level, by each country's telecommunications administration (the FCC in the U.S. , for example) subject to international agreements.
Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, "ham" Radio , etc. LIMITATIONS OF AM RADIO Because of its susceptibility to atmospheric interference and generally lower-fidelity sound, AM broadcasting is better suited to Talk Radio and News programming, while Music Radio and Public Radio mostly shifted to FM Broadcasting in the late 1960s and 1970s . Medium wave and short wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by Groundwave , refracting around the curve of the earth over a distance up to a few hundred Mile s (or Kilometers ) from the signal transmitter. However, after sunset, changes in the Ionosphere cause AM signals to travel by Skywave , enabling AM radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. (Such stations are commonly referred to as Daytimer s.) Some other radio stations are granted Clear Channel rights, meaning that they broadcast on frequencies whose use is more restricted and thus relatively unaffected by interference from other stations Nowadays relatively few stations enjoy clear channel status. The hobby of listening to long distance signals is known as DX or DX'ing, from an old Telegraph Abbreviation for "distance unknown". Several Non-profit Hobby ist clubs are devoted exclusively to DXing The AM Broadcast Band , including the National Radio Club and International Radio Club Of America . Similarly, people listening to short wave transmissions are SWLing . AM radio signals can be severly disrupted in large urban centres by concrete bridges with metal reinforcements, other Faraday Cage structures, tall buildings and sources of Radio Frequency Interference (RFI) and electrical noise. As a result, AM radio in many countries has lost its dominance as a music broadcasting service, and in many cities is now relegated to news, sports, religious and Talk Radio stations although some musical genres particularly Country, Oldies, Nostalgia and Ethnic/World Music survive on AM particularly in areas where FM frequencies are in short supply or in thinly populated or mountainous areas where FM coverage is poor. OTHER DISTRIBUTION METHODS Stereo transmissions are possible (see AM Stereo ), and there is work underway to add Digital Radio services to currently existing AM transmissions. In the United States , the IBiquity company is developing a proprietary standard for medium wave transmissions, while Digital Radio Mondiale is a more open effort often used on the Shortwave Bands , and can be used alongside many AM broadcasts. While FM radio can also be received by Cable , AM radio generally cannot be, although an AM station can be converted into an FM cable signal. In Canada , cable operators that offer FM cable services are required by the CRTC to distribute all locally available AM stations in this manner. In Switzerland a system known as "wire broadcasting" transmits AM signals over telephone lines in the Longwave band SEE ALSO
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