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Radio broadcasting is the transmission of electromagnetic radiation (radio waves) to receivers over a wide territory. Most broadcasts are audio signal (sound), sometimes with embedded metadata. Listeners need a Radio receiver to pick up these signals. "Terrestrial" broadcasts, including AM, FM and DAB stations, originate signals from a land-based transmitter, while "satellite radio" signals originate from a satellite in Earth orbit.
Individual stations either create their own programming, or are affiliated with a radio network that provides content, either in broadcast syndication or by , or both. The most common transmission technologies are analog signal and . Analog radio uses one of two modulation methods: amplitude modulation, used by AM radio, or frequency modulation, for FM radio. A newer technique, digital radio stations, transmit using one of several different digital audio standards, such as DAB (Digital Audio Broadcasting), HD radio, or DRM (Digital Radio Mondiale).
Some early inventors foresaw wireless transmission's potential. In 1902, Nathan Stubblefield, who had developed wireless transmissions using ground conduction, envisioned that: "...any one having a receiving instrument, which would consist merely of a telephone receiver and a few feet of wire, and a signaling gong, could, upon being signaled by a transmitting station in Washington, or nearer, if advisable, be informed of weather news. Eventually it will be used for the general transmission of news of every description." "Telephoning Without Wires" by Trumbull White, Our Wonderful Progress: The World's Triumphant Knowledge and Works, Book 2, "The World's Science and Invention", 1902, pages 297–302.
The earliest radio broadcasting stations transmitted radiotelegraphy dots-and-dashes, for such things as time signals and weather reports, or provided news summaries intended for inclusion in shipboard newspapers. This had limited audiences, because of the need to understand Morse code. Because there was no way to collect fees from listeners, these stations were commonly operated by national governments. Beginning in 1904, the U.S. Navy broadcast daily time signals and weather reports, and a Canadian Marconi station in Camperdown, Nova Scotia began transmitting time signals in 1907. "The First Wireless Time Signal" by Captain J. L. Jayne, The American Jeweler, October 1912, page 411. In Europe, a station located at the Eiffel Tower in Paris, France, transmitted time signals that were audible throughout the continent. "The Eiffel Tower and its Early Radio" by LLoyd Butler, March 2013.
Although most radio stations during the first two decades of the 1900s employed radiotelegraphic transmissions, there was also experimental development of audio transmissions, mostly using "amplitude modulation" (AM) signals. The first AM technologies included high-frequency spark, alternator, and arc transmitters. However, it was not until the development of vacuum-tube (also known as "valve") transmitters that widespread audio broadcasting became practical. In addition, most early experimenters worked to create radiotelephone systems for private communication, and few were interested in broadcasting information and entertainment to general audiences.
Early examples of audio broadcasts included:
The outbreak of World War One largely suspended the development of civilian radio. However, during this period major improvements were made in vacuum-tube technology, which went into service after the end of wartime restrictions. All of the following examples used vacuum-tube transmitters:
Effective December 1, 1921, the U.S. Department of Commerce issued regulations formally establishing a broadcasting service, and by the end of 1922, there were over 500 licensed stations. "Miscellaneous: Amendments to Regulations", Radio Service Bulletin, January 3, 1922, page 10. Canada soon followed, and began issuing broadcasting station licenses in April 1922. "Radio Department: Broadcasting Stations", Winnipeg Evening Tribune, April 25, 1922, page 5 2MT in Great Britain began regular entertainment broadcasts in 1922, and that year the BBC was formed and given a national broadcasting monopoly. It received a Royal Charter in 1926, making it the first national broadcaster in the world, followed by Czech Radio and other European broadcasters in 1923.
In line to ITU Radio Regulations (article1.61) each broadcasting station shall be classified by the service in which it operates permanently or temporarily.
Nielsen Audio, formerly known as Arbitron, the United States–based company that reports on radio audiences, defines a "radio station" as a government-licensed AM or FM station; an HD Radio (primary or multicast) station; an internet stream of an existing government-licensed station; one of the satellite radio channels from XM Satellite Radio or Sirius Satellite Radio; or, potentially, a station that is not government licensed.
The signal is subject to interference from electrical storms (lightning) and other electromagnetic interference (EMI).Based on the "interference" entry of The Concise Oxford English Dictionary, 11th edition, online One advantage of AM radio signal is that it can be detected (turned into sound) with simple equipment. If a signal is strong enough, not even a power source is needed; building an unpowered crystal radio was a common childhood project in the early decades of AM broadcasting.
AM broadcasts occur on airwaves in the medium wave frequency range of 525 to 1,705 Hertz (known as the "standard broadcast band"). The band was expanded in the 1990s by adding nine channels from 1,605 to 1,705 kHz. Channels are spaced every 10 kHz in the Americas, and generally every 9 kHz everywhere else.
AM transmissions cannot be ionospheric propagated during the day due to strong absorption in the D-layer of the ionosphere. In a crowded channel environment, this means that the power of regional channels which share a frequency must be reduced at night or directionally beamed in order to avoid interference, which reduces the potential nighttime audience. Some stations have frequencies unshared with other stations in North America; these are called clear-channel stations. Many of them can be heard across much of the country at night. During the night, absorption largely disappears and permits signals to travel to much more distant locations via ionospheric reflections. However, fading of the signal can be severe at night.
AM radio transmitters can transmit audio frequencies up to 15 kHz (now limited to 10 kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5 kHz or less. At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers. The fidelity of sound equipment subsequently improved considerably, but the receivers did not. Reducing the bandwidth of the receivers reduces the cost of manufacturing and makes them less prone to interference. AM stations are never assigned adjacent channels in the same service area. This prevents the sideband power generated by two stations from interfering with each other. Bob Carver created an AM stereo tuner employing that demonstrated that an AM broadcast can meet or exceed the 15 kHz baseband bandwidth allotted to FM broadcasting stations without objectionable interference. After several years, the tuner was discontinued. Bob Carver had left the company and the Carver Corporation later cut the number of models produced before discontinuing production completely.
As well as on the medium wave bands, amplitude modulation (AM) is also used on the shortwave and long wave bands. Shortwave is used largely for national broadcasters, international propaganda, or religious broadcasting organizations. Shortwave transmissions can have international or inter-continental range depending on atmospheric conditions. Long-wave AM broadcasting occurs in Europe, Asia, and Africa. The ground wave propagation at these frequencies is little affected by daily changes in the ionosphere, so broadcasters need not reduce power at night to avoid interference with other transmitters.
Edwin Howard Armstrong invented wide-band FM radio in the early 1930s to overcome the problem of radio-frequency interference (RFI), which plagued AM radio reception. At the same time, greater fidelity was made possible by spacing stations further apart in the radio frequency spectrum. Instead of 10 kHz apart, as on the AM band in the US, FM channels are 200 kHz (0.2 MHz) apart. In other countries, greater spacing is sometimes mandatory, such as in New Zealand, which uses 700 kHz spacing (previously 800 kHz). The improved fidelity made available was far in advance of the audio equipment of the 1940s, but wide interchannel spacing was chosen to take advantage of the noise-suppressing feature of wideband FM.
Bandwidth of 200 kHz is not needed to accommodate an audio signal — 20 kHz to 30 kHz is all that is necessary for a narrowband FM signal. The 200 kHz bandwidth allowed room for ±75 kHz signal deviation from the assigned frequency, plus guard bands to reduce or eliminate adjacent channel interference. The larger bandwidth allows for broadcasting a 15 kHz bandwidth audio signal plus a 38 kHz stereo "subcarrier"—a piggyback signal that rides on the main signal. Additional unused capacity is used by some broadcasters to transmit utility functions such as background music for public areas, GPS auxiliary signals, or financial market data.
The AM radio problem of interference at night was addressed in a different way. At the time FM was set up, the available frequencies were far higher in the spectrum than those used for AM radio - by a factor of approximately 100. Using these frequencies meant that even at far higher power, the range of a given FM signal was much shorter; thus its market was more local than for AM radio. The reception range at night is the same as in the daytime. All FM broadcast transmissions are line-of-sight, and ionospheric bounce is not viable. The much larger bandwidths, compared to AM and SSB, are more susceptible to phase dispersion. Propagation speeds are fastest in the ionosphere at the lowest sideband frequency. The celerity difference between the highest and lowest sidebands is quite apparent to the listener. Such distortion occurs up to frequencies of approximately 50 MHz. Higher frequencies do not reflect from the ionosphere, nor from storm clouds. Moon reflections have been used in some experiments, but require impractical power levels.
The original FM radio service in the U.S. was the Yankee Network, located in New England.Halper, Donna L. "John Shepard's FM Stations—America's first FM network." Boston Radio Archives (BostonRadio.org). Regular FM broadcasting began in 1939 but did not pose a significant threat to the AM broadcasting industry. It required purchase of a special receiver. The frequencies used, 42 to 50 MHz, were not those used today. The change to the current frequencies, 88 to 108 MHz, began after the end of World War II and was to some extent imposed by AM broadcasters as an attempt to cripple what was by now realized to be a potentially serious threat.
FM radio on the new band had to begin from the ground floor. As a commercial venture, it remained a little-used audio enthusiasts' medium until the 1960s. The more prosperous AM stations, or their owners, acquired FM licenses and often broadcast the same programming on the FM station as on the AM station ("simulcasting"). The FCC limited this practice in the 1960s. By the 1980s, since almost all new radios included both AM and FM tuners, FM became the dominant medium, especially in cities. Because of its greater range, AM remained more common in rural environments.
The broadcasting regulators of the United States and Canada have chosen to use HD radio, an in-band on-channel system that puts digital broadcasts at frequencies adjacent to the analog broadcast. HD Radio is owned by a consortium of private companies that is called iBiquity. An international non-profit consortium Digital Radio Mondiale (DRM), has introduced the public domain DRM system, which is used by a relatively small number of broadcasters worldwide.
Governments typically have different motivations for funding international broadcasting. One clear reason is for ideological, or propaganda reasons. Many government-owned stations portray their nation in a positive, non-threatening way. This could be to encourage business investment in or tourism to the nation. Another reason is to combat a negative image produced by other nations or internal dissidents, or insurgents. Radio RSA, the broadcasting arm of the apartheid South African government, is an example of this. A third reason is to promote the ideology of the broadcaster. For example, a program on Radio Moscow from the 1960s to the 1980s was What is Communism?
A second reason is to advance a nation's foreign policy interests and agenda by disseminating its views on international affairs or on the events in particular parts of the world. During the Cold War the American Radio Free Europe and Radio Liberty and Indian Radio AIR were founded to broadcast news from "behind the Iron Curtain" that was otherwise being censored and promote dissent and occasionally, to disseminate disinformation. Currently, the US operates similar services aimed at Cuba (Radio y Televisión Martí) and the People's Republic of China, Vietnam, Laos and North Korea (Radio Free Asia).
Besides ideological reasons, many stations are run by religious broadcasters and are used to provide religious education, religious music, or worship service programs. For example, Vatican Radio, established in 1931, broadcasts such programs. Another station, such as HCJB or Trans World Radio will carry brokered programming from evangelists. In the case of the Broadcasting Services of the Kingdom of Saudi Arabia, both governmental and religious programming is provided.
In addition, formats change in popularity as time passes and technology improves. Early radio equipment only allowed program material to be broadcast in real time, known as live broadcasting. As technology for sound recording improved, an increasing proportion of broadcast programming used pre-recorded material. A current trend is the automation of radio stations. Some stations now operate without direct human intervention by using entirely pre-recorded material sequenced by computer control.
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