Editing Radio broadcasting (section)

==Types==
[[Image:Radio Transmission Diagram en.svg|thumb|right|260px|Transmission diagram of sound broadcasting (AM and FM)]]
Broadcasting by radio takes several forms. These include [[AM broadcasting|AM]] and [[FM broadcasting|FM]] stations. There are several subtypes, namely [[commercial broadcasting]], [[non-commercial educational]] (NCE) [[public broadcasting]] and [[non-profit]] varieties as well as [[community radio]], student-run [[campus radio]] stations, and [[hospital radio]] stations can be found throughout the world. Many stations broadcast on [[shortwave]] bands using AM technology that can be received over thousands of miles (especially at night). For example, the [[BBC World Service|BBC]], [[Voice of America|VOA]], [[Voice of Russia|VOR]], and [[Deutsche Welle]] have transmitted via shortwave to Africa and Asia. These broadcasts are very sensitive to atmospheric conditions and solar activity.

[[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.<ref>{{cite news|url=http://www.nxtbook.com/nxtbooks/newbay/rw_20081008/index.php|title=What is a Radio Station?|work=[[Radio World]]|page=6}}</ref>

===AM===
{{main|AM broadcasting}}
AM stations were the earliest broadcasting stations to be developed. AM refers to [[amplitude modulation]], a mode of broadcasting radio waves by varying the amplitude of the carrier signal in response to the amplitude of the signal to be transmitted. The medium-wave band is used worldwide for AM broadcasting. Europe also uses the [[long wave]] band. In response to the growing popularity of [[FM broadcasting#Stereo FM|FM stereo]] radio stations in the late 1980s and early 1990s, some [[North America]]n stations began broadcasting in [[AM stereo]], though this never gained popularity and very few receivers were ever sold.

The signal is subject to interference from electrical storms ([[lightning]]) and other [[electromagnetic interference]] (EMI).<ref>Based on the "interference" entry of ''The Concise Oxford English Dictionary'', 11th edition, online</ref> 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|crystal radio receiver]] was a common childhood project in the early decades of AM broadcasting.

AM broadcasts occur on [[North America]]n airwaves in the [[medium wave]] frequency range of 525 to 1,705 [[Hertz|kHz]] (known as the "standard broadcast band"). The band was expanded in the 1990s by adding nine [[Communication channel|channels]] from 1,605 to 1,705&nbsp;kHz. Channels are spaced every 10&nbsp;kHz in the [[Americas]], and generally every 9 kHz everywhere else.

AM transmissions cannot be ionospheric [[radio propagation|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 station]]s. 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&nbsp;kHz (now limited to 10&nbsp;kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5&nbsp;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.<ref>{{cite web | title=Types of Technology, FM vs AM | website=kwarner.bravehost.com | date=July 13, 2012 | url=http://kwarner.bravehost.com/tech.htm | archive-url=https://web.archive.org/web/20120713084431/http://kwarner.bravehost.com/tech.htm | archive-date=July 13, 2012 | url-status=dead | access-date=August 10, 2019}}</ref> [[Bob Carver]] created an [[AM stereo]] tuner employing [[notch filter]]ing that demonstrated that an AM broadcast can meet or exceed the 15&nbsp;kHz baseband bandwidth allotted to [[FM broadcasting|FM]] 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.<ref>{{Cite web |title=Landing Page |url=https://www.bobcarvercorp.com/ |access-date=2025-03-25 |website=BobCarverCorp.com |language=en}}</ref>

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.<ref>{{Cite book|url=https://books.google.com/books?id=HudwCgAAQBAJ|title=Beginning Shortwave Radio Listening|last=Grodkowski|first=Paul|date=August 24, 2015|publisher=Booktango|isbn=9781468964240|language=en}}</ref> 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.

===FM===
{{main|FM broadcasting}}
FM refers to [[frequency modulation]], and occurs on [[VHF]] airwaves in the frequency range of 88 to 108 [[Megahertz|MHz]] everywhere except [[Japan]] and [[Russia]]. Russia, like the former [[Soviet Union]], uses 65.9 to 74&nbsp;MHz frequencies in addition to the world standard. Japan uses the 76 to 90&nbsp;MHz frequency band.

[[Edwin Howard Armstrong]] invented wide-band FM radio in the early 1930s to overcome the problem of [[Electromagnetic interference|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&nbsp;kHz apart, as on the AM band in the US, FM channels are 200&nbsp;kHz (0.2&nbsp;MHz) apart. In other countries, greater spacing is sometimes mandatory, such as in New Zealand, which uses 700&nbsp;kHz spacing (previously 800&nbsp;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 &mdash; 20&nbsp;kHz to 30&nbsp;kHz is all that is necessary for a narrowband FM signal. The 200&nbsp;kHz bandwidth allowed room for ±75&nbsp;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&nbsp;kHz bandwidth audio signal plus a 38&nbsp;kHz [[FM broadcasting#Stereo FM|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, [[Global Positioning System|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&nbsp;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]].<ref>Halper, Donna L. [http://www.bostonradio.org/essays/shepard-fm.html "John Shepard's FM Stations—America's first FM network."] {{Webarchive|url=https://web.archive.org/web/20060212104948/http://www.bostonradio.org/essays/shepard-fm.html |date=February 12, 2006 }} Boston Radio Archives (BostonRadio.org).</ref><ref>{{cite web | title=The Yankee Network in 1936 | website=The Archives @ BostonRadio.org | url=http://www.bostonradio.org/yankee-36.html | access-date=August 10, 2019}}</ref><ref>{{cite web | title=FM Broadcasting Chronology | website=Jeff Miller Pages | date=June 23, 2017 | url=http://jeff560.tripod.com/chronofm.html | access-date=August 10, 2019}}</ref> 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&nbsp;MHz, were not those used today. The change to the current frequencies, 88 to 108&nbsp;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.

===Pirate radio===
{{Main|Pirate radio}}
Pirate radio is illegal or non-regulated radio transmission. It is most commonly used to describe illegal broadcasting for entertainment or political purposes. Sometimes it is used for illegal two-way radio operation. Its history can be traced back to the unlicensed nature of the transmission, but historically there has been occasional use of sea vessels—fitting the most common perception of a pirate—as broadcasting bases.
Rules and regulations vary largely from country to country, but often the term pirate radio describes the unlicensed broadcast of FM radio, AM radio, or shortwave signals over a wide range. In some places, radio stations are legal where the signal is transmitted, but illegal where the signals are received—especially when the signals cross a national boundary. In other cases, a broadcast may be considered "pirate" due to the type of content, its transmission format, or the transmitting power (wattage) of the station, even if the transmission is not technically illegal (such as a webcast or an amateur radio transmission). Pirate radio stations are sometimes referred to as bootleg radio or clandestine stations.

===Terrestrial digital radio===
{{main|Digital audio broadcasting|HD radio|ISDB|Digital Radio Mondiale}}

[[Digital radio]] broadcasting has emerged, first in [[Europe]] (the [[United Kingdom|UK]] in 1995 and [[Germany]] in 1999), and later in the United States, France, the Netherlands, South Africa, and many other countries worldwide. The simplest system is named DAB Digital Radio, for [[Digital Audio Broadcasting]], and uses the [[public domain]] [[Digital Audio Broadcasting|EUREKA 147]] (Band III) system. DAB is used mainly in the UK and South Africa. Germany and the Netherlands use the DAB and DAB+ systems, and France uses the L-Band system of DAB Digital Radio.

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.