Editing AM broadcasting (section)

==Technical information==

AM radio technology is simpler than later transmission systems. An AM receiver detects amplitude variations in the [[electromagnetic radiation|radio wave]]s at a particular frequency, then amplifies changes in the signal [[voltage]] to operate a [[loudspeaker]] or [[earphone]]. However, the simplicity of AM transmission also makes it vulnerable to "static" ([[radio noise]], [[radio frequency interference]]) created by both natural atmospheric electrical activity such as lightning, and electrical and electronic equipment, including fluorescent lights, motors and vehicle ignition systems. In large urban centers, AM radio signals can be severely disrupted by metal structures and tall buildings. As a result, AM radio tends to do best in areas where [[FM broadcast band|FM frequencies]] are in short supply, or in thinly populated or mountainous areas where FM coverage is poor. Great care must be taken to avoid mutual interference between stations operating on the same frequency. In general, an AM transmission needs to be about 20 times stronger than an interfering signal to avoid a reduction in quality, in contrast to FM signals, where the "[[capture effect]]" means that the dominant signal needs to only be about twice as strong as the interfering one.

To allow room for more stations on the mediumwave broadcast band in the United States, in June 1989 the FCC adopted a [[National Radio Systems Committee]] (NRSC) standard that limited maximum transmitted audio [[Bandwidth (signal processing)|bandwidth]] to 10.2&nbsp;kHz, limiting occupied bandwidth to 20.4&nbsp;kHz. The former audio limitation was 15&nbsp;kHz resulting in [[Bandwidth (signal processing)|bandwidth]] of 30&nbsp;kHz. Another common limitation on AM fidelity is the result of receiver design, although some efforts have been made to improve this, notably through the [[AMAX]] standards adopted in the United States.

===Broadcast band frequencies===
AM broadcasts are used on several frequency bands. The allocation of these bands is governed by the [[International Telecommunication Union|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.

The frequency ranges given here are those that are allocated to stations. Because of the bandwidth taken up by the [[sideband]]s, the range allocated for the band as a whole is usually about 5&nbsp;kHz wider on either side.

{{anchor|AM_longwave_band}}

====Longwave broadcasting====
''[[Longwave]]'' (also known as [[Low frequency]] (LF)) (148.5&nbsp;[[Kilohertz|kHz]] – 283.5&nbsp;kHz) Broadcasting stations in this band are assigned transmitting frequencies in the range 153&nbsp;kHz – 279&nbsp;kHz, and generally maintain 9&nbsp;kHz spacing. 
Longwave assignments for broadcasting only exist in [[ITU region|ITU Region&nbsp;1]] (Europe, Africa, and northern and central Asia) and are not allocated elsewhere. Individual stations have coverage measured in the hundreds of kilometers; however, there is only a very limited number of available broadcasting slots.

Most of the earliest broadcasting experiments took place on [[longwave]] frequencies; however, complaints about interference from existing services, particularly the military, led to most broadcasting moving to higher frequencies.

====Medium-wave broadcasting{{anchor|commercial_AM_bands}}====
''[[Medium wave]]'' (also known as [[Medium frequency]] (MF)), is by far the most commonly used AM broadcasting band. In ITU Regions&nbsp;1 and 3, transmitting frequencies run from 531&nbsp;kHz – 1602&nbsp;kHz, with 9&nbsp;kHz spacing (526.5&nbsp;kHz – 1606.5&nbsp;kHz), and in ITU Region&nbsp;2 (the Americas), transmitting frequencies are 530&nbsp;kHz – 1700&nbsp;kHz, using 10&nbsp;kHz spacing (525&nbsp;kHz – 1705&nbsp;kHz), including the ITU [[AM expanded band|Extended AM broadcast band]], authorized in Region&nbsp;2, between 1605&nbsp;kHz and 1705&nbsp;kHz, previously used for police radio.<ref>{{cite report |title=Federal Communications Commission rules |at=47 CFR §2.106}}</ref>

====Shortwave broadcasting====
''[[Shortwave]]'' (also known as [[High frequency]] (HF)) transmissions range from approximately 2.3 to 26.1&nbsp;MHz, divided into 14&nbsp;broadcast bands. Shortwave broadcasts generally use a narrow 5&nbsp;kHz channel spacing. Shortwave is used by audio services intended to be heard at great distances from the transmitting station. The long range of shortwave broadcasts comes at the expense of lower [[High fidelity|audio fidelity]].

Most broadcast services use AM transmissions, although some use a modified version of AM such as [[Single-sideband modulation]] (SSB) or an AM-compatible version of SSB such as "SSB with carrier reinserted".

====VHF AM broadcasting====
Beginning in the mid-1930s, the United States evaluated options for the establishment of broadcasting stations using much higher transmitting frequencies. In October&nbsp;1937, the FCC announced a second band of AM stations, consisting of 75&nbsp;channels spanning from 41.02 to 43.98&nbsp;MHz, which were informally called ''[[Apex (radio band)|Apex]]''.

The 40&nbsp;kHz spacing between adjacent frequencies was four times that of the 10&nbsp;kHz spacing used on the standard AM broadcast band, which reduced adjacent-frequency interference, and provided more bandwidth for high-fidelity programming. However, this band was eliminated effective 1&nbsp;January 1941, after the FCC determined that establishing a band of FM stations was preferable.<ref>[http://www.theradiohistorian.org/Apex/Apex1.htm "America's Apex Broadcasting Stations of the 1930s"] by John Schneider, ''Monitoring Times Magazine'', December 2010. (theradiohistorian.com)</ref>

===Other distribution methods===
{{Main|Carrier current}}
Beginning in the mid-1930s, starting with "[[WBRU#Beginnings|The Brown Network]]" at Brown University in Providence, Rhode Island, a very low power broadcasting method known as [[carrier current]] was developed, and mostly adopted on U.S. college campuses. In this approach AM broadcast signals are distributed over electric power lines, which radiate a signal receivable at a short distance from the lines.<ref>''The Gas Pipe Networks'' by Louis M. Bloch, Jr, 1980.</ref> In Switzerland a system known as "wire broadcasting" (''Telefonrundspruch'' in German) transmitted AM signals over telephone lines in the [[longwave]] band until 1998, when it was shut down.<ref>{{cite web|url=http://www.biennophone.ch/telefonrundspruch.htm|title=Sammlung alter Biennophone-Radios|publisher=Biennophone.ch|access-date=23 July 2017}}</ref> In the UK, [[Rediffusion]] was an early pioneer of AM radio cable distribution.

Hybrid digital broadcast systems, which combine (mono analog) AM transmission with digital sidebands, have started to be used around the world. In the United States, [[iBiquity]]'s proprietary [[HD Radio]] has been adopted and approved by the FCC for medium wave transmissions,<ref>[https://www.fcc.gov/media/radio/digital-radio "Digital Radio"] (fcc.gov)</ref> while [[Digital Radio Mondiale]] is a more open effort often used on the [[shortwave bands]], and can be used alongside many AM broadcasts. Both of these standards are capable of broadcasting audio of significantly greater fidelity than that of standard AM with current bandwidth limitations, and a theoretical frequency response of 0–16&nbsp;kHz, in addition to stereo sound and text data.

===Microbroadcasting===
{{see also|Low-power broadcasting}}
Some [[microbroadcasting|microbroadcasters]], especially those in the United States operating under the FCC's [[Part 15]] rules,<ref>{{cite book   | title = Localist Movements in a Global Economy Sustainability, Justice, and Urban Development in the United StatesStates | author1= David J. Hess | author2 = Professor David J Hess | author3 = Robert Gottlieb |publisher = MIT Press | location=   |url = https://books.google.com/books?id=BS7rsOQv5MUC | date = 2009  | page =  199 | isbn = 9780262012645  | access-date = 7 February 2021}}</ref> and [[pirate radio]] operators on mediumwave and shortwave, achieve greater range than possible on the [[FM broadcast band|FM band]]. On mediumwave these stations often transmit on 1610&nbsp;kHz to 1710&nbsp;kHz. Hobbyists also use low-power AM (LPAM) transmitters to provide programming for vintage radio equipment in areas where AM programming is not widely available or does not carry programming the listener desires; in such cases the transmitter, which is designed to cover only the immediate property and perhaps nearby areas, is connected to a computer, an FM radio or an MP3 player. Microbroadcasting and pirate radio have generally been supplanted by streaming audio on the Internet, but some schools and hobbyists still use LPAM transmissions.