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AM broadcasting is the process of radio broadcasting using amplitude modulation (AM). AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is authorized in the medium wave band worldwide, and also in parts of the long wave and short wave bands. Radio broadcasting was made possible by the invention of the amplifying vacuum tube, the Audion (triode), by Lee de Forest in 1906, which led to the development of inexpensive vacuum tube AM radio receivers and transmitters during World War I. Commercial AM broadcasting developed from amateur broadcasts around 1920, and was the only commercially important form of radio broadcasting until FM broadcasting began after World War II. This period is known as the "Golden Age of Radio". Today, AM competes with FM, as well as with various digital radio broadcasting services distributed from terrestrial and satellite transmitters. In many countries the higher levels of interference experienced with AM transmission have caused AM broadcasters to specialize in news, sports and talk radio, leaving transmission of music mainly to FM and digital broadcasters.
The technology of amplitude modulation (AM) radio transmission was developed during the two decades from 1900 to 1920. Before this, the first radios transmitted information by wireless telegraphy, in which the radio signal didn't carry audio (sound) but was switched on and off to create pulses that carried text messages in Morse code. This was used for private person-to-person communication and message traffic, such as telegrams.
The entrepreneurs who developed AM "radiotelephone" transmission did not anticipate broadcasting voice and music into people's homes. The term "broadcasting", borrowed from agriculture, was coined for this new activity (by either Frank Conrad or RCA historian George Clark) around 1920. Prior to 1920 there was no concept of "broadcasting", or that radio listeners could be a mass market for entertainment. Promoters saw the practical application for AM as similar to the existing communication technologies of wireless telegraphy, telephone, and telegraph: two-way person-to-person commercial voice service, a wireless version of the telephone. Although there were a number of experimental broadcasts during this period, these were mostly to provide publicity for the inventor's products. True radio broadcasting didn't begin until around 1920, when it sprang up spontaneously among amateur stations. AM remained the dominant method of broadcasting for the next 30 years, a period called the "Golden Age of Radio", until FM broadcasting started to become widespread in the 1950s. AM remains a popular, profitable entertainment medium today and the dominant form of broadcasting in some countries such as Australia and Japan.
The first AM voice transmission was made by Canadian researcher Reginald Fessenden on 23 December 1900, using a spark gap transmitter. Fessenden is a significant figure in the development of AM radio. He realized that the damped waves produced by the existing spark transmitters, which transmitted text data by wireless telegraphy, could not be used to transmit sound, and continuous wave transmitters were needed. He helped develop one of the first - the Alexanderson alternator. He also discovered the principle on which AM modulation is based, heterodyning, and invented one of the first detectors able to rectify and receive AM, the electrolytic detector or "liquid baretter", in 1902.
The early experiments in AM transmission, conducted by Fessenden, Valdemar Poulsen, Ernst Ruhmer, Quirino Majorana, Charles Herrold, and Lee de Forest, were hampered by the lack of a technology for amplification. The first practical continuous wave AM transmitters were based on versions of the Poulsen arc transmitter invented in 1903, and the huge, expensive Alexanderson alternator, developed 1906-1910. The modifications necessary to transmit AM were clumsy and resulted in very low audio quality. Modulation was usually accomplished by a carbon microphone inserted directly in the antenna wire. The limited power handling ability of the microphone severely limited the power of the first radiotelephones. At the receiving end, the unamplified crystal radio receivers then in use could not drive loudspeakers, only earphones, so only one member of a family could listen at a time.
The discovery in 1912 of the amplifying ability of the Audion (triode) vacuum tube, invented in 1906 by Lee De Forest, solved these problems. The vacuum tube feedback oscillator invented in 1912 by Alexander Meissner and Edwin Armstrong, was a cheap source of continuous waves and could be easily modulated to make an AM transmitter. Nongovernmental radio transmission was prohibited in many countries during World War 1, but AM radio technology advanced greatly due to wartime research, and after the war the availability of cheap tubes sparked a great increase in the number of amateur radio stations experimenting with AM transmission of news or music, giving people more to listen to. New vacuum tube receivers coming on the market could power loudspeakers, so the entire family could sit and listen together, and people could dance to broadcast music. Vacuum tubes remained the central technology of radio for 50 years, until transistors replaced them in the 1960s, and they are still used in broadcast transmitters.
These changes caused radio listening to evolve explosively around 1919-1922 from a high-tech hobby to a hugely popular social and family pastime, the first electronic mass entertainment medium. In the US, the first broadcast stations were hobby and voluntary efforts without explicit advertising, started by a variety of local organizations: amateurs, local businesses looking for promotion, newspapers, schools, clubs, political parties, and churches. Some naval radio stations broadcast programs of music to the public at certain hours. Later businesses learned to use this new medium to sell products, paying for on-air commercial advertising. US radio broadcasting developed into a private, profitmaking business, with minimal government control on content.
In Europe, broadcasting took a different course. Radio transmission had always been more tightly controlled by government in this region, partly because countries were smaller and closer together; for example, in the UK receiving equipment as well as transmitters had to be licensed. There was a feeling in countries like the UK and France that the radio spectrum was a national resource which should not be surrendered to private interests, motivated by profit, who would pander solely to the desire for entertainment. Radio should serve higher purposes of public information and education. In addition, totalitarian countries for political reasons kept mass communications media under government control. So in much of Europe, broadcasting developed as a government-owned or government-supervised monopoly. It was largely funded not by on-air commercial advertising as in the US, but by taxes on sales of radios, and user fees in the form of an annual "receiver license" that anyone owning a radio had to buy.
Who made the "first" radio broadcast is a contentious issue. In the chaotic, freewheeling, experimental atmosphere of early AM wireless, it is difficult to draw a distinction between "private" and "public" transmissions. In many cases, radio listeners tuned into experimental transmissions by the first stations developing AM modulation, and the stations began to cater to their unexpected audience with news and music. Some of the early milestones:
World War I brought home to nations the strategic importance of long-distance radio; in addition to its military uses in keeping contact with its fleets and overseas forces, a country that didn't have radio could be isolated by an enemy cutting its submarine telegraph cables. In the US, before the war, the radio industry was fragmented by patent monopolies held by competing giant firms, so the best long-range radio technology was owned by two European firms: the British Marconi Co. and the German Telefunken. At its entry into the war in 1917, the US government temporarily took control of the entire US radio industry for the war effort, including the transatlantic wireless stations of these foreign firms. After the war, due to fear of foreign ownership of the US radio industry, there was an abortive effort to create a federal radio monopoly. Instead, in 1919 the US government brokered a patent cross-licensing and market-sharing agreement between the competing US corporate giants AT&T, Westinghouse, United Fruit, and General Electric. Foreign firms were forbidden to own US radio stations, and US assets of Marconi and Telefunken were sold to a newly created firm, RCA. AT&T, Westinghouse, and GE would manufacture radio equipment, and RCA would be the marketing and transmitting arm. This "radio group" oligopoly controlled the US radio industry into the 1940s.
As the US audience for "broadcasting" grew in 1919-22, it caught the interest of the big radio corporations, and they began buying stations. They established "flagship" stations in major cities to promote their corporate image, which during this period broadcast top-quality entertainment and news without advertising.
Since longwave radio frequencies were used for international wireless telegraphy, broadcasting stations were mostly given the medium waves, whose limited range restricted them to local audiences. Corporations around 1922 realized that long distance telephone lines, another innovation made possible around 1915 by the vacuum tube, could be used to link local radio stations into networks (the word "chains" was used until the 1930s) broadcasting the same content, giving corporations a nationwide audience.
In the US, the nationwide telephone carrier AT&T was the first to create a network and take the radical step of commercial advertising. It developed a broadcasting model based on its telephony business: "toll" broadcasting. Its flagship station, WEAF in New York, in August 1922 was first to air commercial advertising, selling half-hour and hour blocks of airtime to commercial "sponsors" that developed entertainment shows containing commercial messages. It had a monopoly on quality telephone lines, and by 1924 had linked 12 stations in Eastern cities into a "chain". RCA and Westinghouse attempted to organize their own network around their flagship WJZ, but were hampered by AT&T's refusal to lease them lines. In 1925 court decisions stripped AT&T of its monopoly over broadcasting, and it decided to get out of radio. AT&T sold WEAF to RCA, which formed the nucleus of the new NBC network. In 1927, to reduce the "chaos" on the airwaves, the government came down on the side of the advertising model, establishing two classes of broadcast licenses: the "A" or "general public interest" stations which sold time impartially to anyone, received favorable frequency assignments and flourished, and the "B" or nonprofit "propaganda" stations, mainly special-interest, political or religious stations which represented a point of view, were phased out.
The adoption of the commercial sponsorship model made radio broadcasting profitable, bringing in a lucrative stream of income that could be used to buy top-quality talent. The new business of advertising agencies acted as middlemen, and by the late 1920s modern radio advertisements were developed. Networks began to see their true product as their audience, and began to tailor shows to bring in specific demographics desired by their advertisers. By the 1930s, most of the radio stations in the country were affiliated with networks owned by two companies, NBC and CBS. In 1934, a third, the Mutual Radio Network was formed as a cooperative owned by its stations.
The other country which pioneered broadcasting, the UK, and its national network the BBC, became the prototype for state-managed monopoly broadcasting. The abrupt shutdown of the 1920 UK experimental Marconi broadcasts had caused increasing pressure on the Post Office to allow broadcasting. The government wanted to avoid the "chaotic" US experience, but also feared a monopoly by the giant Marconi. On 18 October 1922 it allowed 6 large radio manufacturers to form a consortium, the British Broadcasting Company, which was given a monopoly on broadcasting, supported by a tax on radio sets and a license fee on receivers collected by the Post Office. Initially its 8 stations were allowed regional autonomy, but its visionary general manager, John Reith, centralized production in London and lobbied for the removal of advertising and commercial interests. During Britain's General Strike of 1926, when the newspapers were shut down, the BBC was first allowed to broadcast daytime news, and the country was impressed by its impartial reporting. In late 1926 Reith's proposals were adopted, the BBC was nationalised, and an independent nonprofit chartered corporation was formed, the British Broadcasting Corporation supported solely by a 10 shilling receiver license fee.
During the 1920s AM broadcasting extended to the rest of the world. In general, the Middle East, Asian, and African countries adopted the European model of centralized government-run radio networks, while Latin America adopted the US model of private commercial broadcasting.
The discovery in the 1920s of the "skip" or "skywave" propagation mechanism, in which high frequency radio waves are reflected back to Earth beyond the horizon by the ionosphere, made the shortwave frequencies above 1 MHz, previously considered useless, a useful band for long distance broadcasting.
During the 1920s, '30s, and '40s, a period called the "Golden Age of Radio", AM radio was the main source of home entertainment, filling a role similar to the one television played until the Internet started to replace it in the 2010s. This was a big change in people's lives; for the first time people were getting entertainment from outside the home. Instead of having to settle for more traditional forms of entertainment such as oral storytelling and music from family members, they could listen to Bing Crosby's crooning, a BBC Shakespeare play, or a baseball game at New York's Ebbets Field. New forms of entertainment were created for the new medium, many of which later migrated to television: radio plays, mystery serials, soap operas, quiz shows, variety hours, situation comedies, children's shows. Radio news kept people up-to-date, and remote reporting allowed them to be vicariously "present" at notable events, such as the famous 1937 Hindenburg disaster.
In the 1920s the home radio evolved from a forbidding technological device which was esthetically unattractive and difficult to operate, to a consumer item, a piece of furniture, housed in an attractive wooden cabinet, with simple controls designed for anyone to operate, which occupied a place of honor in the living room. Prior to the introduction of the high-fidelity, long-playing record in the late 1940s, AM radio offered the highest sound quality available in a home audio device. Luxury models offered large speakers, "electric eye" tuning (a special type of vacuum tube, which provided a visual aid in tuning), mechanical push-button "memory" of favorite stations, sometimes with booklets of adhesive labels for the buttons with station call letters, and -- an inexpensive but impressive feature -- shortwave bands that allowed access to distant, often foreign, stations. Accessory, then factory-installed radios became available for cars.
Radio eased the isolation of rural life, and allowed people on farms and in towns to keep up with what was happening in the cities. Politicians could speak to millions of citizens at a time; during the Depression Americans gathered around their radios to listen to Franklin Roosevelt's "fireside chats". This period saw the rise of radio propaganda as a powerful tool of governments, contributing to the rise of fascist and communist ideologies.
AM radio was often noisy. There was no protection from static created by lightning, electrical equipment, and other sources of signal pollution. Especially at night, conflict between nearby and distant stations using a single frequency was common, and required many smaller stations to operate at much reduced power after sundown. Finally, the 10 kilohertz minimum separation between stations in the United States limited fidelity to sounds much lower than the upper limit of human hearing, and the advent of high-fidelity recording equipment created a demand for high-fidelity radio.
As a result of these shortcomings, especially the noise issue, RCA in 1934 hired Edwin Howard Armstrong to test his FM broadcasting system, which started to be deployed in the 1940s, but because of a channel allocation change in 1946 would not achieve dominance over AM until the end of the 1970s.
In the 1940s two new broadcast media evolved in the US which competed with AM: FM radio and television. By the 1950s, the dominance of AM radio over home entertainment was ended. Television replaced AM radio as an evening family pastime; instead of sitting and listening to the radio, families would watch television. By the 1970s FM radio, due to its superior audio quality, attracted serious audiophiles.
The AM radio industry suffered a serious loss of audience and advertising revenue during this time, and the value of an AM broadcast license was eventually to decline substantially. The industry coped with this by developing new "narrowcasting" strategies. Network broadcasting gave way to format broadcasting; instead of broadcasting the same programs all over the country, AM stations specialized in different "formats" which appealed to different audience segments: regional and local news, sports, "talk" programs, programs targeted at minorities. "Talk radio", which avoided the need for the broadcaster to pay music royalties, appeared during this period as a consequence of the less expensive "air time", and the need to develop alternative programming, at reasonable cost, to replace the lost network programming. Rather than live music, stations played cheaper recorded music, and developed the "Top 40" format, which capitalized on (and created) the popularity of new rhythm and blues and rock music.
Listening habits changed in the 1960s due to the introduction of the revolutionary transistor radio, made possible by the invention of the transistor in 1946, which greatly reduced power requirements and allowed people to listen to radio anywhere using only small batteries. Radio became a ubiquitous "companion medium" which people could take with them, and listen while at work, driving, gardening, or at the beach.
In the late 1970s, in an unsuccessful effort to stem the exodus of the music audience to FM, the US AM radio industry developed technology for broadcasting in stereo. Stereo is the standard in the music recording industry, and FM broadcasting had adopted a stereo standard early, in 1961. The technology was challenging because of the narrow 20 kHz bandwidth of the AM channel, and the need for backward compatibility with non-stereo AM receivers. In 1975 the US Federal Communications Commission requested proposals for AM stereo standards, and four competing standards were submitted: Harris Corporation's V-CPM (Variable angle Compatible Phase Multiplex), Magnavox's PMX, Motorola's C-QUAM (Compatible Quadrature Amplitude Modulation), and Kahn-Hazeltine independent sideband system. All except the Kahn-Hazeltine system used variations on the same idea: the mono (Left + Right) signal was transmitted in the amplitude modulation as before, while the stereo (Left − Right) information was transmitted by phase modulation.
In 1980 the FCC chose the Magnavox PMX system as the US standard. The FCC was savagely criticized by the other contenders, and lawsuits erupted. In 1982, the FCC reversed its decision and decided not to enforce a standard but allow multiple systems, to "let the marketplace decide". Meanwhile, other nations adopted AM stereo, many choosing Motorola's C-QUAM. Their choice of a single standard rather than allowing competing standards as the US, resulted in greater acceptance of AM stereo in these markets. In 1993, the FCC made C-QUAM system the US standard.
Globally, the adoption of stereo broadcasting was never great, and declined after 1990. With the continued migration of AM stations away from music to news, sports, and talk formats, receiver manufacturers saw little reason to adopt the more expensive stereo tuners, and thus radio stations have little incentive to upgrade to stereo transmission.
AM radio technology is simpler than frequency modulated (FM) radio, Digital Audio Broadcasting (DAB), satellite radio or HD (digital) radio. An AM receiver detects amplitude variations in the radio waves at a particular frequency. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification, and required no power source other than the radio signal itself.
In North American broadcasting practice, transmitter power input to the antenna for commercial AM stations ranges from about 250 to 50,000 watts. Experimental licenses were issued for up to 500,000 watts radiated power, for stations intended for wide-area communication during disasters. One such superstation was Cincinnati station WLW, which used such power on occasion before World War II. WLW's superpower transmitter still exists at the station's suburban transmitter site, but it was decommissioned in the early 1940s and no current commercial broadcaster in the U.S. or Canada is authorized for such power levels. Some other countries do authorize higher power operation (for example the Mexican station XERF formerly operated at 250,000 watts). Antenna design must consider the coverage desired and stations may be required, based on the terms of their license, to directionalize their transmitted signal to avoid interfering with other stations operating on the same frequency.
Medium-wave (medium frequency, MF) and short-wave (high frequency, HF) radio signals act differently during daytime and nighttime. During the day, MF signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred kilometers from the signal transmitter. However, after sunset, changes in the ionosphere cause MF signals to travel by skywave, enabling 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 the medium wave 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 daytimers. In Australia medium wave stations are not required to reduce their power at night and consequently stations such as the 50,000-watt 3LO can be heard in some parts of New Zealand at night.
In the United States and Canada, some radio stations are granted clear channel status, meaning that few if any other stations have been granted permission to broadcast on or near their frequency. This allows an extended coverage area when skywave propagation takes over at night, starting at or near local sunset. Relatively few stations enjoy clear-channel status; most local MW stations rely on ground-wave coverage only, limiting their target market to their own local area. Non-clear channel stations typically have reduced coverage at night, due to noise and a mish-mash of other stations propagating in via skywave after dark. The area covered by a local station at night without significant skywave interference is known as the nighttime interference-free (NIF) contour, and is typically specified in mV/m (signal strength). The higher the NIF value, the stronger the local signal must be to override nighttime interference, resulting in a smaller coverage area and fewer listeners able to hear the station without interference.
The hobby of listening to long distance signals is known as DX'ing, from an old telegraph abbreviation for "distance". Several nonprofit hobbyist clubs are devoted 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 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. The frequency ranges given here are those that are allocated to stations. Because of the bandwidth taken up by the sidebands, the range allocated for the band as a whole is usually about 5 kHz wider on either side.
Frequencies between the broadcast bands are used for other forms of radio communication, and are not broadcast services intended for reception by the general public.
Because of its relatively low audio quality due to audio bandwidth limitations, and its susceptibility to atmospheric and electrical interference, AM broadcasting now attracts mainly talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1970s in the developed countries. However, in the late 1960s and 1970s, top 40 rock and roll stations in the U.S. and Canada such as WABC and CHUM transmitted highly processed and extended audio to 11 kHz, successfully attracting huge audiences. In the UK during the 1980s, the national speech station, BBC Radio 4, had an FM location, whereas BBC Radio 1, a music station, was confined to AM broadcasts. Frequency response is typically 40 Hz–5 kHz with a 50 dB Signal to noise(S/N) ratio.
The limitation on AM fidelity comes partly from current receiver design, and efforts have been made to improve this, notably the AMAX standards. Moreover, to fit more transmitters on the MW broadcast band in the United States, maximum transmitted audio bandwidth is limited to 10.2 kHz by a National Radio Systems Committee (NRSC) standard adopted by the FCC in June 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz.
AM radio signals can be severely disrupted in large urban centres by metal structures, tall buildings and sources of radio frequency interference (RFI) and electrical noise, such as electrical motors, fluorescent lights, or lightning. 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. Some musical genres – particularly country, oldies, nostalgia and ethnic music – survive on AM, especially in areas where FM frequencies are in short supply or in thinly populated or mountainous areas where FM coverage is poor.
Beginning in the 1950s, carrier current distribution was used. In this modality the AM broadcast signal is not transmitted via antenna, but via electric power cables, which radiate a signal receivable at a short distance from wherever the cables run. The signal is normally blocked by power transformers, so the range depends on the distance before a transformer is encountered. The only surviving use of this technology in the U.S. is in college and high school radio, and for highway emergency warnings, for which limited range is adequate, and whose signs direct drivers to an AM frequency – e.g., "Tune to 1680 AM when flashing" – to receive an emergency message.
While uncommon, AM stereo transmissions are possible using a variety of means. In addition, hybrid digital broadcast systems, which combine (mono analog) AM and digital broadcasting technology, are now being 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, 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 kHz, in addition to stereo sound and text data.
While FM radio can also be received by cable, AM radio generally is not available, although AM stations are sometimes converted into FM cable signals. 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" (Telefonrundspruch in German) transmitted AM signals over telephone lines in the longwave band until 1998, when it was shut down. In the UK, Rediffusion was an early pioneer of AM radio cable distribution.
Some microbroadcasters and pirate radio broadcasters, especially those in the United States under the FCC's Part 15 rules, broadcast on AM to achieve greater range than is possible on the FM band. On mediumwave (AM), such radio stations are often found between 1610 kHz and 1710 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, and FM radio or an MP3 player. Microbroadcasting and pirate radio have been almost completely replaced by streaming audio on the Internet, but some schools or hobbyists still use LPAM as a means of broadcasting.