AM broadcasting is a radio broadcasting technology, which employs
amplitude modulation (AM) transmissions. It was the first method
developed for making audio radio transmissions, and is still used
worldwide, primarily for medium wave (also known as "AM band")
transmissions, but also on the longwave and shortwave radio bands.
The earliest experimental AM transmissions were begun in the early
1900s. However, widespread
AM broadcasting was not established until
the 1920s, following the development of vacuum tube receivers and
transmitters. AM radio remained the dominant method of broadcasting
for the next 30 years, a period called the "Golden Age of Radio",
until television broadcasting became widespread in the 1950s and
received most of the programming previously carried by radio.
Subsequently, AM radio's audiences have also greatly shrunk due to
competition from FM (frequency modulation) radio, Digital Audio
Broadcasting (DAB), satellite radio, HD (digital) radio and Internet
AM transmissions are much more susceptible to interference than FM and
digital signals, and often have limited audio fidelity. Thus, AM
broadcasters tend to specialise in spoken-word formats, such as talk
radio, all news and sports, leaving the broadcasting of music mainly
to FM and digital stations.
AM and FM modulated signals for radio. AM (
Amplitude Modulation) and
Frequency Modulation) are types of modulation (coding). The
electrical signal from program material, usually coming from a studio,
is mixed with a carrier wave of a specific frequency, then broadcast.
In the case of AM, this mixing (modulation) is done by altering the
amplitude (strength) of the carrier wave, proportional to the original
signal. In contrast, in the case of FM, it is the carrier wave's
frequency that is varied. A radio receiver contains a demodulator that
extracts the original program material from the broadcast wave.
1.1 Early broadcasting development
1.2 Early amplitude modulation (AM) transmitter technologies
1.2.2 Arc transmitters
Vacuum tube transmitters
1.4 Early experimental broadcasts
1.5 Organized broadcasting
1.6.1 United States
1.6.2 United Kingdom
1.7 "Golden Age of Radio"
1.8 Decline in popularity
1.9 AM stereo
2 Technical information
2.1 Broadcast band frequencies
2.1.2 Medium-wave broadcasting
2.1.4 VHF AM broadcasting
2.2 Other distribution methods
3 See also
Main article: History of radio
Early broadcasting development
One of the earliest radio broadcasts, French soprano Mariette Mazarin
singing into Lee de Forest's arc transmitter in New York City on
February 24, 1910.
Lee de Forest
Lee de Forest used an early vacuum-tube transmitter to broadcast
returns for the Hughes-Wilson presidential election returns on
November 7, 1916, over 2XG in New York City. Pictured is engineer
The idea of broadcasting — the unrestricted transmission of signals
to a widespread audience — dates back to the founding period of
radio development, even though the earliest radio transmissions,
originally known as "Hertzian radiation" and "wireless telegraphy",
used spark-gap transmitters that could only transmit the
dots-and-dashes of Morse code. In October 1898 a London publication,
The Electrician, noted that "there are rare cases where, as Dr.
[Oliver] Lodge once expressed it, it might be advantageous to 'shout'
the message, spreading it broadcast to receivers in all
directions". However, it was recognized that this would involve
significant financial issues, as that same year The Electrician also
commented "did not Prof. Lodge forget that no one wants to pay for
shouting to the world on a system by which it would be impossible to
prevent non-subscribers from benefiting gratuitously?"
On January 1, 1902,
Nathan Stubblefield gave a short-range "wireless
telephone" demonstration, that included simultaneously broadcasting
speech and music to seven locations throughout Murray, Kentucky.
However, this was transmitted using induction rather than radio
signals, and although Stubblefield predicted that his system would be
perfected so that "it will be possible to communicate with hundreds of
homes at the same time", and "a single message can be sent from a
central station to all parts of the United States", he was unable to
overcome the inherent distance limitations of this technology.
The earliest public radiotelegraph broadcasts were provided as
government services, beginning with daily time signals inaugurated on
January 1, 1905, by a number of U.S. Navy stations. In Europe,
signals transmitted from a station located on the
Eiffel tower were
received throughout much of Europe. In both the United States and
France this led to a small market of receiver lines designed geared
for jewelers who needed accurate time to set their clocks, including
the Ondophone in France, and the De Forest RS-100 Jewelers Time
Receiver in the United States The ability to pick up time signal
broadcasts, in addition to
Morse code weather reports and news
summaries, also attracted the interest of amateur radio enthusiasts.
Early amplitude modulation (AM) transmitter technologies
It was immediately recognized that, much like the telegraph had
preceded the invention of the telephone, the ability to make audio
radio transmissions would be a significant technical advance. Despite
this knowledge, it still took two decades to perfect the technology
needed to make quality audio transmissions. In addition, the telephone
had rarely been used for distributing entertainment, outside of a few
"telephone newspaper" systems, most of which were established in
Europe. With this in mind, most early radiotelephone development
envisioned that the device would be more profitably developed as a
"wireless telephone" for personal communication, or for providing
links where regular telephone lines could not be run, rather than for
the uncertain finances of broadcasting.
Nellie Melba making a broadcast over the Marconi
radio station in England on 15 June 1920
Farmer listening to U.S. government weather and crop reports using a
crystal radio. Public service government time, weather, and farm
broadcasts were the first radio "broadcasts".
A family listening to an early broadcast using a crystal radio
receiver in 1922. Crystal sets, used before the advent of vacuum tube
radios in the 1920s, could not drive loudspeakers, so the family had
to listen on earphones.
The person generally credited as the primary early developer of AM
technology is Canadian-born inventor Reginald Fessenden. The original
spark-gap radio transmitters were impractical for transmitting audio,
since they produced discontinuous pulses known as "damped waves".
Fessenden realized that what was needed was a new type of radio
transmitter that produced steady "undamped" (better known as
"continuous wave") signals, which could then be "modulated" to reflect
the sounds being transmitted.
Fessenden's basic approach was disclosed in U.S. Patent 706,737, which
he applied for on May 29, 1901, and was issued the next year. It
called for the use of a high-speed alternator (referred to as "an
alternating-current dynamo") that generated "pure sine waves" and
produced "a continuous train of radiant waves of substantially uniform
strength", or, in modern terminology, a continuous-wave (CW)
transmitter. Fessenden began his research on audio transmissions
while doing developmental work for the United States Weather Service
on Cobb Island, Maryland. Because he did not yet have a
continuous-wave transmitter, initially he worked with an experimental
"high-frequency spark" transmitter, taking advantage of the fact that
the higher the spark rate, the closer a spark-gap transmission comes
to producing continuous waves. He later reported that, in the fall of
1900, he successfully transmitted speech over a distance of about 1.6
kilometers (one mile), which appears to have been the first
successful audio transmission using radio signals. However, at this
time the sound was far too distorted to be commercially practical.
For a time he continued working with more sophisticated high-frequency
spark transmitters, including versions that used compressed air, which
began to take on some of the characteristics of arc-transmitters.
Fessenden attempted to sell this form of radiotelephone for
point-to-point communication, but was unsuccessful.
Fessenden's work with high-frequency spark transmissions was only a
temporary measure, since his ultimate plan for creating an
audio-capable transmitter was to take a basic electrical alternator,
which normally rotated at speeds that produced alternating current of
at most a few hundred cycles-per-second (Hz), and greatly increase its
rotational speed, in order to create electrical currents of
tens-of-thousands of cycles-per-second (kHz), thus producing a steady
continuous-wave transmission when connected to an aerial. The next
step, adopted from standard wire-telephone practice, was to insert a
simple carbon microphone into the transmission line, in order to
modulate the carrier wave signal to produce AM audio transmissions.
However, it would take many years of expensive development before even
a prototype alternator-transmitter would be ready, and a few years
beyond that for high-power versions to become available.
Fessenden worked with General Electric's (GE) Ernst F. W.
Alexanderson, who in August 1906 delivered an improved model which
operated at a transmitting frequency of approximately 50 kHz,
although at low power. The alternator-transmitter achieved the goal of
transmitting quality audio signals, but the lack of any way to amplify
the signals meant they were somewhat weak. On December 21, 1906,
Fessenden made an extensive demonstration of the new
alternator-transmitter at Brant Rock, Massachusetts, showing its
utility for point-to-point wireless telephony, including
interconnecting his stations to the wire telephone network. As part of
the demonstration, speech was transmitted 18 kilometers (11 miles) to
a listening site at Plymouth, Massachusetts.
An American Telephone Journal account of the December 21
alternator-transmitter demonstration included the statement that "It
is admirably adapted to the transmission of news, music, etc. as,
owing to the fact that no wires are needed, simultaneous transmission
to many subscribers can be effected as easily as to a few",
echoing the words of a handout distributed to the demonstration
witnesses, which stated "[Radio] Telephony is admirably adapted for
transmitting news, stock quotations, music, race reports, etc.
simultaneously over a city, on account of the fact that no wires are
needed and a single apparatus can distribute to ten thousand
subscribers as easily as to a few. It is proposed to erect stations
for this purpose in the large cities here and abroad." However,
other than two holiday transmissions reportedly made shortly after
these demonstrations, Fessenden does not appear to have conducted any
radio broadcasts for the general public, or to have even given
additional thought about the potential of a regular broadcast service,
and in a 1908 article providing a comprehensive review of the
potential uses for his radiotelephone invention, he made no references
Because there was no way to amplify electrical currents at this time,
modulation was usually accomplished by a carbon microphone inserted
directly in the antenna wire. This meant that the full transmitter
power flowed through the microphone, and even using water cooling, the
power handling ability of the microphones severely limited the power
of the transmissions. Ultimately only a small number of large and
powerful Alexanderson alternators would be developed. However, they
would be almost exclusively used for long-range radiotelegraph
communication, and occasionally for radiotelephone experimentation,
but were never used for general broadcasting.
Almost all of the continuous wave AM transmissions made prior to 1915
were made by versions of the arc converter transmitter, which had been
initially developed by
Valdemar Poulsen in 1903. Arc transmitters
worked by producing a pulsating electrical arc in an enclosed hydrogen
atmosphere. They were much more compact than alternator transmitters,
and could operate on somewhat higher transmitting frequencies.
However, they suffered from some of the same deficiencies. The lack of
any means to amplify electrical currents meant that, like the
alternator transmitters, modulation was usually accomplished by a
microphone inserted directly in the antenna wire, which again resulted
in overheating issues, even with the use of water-cooled microphones.
Thus, transmitter powers tended to be limited. The arc was also
somewhat unstable, which reduced audio quality. Experimenters who used
arc transmitters for their radiotelephone research included Ernst
Ruhmer, Quirino Majorana, Charles "Doc" Herrold, and Lee de Forest.
Vacuum tube transmitters
Advances in vacuum tube technology (called "valves" in British usage),
especially after around 1915, revolutionized radio technology. Vacuum
tubes devices could be used to amplify electrical currents, which
overcame the overheating issues of needing to insert microphones
directly in the transmission antenna circuit.
Vacuum tube transmitters
also provided high-quality AM signals, and could operate on higher
transmitting frequencies than alternator and arc transmitters.
Non-governmental radio transmissions were prohibited in many countries
during World War I, but AM radiotelephony technology advanced greatly
due to wartime research, and after the war the availability of tubes
sparked a great increase in the number of amateur radio stations
experimenting with AM transmission of news or music. Vacuum tubes
remained the central technology of radio for 40 years, until
transistors began to dominate in the 1960s, and are still used in the
highest power broadcast transmitters.
1938 Zenith Model 12-S vacuum-tube console radio, capable of picking
up mediumwave and shortwave AM transmissions. "All Wave" receivers
could also pick up the third AM band, longwave stations.
Unlike telegraph and telephone systems, which used completely
different types of equipment, most radio receivers were equally
suitable for both radiotelegraph and radiotelephone reception. In 1903
and 1904 the electrolytic detector and thermionic diode (Fleming
valve) were invented by
Reginald Fessenden and John Ambrose Fleming,
respectively. Most important, in 1904–1906 the crystal detector, the
simplest and cheapest AM detector, was developed by G. W. Pickard.
Homemade crystal radios spread rapidly during the next 15 years,
providing ready audiences for the first radio broadcasts. One
limitation of crystals sets was the lack of amplifying the signals, so
listeners had to use earphones, and it required the development of
vacuum-tube receivers before loudspeakers could be used. The dynamic
cone loudspeaker, invented in 1924, greatly improved audio frequency
response over the previous horn speakers, allowing music to be
reproduced with good fidelity. AM radio offered the highest sound
quality available in a home audio device prior to the introduction of
the high-fidelity, long-playing record in the late 1940s.
Listening habits changed in the 1960s due to the introduction of the
revolutionary transistor radio, which made possible by the invention
of the transistor in 1946. Their compact size — small enough to fit
in a shirt pocket — and lower power requirements, compared to vacuum
tubes, meant that for the first time radio receivers were readily
portable. The transistor radio became the most widely used
communication device in history, with billions manufactured by the
Radio became a ubiquitous "companion medium" which people could
take with them anywhere they went.
Early experimental broadcasts
The demarcation between what is considered "experimental" and
"organized" broadcasting is largely arbitrary. Listed below are some
of the early AM radio broadcasts, which, due to their irregular
schedules and limited purposes, can be classified as "experimental":
Christmas Eve, 1906 Until the early-1930s, it was generally accepted
that Lee de Forest's series of demonstration broadcasts begun in 1907
were the first transmissions of music and entertainment by radio.
However, in 1932 an article prepared by Samuel M. Kintner, a former
associate of Reginald Fessenden, asserted that Fessenden had actually
conducted two earlier broadcasts. This claim was based solely on
information included in a January 29, 1932, letter that Fessenden had
sent to Kintner. (Fessenden subsequently died five months before
Kintner's article appeared). In his letter, Fessenden reported that,
on the evening of December 24, 1906 (Christmas Eve), he had made the
first of two broadcasts of music and entertainment to a general
audience, using the alternator-transmitter at Brant Rock,
Massachusetts. Fessenden remembered producing a short program that
included playing a phonograph record, followed by his playing the
violin and singing, and closing with a bible reading. He also stated
that a second short program was broadcast on December 31 (New Year's
Eve). The intended audience for both transmissions was primarily
shipboard radio operators along the Atlantic seaboard. Fessenden
claimed these two programs had been widely publicized in advance, with
Christmas Eve broadcast heard "as far down" as Norfolk, Virginia,
while the New Year Eve's broadcast had been received in the West
Indies. However, extensive efforts to verify Fessenden's claim
during both the 50th and 100th anniversaries of the claimed
broadcasts, which included reviewing ships' radio log accounts and
other contemporary sources, have so far failed to confirm that these
reported holiday broadcasts actually took place.
Lee de Forest
Lee de Forest conducted a series of test broadcasts
beginning in 1907, and was widely quoted promoting the potential of
organized radio broadcasting. Using a series of arc transmitters, he
made his first entertainment broadcast on February 1907, transmitting
electronic telharmonium music from his "Highbridge" laboratory station
in New York City. This was followed by tests that included, in the
fall, Eugenia Farrar singing "I Love You Truly". Additional
promotional events in New York included live performances by famous
Metropolitan Opera stars such as Mariette Mazarin, Geraldine Farrar,
and Enrico Caruso. He also broadcast phonograph music from the Eiffel
Tower in Paris. His company equipped the U.S. Navy's Great White Fleet
with experimental arc radiotelephones for their 1908 around-the-world
cruise, and the operators broadcast phonograph music as the ships
entered ports like San Francisco and Honolulu.
June 1910 In a June 23, 1910, notarized letter that was published in a
catalog produced by the Electro Importing Company of New York, Charles
"Doc" Herrold reported that, using one of that company's spark coils
to create a "high frequency spark" transmitter, he had successfully
broadcast "wireless phone concerts to local amateur wireless men".
Herrold lived in San Jose, California.
1913 Robert Goldschmidt began experimental radiotelephone
transmissions from the Laeken station, near Brussels, Belgium, and by
March 13, 1914 the tests had been heard as far away as the Eiffel
Tower in Paris.
January 15, 1920
Broadcasting in the UK began with impromptu news and
phonograph music over 2MT, the 15 kW experimental tube
transmitter at Marconi's factory in Chelmsford, Essex, at a frequency
of 120 kHz. On June 15, 1920, the
Daily Mail newspaper sponsored
the first scheduled British radio concert, by the famed Australian
opera diva Nellie Melba. This transmission was heard throughout
much of Europe, including in Berlin, Paris, The Hague, Madrid, Spain,
Chelmsford continued broadcasting concerts with noted
performers. A few months later, in spite of burgeoning popularity, the
government ended the broadcasts, due to complaints that the station's
longwave signal was interfering with more important communication, in
particular military aircraft radio.
People who weren't around in the Twenties when radio exploded can't
know what it meant, this milestone for mankind. Suddenly, with radio,
there was instant human communication. No longer were our homes
isolated and lonely and silent. The world came into our homes for the
first time. Music came pouring in. Laughter came in. News came in. The
world shrank, with radio.
— Red Barber, sportscaster, 
In July 1912, Charles "Doc" Herrold began weekly broadcasts in San
Jose, California, using an arc transmitter.
Broadcasting in Germany began 1922 as a Post Office monopoly on a
subscription basis, using sealed receivers which could only receive
Following World War I, the number of stations providing a regular
broadcasting service greatly increased, primarily due to advances in
vacuum-tube technology. In response to ongoing activities, government
regulators eventually codified standards for which stations could make
broadcasts intended for the general public, for example, in the United
States formal recognition of a "broadcasting service" came with the
establishment of regulations effective December 1, 1921, and
Canadian authorities created a separate category of "radio-telephone
broadcasting stations" in April 1922. However, there were numerous
cases of entertainment broadcasts being presented on a regular
schedule before their formal recognition by government regulators.
Some early examples include:
July 21, 1912 The first person to transmit entertainment broadcasts on
a regular schedule appears to have been Charles "Doc" Herrold, who
inaugurated weekly programs, using an arc transmitter, from his
Wireless School station in San Jose, California. The broadcasts
continued until the station was shut down due to the entrance of the
United States into World War I in April 1917.
March 28, 1914 The Laeken station in Belgium, under the oversight of
Robert Goldschmidt, inaugurated a weekly series of concerts,
transmitted at 5:00 p.m. on Saturday evenings. These continued for
about four months until July, and were ended by the start of World War
I. In August 1914 the Laeken facilities were destroyed, to keep
them from falling into the hands of invading German troops.
November 1916 De Forest perfected "Oscillion" power vacuum tubes,
capable of use in radio transmitters, and inaugurated daily broadcasts
of entertainment and news from his New York "Highbridge" station, 2XG.
This station also suspended operations in April 1917 due to the
prohibition of civilian radio transmissions following the United
States' entry into World War I. Its most publicized program was
the broadcasting of election results for the Hughes-Wilson
presidential election on November 7, 1916, with updates provided by
wire from the
New York American
New York American offices. An estimated 7,000 radio
listeners as far as 200 miles (320 kilometers) from New York heard
election returns interspersed with patriotic music.
April 17, 1919 Shortly after the end of World War I, F. S.
McCullough at the Glenn L. Martin aviation plant in Cleveland, Ohio,
began a weekly series of phonograph concerts. However, the
broadcasts were soon suspended, due to interference complaints by the
November 6, 1919 The first scheduled (pre-announced in the press)
Dutch radio broadcast was made by Nederlandsche
PCGG at The Hague, which began regular concerts broadcasts. It
found it had a large audience outside the Netherlands, mostly in the
UK. (Rather than true AM signals, at least initially this station used
a form of narrowband FM, which required receivers to be slightly
deturned in order to receive the signals using slope detection.)
Late 1919 De Forest's New York station, 2XG, returned to the airwaves
in late 1919 after having to suspend operations during World War
I. The station continued to operate until early 1920, when it was
shut down because the transmitter had been moved to a new location
May 20, 1920 Experimental Canadian Marconi station XWA (later CFCF,
deleted in 2010 as CINW) in Montreal began regular broadcasts, and
claims status as the first commercial broadcaster in the world.
June 1920 De Forest transferred 2XG's former transmitter to San
Francisco, California, where it was relicensed as 6XC, the "California
Theater station". By June 1920 the station began transmitting
daily concerts. De Forest later stated that this was the "first
radio-telephone station devoted solely" to broadcasting to the
August 20, 1920 On this date the
Detroit News began daily
transmissions over station 8MK (later WWJ), located in the newspaper's
headquarters building. The newspaper began extensively publicizing
station operations beginning on August 31, 1920, with a special
program featuring primary election returns. Station management
later claimed the title of being where "commercial radio broadcasting
November 2, 1920 Beginning on October 17, 1919, Westinghouse
Frank Conrad began broadcasting recorded and live music on a
semi-regular schedule from his home station, 8XK in Wilkinsburg,
Pennsylvania. This inspired his employer to begin its own ambitious
service at the company's headquarters in East Pittsburgh,
Pennsylvania. Operations began, initially with the call sign 8ZZ, with
an election night program featuring election returns on November 2,
1920. As KDKA, the station adopted a daily schedule beginning on
December 21, 1920. This station is another contender for the title
of "first commercial station".
A live radio play being broadcast at NBC studios in New York. Most
1920s through 1940s network programs were broadcast live.
Because most longwave radio frequencies were used for international
radiotelegraph communication, a majority of early broadcasting
stations operated on mediumwave frequencies, whose limited range
generally restricted them to local audiences. One method for
overcoming this limitation, as well as a method for sharing program
costs, was to create radio networks, linking stations together with
telephone lines to provide a nationwide audience.
In the U.S., the American Telephone and Telegraph Company (AT&T)
was the first organization to create a radio network, and also to
promote commercial advertising, which it called "toll" broadcasting.
Its flagship station, WEAF (now WFAN) in New York City, sold blocks of
airtime to commercial sponsors that developed entertainment shows
containing commercial messages. AT&T held a monopoly on quality
telephone lines, and by 1924 had linked 12 stations in Eastern cities
into a "chain". The
Radio Corporation of America (RCA), General
Electric and Westinghouse organized a competing network around its own
flagship station, RCA's WJZ (now WABC) in New York City, but were
hampered by AT&T's refusal to lease connecting lines or allow them
to sell airtime. In 1926 AT&T sold its radio operations to RCA,
which used them to form the nucleus of the new NBC network. By the
1930s, most of the major radio stations in the country were affiliated
with networks owned by two companies, NBC and CBS. In 1934, a third
national network, the
Mutual Radio Network
Mutual Radio Network was formed as a cooperative
owned by its stations.
A BBC receiver licence from 1923. The British government required
listeners to purchase yearly licences, which financed the stations.
A second country which quickly adopted network programming was the
United Kingdom, and its national network quickly became a prototype
for a state-managed monopoly of broadcasting. A rising interest in
radio broadcasting by the British public pressured the government to
reintroduce the service, following its suspension in 1920. However,
the government also wanted to avoid what it termed the "chaotic" U.S.
experience of allowing large numbers of stations to operate with few
restrictions. There were also concerns about broadcasting becoming
dominated by the Marconi company. Arrangements were made for six
large radio manufacturers to form a consortium, the British
Broadcasting Company (BBC), established on 18 October 1922, which was
given a monopoly on broadcasting. This enterprise was supported by a
tax on radio sets sales, plus an annual license fee on receivers,
collected by the Post Office. Initially the eight stations were
allowed regional autonomy. In 1927, the original broadcasting
organization was replaced by a government chartered British
Broadcasting Corporation. an independent nonprofit supported
solely by a 10 shilling receiver license fee. A mixture of
populist and high brow programmes were carried by the National and
"Golden Age of Radio"
Main article: Golden Age of Radio
When broadcasting began in 1920, music was played on air without
regard to its copyright status. Music publishers challenged this
practice as being copyright infringement, which for a time kept many
popular tunes off the air, and this 1925 editorial cartoon shows a
rich publisher muzzling two radio performers. The radio industry
eventually agreed to make royalty payments.
The period from the 1920s to the 1940s is often called the "Golden Age
of Radio". During this period AM radio was the main source of home
entertainment, until it was replaced by television. For the first time
entertainment was provided from outside the home, replacing
traditional forms of entertainment such as oral storytelling and music
from family members. New forms were created, including radio plays,
mystery serials, soap operas, quiz shows, variety hours, situation
comedies and children's shows.
Radio news, including remote reporting,
allowed listeners to be vicariously present at notable events.
Radio greatly eased the isolation of rural life. Political officials
could now speak directly to millions of citizens. One of the first to
take advantage of this was American president Franklin Roosevelt, who
became famous for his fireside chats during the Great Depression.
However, broadcasting also provided the means to use propaganda as a
powerful government tool, and contributed to the rise of fascist and
Decline in popularity
In the 1940s two new broadcast media, FM radio and television, began
to provide extensive competition with the established broadcasting
services. The AM radio industry suffered a serious loss of audience
and advertising revenue, and coped by developing new strategies.
Network broadcasting gave way to format broadcasting: instead of
broadcasting the same programs all over the country, stations
individually adopted specialized formats which appealed to different
audiences, such as regional and local news, sports, "talk" programs,
and programs targeted at minorities. Instead of live music, most
stations began playing less expensive recorded music. However, the
ongoing development of alternative, and superior, transmission
systems, including Digital Audio
Broadcasting (DAB), satellite radio,
and HD (digital) radio, continued the decline of the popularity of the
traditional broadcast technologies. These new options, including the
Internet streaming, particularly resulted in the
reduction of shortwave transmissions, as international broadcasters
found ways to more easily reach their audiences.
Main article: AM stereo
In the late 1970s, spurred by the exodus of musical programming to FM
stations, the AM radio industry in the United States developed
technology for broadcasting in stereo. (
FM broadcasting in the United
States adopted a common stereo standard in 1961.) The effort was
challenging due to the need to limit the transmissions to a
20 kHz bandwidth, while also making the transmissions backward
compatible with existing non-stereo receivers. In 1975 the U.S.
Federal Communications Commission
Federal Communications Commission (FCC) 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 selected the
Magnavox PMX system as the U.S. standard,
but this choice was immediately contested by the losing applicants in
the courts. In 1982, the FCC reversed its decision and decided not to
enforce a common standard, but instead allow multiple systems to be
installed, in order to "let the marketplace decide". Other nations
adopted AM stereo, most commonly choosing Motorola's C-QUAM. The
choice of a single standard, rather than allowing competing standards
as the U.S., resulted in greater acceptance of
AM stereo in these
markets. In 1993, the FCC again updated its policy, now making the
C-QUAM system the U.S. standard, however, it has been subsequently
installed by very few stations. There was a limited adoption of AM
stereo worldwide, and interest 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 later transmission systems. An AM
receiver detects amplitude variations in the radio waves 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 centres, 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 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.
In order 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 to 10.2 kHz, limiting occupied
bandwidth to 20.4 kHz. The former audio limitation was
15 kHz resulting in bandwidth of 30 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 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
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.
Longwave (also known as
Low frequency (LF)) (148.5-283.5 kHz)
Broadcasting stations in this band are assigned transmitting
frequencies in the range 153–279 kHz., and generally maintain
9 kHz spacing.
Longwave assignments for broadcasting only exist
in ITU Region 1 (Europe, Africa, and northern and central Asia),
and is not allocated elsewhere. Individual stations have coverage
measured in the hundreds of kilometers, however there are 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 (also known as
Medium frequency (MF)), which is by far the
most commonly used
AM broadcasting band. In ITU Regions 1 and 3,
transmitting frequencies run from 531 to 1602 kHz, with
9 kHz spacing (526.5–1606.5 kHz), and in ITU Region 2
(the Americas), transmitting frequencies are 530 to 1700 kHz,
using 10 kHz spacing (525–1705 kHz), including the ITU
Extended AM broadcast band, authorized in Region 2, between 1605
and 1705 kHz, previously used for police radio.
Shortwave (also known as
High frequency (HF)) transmissions range from
approximately 2.3 to 26.1 MHz, divided into 14 broadcast
Shortwave broadcasts generally use a narrow 5 kHz channel
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 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 1937, the FCC announced a
second band of AM stations, consisting of 75 channels spanning
from 41.02 to 43.98 MHz, which were informally called Apex.
The 40 kHz spacing between adjacent frequencies was four times
that of the 10 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 January 1941, after the FCC determined
that establishing a band of FM stations was preferable.
Other distribution methods
Main article: Carrier current
Beginning in the mid-1930s, starting with "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. 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.
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
Digital Radio Mondiale
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.
See also: Low-power broadcasting
Some microbroadcasters, especially those in the United States
operating under the FCC's
Part 15 rules, and pirate radio operators on
mediumwave and shortwave, achieve greater range than possible on the
FM band. On mediumwave these stations often transmit on 1610 kHz
to 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, 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.
Amplitude Modulation Signalling System, a digital system for adding
low bitrate information to an AM broadcast signal.
MW DXing, the hobby of receiving distant AM radio stations on the
History of radio
Extended AM broadcast band
CAM-D, a hybrid digital radio format for AM broadcasting
List of 50 kW AM radio stations in the United States
Lists of radio stations in North America
Oldest radio stations
^ "Wireless Telegraphy", The Electrician (London), October 14, 1898,
Telegraphy at the Physical Society, The Electrician
(London), January 28, 1898, pages 452-453.
^ "Kentucky Inventor Solves Problem of Wireless Telephony", The Sunny
South, March 8, 1902, page 6.
^ "The First Wireless Time Signal" (letter from Captain J. L. Jayne),
Electrician and Mechanic, January 1913, page 52. (Reprinted from The
American Jeweler, October 1912, page 411.)
^ "Vest-Pocket Wireless Receiving Instrument", Electrical Review and
Western Electrician, April 11, 1914, page 745.
Radio Apparatus" (advertisement),
Radio Amateur News, October 1919,
^ U.S. Patent 706,737, submitted May 29, 1901, and issued August 12,
1902, to Reginald Fessenden.
^ "Experiments and Results in Wireless Telephony", by John Grant, The
American Telephone Journal, January 26, 1907, pages 49-51.
^ The Continuous Wave by Hugh G. J. Aitken, 1985, page 61.
^ Aitken (1985), page 62.
^ "Fessenden, Reginald A. ''Inventing the Wireless Telephone and the
Future''". Ewh.ieee.org. Retrieved 2017-07-22.
^ Aitken (1985), page 69.
^ a b "Experiments and Results in Wireless Telephony" by John Grant,
The American Telephone Journal. Part I: January 26, 1907, pages 49-51;
Part II: February 2, 1907, pages 68-70, 79-80.
^ "Dec. 21, 1906: A Very Significant Date in Radio" by James E.
O'Neal, December 22, 2016. (radioworld.com)
^ "Wireless Telephony" by Reginald A. Fessenden, Transactions of the
American Institute of Electrical Engineers, Vol. XXVII (1908), Part I,
^ "Method of Producing Alternating Currents With a High Number of
Vibrations" U.S. patent 789,449, filed June 10, 1903, and granted May
9, 1905, to Valdemar Poulsen.
^ "The Versatile Audion" by H. Winfield Secor, Electrical
Experimenter, February 1920, pages 1000-1001, 1080-1083.
^ McNicol, Donald (1946) Radio's Conquest of Space, p. 336-340
^ "Pittsburgh's Contributions to Radio" by S. M. Kintner, Proceedings
of the Institute of
Radio Engineers, December 1932, pages 1849-1862.
^ Fessenden: Builder of Tomorrows by Helen Fessenden, 1940, pages
^ "Fessenden — The Next Chapter" by James E. O'Neal,
December 23, 2008. (radioworld.com)
^ "Fessenden, World's First Broadcaster?" by James E. O'Neal, Radio
World, October 25, 2006. (radioworld.com)
^ Father of
Radio by Lee de Forest, 1950, page 225.
^ I Looked and I Listened by Ben Gross, 1954, page 48.
^ "Mysterious Voices Startled Him: Wizard Isbell Thought He Heard
Angels Talking", Hawaiian Star, November 25, 1908, page 1.
^ Electro Importing Company catalog page, reproduced in Charles
Herrold, Inventor of
Broadcasting by Gordon Greb and Mike Adams,
2003, page 6.
^ "De la T.S.F. au Congo-Belge et de l'école pratique de Laeken aux
concerts radiophoniques" (Wireless in the Belgian Congo and from the
Laeken Training School to
Radio Concerts) by Bruno Brasseur, Cahiers
d'Histoire de la Radiodiffusion, Number 118, October-December 2013.
^ "A Newspaper's Use of the
Radio Phone", The Wireless Age, November
1920, page 10.
^ The History of
Broadcasting in the United Kingdom: Vol. I: The Birth
Broadcasting by Asa Briggs, 1961, pages 49-50.
^ The Broadcasters by Red Barber, 1970, pages 11-12.
^ "Miscellaneous: Amendments to Regulations",
Radio Service Bulletin,
January 3, 1922, page 10.
Broadcasting Stations", Winnipeg Evening Tribune,
April 25, 1922, page 5.
^ "Will Give Concert by Wireless Telephone", San Jose Mercury Herald,
July 21, 1912, page 27.
^ "Hear Tenor Through Wireless", Washington Evening Star, 29 March
1914, Part one, page 2.
^ "20th Anniversary of First Broadcast" by Raymond Braillard, The
(Singapore) Straits Times, 25 April 1934, page 17 (reprinted from the
Broadcasting Corporation's World-Radio, 30 March 1934, page
^ "Wireless Transmission of News". Telephony. Chicago: Telephony
Publishing Co. 71 (27): 32–33. December 10, 1916. Retrieved December
^ "Election Returns Flashed by
Radio to 7,000 Amateurs", The
Electrical Experimenter, January, 1917, page 650. (archive.org)
^ "Hear Caruso Sing by Wireless Thursday!", Cleveland Plain Dealer,
April 17, 1919, page 1.
^ "Stop Wireless Concerts Here", Cleveland Plain Dealer, May 29, 1919,
^ "Communications Commentary: PCGG", Electronics & Wireless World,
February 1986, page 26.
^ "Foot Ball Score—Via Wireless Telephone" by Morris Press, Radio
Amateur News, December 1919, pages 295, 321.
^ "Wireless Concert Given for Ottawa", Montreal Gazette, May 21, 1920,
^ "Ninth California Theatre Concert", Pacific Coast Musical Review,
May 29, 1920, page 9.
^ "Electrical Home Visitors to Hear Wireless Concert", San Francisco
Chronicle, June 20, 1920, page 8.
^ "'Broadcasting' News by Radiotelephone" (letter from Lee de Forest),
Electrical World, April 23, 1921, page 936.
^ "The News Radiophone To Give Vote Results", Detroit News, August 31,
1920, pages 1-2.
^ WWJ advertisement,
Broadcasting Magazine, August 20, 1945, page 31.
Radio Amateur: Wireless Telephone Here" by C. E. Urban,
Pittsburgh Gazette Times, Sixth section, page 13.
^ "To Give Election Results by Radio", Cleveland Plain Dealer, October
28, 1920, page 10.
^ KDKA, The Wireless Age, August 1922, page 40.
^ "Announcing the National
Broadcasting Company, Inc."
(advertisement), Reading (Pennsylvania) Eagle, September 13, 1926,
^ Hilmes, Michele (2011). Network Nations: A Transnational History of
British and American Broadcasting. Routledge. p. 6.
Radio Fans to Pay Tribute to John Bull", Popular Radio, November
1922, page 222.
^ "A Concise History of British Radio, 1922-2002". google.com.
^ a b "A Concise History of British Radio, 1922-2002".
^ "Whatever Happened to
Shortwave Radio?" by James Careless, March 8,
^ a b c "AM Stereo Broadcasting" (fcc.gov)
Federal Communications Commission
Federal Communications Commission rules, 47 CFR §2.106
^ "America's Apex
Broadcasting Stations of the 1930s" by John
Schneider, Monitoring Times Magazine, December 2010.
^ The Gas Pipe Networks by Louis M. Bloch, Jr, 1980.
^ "Sammlung alter Biennophone-Radios". Biennophone.ch. Retrieved 23
^ "Digital Radio" (fcc.gov)
Analog and digital audio broadcasting
VHF (low / mid / high)
L band (UHF)
Commercial radio providers
Sirius XM Holdings
MPEG-1 Audio Layer II
Audio data compression
Audio signal processing
AM stereo formats)
AM expanded band
Error detection and correction
FM broadcast band
Shortwave relay station
History of radio
Comparison of radio systems
Cable protection system
Prepay mobile phone
The Telephone Cases
Timeline of communication technology
Undersea telegraph line
Edwin Howard Armstrong
John Logie Baird
Alexander Graham Bell
Jagadish Chandra Bose
Lee de Forest
Erna Schneider Hoover
Charles K. Kao
Alexander Stepanovich Popov
Johann Philipp Reis
Vladimir K. Zworykin
Free-space optical communication
Network switching (circuit
Public Switched Telephone
World Wide Web