FM broadcasting is a method of radio broadcasting using frequency
modulation (FM) technology. Invented in 1933 by American engineer
Edwin Armstrong, it is used worldwide to provide high-fidelity sound
over broadcast radio.
FM broadcasting is capable of better sound
quality than AM broadcasting, the chief competing radio broadcasting
technology, so it is used for most music broadcasts. FM radio stations
VHF frequencies. The term "FM band" describes the frequency
band in a given country which is dedicated to FM broadcasting.
1 Broadcast bands
2.2 Pre-emphasis and de-emphasis
2.6 Other subcarrier services
3 Reception distance
4 Adoption of FM broadcasting
4.1 United States
4.2.1 United Kingdom
4.4 New Zealand
4.5 Trinidad and Tobago
4.7 Other countries
ITU Conferences about FM
FM broadcasting switch-off
6 Small-scale use of the FM broadcast band
6.1 Consumer use of FM transmitters
6.3 Clandestine use of FM transmitters
7 See also
FM broadcasting by country
FM broadcasting (technical)
9 External links
Main article: FM broadcast band
Throughout the world, the
FM broadcast band
FM broadcast band falls within the
of the radio spectrum. Usually 87.5 to 108.0 MHz is used, or
some portion thereof, with few exceptions:
In the former Soviet republics, and some former Eastern Bloc
countries, the older 65.8–74 MHz band is also used. Assigned
frequencies are at intervals of 30 kHz. This band, sometimes
referred to as the
OIRT band, is slowly being phased out in many
countries. In those countries the 87.5–108.0 MHz band is
referred to as the CCIR band.
In Japan, the band 76–95 MHz is used.
The frequency of an FM broadcast station (more strictly its assigned
nominal center frequency) is usually an exact multiple of
100 kHz. In most of South Korea, the Americas, the Philippines
and the Caribbean, only odd multiples are used. In some parts of
Greenland and Africa, only even multiples are used. In the UK
odd or even are used. In Italy, multiples of 50 kHz are used.
There are other unusual and obsolete
FM broadcasting standards in some
countries, including 1, 10, 30, 74, 500, and 300 kHz. However, to
minimise inter-channel interference, stations operating from the same
or geographically close transmitter sites tend to keep to at least a
500 kHz frequency separation even when closer frequency spacing
is technically permitted, with closer tunings reserved for more
distantly spaced transmitters, as potentially interfering signals are
already more attenuated and so have less effect on neighboring
FM has better rejection of static (RFI) than AM. This was shown in a
dramatic demonstration by
General Electric at its New York lab in
1940. The radio had both AM and FM receivers. With a million-volt arc
as a source of interference behind it, the AM receiver produced only a
roar of static, while the FM receiver clearly reproduced a music
program from Armstrong's experimental FM transmitter in New Jersey.
Crossed dipole antenna of station KENZ's 94.9 MHz, 48 kW transmitter
on Lake Mountain, Utah. It radiates circularly polarized radio waves.
Frequency modulation or FM is a form of modulation which conveys
information by varying the frequency of a carrier wave; the older
amplitude modulation or AM varies the amplitude of the carrier, with
its frequency remaining constant. With FM, frequency deviation from
the assigned carrier frequency at any instant is directly proportional
to the amplitude of the input signal, determining the instantaneous
frequency of the transmitted signal. Because transmitted FM signals
use more bandwidth than AM signals, this form of modulation is
commonly used with the higher (
VHF or UHF) frequencies used by TV, the
FM broadcast band, and land mobile radio systems.
Armstrong's first prototype FM broadcast transmitter, located in the
Empire State Building, New York City, which he used for secret tests
of his system between 1934 and 1935. Licensed as experimental station
W2XDG, it transmitted on 41 MHz at a power of 2 kW
Pre-emphasis and de-emphasis
Random noise has a triangular spectral distribution in an FM system,
with the effect that noise occurs predominantly at the highest audio
frequencies within the baseband. This can be offset, to a limited
extent, by boosting the high frequencies before transmission and
reducing them by a corresponding amount in the receiver. Reducing the
high audio frequencies in the receiver also reduces the high-frequency
noise. These processes of boosting and then reducing certain
frequencies are known as pre-emphasis and de-emphasis, respectively.
The amount of pre-emphasis and de-emphasis used is defined by the time
constant of a simple RC filter circuit. In most of the world a
50 µs time constant is used. In the Americas and South Korea,
75 µs is used. This applies to both mono and stereo
transmissions. For stereo, pre-emphasis is applied to the left and
right channels before multiplexing.
The amount of pre-emphasis that can be applied is limited by the fact
that many forms of contemporary music contain more high-frequency
energy than the musical styles which prevailed at the birth of FM
broadcasting. They cannot be pre-emphasized as much because it would
cause excessive deviation of the FM carrier. Systems more modern than
FM broadcasting tend to use either programme-dependent variable
pre-emphasis; e.g., dbx in the BTSC TV sound system, or none at all.
FM stereo transmission was considered, FM multiplexing of
other types of audio level information was experimented with. Edwin
Armstrong who invented FM was the first to experiment with
multiplexing, at his experimental 41 MHz station W2XDG located on
the 85th floor of the
Empire State Building
Empire State Building in New York City.
These FM multiplex transmissions started in November 1934 and
consisted of the main channel audio program and three subcarriers: a
fax program, a synchronizing signal for the fax program and a
telegraph “order” channel. These original FM multiplex subcarriers
were amplitude modulated.
Two musical programs, consisting of both the Red and Blue Network
program feeds of the NBC
Radio Network, were simultaneously
transmitted using the same system of subcarrier modulation as part of
a studio-to-transmitter link system. In April 1935, the AM subcarriers
were replaced by FM subcarriers, with much improved results.
The first FM subcarrier transmissions emanating from Major
Armstrong’s experimental station KE2XCC at Alpine, New Jersey
occurred in 1948. These transmissions consisted of two-channel audio
programs, binaural audio programs and a fax program. The original
subcarrier frequency used at KE2XCC was 27.5 kHz. The IF
bandwidth was ±5 kHz, as the only goal at the time was to relay
AM radio-quality audio. This transmission system notably used a
75 µs audio pre-emphasis, a technical innovation that became
part of the original FM
Stereo Multiplex Standard.
In the late 1950s, several systems to add stereo to FM radio were
considered by the FCC. Included were systems from 14 proponents
including Crosby, Halstead, Electrical and Musical Industries, Ltd
(EMI), Zenith, and General Electric. The individual systems were
evaluated for their strengths and weaknesses during field tests in
Uniontown, Pennsylvania using KDKA-FM in Pittsburgh as the originating
Crosby system was rejected by the FCC because it was
incompatible with existing subsidiary communications authorization
(SCA) services which used various subcarrier frequencies including 41
and 67 kHz. Many revenue-starved FM stations used SCAs for
“storecasting” and other non-broadcast purposes. The Halstead
system was rejected due to lack of high frequency stereo separation
and reduction in the main channel signal-to-noise ratio. The GE and
Zenith systems, so similar that they were considered theoretically
identical, were formally approved by the FCC in April 1961 as the
FM broadcasting method in the
United States and later
adopted by most other countries. It is important that stereo
broadcasts be compatible with mono receivers. For this reason, the
left (L) and right (R) channels are algebraically encoded into sum
(L+R) and difference (L−R) signals. A mono receiver will use just
the L+R signal so the listener will hear both channels through the
single loudspeaker. A stereo receiver will add the difference signal
to the sum signal to recover the left channel, and subtract the
difference signal from the sum to recover the right channel.
The (L+R) Main channel signal is transmitted as baseband audio limited
to the range of 30 Hz to 15 kHz. The (L−R) signal is
amplitude modulated onto a 38 kHz double-sideband
suppressed-carrier (DSB-SC) signal occupying the baseband range of 23
to 53 kHz.
A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier
frequency and with a precise phase relationship to it, as defined by
the formula below, is also generated. This is transmitted at 8–10%
of overall modulation level and used by the receiver to regenerate the
38 kHz sub-carrier with the correct phase.
The final multiplex signal from the stereo generator contains the Main
Channel (L+R), the pilot tone, and the sub-channel (L−R). This
composite signal, along with any other sub-carriers, modulates the FM
The instantaneous deviation of the transmitter carrier frequency due
to the stereo audio and pilot tone (at 10% modulation) is
displaystyle left[0.9left[ frac A+B 2 + frac A-B 2 sin 4pi
f_ p tright]+0.1sin 2pi f_ p tright]times 75~mathrm kHz
where A and B are the pre-emphasized left and right audio signals and
displaystyle f_ p
=19 kHz is the frequency of the pilot tone. Slight variations in
the peak deviation may occur in the presence of other subcarriers or
because of local regulations.
Another way to look at the resulting signal is that it alternates
between left and right at 38 kHz, with the phase determined by
the 19 kHz pilot signal.
Converting the multiplex signal back into left and right audio signals
is performed by a decoder, built into stereo receivers.
In order to preserve stereo separation and signal-to-noise parameters,
it is normal practice to apply pre-emphasis to the left and right
channels before encoding, and to apply de-emphasis at the receiver
Stereo FM signals are more susceptible to noise and multipath
distortion than are mono FM signals.
In addition, for a given RF level at the receiver, the signal-to-noise
ratio for the stereo signal will be worse than for the mono receiver.
For this reason many stereo FM receivers include a stereo/mono switch
to allow listening in mono when reception conditions are less than
ideal, and most car radios are arranged to reduce the separation as
the signal-to-noise ratio worsens, eventually going to mono while
still indicating a stereo signal is being received.
Louis Dorren invented the Quadraplex system of single
station, discrete, compatible four-channel FM broadcasting. There are
two additional subcarriers in the Quadraplex system, supplementing the
single one used in standard stereo FM. The baseband layout is as
50 Hz to 15 kHz Main Channel (sum of all 4 channels)
(LF+LR+RF+RR) signal, for mono FM listening compatibility.
23 to 53 kHz (sine quadrature subcarrier) (LF+LR) - (RF+RR) Left
minus Right difference signal. This signal's modulation in algebraic
sum and difference with the Main channel is used for 2 channel stereo
23 to 53 kHz (cosine quadrature 38 kHz subcarrier) (LF+RR) -
(LR+RF) Diagonal difference. This signal's modulation in algebraic sum
and difference with the Main channel and all the other subcarriers is
used for the
61 to 91 kHz (sine quadrature 76 kHz subcarrier) (LF+RF) -
(LR+RR) Front-back difference. This signal's modulation in algebraic
sum and difference with the main channel and all the other subcarriers
is also used for the
105 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for
reading services for the blind, background music, etc.
The normal stereo signal can be considered as switching between left
and right channels at 38 kHz, appropriately band limited. The
quadraphonic signal can be considered as cycling through LF, LR, RF,
RR, at 76 kHz.
Early efforts to transmit discrete four-channel quadraphonic music
required the use of two FM stations; one transmitting the front audio
channels, the other the rear channels. A breakthrough came in 1970
KIOI (K-101) in San Francisco successfully transmitted true
quadraphonic sound from a single FM station using the Quadraplex
Special Temporary Authority from the FCC. Following this
experiment, a long term test period was proposed that would permit one
FM station in each of the top 25 U.S. radio markets to transmit in
Quadraplex. The test results hopefully would prove to the FCC that the
system was compatible with existing two-channel stereo transmission
and reception and that it did not interfere with adjacent stations.
There were several variations on this system submitted by GE, Zenith,
RCA, and Denon for testing and consideration during the National
Radio Committee field trials for the FCC. The original
Dorren Quadraplex System outperformed all the others and was chosen as
the national standard for
FM broadcasting in the United
States. The first commercial FM station to broadcast quadraphonic
program content was WIQB (now called WWWW-FM) in Ann Arbor/Saline,
Michigan under the guidance of Chief Engineer Brian Jeffrey Brown.
Various attempts to add analog noise reduction to
FM broadcasting were
carried out in the 1970s and 1980s:
A commercially unsuccessful noise reduction system used with FM radio
in some countries during the late 1970s,
Dolby FM was similar to Dolby
B but used a modified 25 µs pre-emphasis time constant and a
frequency selective companding arrangement to reduce noise. The
pre-emphasis change compensates for the excess treble response that
otherwise would make listening difficult for those without Dolby
A similar system named High Com FM was tested in
July 1979 and December 1981 by IRT. It was based on the Telefunken
High Com broadband compander system, but was never introduced
commercially in FM broadcasting.
Yet another system was the CX-based noise reduction system FMX
implemented in some radio broadcasting stations in the United States
in the 1980s.
Other subcarrier services
Typical spectrum of composite baseband signal
FM broadcasting has included SCA capability since its inception, as it
was seen as another service which licensees could use to create
additional income. Initially the users of SCA services were
private analog audio channels which could be used internally or rented
out, for example Muzak type services.
Radio reading services for the
blind became a common use, and remain so, and there were experiments
with quadraphonic sound. If a station does not broadcast in stereo,
everything from 23 kHz on up can be used for other services. The
guard band around 19 kHz (±4 kHz) must still be maintained,
so as not to trigger stereo decoders on receivers. If there is stereo,
there will typically be a guard band between the upper limit of the
DSBSC stereo signal (53 kHz) and the lower limit of any other
Digital services are now also available. A 57 kHz subcarrier
(phase locked to the third harmonic of the stereo pilot tone) is used
to carry a low-bandwidth digital
Radio Data System signal, providing
extra features such as Alternative
Frequency (AF) and Network (NN).
This narrowband signal runs at only 1,187.5 bits per second, thus is
only suitable for text. A few proprietary systems are used for private
communications. A variant of RDS is the North American RBDS or "smart
radio" system. In
Germany the analog ARI system was used prior to RDS
for broadcasting traffic announcements to motorists (without
disturbing other listeners). Plans to use ARI for other European
countries led to the development of RDS as a more powerful system. RDS
is designed to be capable of being used alongside ARI despite using
identical subcarrier frequencies.
In the United States, digital radio services are being deployed within
the FM band rather than using Eureka 147 or the Japanese standard
ISDB. This in-band on-channel approach, as do all digital radio
techniques, makes use of advanced compressed audio. The proprietary
iBiquity system, branded as "HD Radio", currently is authorized for
"hybrid" mode operation, wherein both the conventional analog FM
carrier and digital sideband subcarriers are transmitted. Eventually,
presuming widespread deployment of HD
Radio receivers, the analog
services could theoretically be discontinued and the FM band become
In the United States, services (other than stereo, quad and RDS) using
subcarriers are sometimes referred to as subsidiary communications
authorization (SCA) services. Uses for such subcarriers include
book/newspaper reading services for blind listeners, private data
transmission services (for example sending stock market information to
stockbrokers or stolen credit card number blacklists to
stores) subscription commercial-free background music
services for shops, paging ("beeper") services, traffic data for
commercial GPS receivers, and providing a program feed for AM
transmitters of AM/FM stations. SCA subcarriers are typically
67 kHz and 92 kHz.
FM radio waves do not travel far beyond the visual horizon, so
reception distances for FM stations are usually limited to
30—40 miles (48.3—64.4 km) They can also be blocked by
hills. This is less than the range of AM radio waves, which because of
their lower frequency can travel as ground waves or reflect off the
ionosphere, so AM radio stations can be received at hundreds
(sometimes thousands) of miles. This is a property of the carrier
wave's typical frequency (and power), not its mode of modulation.
The range of mono FM transmission is related to the transmitter's RF
power, the antenna gain, and antenna height. The U.S. FCC publishes
curves that aid in calculation of this maximum distance as a function
of signal strength at the receiving location.
Many FM stations, especially those located in severe multipath areas,
use extra audio compression to keep essential sound above the
background noise for listeners, occasionally at the expense of overall
perceived sound quality. In such instances, however, this technique is
often surprisingly effective in increasing the station's useful
Adoption of FM broadcasting
One of the first FM radio stations, Edwin Armstrong's experimental
station W2XMN in Alpine, New Jersey, USA. The insets show a part of
the transmitter, and a map of FM stations in 1940
Despite FM having been patented in 1933, commercial FM broadcasting
did not begin until the late 1930s, when it was initiated by a handful
of early pioneer stations including W8HK, Buffalo, New York (now
WGTR and W1XTG/WSRS, both transmitting from Paxton,
Massachusetts (now listed as Worcester, Massachusetts);
W1XSL/W1XPW/WDRC-FM, Meriden, Connecticut (now WHCN);
Alpine, New Jersey
Alpine, New Jersey (owned by Edwin Armstrong
himself, closed down upon Armstrong's death in 1954);
W2XQR/WQXQ/WQXR-FM, New York; W47NV Nashville, Tennessee (signed off
in 1951); W1XER/W39B/WMNE, whose studios were in Boston but whose
transmitter was atop the highest mountain in the northeast United
Mount Washington, New Hampshire
Mount Washington, New Hampshire (shut down in 1948); W9XAO
Milwaukee, Wisconsin (later WTMJ-FM, off air in 1950, returning in
1959 on another frequency). Also of note are
General Electric stations
W2XDA Schenectady and W2XOY New Scotland, New York—two experimental
frequency modulation transmitters on 48.5 MHz—which signed on
in 1939. The two were merged into one station using the W2XOY call
letters on November 20, 1940, with the station taking the WGFM call
letters a few years later, and moving to 99.5 MHz when the FM
band was relocated to the 88–108 MHz portion of the radio
General Electric sold the station in the 1980s, and today
the station is called WRVE.
WEFM (in the Chicago area) and WGFM (in Schenectady, New York) were
reported as the first stereo stations.
The first commercial
FM broadcasting stations were in the United
States, but initially they were primarily used to simulcast their AM
sister stations, to broadcast lush orchestral music for stores and
offices, to broadcast classical music to an upmarket listenership in
urban areas, or for educational programming. By the late 1960s, FM had
been adopted for broadcast of stereo "A.O.R.—'Album Oriented Rock'
Format", but it was not until 1978 that listenership to FM stations
exceeded that of AM stations in North America. During the 1980s and
1990s, Top 40 music stations and later even country music stations
largely abandoned AM for FM. Today AM is mainly the preserve of talk
radio, news, sports, religious programming, ethnic (minority language)
broadcasting and some types of minority interest music. This shift has
transformed AM into the "alternative band" that FM once was. (Some AM
stations have begun to simulcast on, or switch to, FM signals to
attract younger listeners and aid reception problems in buildings,
during thunderstorms, and near high-voltage wires. Some of these
stations now emphasize their presence on the FM dial.)
The medium wave band (known as the AM band because most stations using
it employ amplitude modulation in North America) is
overcrowded in Western Europe, leading to
interference problems and, as a result, many MW frequencies are
suitable only for speech broadcasting.
Belgium, the Netherlands,
Denmark and particularly
Germany were among
the first countries to adopt FM on a widespread scale. Among the
reasons for this were:
The medium wave band in Western
Europe became overcrowded after World
War II, mainly due to the best available medium wave frequencies being
used at high power levels by the Allied Occupation Forces, both for
broadcasting entertainment to their troops and for broadcasting Cold
War propaganda across the Iron Curtain.
After World War II, broadcasting frequencies were reorganized and
reallocated by delegates of the victorious countries in the Copenhagen
Frequency Plan. German broadcasters were left with only two remaining
AM frequencies and were forced to look to FM for expansion.
Public service broadcasters in Ireland and
Australia were far slower
at adopting FM radio than those in either
North America or continental
In the United Kingdom, the
FM broadcasting in 1955, with
three national networks: the Light Programme, Third Programme and Home
Service. These three networks used the sub-band 88.0–94.6 MHz.
The sub-band 94.6–97.6 MHz was later used for
BBC and local
However, only when commercial broadcasting was introduced to the UK in
1973 did the use of FM pick up in Britain. With the gradual clearance
of other users (notably Public Services such as police, fire and
ambulance) and the extension of the FM band to 108.0 MHz between
1980 and 1995, FM expanded rapidly throughout the British Isles and
effectively took over from LW and MW as the delivery platform of
choice for fixed and portable domestic and vehicle-based receivers. In
Ofcom (previously the
Radio Authority) in the UK issues on
demand Restricted Service Licences on FM and also on AM (MW) for
short-term local-coverage broadcasting which is open to anyone who
does not carry a prohibition and can put up the appropriate licensing
and royalty fees. In 2010 around 450 such licences were issued.
When the BBC's radio networks were renamed
Radio 3 and Radio
4 respectively in 1967 to coincide with the launch of
Radio 1, the new
station was the only one of the main four to not have an FM frequency
allocated, which was the case for 21 years. Instead,
Radio 1 shared
Radio 2 FM, on Saturday afternoons, Sunday evenings,
weekday evenings (10pm to midnight) and Bank Holidays. Eventually in
1987 a frequency range of 97.6-99.8 MHz was allocated as police
relay transmitters were moved from the 100 MHz frequency,
starting in London before being broadly completed by 1989.
Italy adopted FM broadcast widely in the early 1970s, but first
experiments made by RAI dated back to 1950, when the "movement for
free radio", developed by so-called "pirates", forced the recognition
of free speech rights also through the use of "free radio media such
as Broadcast transmitters", and took the case to the Constitutional
Court of Italy. The court finally decided in favor of Free Radio. Just
weeks after the court's final decision there was an "FM radio boom"
involving small private radio stations across the country. By the mid
1970s, every city in
Italy had a crowded FM radio spectrum.
Greece was another European country where the FM radio spectrum was
used at first by the so-called "pirates" (both in Athens and
Thessaloniki, the two major Greek cities) in the mid 1970s, before any
national stations had started broadcasting on it; there were many AM
(MW) stations in use for the purpose. No later than the end of 1977,
the national public service broadcasting company EIRT (later also
known as ERT) placed in service its first FM transmitter in the
capital, Athens. By the end of the 1970s, most of Greek territory was
covered by three National FM programs, and every city had many FM
"pirates" as well. The adaptation of the FM band for privately owned
commercial radio stations came far later, in 1987.
FM broadcasting started in Australian capital cities in 1947 on an
"experimental" basis, using an ABC national network feed, consisting
largely of classical music and Parliament, as a programme source. It
had a very small audience and was shut down in 1961 ostensibly to
clear the television band: TV channel 5 (102.250 video carrier) if
allocated would fall within the
VHF FM band (98–108 MHz). The
official policy on FM at the time was to eventually introduce it on
another band, which would have required FM tuners custom-built for
Australia. This policy was finally reversed and
FM broadcasting was
reopened in 1975 using the
VHF band, after the few encroaching TV
stations had been moved. Subsequently, it developed steadily until in
the 1980s many AM stations transferred to FM due to its superior sound
quality and lower operating costs. Today, as elsewhere in the
developed world, most urban Australian broadcasting is on FM, although
AM talk stations are still very popular. Regional broadcasters still
commonly operate AM stations due to the additional range the
broadcasting method offers. Some stations in major regional centres
simulcast on AM and FM bands.
Digital radio using the DAB+ standard
has been rolled out to capital cities.
Like Australia, New Zealand adopted the FM format relatively late. As
was the case with privately owned AM radio in the late 1960s, it took
a spate of 'pirate' broadcasters to persuade a control-oriented,
technology averse government to allow FM to be introduced after at
least five years of consumer campaigning starting in the mid-1970s,
particularly in Auckland. An experimental FM station, FM 90.7, was
Whakatane in early 1982. Later that year, Victoria
University of Wellington's
Radio Active began full-time FM
transmissions. Commercial FM licences were finally approved in 1983,
with Auckland-based 91FM and 89FM being the first to take up the
Broadcasting was deregulated in 1989.
Trinidad and Tobago
Trinidad and Tobago's first FM
Radio station was 95.1FM, now rebranded
as 951 Remix, was launched in March 1976 by the TBC
FM broadcasting began in the late 1960s, carrying several
shows from the One television network which was transferred from the
AM frequency (also known as MW in Turkey). In subsequent years, more
MW stations were slowly transferred to FM, and by the end of the
1970s, most radio stations that were previously on MW had been moved
to FM, though many talk, news and sport, but mostly religious
stations, still remain on MW.
Most other countries implemented
FM broadcasting through 1960s and
expanded their use of FM through the 1990s. Because it takes a large
number of FM transmitting stations to cover a geographically large
country, particularly where there are terrain difficulties, FM is more
suited to local broadcasting than for national networks. In such
countries, particularly where there are economic or infrastructural
problems, "rolling out" a national FM broadcast network to reach the
majority of the population can be a slow and expensive process.
Despite this, mostly in east European counties, national FM broadcast
networks were established in the late 1960s and 1970s. In all
Soviet-dependent countries but GDR, the
OIRT band was used. First
restricted to 68–73 MHz with 100 kHz channel spacing, then
in the 1970s eventually expanded to 65.84–74.00 MHz with
30 kHz channel spacing.
ITU Conferences about FM
The frequencies available for FM were decided by some important
conferences of ITU. The milestone of those conferences is the
Stockholm agreement of 1961 among 38 countries. A 1984 conference
in Geneva made some modifications to the original Stockholm agreement
particularly in the frequency range above 100 MHz.
FM broadcasting switch-off
Further information: FM radio switch-off
Digital audio broadcasting
Small-scale use of the FM broadcast band
Belkin TuneCast II FM microtransmitter
Consumer use of FM transmitters
In some countries, small-scale (Part 15 in
United States terms)
transmitters are available that can transmit a signal from an audio
device (usually an
MP3 player or similar) to a standard FM radio
receiver; such devices range from small units built to carry audio to
a car radio with no audio-in capability (often formerly provided by
special adapters for audio cassette decks, which are becoming less
common on car radio designs) up to full-sized, near-professional-grade
broadcasting systems that can be used to transmit audio throughout a
property. Most such units transmit in full stereo, though some models
designed for beginner hobbyists might not. Similar transmitters are
often included in satellite radio receivers and some toys.
Legality of these devices varies by country. The U.S. Federal
Communications Commission and
Industry Canada allow them. Starting on
1 October 2006, these devices became legal in most countries in the
European Union. Devices made to the harmonised European specification
became legal in the UK on 8 December 2006.
FM broadcast band
FM broadcast band is also used by some inexpensive wireless
microphones sold as toys for karaoke or similar purposes, allowing the
user to use an FM radio as an output rather than a dedicated amplifier
and speaker. Professional-grade wireless microphones generally use
bands in the
UHF region so they can run on dedicated equipment without
Some wireless headphones transmit in the FM broadcast band, with the
headphones tunable to only a subset of the broadcast band.
Higher-quality wireless headphones use infrared transmission or UHF
ISM bands such as 315 MHz, 863 MHz, 915 MHz, or
2.4 GHz instead of the FM broadcast band.
Low-power transmitters such as those mentioned above are also
sometimes used for neighborhood or campus radio stations, though
campus radio stations are often run over carrier current. This is
generally considered a form of microbroadcasting. As a general
rule,[vague] enforcement towards low-power FM stations is stricter
than with AM stations, due to problems such as the capture
effect, and as a result, FM microbroadcasters
generally do not reach as far as their AM competitors.
Clandestine use of FM transmitters
FM transmitters have been used to construct miniature wireless
microphones for espionage and surveillance purposes (covert listening
devices or so-called "bugs"); the advantage to using the FM broadcast
band for such operations is that the receiving equipment would not be
considered particularly suspect. Common practice is to tune the bug's
transmitter off the ends of the broadcast band, into what in the
United States would be TV channel 6 (<87.9 MHz) or aviation
navigation frequencies (>107.9 MHz); most FM radios with
analog tuners have sufficient overcoverage to pick up these
slightly-beyond-outermost frequencies, although many digitally tuned
radios have not.
Constructing a "bug" is a common early project for electronics
hobbyists, and project kits to do so are available from a wide variety
of sources. The devices constructed, however, are often too large and
poorly shielded for use in clandestine activity.
In addition, much pirate radio activity is broadcast in the FM range,
because of the band's greater clarity and listenership, the smaller
size and lower cost of equipment.
FM broadcasting by country
FM broadcasting in Australia
FM broadcasting in Canada
FM broadcasting in Egypt
FM broadcasting in India
FM broadcasting in Japan
FM broadcasting in New Zealand
FM broadcasting in Pakistan
FM broadcasting in the UK
FM broadcasting in the United States
FM broadcasting (technical)
AM stereo (related technology)
FM broadcast band
Long-distance FM reception (FM DX)
Ripping music from FM broadcasts
Radio Data System)
List of broadcast station classes
Lists of radio stations in North America
History of radio
Oldest radio station
^ "Transmission standards for FM sound broadcasting at VHF".
BS.450. International Telecommunications Union. pp. 4–5.
^ RWO. "How FM
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