FM broadcasting | Wikipedia audio article

FM broadcasting | Wikipedia audio article


FM broadcasting is a method of radio broadcasting
using frequency modulation (FM) technology. Invented in 1933 by American engineer Edwin
Armstrong, wide-band FM is used worldwide to provide high-fidelity sound over broadcast
radio. FM broadcasting is capable of better sound quality than DAB/+ radio, and AM broadcasting
under normal listening conditions, so it is used for most music broadcasts. Theoretically
wideband AM can offer equally good sound quality, provided the reception conditions are ideal.
FM radio stations use the VHF frequencies. The term “FM band” describes the frequency
band in a given country which is dedicated to FM broadcasting.==Broadcast bands==Throughout the world, the FM broadcast band
falls within the VHF part 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. Where the OIRT band is used, 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 a multiple of 100 kHz. In most of South Korea, the Americas, the Philippines
and the Caribbean, only odd multiples are used. Some other countries follow this plan
because of the import of vehicles, principally from the USA, with radios that can only tune
to these frequencies. In some parts of Europe, Greenland and Africa, only even multiples
are used. In the UK odd or even are used. In Italy, multiples of 50 kHz are used. In
most countries the maximum permitted frequency error of the unmodulated carrier is specified,
which typically should be within 2000 Hz of the assigned frequency.There are other unusual
and obsolete FM broadcasting standards in some countries, with non standard spacings
of 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. The ITU publishes Protection Ratio graphs which give the minimum spacing between
frequencies based on their relative strengths. Only broadcast stations with large enough
geographic separations between their coverage areas can operate on close or the same frequencies.==Technology=====
Modulation===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.
The maximum frequency deviation of the carrier is usually specified and regulated by the
licensing authorities in each country. For a stereo broadcast, the maximum permitted
carrier deviation is invariably ±75 kHz, although a little higher is permitted in the
United States when SCA systems are used. For a monophonic broadcast, again the most common
permitted maximum deviation is ±75 kHz. However, some countries specify a lower value for monophonic
broadcasts, such as ±50 kHz.===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 use of pre-emphasis becomes a problem because of 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. Pre-emphasizing these high frequency sounds would cause excessive
deviation of the FM carrier. Modulation control (limiter) devices are used to prevent this.
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.===Stereo FM===
Long before 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 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 station. The 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 standard
stereo 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±2 Hz 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 identify a stereo transmission and 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 transmitter. The instantaneous deviation of the transmitter
carrier frequency due to the stereo audio and pilot tone (at 10% modulation) is [ 0.9 [ A
+ B 2 + A
− B 2 sin
⁡ 4
π f p t ] +
0.1 sin
⁡ 2
π f p t ] ×
75 k
H z {\displaystyle \left[0.9\left[{\frac {A+B}{2}}+{\frac
{A-B}{2}}\sin 4\pi f_{p}t\right]+0.1\sin 2\pi f_{p}t\right]\times 75~\mathrm {kHz} }
where A and B are the pre-emphasized left and right audio signals and f p {\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. Most stereo encoders
use this switching technique to generate the 38 kHz subcarrier, but practical encoder designs
need to incorporate circuitry to deal with the switching harmonics. Converting the multiplex
signal back into left and right audio signals is performed by a decoder, built into stereo
receivers. Again, the decoder can use a switching technique to recover the left and right channels.
In addition, for a given RF level at the receiver, the signal-to-noise ratio and multipath distortion
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. As with monaural transmission, it is normal practice to apply
pre-emphasis to the left and right channels before encoding and to apply de-emphasis at
the receiver after decoding. In the U.S. beginning around 2010 using single
sideband subcarriers for stereo was proposed. It is theorized to be more power efficient
and have less noise at the receiver. A handful of radio stations around the country are broadcasting
stereo in this way. It may not be compatible with very old receivers, but it is claimed
most newer receivers cannot tell the difference.===Quadraphonic FM===
In 1969, 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 follows: 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 listener compatibility. 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 Quadraphonic listener.
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 Quadraphonic listener. 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 when KIOI (K-101) in San Francisco successfully transmitted
true quadraphonic sound from a single FM station using the Quadraplex system under 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
Quadraphonic Radio Committee field trials for the FCC. The original Dorren Quadraplex
System outperformed all the others and was chosen as the national standard for Quadraphonic
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.===Noise reduction===
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 decoders.
A similar system named High Com FM was tested in Germany between 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===FM broadcasting has included subsidiary communications
authorization (SCA) services capability since its inception, as it was seen as another service
which licensees could use to create additional income. Use of SCAs was particularly popular
in the US, but much less so elsewhere. Uses for such subcarriers include Radio reading
services for the blind, which became common and remain so, 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, non-native language programming, and providing a program feed for AM transmitters
of AM/FM stations. SCA subcarriers are typically 67 kHz and 92 kHz. Initially the users of
SCA services were private analog audio channels which could be used internally or leased,
for example Muzak type services. 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 subcarrier. 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 station name, Alternative Frequency (AF), traffic data for commercial GPS receivers
and Radio text (RT). 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 to alert motorists that traffic announcements were
being broadcast (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 and Canada, 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 all digital.===Transmission power===
The output power of a FM broadcasting transmitter is one of the parameters that governs how
far a transmission will cover. The other important parameters are the height of the transmitting
antenna and the Antenna gain. Transmitter powers should be carefully chosen so that
the required area is covered without causing interference to other stations further away.
Practical transmitter powers range from a few milliwatts to 80 kW. As transmitter powers
increase above a few kilowatts, the operating costs become high and only viable for large
stations.===Reception distance===
VHF Radio waves usually do not travel far beyond the visual horizon, so reception distances
for FM stations are typically limited to 30–40 miles (48–64 km). They can also be blocked
by hills and to a lesser extent by buildings. The knife edge effect can permit reception
where there is no direct line of sight between broadcaster and receiver. The reception can
vary considerably depending on the position. One example is the Učka mountain range, which
makes constant reception of Italian signals from Veneto and Marche possible in a good
portion of Rijeka, Croatia, despite the distance being over 200 km. Other radio propagation
effects such as tropospheric ducting and Sporadic E can occasionally allow distant stations
to be intermittently received over very large distances, but cannot be relied on for commercial
broadcast purposes. Good reception across the country, is one of the main advantages
over DAB/+ radio. This is still 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 FM transmission is related to
the transmitter’s RF power, the antenna gain, and antenna height. Interference from other
stations is also a factor in some places. In the U.S, the FCC publishes curves that
aid in calculation of this maximum distance as a function of signal strength at the receiving
location. Computer modelling is more commonly used for this around the world.
Many FM stations, especially those located in severe multipath areas, use extra audio
compression/processing 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 range..==
History=====
United States===FM broadcasting began in the late 1930s, when
it was initiated by a handful of early pioneer stations including W8HK, Buffalo, New York
(now WTSS); W1XOJ/WGTR and W1XTG/WSRS, both transmitting from Paxton, Massachusetts (now
listed as Worcester, Massachusetts); W1XSL/W1XPW/WDRC-FM, Meriden, Connecticut (now WHCN); W2XMN/KE2XCC/WFMN,
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 States, 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 spectrum.
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.)===Europe===
The medium wave band (known as the AM band because most stations using it employ amplitude
modulation) was 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 Europe.====United Kingdom====
In the United Kingdom, the BBC began 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 commercial
services. 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 addition, 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 2, 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 airtime with 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====
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====
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.===Australia===
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.===New Zealand===
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 broadcast in 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 offer.[1]. 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 Radio Network.===Turkey===
In Turkey, 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.===Other countries===
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==In 2017, Norway became the first country so
far to completely switch to Digital audio broadcasting, the exception being some local
stations remaining on FM until 2022. The turnover to DAB+ meant that especially rural areas
obtained a far more diverse radio content compared to the FM-only period, as several
new radio stations had started transmissions on DAB+ in the years before the FM switch-off.==Small-scale use of the FM broadcast band
=====
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.The
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 broadcast interference.
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.===Microbroadcasting===
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, 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
(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.==See also=====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 broadcasting AM stereo (related technology)
FM broadcast band FM stereo
Frequency modulation Long-distance FM reception (FM DX)
Ripping music from FM broadcasts RDS (Radio Data System)===Lists===
List of broadcast station classes Lists of radio stations in North America===History===
History of radio Oldest radio station===Bands===
Band I Band II
Band III

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