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AM and FM modulated signals for radio. AM () and FM () are types of
(coding). The electrical signal from program material, usually coming from a , is mixed with a
of a specific frequency, then broadcast. In the case of AM, this mixing (modulation) is done by altering the
of the carrier wave with time, according to the original signal. In the case of FM, it is the
of the carrier wave that is varied. A radio receiver (a "radio") contains a demodulator that
the original program material from the broadcast wave.
FM Radio Broadcasting : Big Picture in Full Electromagnetic Spectrum
A commercial 35 kW FM radio transmitter built in the late 1980s. It belongs to FM radio station KWNR in Las Vegas, NV, USA, and broadcasts at a frequency of 95.5 MHz.
FM broadcasting is
(FM) technology. Invented in 1933 by American engineer , it is used worldwide to provide
sound over broadcast . FM broadcasting is capable of better sound quality than , the chief competing radio broadcasting technology, so it is used for most music broadcasts.
frequencies. The term "FM band" describes the
band in a given country which is dedicated to FM broadcasting.
Broadcast bands[]
Main article:
Throughout the world, the FM broadcast band falls within the
part of the . Usually 87.5 to 108.0 MHz is used, or some portion thereof, with few exceptions:
In the , and some former
countries, the older 65–74 MHz band is also used. Assigned frequencies are at intervals of 30 kHz. This band, sometimes referred to as the
band, is slowly being phased out in many countries. In those countries the 87.5–108.0 MHz band is referred to as the
In , 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 , , the
and the , only odd multiples are used. In some parts of ,
and , only even multiples are used. In the UK odd or even are used. In , 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 frequencies.[]
FM has better rejection of static () than AM. This was shown in a dramatic demonstration by
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.
or FM is a form of modulation which conveys information by varying the older
or AM varies the amplitude of the carrier, with its frequency remaining constant. With FM,
from the assigned
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
than AM signals, this form of modulation is commonly used with the higher ( or ) frequencies used by , the , and .
Armstrong's first prototype FM broadcast transmitter, located in the , 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
has a triangular
distribution in an FM system, with the effect that noise occurs predominantly at the highest audio
within the . This can be offset, to a limited extent, by boosting the high frequencies before
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
and , respectively.
The amount of pre-emphasis and de-emphasis used is defined by the
of a simple
circuit. In most of the world a 50 us time constant is used. In the Americas and , 75 us is used. This applies to both
transmissions. For stereo, pre-emphasis is applied to the left and right channels before .
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 . Systems more modern than FM broadcasting tend to use either programme-dependent variable pre- e.g.,
TV sound system, or none at all.
Long before FM stereo transmission was considered, FM multiplexing of other types of audio level information was experimented with. The original station to experiment with multiplexing was W2XDG (New York City) on 41 MHz, located on the 85th floor of the .
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 like 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 York 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 microsecond audio pre-emphasis, a technical innovation that became part of the original FM Stereo Multiplex Standard.
In the late 1950s, several systems to add
to FM radio were considered by the . Included were systems from 14 proponents including Crosby, Halstead, Electrical and Musical Industries, Ltd (), Zenith, and General Electric. The individual systems were evaluated for their strengths and weaknesses during field tests in
in Pittsburgh as the originating station. The
was rejected by the FCC because it was incompatible with existing
(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
limited to the range of 30 Hz to 15 kHz. The (L-R) signal is amplitude modulated onto a 38 kHz
(DSB-SC) signal occupying the baseband range of 23 to 53 kHz.
A 19 kHz , at exactly half the 38 kHz
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
level and used by the receiver to regenerate the 38 kHz
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
{\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
{\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 after decoding.
Stereo FM signals are more susceptible to
and multipath
than are mono FM signals.
In addition, for a given RF level at the receiver, the
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.
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 (cosine quadrature subcarrier) (LF+LR) - (RF+RR) Left minus Right difference signal. This signal's modulation in algebraic sum and difference with the Main channel was used for 2 channel stereo listener compatibility.
23 to 53 kHz (sine quadrature 38 kHz subcarrier) (LF+RF) - (LR+RR) Front minus Back difference signal. 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 (cosine quadrature 76 kHz subcarrier) (LF+RR) - (LR+RF) Diagonal difference signal. 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.
95 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for reading services for the blind, background music, etc.
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
(now called ) in / under the guidance of Chief Engineer Brian Jeffrey Brown.
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
out, for example
type services.
became a common use, and remain so, and there were
sound. If a station does not broadcast in stereo, everything from 23 kHz on up can be used for other services. The
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.
services are now also available. A 57 kHz subcarrier ( to the third
of the stereo pilot tone) is used to carry a low-bandwidth digital
signal, providing extra features such as
(AF) and Network (NN). This
signal runs at only 1,187.5 , thus is only suitable for text. A few
systems are used for private communications. A variant of
is the North American
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.
services are being deployed within the FM band rather than using
or the Japanese standard . This
approach, as do all
techniques, makes use of advanced . The proprietary
as "", currently is authorized for "hybrid" mode operation, wherein both the conventional analog FM carrier and digital
subcarriers are transmitted. Eventually, presuming widespread deployment of
receivers, the analog services could theoretically be discontinued and the FM band become all digital.
In the United States, services (other than stereo, quad and RDS) using subcarriers are sometimes referred to as
(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 and providing a program feed for AM transmitters of AM/FM stations. SCA subcarriers are typically 67 kHz and 92 kHz.
A commercially unsuccessful
system used with FM radio in some countries during the late 1970s, Dolby FM was similar to
but used a modified 25 us pre-emphasis time constant and a frequency selective
arrangement to reduce noise.
A similar system named
was tested in Germany between July 1979 and December 1981 by . It was based on the
broadband compander system, but never introduced commercially in FM broadcasting.
Further information:
The range of mono FM transmission is related to the 's RF power, the , and . The U.S. FCC publishes curves that aid in calculation of this maximum distance as a function of signal strength at the receiving location.
For stereo FM, the range is somewhat[] reduced. This is due to the need to lower the
of the main (sum) signal to accommodate the presence of the 38 kHz DSB-SC (double side-band suppressed-carrier) subcarrier and 19 kHz pilot tone.
Many FM stations, especially those located in severe multipath areas, use extra
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.[]
One of the first FM radio stations, '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 ); //,
(now listed as ); //, Meriden, Connecticut (now WHCN); W2XMN/KE2XCC/WFMN, Alpine, New Jersey (owned by Edwin Armstrong himself, closed down upon Armstrong's death in 1954); , New Y W47NV Nashville, Tennessee (signed off in 1951); //WMNE, whose studios were in Boston but whose transmitter was atop the highest mountain in the northeast United States, ,
(shut down in 1948); W9XAO Milwaukee, Wisconsin (later , 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
to the 88-108 MHz portion of the radio spectrum. General Electric sold the station in the 1980s, and today the station is called .
On June 1, 1961, at 12:01 a.m. (EDT),
became the first FM station in the United States to broadcast in stereo.
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 by fans of "Alternative Rock" music ("A.O.R.—'' Format"), but it wasn't 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.)
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.
and particularly
were among the first countries to adopt FM on a widespread scale. Among the reasons for this were:
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
and for broadcasting
propaganda across the .
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
and Australia were far slower at adopting FM radio than those in either
or continental .
In the , the
began FM broadcasting in 1955, with three national networks: the ,
and . 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,
(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 ,
respectively in
to coincide with the launch of , 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
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.
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.
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 starte 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 started in
in 1947 but did not catch on and was shut down in 1961 to expand the
band: some TV stations were allocated within the VHF band (98-108MHz). 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. 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, , was broadcast in
in early 1982. Later that year, 's
began full-time FM transmissions. Commercial FM licences were finally approved in 1983, with Auckland-based
being the first to take up the offer.. Broadcasting was deregulated in 1989.
In , 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, then in the 1970s eventually expanded to 65–74 MHz.
The frequencies available for FM were decided by some important conferences of . 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.
TuneCast II FM microtransmitter
In some countries, small-scale ( in United States terms) transmitters are available that can transmit a signal from an audio device (usually an
or similar) to a standard FM 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
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
receivers and some toys.
Legality of these devices varies by country. The
allow them. Starting on 1 October 2006, these devices became legal in most countries in the . Devices made to the harmonised European specification became legal in the
on 8 December 2006.
The FM broadcast band is also used by some inexpensive wireless
sold as toys for
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
region so they can run on dedicated equipment without broadcast interference.
Some wireless
transmit in the FM broadcast band, with the headphones tunable to only a subset of the broadcast band. Higher-quality wireless headphones use
such as 315 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 . This is generally considered a form of . As a general rule,[] enforcement towards low-power FM stations is stricter than with AM stations, due to problems such as the ,[] and as a result, FM microbroadcasters generally do not reach as far as their AM competitors.
FM transmitters have been used to construct miniature wireless microphones for
purposes ( 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
activity is broadcast in the FM range, because of the band's greater clarity and listenership, the smaller size and lower cost of equipment.
(related technology)
. ITU Rec. BS.450. . pp. 4–5.
FCC FM Stereo Final Report and Order
"Stereophonic Broadcasting: Technical Details of Pilot-tone System", Information Sheet 1604(4),
Engineering Information Service, June 1970
. USPTO 2015.
Includes tips for multipath & fringe problems.
, , January 3, 1973
. archive.org.
Mielke, E.-J. (1977). Einfluss des Dolby-B-Verfahrens auf die ?bertragungsqualit?t im UKW-H?rrundfunk. Rundfunktechnische Mitteilungen, Vol 21, pp 222 - 228.
IRT (). IRT Technical Report 55/81. Prüfung eines modifizierten HIGH COM-Kompanders für den Einsatz bei der RF-?bertragung im UKW-H?rfunk.
. Radio Rewind 2013.
. Ofcom. 23 November .
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