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FM broadcasting is a broadcast technology pioneered by Edwin Howard Armstrong that uses frequency modulation (FM) to provide high-fidelity sound over broadcast radio.

Terminology

The term "FM band" is effectively shorthand for "frequency band in which FM is used for broadcasting". This term can upset purists because it conflates a modulation scheme with a range of frequencies.

The term "VHF" (Very High Frequency) was previously in common use in Europe. "UKW", which stands for Ultrakurzwellen (ultra short wave) in German, is still widely used in Germany, as is "UKV" (Ultrakortvåg) in Sweden and Slovenia (ultra kratki valovi), and "UKF" (Ultrakrótkie fale) in Polish.

Broadcast bands

Throughout the world, the 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 65–74 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 76–90 MHz is used.

The frequency of an FM broadcast station (more strictly its assigned nominal centre frequency) is usually an exact multiple of 100 kHz. In most of the Americas and the Caribbean, only odd multiples are used. In some parts of Europe, Greenland and Africa, only even multiples are used. In Italy, multiples of 50 kHz are used. There are other unusual and obsolete standards in some countries, including 0.001, 0.01, 0.03, 0.074, 0.5, and 0.3 MHz.

For more information on FM frequency allocations, see FM broadcast band.

Modulation characteristics

Modulation

Frequency modulation (FM) is a form of modulation which conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. This form of modulation is commonly used in the FM broadcast band.

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 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 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 North America 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

In the late 1950s, several systems to add stereo to FM radio were considered by the FCC. Included were systems from 14 proponents including Crosley, Halstead, Electrical and Musical Industries, Ltd (EMI), Zenith Electronics Corporation 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 Crosley system was rejected by the FCC because it degraded the signal-to-noise ratio of the main channel and did not perform well under multipath RF conditions. In addition, it did not allow for SCA services because of its wide FM sub-carrier bandwidth. 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 USA and later adopted by most other countries.

It is important that stereo broadcasts should 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 in 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 in the range of 30 Hz to 15 kHz. The (L−R) Sub-channel signal is modulated onto a 38 kHz double-sideband suppressed carrier (DSBSC) 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 transmitter.

The instantaneous deviation of the transmitter carrier frequency due to the stereo audio and pilot tone (at 10% modulation) is

: $\left [ 0.9 \left [ \frac{A+B}{2} + \frac{A-B}{2}\sin4\pi f_pt \right ] + 0.1\sin2\pi f_pt \right ] \times 75~\mathrm{kHz}$

where A and B are the pre-emphasized Left and Right audio signals and $f_p$ =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.

Converting the multiplex signal back into left and right audio signals is performed by a stereo decoder, which is 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 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 FM stereo 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.

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+LB+RF+RB) signal, for mono FM listening compatibility.

23 to 53 kHz (cosine quadrature subcarrier) (LF+LB) - (RF+RB) 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) - (LB+RB) 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+RB) - (LB+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 WIQB (now called WWWW-FM) in Ann Arbor/Saline, Michigan under the guidance of Chief Engineer Brian Brown.

Other subcarrier services

The subcarrier system has been further extended to add other services. Initially these were private analog audio channels which could be used internally or rented out. Radio reading services for the blind are also still common, and there were experiments with quadraphonic sound. If stereo is not on a station, 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 Alternative Frequency (AF) and Network (NN). This narrowband signal runs at only 1187.5 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 all digital.

In the USA services (other than stereo, quad and RDS) using subcarriers are sometimes referred to as subsidiary communications authorisation (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.

Dolby FM

A commercially unsuccessful noise reduction system used with FM radio in some countries during the late 1970s, Dolby FM used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise. See: Dolby noise reduction system.

Distance covered by an FM stereo transmission

The range of an FM mono transmission is related to the transmitter RF power, the antenna gain and antenna height. The FCC (USA) publishes curves that aid in calculation of this maximum distance as a function of signal strength at the receiving location.

For FM stereo, the maximum distance covered is significantly reduced. This is due to the presence of the 38 kHz subcarrier modulation. Vigorous audio processing improves the coverage area of an FM stereo station.

Adoption of FM broadcasting worldwide

Despite FM having been patented in 1933, commercial FM broadcasting did not begin until 1939, when it was initiated by WRVE, the FM station of General Electric's main factory in Schenectady, NY. In countries outside of Europe it took many years for FM to be adopted by the majority of radio listeners.

The first commercial FM broadcasting stations were in the United States, but initially they were primarily used to broadcast classical music to an upmarket listenership in urban areas, and for educational programming. By the late 1960s FM had been adopted by fans of "Alternative Rock" music ("A.O.R. - 'Album Oriented Rock' 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.

Europe

The medium wave band (known as AM 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 Europe.

Australia

FM started in Australia in 1947 but did not catch on and was shut down in 1961 to expand the television band. It was not reopened until 1975. Subsequently, it developed steadily until in the 1980s many AM stations transferred to FM because of its superior sound quality. Today, as elsewhere in the developed world, most Australian broadcasting is on FM, although AM talk stations are still very popular.

New Zealand

Like Australia, New Zealand adopted the FM format relatively late. 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 approved in 1983, with Auckland-based 91FM and 89FM being the first to take up the offer.

United Kingdom

In the United Kingdom, the BBC began FM broadcasting in 1955, with three national networks carrying the Light Programme, Third Programme and Home Service (renamed Radio 2, Radio 3 and Radio 4 respectively in 1967). 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. 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 2006, almost 500 such licenses were issued.

Other countries

Most other countries 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.

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.

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 may not. Similar transmitters are often included in satellite radio receivers and some toys.

Legality of these devices varies by country. The FCC in the US 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 radio microphones

The FM broadcast band can also be used by some inexpensive wireless microphones, but professional-grade wireless microphones generally use bands in the UHF region so they can run on dedicated equipment without broadcast interference. Such inexpensive wireless microphones are generally 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.

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 AM stations due to issues 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); most FM radios with analog tuners have sufficient overcoverage to pick up these slightly-beyond-outermost frequencies, although many digitally tuned radios do 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.

References

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