FM broadcasting

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

Broadcast bands

The original FM broadcast band in the United States in the early-1940s was on 42 to 50 MHz with 0.2 MHz channel spacing. This band was abandoned after World War II and is now allocated to a seldom-used two-way communications service.

The name "FM band" is misleading, since one can transmit FM on any frequency. All of these bands mentioned are in the VHF band which extends from 30 MHz to 300 MHz. Nevertheless "the FM band" is usually understood to refer to FM sound broadcast band (The terms "VHF" and "UKW" were onetime popular in parts of Europe).

Broadcast bands around the world

Throughout the world, the broadcast band is 87.5 to 108.5 MHz (or some portion thereof: in the U.S. it is 87.9 to 107.9 MHz, in mainland China it is 92 to 108 MHz). Japan is the only exception, using exclusively its own unique 76 to 90 MHz band with 0.1 MHz channel spacing.

In the former Soviet republics, and some Eastern Bloc nations, an additional older band from 65.9 to 74 MHz is also used. Assigned frequencies are multiples of 30 kHz. This band, sometimes referred to as the OIRT band is slowly being phased out in many countries.

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 Americas and Caribbean only odd multiples are used. In some of Europe, Greenland and Africa only even multiples are used. In Italy, multiples of 50 kHz are used. There are other unusual and obsolescent standards in some countries including 0.001, 0.01, 0.03, 0.074, and 0.3 MHz.

Technical characteristics

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, 75 µs is used. This applies to both mono and stereo transmissions. And is applied to baseband audio (not the subcarriers)

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 program-dependent variable pre-emphasis (e.g. J.17) or none at all.)

Dolby FM

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

FM stereo

The Zenith-GE pilot tone multiplex system was added to FM radio in the early 1960s to allow FM stereo and has become the standard method in most countries.

It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are matrixed into sum (M) and difference (S) signals, i.e. M = (L+R)/2 and S = (L−R)/2. A mono receiver will just use the M signal. A stereo receiver will matrix the M and S signals to recover L and R: L = M+S and R = M−S.

The M signal is transmitted as baseband audio in the range 30 Hz to 15 kHz. The S signal is amplitude-modulated onto a 38 kHz suppressed carrier to produce a double-sideband suppressed carrier (DSBSC) signal in the range 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz subcarrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 8-10% of overall modulation level and used by the receiver to regenerate the 38 kHz subcarrier with the correct phase.

The final multiplex signal from the stereo generator is the sum of the baseband audio (M), the pilot tone, and the DSBSC subcarrier (S). This multiplex, along with any other subcarriers, modulates the FM transmitter.

Converting the multiplex signal back to left and right is performed by a stereo decoder, which is built into stereo receivers.

It is normal practice to apply pre-emphasis to the left and right channels before matrixing, and to apply de-emphasis at the receiver after matrixing.

Stereo FM signals are far more suceptable to noise and multipath distortion than mono FM signals. For this reason many FM stereo receivers include a stereo/mono switch to allow listening in mono when reception conditions are less than ideal.

A short lived quadrophonic version of the Zenith-GE system used an additional subcarrier at 76 kHz.

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 there is no stereo on a station, everything from 23 kHz on up can be used for other services. The guard band around 19kHz (±4kHz) 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 Alternate 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 varient of RDS is the North American RBDS or "Smart radio" system while in Germany a system called ARI is used for broadcasting traffic announcements to motorists (without disturbing other listeners) RDS is designed to be capable of being used alongside ARI despite using identical subcarrier frequencies.

The United States is the only country attempting to put digital radio onto FM rather than using EUREKA 147 like most other countries (including Canada), or ISDB like Japan. This in-band on-channel approach results in highly-compressed audio, and blocks any opportunity for new stations to broadcast. The proprietary iBiquity system, branded as "HD Radio", uses subcarriers and extends out somewhat into the sidebands. The hybrid digital (hence "HD") system can later take the bandwidth used by the current analog stereo system, and eventually go all-digital, though this would shut out every existing analog radio.

In the USA services (other than Stereo, Quad and RDS) using subcarriers are sometimes referred to as SCA (subsidiary communications authorisation) 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 programme feed for AM transmitters of AM/FM stations. SCA subcarriers are typically 67 kHz and 92 kHz.

Adoption of FM broadcasting worldwide

Despite being developed in the 1940s, FM broadcasting took a long time to be adopted by the majority of radio listeners.

The first 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 educational programming. By the late 1960s FM had been adopted by fans of "alternative rock" music, but it wasn't until 1978 (the first year that listenership to FM stations exceeded that of AM stations) that FM became mainstream. 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, religious programming, ethnic (minority language) broadcasting and some types of minority interest music. Ironically, this shift has transformed AM into the "alternative band" that FM once was.

Belgium, the Netherlands, Denmark and particularly West Germany were among the first countries to adopt FM on a widespread scale. Among the reasons for this were:

1) The medium wave band in Western Europe is heavily overcrowded leading to severe interference problems as a result most MW frequencies are only suitable for speech broadcasting.

2) Particularly in Germany after WW2 the best available medium wave frequencies were used by the allied occupation forces both for broadcasting entertainment to their troops and for broadcasting cold war propaganda across the Iron curtain

3) The regional structure of German broadcasting meant that the few remaining AM frequencies available for civilian domestic broadcasting fell far short of the number required and the broadcasters looked to FM as an alternative.

Ireland the United Kingdom and Australia were far slower at adopting FM radio than either North America or continental Europe nevertheless by the end of the 1980s most broadcasting in these countries was on FM

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 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.

Non-broadcast uses

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 and 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. The United Kingdom's Ofcom currently prohibits them, but on 14 July 2006 published a consultation document proposing to allow them subject to a maximum ERP of 50 nW.

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

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 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, due to the band's greater clarity and listenership.

External links

*An Introduction to FM MPX
*Stereo for Dummies Many graphs that show waveforms at different points in the FM Multiplex process
*Listen to Live FM Radio Tune a number of FM radios around the world and get live audio (Pay Site)
*CIA Factbook list of stations worldwide

FM broadcasting: Encyclopedia BETA

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