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FM broadcasting

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FM radio 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 42-50 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 within the VHF range. All of these bands mentioned are in the VHF band which extends from 30MHz to 300MHz.

Throughout the world, 87.5-108 MHz (or some portion thereof, in the U.S. is 88.1-107.9 MHz) is used as a broadcast band, with one very notable exception: Japan, which uses its own unique 76-90 MHz band with 0.1 MHz channel spacing.

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 the Americas and Caribbean only odd multiples are used. In Italy, multiples of 50 kHz are used.

The bandwidth of an FM transmission is normally somewhat wider than these figures, and depends on whether stereo is used and the manner in which the peak deviation is regulated. In the UK, for example, broadcast licences specify that the nominal bandwidth of the transmission is 270 kHz.

In many countries the minimum spacing for stations intended to serve overlapping areas is 400 kHz.

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 older band, sometimes referred to as the OIRT band is slowly being phased out in many countries.

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.

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.)

FM stereo

The Zenith-GE pilot tone multiplex system was added to FM radio in the early 1960s to allow FM stereo.

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.

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 23kHz 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 57kHz 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.

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.

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; while the FCC in the US and Industry Canada allow them, the United Kingdom's Ofcom prohibits them as an intrusion on licensed broadcasting.

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 recieving 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.

See also

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