Small-scale use of the FM broadcast band
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. Starting on 1 Oct 2006 these devices became legal in most
countries in the European Union. Devices made to the harmonised European
5/5/07
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.
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.
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
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
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
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.
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 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. dbx in the BTSC TV sound system or none at all.
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 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. dbx in the BTSC TV sound system or none at all.
Broadcast bands around the world
Throughout the world, the broadcast band is 87.5 to 108.0 MHz, or some portion thereof. In the U.S. it is 87.9 to 107.9 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 at intervals 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 centre frequency) is usually an exact multiple of 100 kHz. In most of the Americas and Caribbean only odd multiples are used. In some parts of Europe, Greenland and Africa only even multiples are used. In Italy, "half-channel"
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.
For more information on FM frequency allocations, see FM broadcast band.
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 at intervals 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 centre frequency) is usually an exact multiple of 100 kHz. In most of the Americas and Caribbean only odd multiples are used. In some parts of Europe, Greenland and Africa only even multiples are used. In Italy, "half-channel"
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.
For more information on FM frequency allocations, see FM broadcast band.
Broadcast bands
The original FM broadcast band in the United States until 1946 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 fixed, mobile, and land mobile radio services.
The name "FM band" is misleading, since one can transmit FM on any frequency. All of these bands mentioned are in the VHF (very high frequency) band which extends from 30 MHz to 300 MHz. Nevertheless "the FM band" is usually understood to refer to FM sound broadcast Band II. The technically correct terms "VHF" were previously in common use within the EU ("UKW", Ultrakurzwellen = Ultra short wave, still widely used in Germany).
The name "FM band" is misleading, since one can transmit FM on any frequency. All of these bands mentioned are in the VHF (very high frequency) band which extends from 30 MHz to 300 MHz. Nevertheless "the FM band" is usually understood to refer to FM sound broadcast Band II. The technically correct terms "VHF" were previously in common use within the EU ("UKW", Ultrakurzwellen = Ultra short wave, still widely used in Germany).
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