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Types of radio |
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Types
The best known are the AM and FM stations;
these include both commercial, public and
nonprofit varieties as well as student-run
campus radio stations and hospital radio
stations can be found throughout the
developed world.
Although now being eclipsed by
internet-distributed radio, there are many
stations that broadcast on shortwave bands
using AM technology that can be received
over thousands of miles (especially at
night). For example, the BBC has a full
schedule transmitted via shortwave. These
broadcasts are very sensitive to atmospheric
conditions and sunspots.
AM
AM stations were the earliest broadcasting
stations to be developed. AM refers to
amplitude modulation, a mode of broadcasting
radio waves by varying the amplitude of the
carrier signal in response to the amplitude
of the signal to be transmitted.
One of the advantages of AM is that its
unsophisticated signal can be detected
(turned into sound) with simple equipment.
If a signal is strong enough, not even a
power source is needed; building an
unpowered crystal radio receiver was a
common childhood project in the early years
of radio.
AM broadcasts occur on North American
airwaves in the medium wave frequency range
of 530 to 1700 kHz (known as the "standard
broadcast band"). The band was expanded in
the 1990s by adding nine channels from 1620
to 1700 kHz. Channels are spaced every 10
kHz in the Americas, and generally every 9
kHz everywhere else.
Many countries outside of the U.S. use a
similar frequency band for AM transmissions.
Europe also uses the long wave band. In
response to the growing popularity of FM
radio stereo radio stations in the late
1980s and early 1990s, some North American
stations began broadcasting in AM stereo,
though this never gained popularity, and
very few receivers were ever sold.
AM radio has some serious shortcomings.
The signal is subject to interference from
electrical storms (lightning) and other EMI.
Fading of the signal can be severe at night.
AM signals exhibit diurnal variation,
traveling much longer distances at night. In
a crowded channel environment this means
that the power of regional channels which
share a frequency must be reduced at night
or directionally beamed in order to avoid
interference, which reduces the potential
nighttime audience. Some stations have
frequencies unshared with other stations in
North America; these are called clear
channel stations. Many of them can be heard
across much of the country at night. (This
is not to be confused with Clear Channel
Communications, merely a brand name, which
currently owns many U.S. radio stations.)
AM radio transmitters can transmit audio
frequencies up to 15 kHz (now limited to 10
kHz in the US due to FCC rules designed to
reduce interference), but most receivers are
only capable of reproducing frequencies up
to 5 kHz or less. At the time that AM
broadcasting began in the 1920s, this
provided adequate fidelity for existing
microphones, 78 rpm recordings, and
loudspeakers. The fidelity of sound
equipment subsequently improved
considerably, but the receivers did not.
Reducing the bandwidth of the receivers
reduces the cost of manufacturing and makes
them less prone to interference. AM stations
are never assigned adjacent channels in the
same service area. This prevents the
sideband power generated by two stations
from interfering with each other. Bob Carver
created an AM stereo tuner employing notch
filtering that demonstrated that an AM
broadcast can meet or exceed the 15 kHz
base-band bandwidth allocated to FM stations
without objectionable interference. After a
several years, the tuner was discontinued.
Bob Carver had left the company and the
Carver Corporation later cut the number of
models produced before discontinuing
production completely. AM stereo broadcasts
declined with the advent of HD Radio.
FM
FM refers to frequency modulation, and
occurs on VHF airwaves in the frequency
range of 88 to 108 MHz everywhere (except
Japan and Russia). Japan uses the 76 to 90
MHz band. FM stations are much more popular
in economically developed regions, such as
Europe and the United States, especially
since higher sound fidelity and stereo
broadcasting became common in this format.
FM radio was invented by Edwin H. Armstrong
in the 1930s for the specific purpose of
overcoming the interference (static) problem
of AM radio, to which it is relatively
immune. At the same time, greater fidelity
was made possible by spacing stations
further apart. Instead of 10 kHz apart, they
are 200 kHz apart. (For example, the
difference between the lowest current FM
frequency in the U.S., 88.1 MHz, and the
next lowest, 88.3 MHz. This was far in
advance of the audio equipment of the 1940s,
but wide interchannel spacing was chosen to
take advantage of the noise-suppressing
feature of wideband FM.
In fact, 200 kHz is not needed to
accommodate an audio signal — 20 kHz to 30
kHz is all that is necessary for a
narrowband FM signal. The 200 kHz bandwidth
allowed room for ±75 kHz signal deviation
from the assigned frequency plus a 20 kHz
guardband to eliminate adjacent channel
interference. The larger bandwidth allows
for broadcasting a 15 kHz bandwidth audio
signal plus a 38 kHz stereo "subcarrier" — a
piggyback signal that rides on the main
signal. Additional unused capacity is used
by some broadcasters to transmit utility
functions such as background music for
public areas, GPS auxiliary signals, or
financial market data.
The AM radio problem of interference at
night was addressed in a different way. At
the time FM was set up, the available
frequencies were far higher in the spectrum
than those used for AM radio - by a factor
of approximately 100. Using these
frequencies meant that even at far higher
power, the range of a given FM signal was
much lower, thus its market was more local
than for AM radio. The reception range at
night is the same as at daytime.
The original FM radio service in the U.S.
was the Yankee Network, located in New
England. See [5] [6] [7]. Broadcasting began
in the early 1940s but did not pose a
significant threat to the AM broadcasting
industry. It required purchase of a special
receiver. The frequencies used were not
those used today: 42 to 50 megahertz. The
change to the current frequencies, 88 to 108
megahertz, began after the end of World War
II, and it was to some extent imposed by AM
radio owners so as to attempt cripple what
was by now realized to be a potentially
serious threat.
FM radio on the new band had to begin from
the ground floor. As a commercial venture it
remained a little-used audio enthusiast's
medium until the 1960s. The more prosperous
AM stations, or their owners, acquired FM
licenses and often broadcast the same
programming on the FM station as on the AM
station (simulcasting). The FCC limited this
practice in the 1970s. By the 1980s, since
almost all new radios included both AM and
FM tuners (without any government mandate),
FM became the dominant medium, especially in
cities. Because of its greater range, AM
remained more common in rural environments.
Digital
Digital radio broadcasting has emerged,
first in Europe (the UK in 1995 and Germany
in 1999), and later in the United States.
The European system is named DAB, for
Digital Audio Broadcasting, and uses the
public domain EUREKA 147 system. In the
United States, the IBOC system is named HD
Radio and owned by a consortium of private
companies called iBiquity. An international
non-profit consortium Digital Radio Mondiale
(DRM), has introduced the public domain DRM
system.
It is expected that for the next 10 to 20
years, all these systems will co-exist,
while by 2015 to 2020 digital radio may
predominate, at least in the developed
countries.
Satellite Radio
Satellite radiobroadcasters are slowly
emerging, but the enormous entry costs of
space-based satellite transmitters, and
restrictions on available radio spectrum
licenses has restricted growth of this
market. In the USA and Canada, just two
services, XM Satellite Radio and Sirius
Satellite Radio exist while WorldSpace
Satellite Radio is available in Asia,
Europe, Africa and some parts of Americas.
From
Wikipedia. |
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