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Definition: Broadcasting is
defined as the
transmission of sound or images to a large
number of receivers by radio or television.
Every radio stations needs to
have a radio transmitting system to
broadcast the audio content produced by the
studios and audio devices of the station.
The transmitting system basically comprises
of a radio transmitter and an antenna
system. Additionally, there may be other
devices to enhance the audio quality or the
radio signals themselves. The transmitter
converts the audio into radio signals which
is then fed to the antenna system for
wireless transmission. The antenna then
radiates the radio signal at a particular
preset and licensed frequency which known as
the broadcasting frequency of that radio
station. The frequency is specified in Mega
Hertz (MHz).
The signal transmitted by the
transmitter and antenna system may reach far
away from the transmission site and coverage
area depends upon the power of transmitter,
location of transmitter, antenna gain,
losses in between antenna and transmitter
and surrounding terrain. The pattern of
signal developed or formed after
transmitting the signal at particular
frequency is known as Radiation Pattern.
A radio receiver when tuned
to this particular frequency will receive
the signal and convert it to audible signal.
A radio set may or may not require an
antenna to receive the signals. A radio also
may not be able to catch the signals if the
transmitted signals are very weak or doesn’t
meet the receiving threshold of the radio
set. This is generally defined as the
sensitivity of the radio set which
determines the reception quality of that
particular radio set. The strength or level
of the signal received by a standard radio
receiver at a certain point is called Signal
Strength which is measured in dBµV (decibels
micro Volts) or dBm (decibel meter). The
signal strength charts show signal strength
as dBµV/m (dB in short) (eg. 30 dB). The
higher the number the stronger is the
signal.
1. Radio Transmitter
Radio transmitter can refer
to an entire radio system of multiple
equipments responsible for transmitting a
signal, or just the particular bits of such
a system that amplify the signal up to a
high enough level to be received by some
radio receiver once radiated or transmitted
with an antenna.
The first thing a transmitter
needs is a carrier signal. The carrier
signal is the radio frequency which will be
broadcast by the transmitter. For us, all
the frequency numbers that we use to
identity the radio stations are the carrier
frequencies. For example, if a radio station
broadcasts at 91.2 MHz (Mega Hertz), this
frequency is the carrier frequency that will
carry the audio content produced by the
radio station.
The audio content refers to
something that we're actually trying to
transmit, such as music, voice or anything
audible. This is modulated or inserted onto
the carrier frequency in one of two ways:
1.
Frequency Modulation (FM)
2.
Amplitude Modulation (AM)
After the transmitter
receives the audio content, a transmitter
normally converts the audio signals into
radio signals which can be radiated or
broadcast by the antenna attached to the
transmitter. Additional functions or
equipments may come along with the process
of transmission such as exciter, compressor,
limiter, audio processor, combiner, filters,
amplifiers which have their own usage of
providing the better output in terms of
audio quality and radio signal strength.
2. AM and FM radio
frequencies
The AM carrier frequencies
are in the frequency range of 535 to 1605
kilo Hertz (kHz). These carrier frequencies
are assigned at a difference of 10 kHz
intervals. The FM radio band is from 88 to
108 MHz which is generally licensed from
87.5 MHz to 108.0 MHz for radio broadcasting
stations. USA licenses from 88.1 MHz to
108.1 MHz which is also adopted by many
countries in the world. Japan is the only
country that uses 76-90 MHz band for FM
radio broadcasting.
The FM stations are generally
assigned with center frequencies at 200 kHz
(or 0.2 MHz) separation starting at 88.0
MHz, which allows broadcasting frequencies
for a maximum of 100 stations. This means
that the radio stations could broadcast in
the frequencies of 90.0 MHz, 90.2 MHz, 90.4
MHz, 90.6 MHz and so on. These FM stations
have a 75 kHz maximum deviation from the
center frequency, which leaves 25 kHz upper
and lower "guard bands" to minimize
interaction with the adjacent frequency
bands.
(Continued...)
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