To broadcast means to cast something in many different directions at the same time. A
radio or television broadcast is a program that is transmitted over airwaves for public
reception by anyone with a receiver tuned to the right signal channel.
A radio program consists of speech, music and other sounds. These sounds can be either
live or pre-recorded. To understand broadcasting, it is necessary to know what sound is.
All sounds consist of vibrations traveling through the air as sound waves. When the waves
reaches a persons ear, the person hears the original sound.
During a typical radio broadcast an electric current is run through the microphone. The
sound waves disturb the current in the microphone creating vibrations in the current that
matches the sound. These electric waves eventually make up the broadcast. The electric
waves representing the sounds produced travel over wires to a control board with many
switches where the sound may be edited by increasing the volume or by making other
adjustments. The modified electric waves then make their way to the transmitter.
The transmitter strengthens the waves representing the broadcast before producing a
carrier wave that it combines with the electric waves. This combination is the radio
signal that eventually reaches our radios.
The transmitter then sends the radio signal to an antenna, which then sends the signal out
into the air as radio waves. Antennas are usually located high up and away from tall
buildings so as to prevent obstruction of these waves. Radio waves cannot be seen, heard
or felt in any way, but radio receivers can pick them up and turn them into sounds.
There are two different ways of transmitting radio signals.
Amplitude modulation is the method of combining the carrier wave and electric wave such
that the amplitude of the carrier waves varies to match the change in the electric waves.
Frequency modulation is the method of combining the carrier and electric wave so the
frequency of the carrier waves changes to match that of the electric waves.
An antenna sends out two kinds of AM radio waves ground waves and sky waves. Ground
waves travel directly to a persons home via the earth, while sky waves travel up
into the sky, hit the ionosphere, and are reflected back to the earth. This enables AM
broadcasts to be received at great distances from the place of broadcast.
An FM radio antenna sends out waves that travel the same direction as AM radio waves, but
these waves are not reflected. Therefore, FM transmissions cannot reach as far as AM
transmissions. The advantages of FM broadcasts are that they are not as affected by static
as much, and that they produce a truer reproduction of the original sound.
Another factor affecting the distance a radio transmission travels is the power of the
transmitter. The power of AM transmitters range from 250 watts (serving only one or two
towns) to 50,000 watts (can be heard over 1,000 miles away). FM transmitters range from
100 watts (15 miles broadcast) to 100,000 watts (65 miles). This shows how FM radio
signals still travel less than 10% the distance of AM stations with ½ the power.
Each station broadcasts on a different frequency, which keeps stations from interfering
with each others broadcasts.
AM stations broadcast on frequencies from 535 to 1605 kilohertz, while FM stations
broadcast on frequencies between 88 to 108 megahertz. Programs carried by radio waves
travel at the speed of light, instead of the slower speed of sound.
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Television broadcasts are similar to radio broadcasts. The television frequency signal is
boosted to carry the picture information through the air.
Transmission of video signals is based on amplitude modulation, with high-frequency
electromagnetic waves acting as the carrier waves to be modulated. This produces the video
part of the television signal, which is then amplified to a power of 1,000 to 10,000
The audio signals are transmitted by frequency modulation. The transmitter then combines
the audio and video signals to produce the television signal. A wire called the
transmission line then carries the television signal to the transmitting antenna, which
releases the signal into the air. Television signals also travel at the speed of light,
with the maximum range of most signals being between 75-150 miles.
Just like radio stations, television stations in the same area broadcast on different
frequencies so their signals do not interfere with one another. The group of frequencies
on which a particular station broadcasts is known as a channel.
Currently, certain websites deliver original or redistributed broadcasts from radio or
television stations using streaming technologies. Web users can visit these sites, and
tune in to these broadcasts by using a program like RealPlayer.
Web broadcasts are available to anyone, with programming scheduled at times, or
pre-packaged transmissions delivered on demand. The advantage of about web broadcasts is
that no matter where in the world you are you can listen to a particular broadcast
whenever you have a connection to the Internet. This is unlike television and radio
broadcasts that can only travel certain distances, and very often need to be distributed
through the various cable systems.