Infrared light, microwaves and radio waves are all used to
transmit information such as computer data, telephone
calls and TV signals.
Infrared light
Information such as computer data and telephone calls can be converted into infrared signals and transmitted by
optical fibres. Optical fibres are able to carry more information than an ordinary cable of the same thickness. In
addition the signals they carry do not weaken so much over long distances. Television remote controls use infrared
light to transmit coded signals to the television set in order to, for example, change channels or adjust the volume.
Microwaves
Microwave radiation can be used to transmit signals such as
mobile phone calls. Microwave transmitters and receivers on
buildings and masts communicate with the mobile telephones
which are in their range.
Certain microwave radiation wavelengths pass
through the Earth’s atmosphere and can be used to
transmit information to and from satellites in orbit.
Radio waves
Radio waves are used to transmit television and radio programmes.
Longer wavelength radio waves are reflected from an electrically charged
layer of the upper atmosphere. This means they can reach receivers that
are not in the line of sight because of the curvature of the Earth’s surface.
Carrying analogue and digital information
Analogue and digital
Before a sound or piece of information is transmitted, it is encoded in the
transmitter in one of the ways described below - analogue or digital. The
receiver must then decode the signal to produce a copy of the original
information or sound.
Analogue signals vary
continuously in amplitude,
frequency or both.
Digital signals are a series of pulses with two states - on
(shown by the symbol ‘1’) or off (shown by the symbol ‘0’). Digital
signals carry more information per second than analogue
signals and they maintain their quality better over long
distances.
Noise
All signals become weaker as they travel long distances. They may
also pick up random extra signals. This is called noise, and it is heard
as crackles and hiss on radio programmes. Noise may also cause an
internet connection to drop, or slow down as the modem tries to
compensate.
An important advantage of digital signals over analogue signals is that if the original signal has
been affected by noise it can be recovered more easily. In analogue signals, when the signal is
amplified to return to its original height, noise gets amplified as well.
Analogue vs. digital - Higher tier
Analogue signals
Noise adds extra random information to analogue signals. Each time the
signal is amplified the noise is also amplified. Gradually, the signal
becomes less and less like the original signal. Eventually, it may be
impossible to make out the music in a radio broadcast from the
background noise, for example.
Digital signals
Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than
the 'on' states of the digital signal. As a result, the electronics in the amplifiers can ignore the noise and it does not
get passed along. This means that the quality of the signal is maintained. This is one reason why television and
radio broadcasters are gradually changing from analogue to digital transmissions. They can also squeeze in more
programmes because digital signals can carry more information per second than analogue signals. Another
advantage of digital signals is that information can be stored and processed by computers.
Coding and storing information
Coding
Coding involves converting information from one form to
another. All types of information can be coded into a
digital signal.
Digital signals are a series of pulses consisting of
just two states, ON (1) or OFF (0). There are no
values in between. The sound is converted into a
digital code of 0s and 1s, and this coded information
controls the short bursts of waves produced by a
source.
When waves are received, the pulses are
decoded to produce a copy of the original sound
or image.
Amount of information
The amount of information needed to
store an image or sound is measured in
bytes (B).
A megabyte is larger than a byte, and a gigabyte is larger than a megabyte.
To store one minute’s worth of music it would take about 1
megabyte, to store an average two hour movie it would
take 1.5 gigabytes.
In general, the more information that
is stored about an image or sound, the
higher the quality.