Transmission of information

In the modern world information is transmitted over large distances. Think of all the things that you use that transmit or receiver information – television, radio, mobile phones, computers (via the internet) and telephone.


The diagram in Figure 1 shows a very simple version of the digital transmission of information.
(a) the input – perhaps a microphone or camera
(b) coder – turns analogue signals into digital signals
(c) transmitter – transmits the digital signal
(d) receiver – receives the digital signal
(e) decoder – turns the digital signal to an analogue signal
(f) amplifier – amplifies the analogue signal
(g) output – loudspeaker or TV

Why do we need an amplifier?
There may be a long distance between the transmitter and receiver – this might be hundreds of miles for transmitting between points on the Earth, thousands of miles if a satellite is used to relay the transmission or even millions of miles in the case of a signal coming back to Earth from a spacecraft flying past Pluto at the outer reaches of the Solar System.

As the original signal travels out from the transmitter it gets weaker. This is because it is spreading out and the amount of energy falling on every square metre of a receiver gets less and less.

Double the distance and the energy is reduced to one quarter. (See Figure 2).
Receiver 2 receives only a quarter of the energy received by receiver 1.

Another reason for the signal getting weaker is that some of the energy may be absorbed by the material that it is passing through.

A beam of sunlight will be weaker if viewed through a number of panes of dirty glass and light received from a street lamp is less if it is a foggy or misty evening.(Photo credit: Konstantin Klagin) In the same way some radio signals can be absorbed by the atmosphere.

When the Mariner spacecraft reached Mars in 1965 its transmitter was sending information back to Earth at a power of 10 W. At that point Mars was more than 210 million kilometres from the Earth so that power of the signal received at tracking station at Goldstone in California was 10^-19 W. This means that the signal strength had dwindled from 10 watts at Mars to a tenth of a million million millionth of a watt at Earth.

Because the signal reaching the receiver is weak it needs to be made stronger. This is done using an amplifier. An amplifier will increase the intensity of a signal that reaches it.

All signals will pick up some background noise during transmission and on their 'journey' from the transmitter to the receiver. Unfortunately amplification also amplifies the noise as well as the signal. This is a real problem for analogue signals. However with a digital signal even with noise the on-off nature of the pulses can still be recognised, the signal can be "cleaned up" and so a much clearer output is possible than with analogue.