Telecommunication Channels

Lessons

Senior 3 · Physics

Telecommunication Channels

4 lessons

Lesson 1 of 4

Communication and the Communication System

25 min

A voice that crosses the country in a blink

When a relative in Rusizi phones you in Kigali, you hear her voice almost at the instant she speaks. On the way, her voice has been turned into an electrical signal in her phone, sent out as radio waves to a mast, carried as pulses of light along glass fibres and as radio waves again, and turned back into sound in your phone. Shouting could never carry that far: sound in air fades within a few hundred metres and would take minutes to arrive. This unit is about the paths, or channels, that carry information over long distances, and about the signals that travel along them.

In S2 ICT you met cables and wireless links as network media. Here you will look at the physics: what a signal is, why it weakens and picks up noise, and how waves are made to carry speech, music and data.

Definition

Communication: the transfer of information (a message) from a source to a destination. Example: a teacher explaining a sum to the class.

Telecommunication: communication over a distance by electrical signals, radio waves or light. Example: a phone call or a television broadcast.

Signal: a quantity that changes with time to carry information, such as a varying voltage, a radio wave or flashes of light.

Channel: the path along which a signal travels from transmitter to receiver, such as a pair of wires, a coaxial cable, an optical fibre or radio waves through the air.

Experiment: a string telephone

Apparatus: two paper cups (or clean tins with no sharp edges), about 10 m of thin string, and a nail to pierce the cup bottoms.

Thread the string through the bottom of each cup and knot it inside.
Two pupils walk apart until the string is tight. One speaks into a cup while the other holds the second cup to an ear.
Try it with the string slack, then pinch the string halfway along with your fingers.
Try it with a longer string, and then with other pupils talking loudly near the listener.

What happens: the voice is clear only when the string is tight. Pinching the string stops the message completely. A longer string gives a fainter voice, and nearby talk makes the message harder to follow.

Explanation: the speaker's cup changes the sound into vibrations of the string; it acts as a transmitter. The string is the channel. The listener's cup turns the vibrations back into sound; it is the receiver. A slack or pinched string cannot carry the vibrations, so the channel is broken. Energy is lost along the string, so the signal gets weaker with distance: this is attenuation. The other pupils' voices are unwanted sound mixed with the message: this is noise. Every telecommunication system, however advanced, has these same parts and the same two enemies.

A string telephone, from Th. du Moncel's book Le Téléphone (Paris, 1880). The man speaks into one end and the woman listens at the other; the tight string carries the vibrations between them. Engraving, public domain, via Wikimedia Commons

The parts of a communication system

Information source: where the message starts: speech, music, a picture, text or computer data.
Transmitter: changes the message into a signal that suits the channel. It contains an input transducer such as a microphone or camera, which turns sound or light into an electrical signal, and circuits that encode or modulate it (lessons 2 and 4) and amplify it.
Channel: carries the signal. On the way the signal is weakened (attenuated), and noise is added.
Receiver: picks up the weak signal, amplifies it, recovers the message (it decodes or demodulates), and uses an output transducer such as a loudspeaker or a screen to turn it back into sound or light.
Destination: the person or machine the message is for.

Definition

Transducer: a device that changes energy from one form to another. A microphone changes sound into an electrical signal; a loudspeaker changes an electrical signal into sound.

Attenuation: the gradual loss of strength of a signal as it travels along a channel.

Noise: unwanted random signals that add to the message signal in a communication system, for example the hiss on a badly tuned radio or interference from a nearby motor.

System	Source	Transmitter	Channel	Receiver	Destination
Radio broadcast	Presenter's voice	Microphone and radio transmitter	Radio waves through the air	Radio set	Listener
Phone call	Caller's voice	Caller's phone	Radio waves, cables and fibres	Other phone	Person called
Text message	Typed text	Sender's phone	Radio waves, cables and fibres	Receiving phone	Reader
Television	Studio scene and sound	Cameras, microphones and transmitter	Radio waves, satellite or cable	Television set	Viewers

One way or both ways

A broadcast is simplex: listeners cannot reply through their radios. A walkie-talkie user must stop talking and release a button to listen, so it is half duplex. A telephone lets both people talk and hear at the same time: it is full duplex.

Worked example 1: naming the parts

A football commentator speaks into a microphone at Amahoro Stadium, and a farmer hears the match on a small radio in her field. Identify the parts of this communication system and its mode.

Source: the commentator's voice. Transmitter: the microphone (input transducer) and the radio station's transmitter. Channel: radio waves through the air. Receiver: the farmer's radio, whose loudspeaker is the output transducer. Destination: the farmer. The farmer cannot answer through her radio, so the mode is simplex.

Worked example 2: why not just shout?

A radio transmitter is 60 km from a listener. Compare the time taken by the radio signal ( $3.00 \times 10^{8}\,\mathrm{m\,s^{-1}}$ ) with the time sound would take in air (about $340\,\mathrm{m\,s^{-1}}$ ).

t_{\text{radio}} = \frac{d}{v} = \frac{60\,000}{3.00 \times 10^{8}} = 2.0 \times 10^{-4}\,\mathrm{s} = 0.20\,\mathrm{ms}

t_{\text{sound}} = \frac{60\,000}{340} = 176\,\mathrm{s} \approx 2.9\ \text{minutes}

And long before 60 km, the sound would have faded below the noise. Radio waves are almost a million times faster, and can be received far away.

Warning

Common mistakes

Calling the radio set or phone "the channel". The channel is the path between transmitter and receiver: the cable, the fibre or the radio waves.
Thinking noise means only sound. In telecommunication, noise is any unwanted signal: electrical crackle, hiss, or interference from another transmitter.
Forgetting the transducers. The microphone and the loudspeaker are part of the transmitter and the receiver.

Tip

S, T, C, R, D. Source, Transmitter, Channel, Receiver, Destination: "Some Teachers Can Really Dance". Noise always comes in at the Channel.

Introduction to Analog and Digital Communication: the basic block diagram (ALL ABOUT ELECTRONICS). Watch for the role of each block, and how the channel attenuates the signal and adds noise. The video also introduces analogue and digital signals, the topic of lesson 2.

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Practice

Define telecommunication and give two examples.
Draw and label the block diagram of a communication system, and show where noise enters.
In a string telephone, name the transmitter, the channel and the receiver. Why must the string be tight?
A pupil watches a football match on television at home. Identify the source, transmitter, channel, receiver and destination.
State whether each is simplex, half duplex or full duplex: (a) a video call, (b) a television broadcast, (c) a police walkie-talkie.
Name the input transducer and the output transducer in a telephone, and the energy change in each.
A radio signal travels from a transmitter to a receiver 450 km away. How long does it take?
Explain the difference between attenuation and noise, and say which one an amplifier in the receiver cannot remove.

Show answersHide answers

Communication over a distance by electrical signals, radio waves or light; for example a phone call and a television broadcast.
Information source → transmitter → channel → receiver → destination, with noise shown entering the channel.
The speaker's cup is the transmitter, the string the channel and the listener's cup the receiver. A slack string cannot carry the vibrations from one cup to the other.
Source: the match (pictures and sound). Transmitter: cameras, microphones and the TV transmitter. Channel: radio waves (or a satellite or cable link). Receiver: the television set. Destination: the pupil.
(a) Full duplex; (b) simplex; (c) half duplex.
Input: the microphone, sound to electrical energy. Output: the earpiece (a small loudspeaker), electrical energy to sound.
$t = \frac{450\,000}{3.00 \times 10^{8}} = 1.5 \times 10^{-3}\,\mathrm{s} = 1.5\,\mathrm{ms}$ .
Attenuation is the weakening of the signal; noise is unwanted signal added to it. An amplifier can make a weak signal stronger, but it amplifies the noise that has already been added along with it, so it cannot remove noise.

Key points

A communication system has a source, transmitter, channel, receiver and destination; noise is added mainly in the channel.
Transducers change energy from one form to another: microphones at the transmitter, loudspeakers and screens at the receiver.
Signals are weakened by attenuation and spoiled by noise.
Modes: simplex (one way), half duplex (both ways, in turn) and full duplex (both ways at once).

Sources and credits

Image: Fig 7 Le Telephone by T du Moncel Paris 1880.jpg, engraving from Th. du Moncel, Le Téléphone (Paris, 1880), scanned by Benoît Bonnafoux, Wikimedia Commons, public domain.
Video: Introduction to Analog and Digital Communication, The Basic Block Diagram of Communication System, ALL ABOUT ELECTRONICS, embedded from YouTube.

Written by Ganzaa: the lesson text, both diagrams, the table, worked examples and practice exercises.

1A voice that crosses the country in a blink

2Experiment: a string telephone

3The parts of a communication system

4One way or both ways

A voice that crosses the country in a blink

Experiment: a string telephone

The parts of a communication system

One way or both ways