Chapter 3 - Signalling
3.1 Digital revolution and the death of distance
Learning Outcomes:
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The world has seen an explosive growth of signalling capacity.
A signal channel has a capacity; a maximum rate at which it can transmit information.
Digital signals have the advantage that
 they can be regenerated easily, reducing the effects of noise
 they can readily be switched
 they can be processed and encoded
 they can represent different kinds of signal in a uniform way
Analogue signals are made digital by sampling. The sampling must be done fast enough
to reproduce the highest important frequencies in the signal.
Samples can be digitised with different resolutions (e.g. 8 bit, 16 bit). N bits gives 2 N
different levels of measured values.
Suitable coding can make digital transmission essentially error free.
Lesson 1 – Digital and analogue
Objectives: To be able to describe ways in which communication has changed over the past
hundred years; to be able to recognise the differences between digital and analogue signals; to
be able to change an analogue signal to a digital one.
Activities:
Choose from the questions in 10S Data on the telecommunications explosion. This is a
scene setter and should only take 10-15mins.
Then demonstrate digital signals using 20D What do digital signals look like? And the
optical fibre kit with a radio or MP3 player. If a microphone is used, this provides a good way
into analogue to digital conversion.
Pose the problem of how to change a smoothly varying analogue signal into a discrete digital
one and introduce the idea of sampling. Try 50E Guess a waveform from a sample. This may
take some time, but do not get tempted to go into sampling rates, or bits and bytes too much at
this stage.
50S given below should be given as homework and leads into the next lesson
Out of class:
Piglet
50S Simple sampling
Pooh
30D Digital recording error
correction
Christopher Robin
Lesson 2 – How to digitise a signal, resolution and noise
Objectives: to be able to describe the advantages of digital signals; to be able to explain how
aliasing occurs and how to avoid it; to know that samples can be digitised with different
resolutions and link this to size of the signal in bits and bytes
Activities:
Demonstrate 70S Looking less often to show how an analogue signal can be degraded if you
don’t sample often enough. Show videos of aliasing eg helicopter blades or car wheels. Look at
60D Mains interference with noise and 70D Electrical noise pickup
Discuss the error correction homework and demonstrate 100D CD with a hole
Time will need to be spent going through examples of signal transmission eg fax/email on p59.
Also questions on p64. This is an essential and often misunderstood part of the course and
often comes up in exams.
Piglet
60S Sampling repetitive
motion
70S Sampling and hearing
Pooh
Christopher Robin
3.2 Signalling with electromagnetic waves
Learning outcomes:
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Communication with electromagnetic waves uses frequencies from a few thousand
hertz to infrared frequencies and above, divided into bands used for different purposes.
Electromagnetic waves can be polarised; the orientation of a detector has to take this
into account.
A signal can be analysed into the frequencies it consists of – its spectrum.
A signal channel has a capacity, the rate at which it can transmit information, measured
in bits per second.
The bandwidth of a signal is the range of frequencies in its spectrum. The larger the
bandwidth the greater the rate of transmission of information.
Noise limits the rate at which information can be transmitted.
Lesson 3 – Using waves for communication
Objectives: To know what the range of radio / TV frequencies used for communication is; to be
able to describe why shorter wavelengths can carry more information
80X Charting the electromagnetic spectrum is a good scene setter and a reminder of large
and small numbers and v=fλ. Then focus in on communication by showing 80O Signal bands
for communication. Discuss how digital signals are sent using a carrier wave and consider
why shorter waves are able to carry more information.
Encourage pupils to try 110H Home experiments with television and radio signals
80S Rate of transmission information limited by noise
230E Making an electronic sound generator
Piglet
90S Using the wave equation
Pooh
100S Hearing better, phoning
sooner
Christopher Robin
Lesson 4 - Polarisation
Objectives: To be able to describe polarisation of waves and to know what waves ca be
polarised and what cannot; to know how polarisation is used in communication.
Acitvities:
120D Polarisation of waves contains the essential demonstrations and should be used with
90O Polarisation. 130E Polarisation by scattering and 140D Polarisation by reflected
light go a little further and will be of interest to pupils.
Introduce then next section with an adapted 170E using the PC and frequency analyser of
picoscope.
Piglet
120S Longitudinal and
trasverse waves
Pooh
130S Polarisation in satellite
communication
Christopher Robin
140S Polarisation in practice
Lesson 5 / 6 – Processing sound
Objectives: To be able to use sound processing software in order to generate an filter waves;
to know that sounds are made from a range of frequencies and that computers can be used to
analyse the frequencies present; to be able to use the term bandwidth in relation to signal
transmission rates; be able to calculate the bandwidth of a signal
Activities:
This computer based lesson asks students to work through a variety of exercise aimed at
familiarising them with sound processing: 160S Filtering sounds, 190S Hearing impairment:
using a digital filter, 210S Cleaning up a sound, 220S Building up a sound.
When students are happy with this, introduce the term bandwidth and try 240S Spectra of
pulses, and 260S Bits per second and bandwidth. Along with the student book, these
provide the basics of bandwidth and transmission rate. Only a brief treatment is necessary
There are also presentations regarding harmonics in music and a frequency analyser software
programme to provide real time views of harmonics.
Piglet
150X Waveform and
frequency spectra
160D Oscilloscope displays of
waveforms
Pooh
Christopher Robin