More Waves –
diffraction and communications
Spread through Chapters 11-14
• use the following units: degree (°), hertz (Hz), metre (m),
metre/second (m/s), second (s), [3.1]
• understand that waves can be diffracted when they pass an edge,
[3.8]
• understand that waves can be diffracted through gaps, and that
the extent of diffraction depends on the wavelength and the
physical dimension of the gap, [3.9]
• understand that light waves are transverse waves which can be
reflected, refracted and diffracted, [3.14]
• understand that sound waves are longitudinal waves and how they
can be reflected, refracted and diffracted, [3.26]
• understand the difference between analogue and digital signals,
[3.23]
• describe the advantages of using digital signals rather than
analogue signals, [3.24]
• describe how digital signals can carry more information, [3.25]
• understand how an oscilloscope and microphone can be used to
display a sound wave, [3.29]
• describe an experiment using an oscilloscope to determine the
frequency of a sound wave, [3.30]
Wave behaviours
• Remember that light and sound are waves?
• 3 wave behaviours:
– Reflection
– Refraction
– diffraction
Diffraction
• Diffraction is the tendency of waves to spread
out when they pass the edge of an obstacle
Wave spreads out into “shadow” region
Diffraction at an edge
• Light can be seen to
diffract around the
edges of an object
– We don’t usually notice
this unless we use laser
light
Diffraction through a gap
• As the gap gets smaller, the wave spreads out more,
increasingly filling in the “shadow” area
• We see the most spreading when the size of the gap is roughly
the same as the wavelength
• With light, the wavelength is very small, so you need a very
small gap before you notice light diffraction
• Diffraction applet to try...
Diffraction through a gap
• Large gap –
little diffraction
• Small gap – lots
of diffraction
All waves diffract
• Why can you hear someone in the corridor
when you can’t see them?
– Sound waves diffract through doorways
– Calculate the wavelength of a 1kHz sound wave
– Would you expect it to diffract through a door
efficiently?
Radio Wave Diffraction
• A dish reflector causes
diffraction of a transmitted
radio wave
– The smaller the dish, the more
the signal spreads out
• Longer wavelengths diffract
more than shorter ones
– Microwave transmission needs
to be line-of-sight, long wave
radio can be received behind
obstructions
Diffraction and interference effects in nature
Seeing Sound
• It can be very useful
to represent sounds
visually
Sound to Electricity
• A microphone converts sound energy to electrical
energy
– (Like a loudspeaker in reverse)
• If we display the electrical signal on an
oscilloscope we can see the waveform
Oscilloscope Traces
• The trace on the screen represents the
changing pressure of the sound wave as it
passes the microphone
Oscilloscope Traces
• Higher frequency sounds have more vibrations
per second
• They produce more compressed traces on the
screen
Measuring Frequency
• Looking at the oscilloscope
trace, we can measure the
frequency (pitch) of a sound
• First, measure the period T
(time for one cycle)
– It may be more accurate to
measure the period for several
cycles and divide by the number
of cycles
• Calculate the frequency in Hz
using f  1
T
T
Amaze your family...
Communicating With Waves
• Waves are often used to carry
messages
– Sound
– Radio
– light
Analogue Versus Digital
• An analogue signal is one which
varies continuously with time and
can take any value.
– Natural phenomena are analogue
• A digital signal is one which can
only ever have one of a predetermined set of values and can
change only at some fixed time
interval.
– Now ubiquitous, thanks to computers
• Why bother to do this? There are
several advantages...
Sound recording
• Sound waves are analogue
• Until around 1980, sound recordings were
generally analogue
• Sound was stored as a groove in a vinyl disc or on
magnetic tape.
Analogue Sound Storage
• Vinyl records
– Edges of the groove are a
representation of the
sound wave
– Diamond stylus moves in
groove, motion is
converted to electrical
signal
• Magnetic tape
– Orientation of magnetic
particles in the tape stores
the sound wave
– Changing magnetic field
induces a voltage in the
playback head
Digital Sound Storage
• The analogue sound signal is
converted to a stream of
numbers during recording
• Compact disc
– The numbers are encoded as tiny
pits in the surface of the disc
– A laser beam is reflected less by
pits, the intensity change is
detected and converted to an
electrical digital signal
• mp3 player
– The digital file is stored in
memory or on a hard disc
• The stream of numbers is
converted back into an analogue
signal and played back
digital in
analogue out
Analogue to digital conversion
• At equal time intervals the amplitude of the analogue
signal is sampled.
• For each time slot it’s value is rounded to the nearest
whole number
Black line: analogue signal
Red profile: digitally sampled signal
Digital values can be read off and
strung together for transmission
• At the receiver the reverse process recovers a
representation of the original analogue signal
Analogue to digital conversion
• The digital signal is only
ever an approximation to
the analogue signal
• To make it a closer
representation you need to
take samples closer
together in time, with a
smaller gap between
allowed levels
16 level, 1 x
sampling
128 level, 16 x
sampling
This digital signal better represents the analogue signal
Binary codes
• By converting the digital
values from decimal to
binary it is possible to
represent the signal by a
string of “1”s and “0”s
• These can be simply
transmitted as “on” or
“off” pulses of the carrier
wave – the simplest
possible signal to send
Which is better, analogue or digital?
• The argument rages on...
– Analogue is “cleaner”
– Digital is always just an
approximation to the original
– Digital doesn’t degrade
• But digital won long ago
– It’s the natural “language” of
computers
– Regeneration and
compression are possible
Signals degrade on transmission
• Whatever the
transmission method, two
things happen to signals:
– They gradually get smaller,
and so harder to detect, as
energy is lost
– They pick up noise and
interference
• The first problem can be
fixed with an amplifier, but
this also amplifies the
unwanted noise
Some digital magic
• When an analogue signal has been degraded
with noise there is nothing you can do to
recover the original signal
• Digital is different, though. As long as you can
decide what is a “1” and what is a “0”, you can
reconstruct the original
A digital regenerator
takes you from this...
back to this...!
So digital provides higher quality...
• ... because you can always get back to your
perfect digital signal, but this isn’t possible
with analogue signals
Digital data compression
• Once a digital file has been produced, it is
often possible to make it smaller.
• Compression can use encoding to:
– Retain all the information (lossless)
• e.g. Zip files
– “throw away” information which isn’t needed
(lossy)
• e.g. jpeg, mp3, mpeg-2 formats
Compression Example - HDTV
• Uncompressed HDTV has 1920x1080 pixels, 1024
RGB levels, 25 frames per second, plus stereo
sound = 1.5 Gbit/s
• The bandwidth available to transmit HDTV is
generally ~10Mbit/s (~150 times smaller and
approx same as for an old analogue TV channel)
• A lot of raw data is stripped out without most
viewers noticing – we are very tolerant of
imperfections in moving pictures and our brains
“fill in” the missing details
Video compression techniques
• Don’t retransmit parts on an image that don’t
change from frame-to-frame, only transmit
differences
• Transmit movement of blocks as a “move”
command, rather than pixel-by-pixel
• Average out small changes of colour within a
frame and send that region as a single colour
block
• These only become noticeable when over-applied
or if there is an error on transmission
Transmission error reveals
compression effects in sky image
Example - jpeg data compression
• The compression
factor increases
looking from left to
right
• Over-compression
leads to noticeable
artefacts and lack
of colour depth
Advantages of Digital Communications
• Compatible with computer data
• After being degraded, it is possible to regenerate
a perfect copy of a digital signal
– An analogue one will always remain degraded
• It is easy to manipulate digital data
– We can apply data compression, encryption, error
correction and other signal processing to digital
signals
• Thanks to data compression techniques, digital
signals can require less bandwidth than analogue
ones
– So more information may be stored or transmitted
digitally than by analogue methods
Increasing digital channel capacity
• To send more data on a digital channel you can:
– Shorten the time interval for a bit (increase the
frequency)
– Use more levels to carry information (move from
binary to ternary, or more…)
(but you have to regenerate more often to keen noise down
and still be able to distinguish one level from another)
– Use more exotic modulation methods (PSK, QAM, SSB
etc can send the same data rate in less bandwidth)
(Up to now the extra complexity has cost more than increasing
the frequency)
• use the following units: degree (°), hertz (Hz), metre (m),
metre/second (m/s), second (s), [3.1]
• understand that waves can be diffracted when they pass an edge,
[3.8]
• understand that waves can be diffracted through gaps, and that
the extent of diffraction depends on the wavelength and the
physical dimension of the gap, [3.9]
• understand that light waves are transverse waves which can be
reflected, refracted and diffracted, [3.14]
• understand that sound waves are longitudinal waves and how they
can be reflected, refracted and diffracted, [3.26]
• understand the difference between analogue and digital signals,
[3.23]
• describe the advantages of using digital signals rather than
analogue signals, [3.24]
• describe how digital signals can carry more information, [3.25]
• understand how an oscilloscope and microphone can be used to
display a sound wave, [3.29]
• describe an experiment using an oscilloscope to determine the
frequency of a sound wave, [3.30]