Elements of Comms Systems V2

8/6/2019 Elements of Comms Systems V2

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Elements of

Analogue and Digital

Communications Systems

Nick Brackenbury

Oct 2010 Nick Brackenbury 1


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Telecommunications definition

Communications at a distance

The transmission of signals over a

long distance

Electrical / electronic

communication



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Signal types

Examples of analogue (natural) signal types are:

Voice, transmission is called telephony

Still image such as photograph or fax

Moving pictures such as TV and Video

Machine and instrument position and control

Examples of digital (man made) signal types are:

Computer data, alphanumeric

Digitised analogue signals



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Analogue and Digital

Analogue signals are continuous and

have an infinite number of variables

Analogue signals are expensive to handle

Analogue signals are prone to noise and

other forms of distortion and corruption

Time seconds

SignalAmplitude

Volts or Amps

DC steady state

DC variable

AC sine wave

AC variablenegative

positive

Example quantisation of analogue waveform into

9 levels to form digital equivalent

One sample is taken every clock pulse

Each sample from +5 volts to -5 volts can be

converted to a binary value forming a bit stream

positive

negative

SignalAmplitude

inVolts Time seconds

1 0 1 1 0 1 0 1

Timeseconds

ONOr

OFF

Analogue signals can be represented by adigital equivalent

Digital electronics is robust and cheap

Digital signals can also represent datacharacters and mathematical values

Digital signals can be easily stored, searched,altered and transmitted



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Transmission media

There are four methods of transporting

telecommunication signals: Wire pair house telephone

Coaxial cable video and aerial

Fibre optic able to support many channels

per cable, underground

Radio able to support one to many

channels through the air



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Cost issues of telecomms channel

Signal bandwidth needs

Distance between originator and destination

Single signal or multi signal

Send only, receive only or both concurrently

Answer back response time

Level of backround noise

Channel distorting effects

Rate of attenuation (signal loss)



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Practical bandwidth examples

Voice over telephone 300 Hz to 3400 Hz

Old style PC modem 56 KHz (compression)

Black & white video 50 KHz to 200 KHz

Colour video 200 KHz to 500 KHz

TeleVision colour 3 MHz

Audio Hi Fidelity 30 Hz to 20 KHz

Machine control 1 Hz to 10 KHz



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Methods of improving channel

bandwidth

Methods of improving the bandwidth of a physical channel are:

Data compression

Techniques to make signal immune to noise

Techniques to make signal immune to distortion

Repeaters to reconstruct the signal to its original form

All of these methods are achieved significantly better by: Converting analogue signals into digital signals

Sending the digital over the transmission medium

Convert the digital signal back into original analogue signal



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Definition of communication

Communication is the transfer of meaningfulinformation from:

one location sender, source, originatorto another destination, receiver

Originator

Voice, video,Movement, keyboard

Destination

Hear, display,Actuate, print

Information, message

Channel for analogueor digital signalling



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Electronic communication

terminology

Simple block diagram of a telecommunication

system sending information in one direction

only

Transmitter ReceiverInformation flow

Transmission link /medium / channel



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Definition of information

INFORMATION is a physical pattern betweenoriginator and destination that has been

assigned an understood meaning. Thepattern must be unique for each uniquemessage. The originator must be capableof sending the message and the

destination must be capable of receivingthe message.



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Information codes

If I say to you it is raining outside you willimmediately have a full and correct image

of this event in your mind. I do not have togive a full description of water dropsfalling from the sky. The words raining

and outside are mutually understood

words, or codes, for this communicationevent.



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Encoding and decoding

Electronic communication systems: Use a form of encoding information

before sending a message

And Use an identical decoding process on the

received message at the destination

originator

Encoded information flow


encoder decoder destination



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The Purpose of encoding

and decoding

The purpose of the encoding / decoding process is to:

Make transmission through the channel possible

Improve efficiency of information flow

Secrecy, only selected destination can decode

Minimise the effects of noise and distortion

Originatingsignal

Modulated carrier


modulation De-modulation

Destinationsignal



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QUIZ on Elements of

Telecommunications

1. Define telecommunications

2. What do analogue signals suffer from but digital signals avoid?

3. In their natural form, are video signals analogue or digital?

4. What transmission media does a house telephone use?

5. Can black & white video be transmitted over an old style modem?

6. Why does a repeating cycle sine wave contain no information?7. Give two reasons for encoding / decoding signals


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Elements of Comms Systems V2