C/W 24th November 2011Recap - Electromagnetic waves for

communicatingKeywords: optical fibre carrier wave modulationObjectives:Must – Which types of electromagnetic waves are used for communication? (C/D)Should – Understand how information is carried using electromagnetic waves. (C/B)Could – Understand how waves are modulated with a carrier wave. (A/A*)

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We are going to see how signals are actually broadcast as a wave.....

Recap on waves

Carrier wave images from text book.

Plenary sketch on your mini white board

The carrier wave

Sound signal to be broadcast

Modulated carrier wave

C/W 24th November 2011Digital Signals

Keywords: Analogue digital Objectives:Must – Know the difference between an analogue and digital signal. (C/D)Should – Understand how digital signals are transmitted (C/B)Could – Understand the advantages of digital signals. (A/A*)

What is an analogue signal?Most of the signals sensed by humans are analogue signals. Everyday examples include sound, light and temperature.

An analogue signal may have any value within a continuous range.

An analogue signal can be converted into an electrical signal. This graph shows how the voltage of an analogue signal varies with time.

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What is a digital signal?Modern communication systems carry information from analogue signals as digital signals.

A digital signal only contains two values: ‘0’ (off) and ‘1’ (on). These are used to encode analogue information.

Unlike analogue, a digital signal does not vary smoothly, and so is described as being a discontinuous signal.

Digital signals can be sent as electrical signals, radio waves or as pulses of light through optical fibres.

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What are the advantages of digital signals?Most forms of communication, such as television and radio, now involve digital signals.

1) No loss of signal quality. Analogue signals lose quality during transmission but digital signals don’t.

This is because digital signals have two main advantages over traditional analogue signals.

2) More information. Digital signals can carry more information than analogue signals and different types, likes pics, and text.

Think about digital television

3) Digital info can be stored and processed by computers

Analogue or digital?

Why do analogue signals lose quality? All types of signals lose strength as they travel. This is called attenuation and means that signals often have to be amplified so they can be used.

original attenuated amplified

It is difficult to distinguish the noise from the signal, because an analogue signal can have any value. This means that an analogue signal is of lower quality by the time it is received.

Amplifying an analogue signal also amplifies the noise.

Signals can also pick up noise (interference).

Why don’t digital signals lose quality?

Digital signals also lose strength and need to be amplified.

During transmission, a digital signal can be regenerated by comparing the amplified signal to a threshold value.

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It is easy to distinguish the noise from a digital signal and so the signal does not lose quality by the time it is received.

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An analogue signal is converted to digital by sampling. A signal at the red line could be coded as off, off, pulse, off, pulse, pulse, off, pulse, off

Answer questions 5 and 6

then 3 and 4p258

http://www.youtube.com/watch?v=1go6ITQXSlQ

C/W 24th November 2011Digital Signals

Keywords: Analogue digital Objectives:Must – Know the difference between an analogue and digital signal. (C/D)Should – Understand how digital signals are transmitted (C/B)Could – Understand the advantages of digital signals. (A/A*)