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Chapter 3. Signalling. Lesson Objectives. Equations. Chapter 3.1.1. Sampling and digitising 1. Signals and Noise. Digital Signals are much less prone to interference and so I theory should produce higher quality signals e.g. Digital TV and Radio . Sampling 1.

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Chapter 3

Chapter 3

Signalling




Sampling and digitising 1

Chapter 3.1.1

Sampling and digitising 1


Signals and noise
Signals and Noise

Digital Signals are much less prone to interference and so I theory should produce higher quality signals e.g. Digital TV and Radio


Sampling 1
Sampling 1

Instead of storing a whole waveform a signal can be sampled (digitised) at regular intervals. As long as the signal is sampled at a high enough frequency the original signal can be reconstructed


Sampling problems 1
Sampling Problems 1

If you sample at the wrong frequency then the original signal cannot be reconstructed accurately .

The optimum sampling frequency is:

2x the highest frequency in the original signal

e.g. a music CD need a highest frequency of 20kHz to be stored so music is sampled at 44.1kHz.


Sampling problems 2
Sampling Problems 2

Another problem is that if you sample at too a low frequency then spurious frequencies called aliases can be created in the reconstructed signal


Wheels moving backwards
Wheels moving backwards

You have probably seen this effect on TV or the cinema when a car moving forwards appears to have wheels that are rotating backwards


Software demo
Software Demo

  • Looking Less Often

  • Activity 70S


Digitising
Digitising

Signals can be digitised by turning the sampled waveform into numbers.

Sampling is done using an analogue to digital converter (ADC)

A Digital to analogue converter (DAC) can reverse the process

The diagram on the left shows that with 3 bits of information up to 8 levels can be stored (3 bits = 8 possible binary numbers)

Using more bits means more levels and a greater resolution

A telephone uses 8 bits = 256 levels for each sample

A high quality CD uses 16 bits = 65536 levels for each sample


Sampling and digitising 2

Chapter 3.1.2 – 3.1.3

Sampling and digitising 2


Signal transmission
Signal Transmission

A fax converts text and pictures into pixels one line at a time. This is slow and requires a lot of data to be transferred

Slow information transfer rate


Signal transmission1
Signal Transmission

E-mail sends less information per page because it can encode letters as numbers instead of pixel by pixel.

This means a faster information transmission time even if we use the same transmission rate (64000 bits per second) as the fax machine


Chapter 3 2

Chapter 3.2

Signalling with EM Waves




Waves all around
Waves all around

Look at the size of TV aerials on the roofs of houses - they give you and idea of the wavelengths being received .

Usually the rods are half a wavelength long – a few cm


Tv ariel s
TV Ariel's

The rods are a few cm long indicating that it is designed to receive waves with wavelengths of a few cm


Polarisation

Chapter 3.2.2 - 3.2.3

Polarisation


Polarisation1
Polarisation

TV and Radio waves are polarised

To pick up a signal the receiving rods must be parallel to the electric field oscillations of the wave

Again looking at TV aerials will tell you the direction of polarisation


Tv ariel s1
TV Ariel's

The short Transverse rods indicate that this ariel is designed to receive waves that are horizontally polarised


Frequency spectrums

Chapter 3.2.3

Frequency Spectrums


Making music
Making Music

If you are a Jazz Maverick and Electro-pop superstar its important that you know how to process sounds and music…









Software demo1
Software Demo

  • Cleaning up a sound

  • Activity 210S


Multiplexing and bandwidth

Chapter 3.2.4

Multiplexing and Bandwidth



Software demo2
Software Demo

  • Bits per second and bandwidth

  • Activity 260S