PCM and Optical fibres

Background information for:

PH 2.2 (f) & (g)

PAM and Sampling

The continuous analogue wave has its amplitude sampled

Sampling must be done at a minimum of twice the rate to avoid aliasing

(a) Voltage

(b) Time

(1) Original signal

(2) PAM Signal

Quantisation

The amplitude (voltage) is measured and assigned to a quantum level

Each signal that falls into a level is then converted into binary code

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Time (s)

Vo

ltag

e (V

)

Quantum level Binary code

9 1001

8 1000

7 0111

6 0110

5 0101

4 0100

3 0011

2 0010

1 0001

0 0000

Companding

Quantised distortion will occur because each level corresponds to a range of voltages

Low-amplitude signals are more seriously affected

The quantum levels are altered by a process known as companding (compressing and expanding)

Continuous Variation

0

1

2

3

4

5

0 1 2 3 4 5 6 7 8 9 10

TDM & FDM

Different stands of information can be sent along the same optical fibre at the same time

A B

1 2

A 1 B 2

As the diagram also implies, different frequencies can be used for different signals

Other methods of multiplexing can be employed

Dispersion

In a clad fibre light is dispersedLight/signal will travel along many different

paths reaching the end at different times The signal becomes spread out The rate of transfer is limited as the information

arrives over a longer period of time

Original pulses

After dispersion

Resultant signal

Multimode fibres

To reduce multipath dispersion, multimode fibres (graded index core) were designed

With modern designs it is possible to get the rays to meet to within 1ns km-1

Monomode fibres

The diameter of the fibre is only a few wavelengths thick

Light only travels parallel to the axisSome dispersion does occur, but is very

limited