Transcript PCM and Optical fibres

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
Quantum level
Binary code
9
1001
8
1000
7
0111
6
0110
5
0101
4
0100
3
0011
2
0010
1
0001
0
0000
5
4
3
2
Voltage (V)
1
0
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
-1
-2
-3
-4
-5
Time (s)
Companding
Continuous Variation
 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)
5
4
3
2
1
0
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
A
1
1
B
2
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 dispersed
Light/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 axis
Some dispersion does occur, but is very
limited