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Transcript
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