Introduction to digital communication (lectures)
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Transcript of Introduction to digital communication (lectures)
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Digital
Communication
Lecture 2
by
Dr. Sutapa Mukherjee
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Pulse Code Modulation (PCM)
What is PCM?
Digital Pulse modulation technique to
represent analog signals by coded group of
digital pulses.
A/D conversion
Applications: digital audio in computers, DVD
and Compact Disc formats, digital telephone
systems.
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PCM
Three steps of PCM
Sampling
Quantization
Encoding
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Sampling
Signal representation by discrete samples at regular
interval (sample period Ts)
Sampling freq. fs = 1/Ts
fs (Sampl freq.) 2fm (highest freq. of message)
(Nyquist Sampling Theorem)
Outcome is a PAM signal with analog values
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Types of Sampling
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QUANTIzATION
Process where sampled values of analog signal are rounded off to the nearest predetermined levels.
Sampling : series of pulses of varying amplitude ranging between two limits Vmin and Vmax.
The amplitude values are infinite between the two limits.
Mapping of infinite values onto a finite set of known values
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quantization
Divide the distance between Vmin and Vmax into L zones, each of height
= (max - min)/L
The midpoint of each zone is assigned a value from 0 to L-1 (resulting in L values)
Each sample falling in a zone is then approximated to the value of the midpoint.
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encoding
Each Predetermined level /zone is assigned a
binary code.
The no. of bits required per sample
n = log2 L
For 8 level, n = log2 8 = 3
The 8 zone (or level) codes are 000, 001, 010,
011, 100, 101, 110, and 111
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Quantisation & encoding
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Pulse Code Modulation
7
5
3
1
0
fsq(t) 0 4 5 5 5 5 7
PCM 000 100 101 101 101 101 111
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PCM
LPLPF
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TX. BLOCK diagram
f(t)+ n(t) fs(t) fsq(t)
PCM
QUANTIZER ENCODERSAMPLER
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