Digital

Communication

Lecture 2

by

Dr. Sutapa Mukherjee

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.

PCM

Three steps of PCM

Sampling

Quantization

Encoding

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

Types of Sampling

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

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.

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

Quantisation & encoding

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

PCM

LPLPF

TX. BLOCK diagram

f(t)+ n(t) fs(t) fsq(t)

PCM

QUANTIZER ENCODERSAMPLER

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