ANALOG & DIGITAL COMMUNICATION

By Engr. Hyder Bux MangrioInstitute of Information & Communication

TechnologiesMehran University of Engineering and Technology,

Jamshoro.

09TL-BATCH

Today's Lecture:27Digital Pulse Modulation

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Most widely used Developed in 1937 at the Paris

Laboratories of AT&T by Alex H. Reeves. PCM is a method of serially transmitting an

approximate representation of an analog signal.

The PCM signal itself is a succession of discrete, numerically encoded binary values derived from digitizing the analog signal

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In pulse-code modulation (PCM), an analogmessage signal is represented by a sequence of coded pulses, which is accomplished byrepresenting the signal in discrete form in both time and amplitude PCM is the most basic form of digital pulse Modulation A PCM system contains three main blocks: PCM transmitter Transmission path Receiver

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Fig. PCM System

Steps of Pulse Code Modulation1. Quantization : The max. expected amplitude

of the analog signal is first "quantized" (it is divided into discrete numerical levels) quantized range = 2 (by the power of no. of converter bits)

Quantized Level= 2n

Where n is number of converter bits

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2. Sampling: The process of determining theinstantaneous voltage at these given intervals of time

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3. Encoding : The "encoded binary value" isproduced representing an approximation of amplitude at the time of the sample.

– The binary value representing the amplitude is then serialized to form the PCM signal and transmitted onto the communication channel.

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Figure : A 3-bit PCM system showing analog to 3-bit digital

Digital signal

Sampling pulses

Digital clock

Serial PCM

output

Analog

input

Pulse

generator

Sampler

Encoder

t

t

Ts

001011101111

1 2 3 000010100110

31

57

1

011 11 0 10 1

2 3

10 04

A(t)

0 4

10

For input signal minimum amplitudeSQR = minimum voltage / quantization noiseFor input signal maximum amplitudeSQR = maximum voltage / quantization noise

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Linear quantizng in PCM systems has two major drawbacks.

(i) The uniform step size means that weak analog signals will have a much poorer S/Nq than the strong signals.

(ii) Systems of wide dynamic range require many ending bits and consequently wide system bandwidth.

Companding Companding is the process of compressing, then

expanding.Or nonlinear encoding/decoding, called companding

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000

001

010

001

100

101

110

111 Linear analog-to-digital converter

transfer characteristic.

N

Full scale, or VFS

Dig

ital o

utpu

t cod

e

q

Sample voltage input(Volts) Vmax

(Volts)

} q0

+ q / 2

- q / 2B. Quantum uncertainty or quantization noise, ± q/2

Fig : Linear ADC characteristic and quantization noise.

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Given a sine wave whose peak-to-peak amplitude utilizes the full range of a 12 bit A/D converter, compute the SQNR.

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delta modulation converts or encode the message into sequence of binary digits.

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Figure: Delta Modulator

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