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PULSE

CODE MODULATION

You

asked for it

and here it

is: everything

you

wanted

to

know

about

pulse -code modulation!

By Rodney

A. Kreuter

IN

ELECTRONICS

TODAY THERE

SEEM TO

BE MORE WAYS

OF

representing

information

than there

are op -amps.

In addi-

tion

to the traditional

methods

of modulation,

such

as AM,

FM, and

on -off keying,

the revolution

in

digital

electronics

has

brought

about many

different ways

of

representing infor-

mation

using simple

pulse techniques.

Some of these

have

been around

longer than

you might

think, but

the popularity

of

digital electronics

has made

it easier

to use them

than in the

past, and

it seems that

many

different systems

are

taking

advantage of

different pulse

-code

techniques.

Take

compact

discs for

example. Traditional

records

used a form of

AM.

The louder

the

music the

broader the

groove. Today

a 16 -bit

code (plus some

bits for

error checking)

are used

to represent

the

amplitude of

a sound.

Now

we

will

explore

the topic in greater

depth. You will

AVERAGE

VALUE

ANALOG

SIGNAL

PULSE

WIDTH

MODULATION

PWM

PULSE

POSITION MODULATION

PPM

PULSE

AMPLITUDE

MODULATION

PAM

PULSE

CODE

MODULATION

PCM

BIT 0

PCM

PCM

PC

BIT 1

BIT

2

BIT 3

5

4

V 3

0

2

1

L 0

T -1

S

-3

-4

-5

1

2

3

1

2 3

find

Fig. 1 helpful

in

understanding

the distinctions

in the

main

methods of pulse

modulation.

Refer

to it as you

read

each topic.

Pulse

-Width Modulation

On -ott keying

has

probably been

around

the longest

of the

traditional

techniques, and

I'll bet many

of you never

stopped

to think

that it is simply

a Pulse

-Width Modulation

(PWM)

technique.

One modulation

system

that uses

it is a lot

better

known as

Morse code.

In Morse

code "elements"

(dits

and

dahs, dots

and dashes, shorts

and longs)

are represented

by

pulses of

two different widths.

Letters

and

numbers

are

composed

of

groups

of

elements.

Another

system

that has

used PWM for

a long

time is the

radio

-controlled

model.

In that system,

the

position of

a

4

2 1

4 5

6

-1

-3

-4 -2 1 2

7

8

9

10 11

12

Fig.

1 -Here

an analog signal is

digitized by the four methods

described.

The

signal

is

divided into 12

time slots sometimes

called

"frames"

or

"cells."

Each

cell

is

further

divided

into

50 periods

of

time which

I will

call

"units."

The average value

is the

average voltage

of

the

analog signal

during a particular

cell.

1 2 ... 49 50 51 52 53 54 55 56 57 58 59 ... 99 100

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