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Chapter 5 Signal Encoding andChapter 5 Signal Encoding and

Modulation TechniquesModulation Techniques

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Encoding and Modulation TechniquesEncoding and Modulation Techniques

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Digital SignalingDigital Signaling

Digital data, digital signal

implest encoding scheme: assign one voltage level to

binary one and another voltage level to binary zero

More comple! encoding schemes: are used to improve

performance (reduce transmission bandwidth and minimize

errors)"

E!amples are #$%&', #$%, Manchester, etc"

Analog data, Digital signal

nalog data, such as voice and video*ften digitized to be able to use digital transmission facility

E!ample: +ulse ode Modulation (+M), which involves

samplingthe analog data periodically and quantizingthe

samples

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Digital Data, Digital SignalDigital Data, Digital Signal

/igital signaldiscrete, discontinuous voltage pulses

each pulse is a signal element

binary data encoded into signal elements

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Periodic signalsPeriodic signals

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Data element: a single binary 0 or 1

Signal element:a voltage pulse of constant amplitude

Unipolar:ll signal elements have the same sign

olar:*ne logic state represented by positive voltage the otherby negative voltage

Data rate:$ate of data ($) transmission in bits per second

Duration or length o! a "it:Time ta2en for transmitter to emit

the bit (Tb304$) Modulation rate:$ate at which the signal level changes,

measured in baud 3 signal elements per second" /epends on

type of digital encoding used"

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Comparison of Encoding Schemes

signal spectrum

'ac2 of high frequencies reduces required bandwidth,

lac2 of dc component allows ac coupling via transformer,

providing isolation,

should concentrate power in the middle of the bandwidth

loc2ing

synchronizing transmitter and receiver with a sync

mechanism based on suitable encoding

error detectionuseful if can be built in to signal encoding

signal interference and noise immunity

cost and comple!ity: increases when increases data rate

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Encoding Schemes

+ositive level (678)

#egative level (&78)

+ositive level (678)

#o line signal (18)

#egative level (&78)

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Encoding Schemes

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Noneturn to !ero"#e$el %N!"#&

Two different voltages for 1 and 0 bits

8oltage constant during bit interval

no transition, i"e" no return to zero voltagemore often, negative voltage for binary one

and positive voltage for binary zero

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Noneturn to !ero INVETED %N!I&

#onreturn to zero inverted on onesonstant voltage pulse for duration of bit

/ata encoded as presence or absence of signaltransition at beginning of bit time

transition (low to high or high to low) denotes binary 0no transition denotes binary 1

E!ample of differential encoding since have9 data represented by changes rather than levels

9 more reliable detection of transition rather than level

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Multile$el 'inar('ipolar Alternate Mar) In$ersion %AMI&

se more than two levels (three levels,positive, negative and no line signal)

5ipolar&M

zero represented by no line signalone represented by positive or negative pulse

one pulses alternate in polarity

no loss of sync if a long string of ones

long runs of zeros still a problemno net dc component

lower bandwidth

easy error detection

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Multile$el 'inar( Issues

dvantages:

#o loss of synchronization if a long string of 0;s occurs, each

introduce a transition, and the receiver can resynchronize on

that transition

#o net dc component, as the 0 signal alternate in voltage

from negative to positive

'ess bandwidth than #$%

+ulse alternating provides a simple mean for error detection

/isadvantagesreceiver distinguishes between three levels: 6, &, 1

a < level system could represent log=< 3 0"7> bits

requires appro!"

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Manchester Encoding

has transition in middle of each bit periodlow to high represents binary one

transition serves as cloc2 and data

high to low represents binary zero

used by EEE >1="< (Ethernet) '# standard

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Scramling

se scrambling to replace sequences that wouldproduce constant voltage

These filling sequences must

produce enough transitions to maintain synchronization

be recognized by receiver @ replaced with original

be same length as original

/esign goals

have no dc componenthave no long sequences of zero level line signal

have no reduction in data rate

give error detection capability

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'ipolar 0ith -"!ero Sustitution%'-!S&

To overcome the drawback of the AMI code that along string of zeros may result in loss of

synchronization, the encoding is amended with the

following rules:

If 8 zeros occurs and the last voltage pulse was positive,

then the 8 zeros are encoded as 000+0+

If zeros occurs and the last voltage pulse was negative,

then the 8 zeros are encoded as 000+0+

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.igh Densit( 'ipolar"/ 1eros %.D'/&

The scheme replaces strings with 4 zeros by sequencescontaining one or two pulses

In each case, the fourth zero is replaced with a code

violation (V)

successive violations are of alternate polarity

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Digital Data, Analog Signal

Main use is public telephone system

has freq range of

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'inar( *requenc( Shift 2e(ing %'*S2&

The most common form of .- is 5inary .- (5.-)

Two binary values represented by two differentfrequencies (f1and f2 )

less susceptible to noise than -used forup to 0=11bps on voice grade lines

high frequency radio (< to

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Multiple *S2 %M*S2&

More than two frequencies (M frequencies) are used

More bandwidth efficient compared to 5.-

More susceptible to noise compared to 5.-

M.- signal:

elementsignalperbitsofnumberL

elementssignaldifferentofnumberM

frequencydifferencetheffrequencycarrierthef

fMiff

here

MitfAts

L

d

c

dci

ii

=

==

==

+=

=

2

)12(

1),2cos()(

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E l

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E3ample

Cithfc#2$%&'(,fd#2$&'(, andM#)(L#3bits), we have the

following frequency assignment for each of the > possible

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Ph Shift 2 i %PS2&

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Phase Shift 2e(ing %PS2&

+hase of carrier signal is shifted to represent data

5inary +- (5+-): two phases represent two

binary digits

0 0 1 1 0 1 0 0 0 1 0

* * % % * 0 * * * % *

1)(),2cos()(

0),2cos(

1),2cos(

0),2cos(

1),2cos()(

==

=

+=

tdtftAd

binarytfA

binarytfA

binarytfA

binarytfAts

c

c

c

c

c

Di+ ti l PS2 %DPS2&

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Di+erential PS2 %DPS2&

n /+-, the phase shift is with reference to the previous bit

transmitted rather than to some constant reference signal 5inary 1:signal burst with the same phase as the previous one

5inary 0:signal burst of opposite phase to the preceding one

* l l PS2 6 d t PS2

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*our"le$el PS25 6uadrature PS2%6PS2&

+

+

+

=

10)4

2cos(

00)4

32cos(

01)4

32cos(

11)4

2cos(

)(

tfA

tfA

tfA

tfA

ts

c

c

c

c

More efficient use of bandwidth if each signal element

represents more than one bit eg" shifts of 4= (D1o) each signal element represents two bits

split input data stream in two @ modulate onto the phase of the carrier

can use > phase angles @ more than one amplitude

D11bps modem uses 0= phase angles, four of which have two

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E3ample of 6PS2 and 76PS2

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E3ample of 6PS2 and 76PS28a$eforms

41101

4

31100

4

31110

41111

:

+./&for

Performance of AS2 *S2 M*S2 PS2

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Performance of AS2, *S2, M*S2, PS2and MPS2

5andwidth Efficiency

-4+-:

M+-:

M.-:

10,1

1

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Performance of M*S2 and MPS2

M.-: increasingMdecreases 5E$ and decreases bandwidth Efficiency

M+-: ncreasingMincreases 5E$ and increases bandwidth efficiency

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6AM Variants

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6AM Variants

Two level - (two different amplitude levels)each of two streams in one of two states

four state system

essentially F+-

.our level - (four different amplitude levels)combined stream in one of 0 states

Aave B and =7 state systems

mproved data rate for given bandwidthbut increased potential error rate