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Digital Data, Analog Signals (5.2)

CSE 3213Fall 2011

April 24, 2023

Digital Data, Analog Signal

main use is public telephone systemhas frequency

range of 300Hz to 3400Hz

uses modem (modulator-demodulator)

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Modulation Techniques

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Amplitude Shift Keying

• Values represented by different amplitudes of carrier• Usually, one amplitude is zero

—i.e. presence and absence of carrier is used• Susceptible to sudden gain changes• Inefficient• Used for voice grade lines (up to 1,200 bps)• Used over optical fiber (e.g., 1 = light pulse; 0 = no

light)

0binary 1binary

0)2sin(

)(tfA

ts c

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Binary Frequency Shift Keying

• Most common form is binary FSK (BFSK)• Two binary values represented by two different

frequencies (near carrier)• Less susceptible to error than ASK• Used for

—Up to 1,200 bps on voice grade lines—High frequency radio (3-30 MHz)—Even higher frequency on LANs using coaxial cable

0binary 1binary

)2sin()2sin(

)(2

1tfAtfA

ts

Multiple FSKeach signalling element represents more

than one bitmore than two frequencies usedmore bandwidth efficientmore prone to error

FSK on Voice-Grade Line

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Phase Shift Keying

• Phase of carrier signal is shifted to represent data• Binary PSK

—Two phases represent two binary digits• Differential PSK

—Phase shifted relative to previous transmission rather than some reference signal

—Binary 0: same phase as previous signal burst—Binary 1: opposite phase to previous signal burst

1binary )2sin(0binary )2sin(

)(tfAtfA

tsc

c

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Differential PSK

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Quadrature PSK

00binary 01binary 10binary 11binary

)4/72sin()4/52sin()4/32sin()4/2sin(

)(

tfAtfAtfAtfA

ts

c

c

c

c

• More efficient: each signal element representing more than one bit—QPSK: each element represents two bits

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PSK Combined with ASK• 9600 bps modem use 12 angles, four of which

have two amplitudes

Performance of Digital to Analog Modulation Schemes

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Performance of Digital to Analog Modulation Schemes• Transmission bandwidth:

— ASK and PSK bandwidth directly related to bit rate— FSK bandwidth related to data rate for lower frequencies, but to

offset of modulated frequency from carrier at high frequencies— Roll-off factor r depends on technique for filtering signal to

establish B

levels of no.;log

)1(:PSK Multilevel

;)1(:PSK;)1(2:FSK

10 factor; rolloff;)1(:ASK

2

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LRLrB

RrBfffffRrfB

rrRrB

T

T

ccT

T

Bandwidth Efficiency for Digital-to-Analog Encoding Schemes R/BT

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Analog Data, Analog Signals (5.4)

CSE 3213

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Analog Data, Analog Signals• Why modulate analog signals?

—Higher frequency can give more efficient transmission—Permits frequency division multiplexing (Chapter 8)

• Combining an input signal m(t) and a carrier of frequency fc to produce a signal s(t) whose bandwidth is centered at fc—m(t): modulating signal—s(t): modulated signal

• Types of modulation:

index modulation);()(')(2cos)(:ModulationFrequency

index modulation);()()(2cos)(:Modulation Phaseindex modulation2cos)](1[)(:Modulation Amplitude

ffcc

ppcc

aca

ntmntttfAtsntmntttfAts

ntftxnts

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Analog Modulation

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Transmission Bandwidth of Analog Modulation Schemes• FM and PM require greater bandwidth than AM.• B is the bandwidth of the original signal.

)( of valuemaximum theis andFMfor 2

PMfor where

)1(2:FM / PM2:AM

tmAB

nAnABB

BB

m

fm

pm

T

T

SummarySignal encoding techniques

digital data, digital signal• NRZ, multilevel binary, biphase • modulation rate, scrambling techniques

analog data, digital signal• PCM, DM

digital data, analog signal• ASK, FSK, BFSK, PSK

analog data, analog signal• AM, FM, PM

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Reading• Chapter 5, Stallings’ book

• Next time: Digital Data Communications Techniques (chapter 6)