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1
Digital Data, Analog Signals (5.2)
CSE 3213Fall 2011
April 18, 2023
Digital Data, Analog Signal
main use is public telephone systemhas frequency
range of 300Hz to 3400Hz
uses modem (modulator-demodulator)
3
Modulation Techniques
4
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()(
tfAts c
5
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
1
tfA
tfAts
Multiple FSK
each signalling element represents more than one bit
more than two frequencies usedmore bandwidth efficientmore prone to error
FSK on Voice-Grade Line
8
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()(
tfA
tfAts
c
c
9
Differential PSK
10
Quadrature PSK
00binary
01binary
10binary
11binary
)4/72sin(
)4/52sin(
)4/32sin(
)4/2sin(
)(
tfA
tfA
tfA
tfA
ts
c
c
c
c
• More efficient: each signal element representing more than one bit—QPSK: each element represents two bits
11
PSK Combined with ASK• 9600 bps modem use 12 angles, four of which
have two amplitudes
Performance of Digital to Analog Modulation Schemes
13
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
12
LRL
rB
RrB
fffffRrfB
rrRrB
T
T
ccT
T
Bandwidth Efficiency for Digital-to-Analog Encoding Schemes R/BT
15
Analog Data, Analog Signals (5.4)
CSE 3213
16
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 Phase
index modulation2cos)](1[)(:Modulation Amplitude
ffcc
ppcc
aca
ntmntttfAts
ntmntttfAts
ntftxnts
17
Analog Modulation
18
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 and
FMfor 2
PMfor where
)1(2:FM / PM
2:AM
tmAB
nAnABB
BB
m
fm
pm
T
T
Summary
Signal encoding techniquesdigital 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
20
Reading• Chapter 5, Stallings’ book
• Next time: Digital Data Communications Techniques (chapter 6)