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� Lecture 3
� Paul Flynn
Modulation
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Frequency Spectrum
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Long Waves, Medium Waves and Short Waves
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Modulation of Analog SignalsModulation of Analog Signals
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
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Figure 5.24 Analog-to-analog modulation
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Types of analog-to-analog modulation
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Amplitude modulation (AM)
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Frequency modulation (FM)
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Phase Modulation (PM)
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Digital modulation
� Three basic methods
� – Amplitude shift keying (ASK)
� – Frequency shift keying (FSK)
� – Phase shift keying (PSK
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Amplitude shift keying (ASK)
� Use different amplitude to represent 0 and 1.
� – Simple, low bandwidth – Sensitive to interference.
� • Multi-path propagation, noise or path loss heavily influence the amplitude.
� – A constant amplitude in wireless environment can not be guaranteed.
� • Used in wired optical communication.
� – A light pulse =1, no light =0.
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Frequency shift keying (FSK)
� Binary FSK (BFSK) – One frequency for 0 and one
� frequency for 1. – needs larger bandwidth
� • Avoid discontinuity – Discontinuity creates high
� frequencies as side effects. – Continuous phase modulation (CPM) can be used.
� • Demodulation: – Use two bandpass filters for 2 frequencies.
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Phase shift keying (PSK)
� Use shift in phase to represent data.
� • Binary PSK (BPSK) – Shift the phase by 180.
� • Synchronization is important
� • More resistant to interference
� • More complex transmitters and receivers.
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Digitizing Analog Signals
1.1.SampleSample
2.2.QuantizeQuantize
3.3.Encode Encode
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Sampling Analog SignalsOriginal
Signal
Sample Times
Sample Values
Reformed Signal
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Digitizing Analog SignalsOriginal
Signal and
Samples
Actual Sample Values
Quantized Sample Values
Reconstructed Reconstructed SignalSignal
Original Original Signal Signal
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Linear QuantizingActual
Sample Values
Quantized Sample Values
Reconstructed Signal
Original Signal
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Linear Quantizing
Value of Input Signal
Value of Output Signal
-1-2-3-4-5 1 2 3 4
1
2
3
4
-1
-2
-3
-4
5
Quantizing Error
(output-input)
Linear: quantizing steps for all segments are the same height.
Linear: quantizing steps for all segments are the same height.
SQRSQRSQRSQR
2-42
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Quantizing Noise
Transmit Receive
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Voice Digitization Process
Pulse Amplitude Modulation
Sampler
Quantizer EncoderAnalog
Input"PAM" Pulses
"PAM" Pulses
PCM Output Signal
Continous Signals
Discrete-time Continous- Amplitude
Signals
Discrete-time Discrete- Amplitude
Signals
Digital bit stream
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Pulse Code ModulationWaveform-Time Domain-Nondifferential
Sample Values
Quantized value of each sample is coded
Quantized value of each sample is coded
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Coding
How many bits are needed to code this many levels?
How many bits are needed to code this many levels?
111
110
101
100
011
010
001
000
Answer: 3 bits for 8 levelsL=2n
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Companding or Non-linear EncodingCompanding = compressing +
expandingWhy companding?Quantization levels not evenly spacedReduces overall signal distortionCan also be done by companding
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Binary Signaling Encoding Schemes1 0 1 1 0 0 0 1 1 0 1
NRZ-L
NRZ-M
NRZ-S
RZ
Biphase-L Manchester
Biphase-M
Biphase-S
Differential Manchester
Delay Modulation
AMI- Bipolar
0 = 1
0 to 1
1 to 0
1=sq
sq 1 & 0
c x on 1 chg on 0
alt mark inversion (B8ZS)
sq wv 1 inv 2nd 1
n
n
n
n
s
s
s
s
n
n
EIA 232
B8Zs
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2B1Q (ISDN)
EACH LEVEL REPRESENTS TWO BITS 00, 01, 10, AND 11
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The digital Loop Signal
0v
-v
+vBipolar NRZ
(EIA-232)
+v
0v
-v
Bipolar AMI
0 1 0 0 1 1 1 0 0 0 1
+v
0v
-v
Actual Signal
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AMI (Alternate Mark Conversion) Violation
AMI
Violation
0 1 1 0 0 1 0 1 1 1 0 1
+v
0v
-v
START OF CONTROL INFO
NO MORE THAN 15 0s IN A ROW