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� Lecture 3
� Paul Flynn
Modulation
Frequency Spectrum
Long Waves, Medium Waves and Short Waves
Modulation of Analog SignalsModulation of Analog Signals
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Figure 5.24 Analog-to-analog modulation
Types of analog-to-analog modulation
Amplitude modulation (AM)
Frequency modulation (FM)
Phase Modulation (PM)
Digital modulation
� Three basic methods
� – Amplitude shift keying (ASK)
� – Frequency shift keying (FSK)
� – Phase shift keying (PSK
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.
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.
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.
Digitizing Analog Signals
1.1.SampleSample
2.2.QuantizeQuantize
3.3.Encode Encode
Sampling Analog SignalsOriginal
Signal
Sample Times
Sample Values
Reformed Signal
Digitizing Analog SignalsOriginal
Signal and
Samples
Actual Sample Values
Quantized Sample Values
Reconstructed Reconstructed SignalSignal
Original Original Signal Signal
Linear QuantizingActual
Sample Values
Quantized Sample Values
Reconstructed Signal
Original Signal
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
Quantizing Noise
Transmit Receive
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
Pulse Code ModulationWaveform-Time Domain-Nondifferential
Sample Values
Quantized value of each sample is coded
Quantized value of each sample is coded
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
Companding or Non-linear EncodingCompanding = compressing +
expandingWhy companding?Quantization levels not evenly spacedReduces overall signal distortionCan also be done by companding
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
2B1Q (ISDN)
EACH LEVEL REPRESENTS TWO BITS 00, 01, 10, AND 11
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
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