� 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