ECE 4371, Fall, 2015
Introduction to Telecommunication
Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 3
Sep. 1st, 2015
Review

Double side band and AM modulation
–
–
–
–
–
–




Time domain equation and figure
Frequency domain equation and figure
Frequency conversion
Modulation index
AM modulation and demodulation
Coherent vs. non-coherent demoludation
QAM and Single side band,
Vestigial side band
FDM system
Analog TV
QAM

AM signal BANDWIDTH : AM signal bandwidth is twice the bandwidth
of the modulating signal. A 5kHz signal requires 10kHz bandwidth for AM
transmission. If the carrier frequency is 1000 kHz, the AM signal spectrum
is in the frequency range of 995kHz to 1005 kHz.

QUADRARTURE AMPLITUDE MODULATION is a scheme that allows
two signals to be transmitted over the same frequency range.

Coherent in frequency
and phase. Expensive

TV for analog

Most modems
Single Sideband (SSB)
• Purpose : to reduce the bandwidth requirement of AM by one-half. This is
achieved by transmitting only the upper sideband or the lower sidebband of
the DSB AM signal.
SSB Frequency
M()
baseband
2B
2B
0

DSB
c
0
 SSB ()
c

c

SSB (Upper sideband)
SSB
c
0
SSB Math
1  sgn( ) 
M  ()  M()U()  M()
 
2


j
1
F 1 {sgn( )}  

jt t
1
2
M()  M() sgn( )

m  ( t )  21 m(t)  21 F 1 {M()}  F 1 {sgn( )}
1 

 m(t)  jm( t ) 

t 

1 1  m( )
where m h ( t )  m( t )   
d
t    t  
 m  (t) 
1
2
1
2
m(t)  jm h (t ) 
m h ( t ) is called the Hilbert transform of m(t).
Similarly, we can show that m  ( t ) 
1
2
1 

 m(t)  jm( t ) 

t 

1
2
M ()
m(t)  jm h (t ) 
M  ()
F  1 {sgn( )}  M H ()   j M() sgn( )
How to generate mh(t) ?
MH ()   jM() sgn( )
M ()
H( )  H
 - jsgn( )
M()
 j

j
H( )  1
M()
Mh()
H()

for   0
for   0
Transfer function of a Hilbert transformer
H(  )
2



2
SSB Hilbert
• SSB signal can be expressed in terms of m(t) and its Hilbert transform
 SSB USB ()  M  (   c )  M  (   c )
M  (   c )  F - 1 {m  ( t )e j t }  F - 1 { 21 (m( t )  jm h ( t ))e j t }
c
c
M  (   c )  F - 1 {m  ( t )e  j t }  F - 1 { 21 (m( t )  jm h ( t ))e  j t
c
c
M  (   c )  M  (   c )  F - 1 { 21 (m( t )  jm h ( t ))e j t 
1 (m( t )  jm ( t ))e  jc t }
h
2
( t )( e jc t  e  jc t )
c
 F -1
21 m(t)( e j t  e j t )  j 21 mh
c
c

 F - 1 m(t) cos( c t )  m h ( t )sin(  c t )
  SSB USB ()  F - 1 m(t) cos( c t )  m h ( t )sin(  c t )
Similarly, we can show that  SSB LSB ()  F - 1 m(t) cos( c t )  m h ( t )sin(  c t );
In general,  SSB ()  F - 1 m(t) cos( c t )  m h ( t )sin(  c t ) (- for USB,  for LSB )
SSB Generator
• Selective Filtering using filters with sharp cutoff characteristics. Sharp cutoff
filters are difficult to design. The audio signal spectrum has no dc
component, therefore , the spectrum of the modulated audio signal has a null
around the carrier frequency. This means a less than perfect filter can do a
reasonably good job of filtering the DSB to produce SSB signals.
• Baseband signal must be bandpass
• Filter design challenges
• No low frequency components
c
0
c

SSB Generator

Phase shift method using Hilbert transformer

Non-causal filter, approximations
x
+
m(t)
cos c )
~
Hilbert
Transformer


2
X
 ssb (t )
+

SSB Demodulation
Synchronous, SSB-SC demodulation
 SSB ( t ) cos(c t )  m( t ) cos(c t )  jm h ( t ) sin( c t )cos(n(c t ) 
A lowpass filter can be used to get
1
2
m(t )(1  cos(c t ))  jm h (t ) sin(2c t )
1 m( t ).
2
SSB+C, envelop detection
 SSB C ( t )  A cos( c t )  m( t ) cos( c t )  m h ( t ) sin(  c t )
An envelope detector can be used to demodulate such SSB signals .
What is the envelope of  SSB C ( t )  ( A  m( t )) cos( c t ))  m h ( t ) sin(  c t )  E( t ) cos( c t  ) ?

{Recall Acos( )  Bsin(  )  A 2  B 2

1
2
cos(  ),   tan -1( B
))
A
E(t)  (( A  m( t )) 2  m h2 ( t )) 2  (( A 2  m 2 ( t ))  m h2 ( t )  2Am( t )) 2
1
m2 ( t )
m (t)
2m( t )
 A1  A 2  Ah 2  A 


 A  m( t )
for A  m(t) , A  m h (t) .
2
The efficiency of this scheme is very low since A has to be large.
1
SSB vs. AM

Since the carrier is not transmitted, there is a reduction by
67% of the transmitted power (-4.7dBm). --In AM @100%
modulation: 2/3 of the power is comprised of the carrier; with
the remaining (1/3) power in both sidebands.

Because in SSB, only one sideband is transmitted, there is a
further reduction by 50% in transmitted power

Finally, because only one sideband is received, the receiver's
needed bandwidth is reduced by one half--thus effectively
reducing the required power by the transmitter another 50%

(-4.7dBm (+) -3dBm (+) -3dBm = -10.7dBm).

Relative expensive receiver
Vestigial Sideband (VSB)
• VSB is a compromise between DSB and SSB. To produce SSB signal from
DSB signal ideal filters should be used to split the spectrum in the middle so
that the bandwidth of bandpass signal is reduced by one half. In VSB system
one sideband and a vestige of other sideband are transmitted together. The
resulting signal has a bandwidth > the bandwidth of the modulating
(baseband) signal but < the DSB signal bandwidth.
DSB
c
c
0

 SSB () SSB (Upper sideband)
c
c
0
 VSB () VSB Spectrum

c
c

Filtering scheme for the generation of VSB modulated wave.
VSB Transceiver
 VSB ()
m(t)
H i ( )
e(t)
 VSB ()
M()
LPF
Ho()
2cos( c t )
2cos( c t )
Transmitter
Receiver
M() is bandlimite d to 2B rad/sec
 VSB ()  [M(   c )  M(   c )]Hi ()
E()  [ VSB (   c )   VSB (   c )]
 [Hi (   c )M(  2 c )  Hi (   c )M()  Hi (   c )M()  Hi (   c )M(  2 c )]
High freq. term
High freq. term
 M()  E()H o ()  Hi (   c )  Hi (   c )M()H o ()
 [Hi (   c )M(  2 c )  Hi (   c )M(  2 c )]Ho ()
Lowpass filter removes this.
Thus we should have
OR
H o () 
Hi (  c )  Hi (  c )Ho ()  1
1
Hi (    c )  Hi (    c )
for
  2B
Other Facts about VSB

Envelope detection of VSB+C

Analog TV:

DSB, SSB and VSB
– DSB bandwidth too high
– SSB: baseband has low
frequency component, receiver
cost
– Relax the filter and baseband
requirement with modest increase
in bandwidth
(a) Idealized magnitude
spectrum of a transmitted TV
signal. (b) Magnitude
response of VSB shaping
filter in the receiver.
Comparison
 Common types & modulated signal
1. AM:
sAM (t )  Ac [1  m(t )]cos(2 f ct )
2. DSB:
sDSB (t )  Ac m(t ) cos(2 f ct )
3. QAM: sQAM (t )  Ac m1 (t ) cos(2 f ct )  Ac m2 (t ) sin(2 f ct )
4. SSB:
sSSB (t )  Ac m(t ) cos(2 f ct ) Ac mˆ (t ) sin(2 f ct )
5. VSB:
sVSB (t )  Ac m(t ) cos(2 f ct )
Ac m(t ) sin(2 f ct )
 Complex domain representation:
s (t )  Re{g (t )e j 2 fct },
complex envelop: g (t )  ?
 Bandwidth: Bm  B  2 Bm (Bm : message bandwdith)
Block diagram of FDM system.
Illustrating the modulation steps in an FDM system
FMA of SSB for Telephone Systems
FMA of SSB for Telephone Systems
FMA of SSB for Telephone Systems
AM Broadcasting

History

Frequency
– Long wave: 153-270kHz
– Medium wave: 520-1,710kHz, AM radio
– Short wave: 2,300-26,100kHz, long distance, SSB, VOA

Limitation
– Susceptibility to atmospheric interference
– Lower-fidelity sound, news and talk radio
– Better at night, ionosphere.
Superheterodyne vs. homodyne

Move all frequencies of different channels to one medium freq.
– In AM receivers, that frequency is 455 kHz,
– for FM receivers, it is usually 10.7 MHz.
– Filter Design Concern
– Accommodate more radio stations
– Edwin Howard Armstrong
Television
Digital Display (CRT)
Analog Display (TV)
Eliminate flicker effects
Deflection Signal and Synchronization
Deflection signal and synchronization signal
525525 30=8.27M
Solar Power and Human Eye
RGB, LIQ
mL=0.3mr+0.59mg+0.11mb
mI=0.6mr+0.28mg-0.32mb
mQ=0.21mr-0.52mg+0.31mb
Bandwidth
VSB and QAM
Comb Filtering
NTSC, PAL, and SECAM

National Television System Committee
– Low complexity, higher vertical color resolution
– 525 line/60Hz(30frames per second)

Phase Alternative Line: PAL
– The phase of the color components is reversed from line to line
– Robust to Multipath, phase distortion
– 625line/50Hz(25 frames per second), slightly larger bandwidth

SECAM
– Requires the receiver to memorize the content of each line

Mono when used for different standards
TV standards in the world
Why is it Changing for Digital TV?
Digital is Better
 No snow, no static, no ghosts

Higher resolution images, and Sound

Additional channels

Frees up Bandwidth for Other Important Needs

More Options
– Improved captions (but not without pain)
– Interactive TV
– TV to your Cell Phone or PDA? (Iphone, Asia)
Resolution
Clearer and More Detailed
ATSC
ATSC
Digital TV
Displays as Pixels

Signal is just a bunch of bits

Define color and intensity of
each point on the screen

Bit stream is heavily
compressed

Captions are also digital
And That’s the News, Tonight
1010011000101101110100110001011011
Summary

Digital TVs Should be Fine
– High Definition Video and Audio
– Some Requirements to Get Captions



Learn where captions will be decoded
Learn how to control
Use the right connectors
– More Options for Captions




Color
Font size and style
Opacity
Analog TVs Will Still Work With Digital/Analog Converter
– Cable and Satellite Subscribers (May have to trade in STB)
– Cable and Satellite Subscribers may still want a D/A Converter for emergencies
– Must Buy Converter Box To View Over The Air Signal on Analog TV



Wait until you know what you need
Don’t wait until the coupons are gone
Only have 90 days after you receive coupon