8 VSB_Generation_and_Detection.pdf

VSB Method
• VSB – Vestigial Side Band – Compromise between DSB & SSB
• In VSB method, the desired sideband is allowed completely and just a small
portion (trace/vestige) of the undesired sideband is allowed
• The transmitted vestige of undesired sideband component compensates for
the loss of wanted sideband
• Bandwidth and power requirement is greater than SSB and lesser than DSB
• Filters doesn’t require sharp cutoff frequency
• Used in transmission of picture signal in TV

VSB Modulator
• To generate the VSB signal, first generate the DSB-SC signal and send it to the
sideband filter
• The modulating signal x(t) is applied to a product modulator
• The output of the carrier oscillator is also applied to the other input of the
product modulator

VSB Modulator
• The output of the product modulator is given as
m(t)  x(t).c(t)
m(t) Vc cos2fct.x(t)    (1)
Applying fourier transform on both sides
• This DSB-SC signal is given to SSB filter which pass the wanted
sideband and vestige of unwanted sideband
• Let the transfer function of the filter be H(f)
• Hence, the spectrum of the VSB modulated signal is given as
2
S( f ) 
V
[X ( f  fc )  X ( f  fc )]H ( f )    (3)
c
2
M( f )
V
[X ( f  fc )  X ( f  fc )]    (2)
c
S( f )  M( f ) H( f )

Spectrum of VSB signal
Spectrum of message signal
Spectrum of VSB signal
• The transmission bandwidth of the VSB modulated wave is given as
BW  fm  fv
fm = Message bandwidth
fv = Width of the vestigial sideband

VSB Demodulator
• The VSB modulated wave is passed through a product modulator
where it is multiplied with the locally generated synchronous carrier

VSB Demodulator
• The output of the product modulator is given as
m(t)  s(t).c(t)
m(t) Vc cos2fct.s(t)    (1)
• Taking Fourier transform on both sides, we get
• Sub Eq.(3) in Eq.(2)
2
S( f ) 
V
[X ( f  fc )  X ( f  fc )]H ( f )    (3)
c

VSB Demodulator
4
2
M ( f ) 
 c
{X ( f )[H( f  fc )  H( f  fc )}   (4)
c c c c
X ( f  2 f )H( f  f )  X ( f  2 f )H( f  f )
c
V2
• The first term in the above expression represents the VSB modulated wave,
corresponding to a carrier frequency of 2fc
• This term will be eliminated by the filter to produce output vo(t)
• The second term in the above expression for M(f) represents the spectrum of
demodulated VSB output
4
V0 ( f )  c
{X ( f )[H( f  fc )  H( f  fc )}   (5)
V2
V2

VSB Demodulator
In order to obtain the undistorted message signal x(t) at the output of
the demodulator, Vo(f) should be a scaled version of X(f) .
For this the transfer function H(f) should satisfy the following conditions :
Where H( fc) is constant .

VSB Modulation − Advantages
 Highly efficient.
 Reduction in bandwidth.
 Filter design is easy as high accuracy is not needed.
 The transmission of low frequency components is possible, without
difficulty.
 Possesses good phase characteristics.

VSB Modulation − Disadvantages
 Bandwidth when compared to SSB is greater.
 Demodulation is complex.

VSB Modulation − Application
 The most prominent and standard application of VSB is for the transmission
of television signals.
 Also, this is the most convenient and efficient technique when bandwidth usage is
considered.

8 VSB_Generation_and_Detection.pdf

8 VSB_Generation_and_Detection.pdf

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8 VSB_Generation_and_Detection.pdf