Describe Vestigial Sideband Transmission (VSB) . Applications of Vestigial Siddeband Transmission

Vestigial Sideband Transmission

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The exact frequency response requirements on the sideband filter in SSB-SC system can be relaxed by allowing a part of the unwanted sideband called vestige to appear in the output of the modulator.

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Due to this, the design of the sideband filter is simplified to a great extent .

But the bandwidth of the system is increased slightly .

To generate a VSB signal, we have to first generate a DSB-SC signal and then pass it through a sideband filter as shown in fig. 1 . This filter will pass the wanted sideband as it is along with a part of unwanted sideband .

block diagram of VSB transmitter

Fig.1 : VSB Transmitter

Frequency Domain Description

Frequency Spectrum

The spectrum of VSB is as shown in fig. 2 .

spectrum of message signal

(a) Spectrum of message signal

Spectrum of VSB signal

(b) Spectrum of VSB Signal

Fig. 2

The spectrum of message signal x(t) has also been shown .

In the frequency spectrum, it is assumed that the upper sideband is transmitted as it is and the lower sideband is modified into vestigial sideband .

Transmission Bandwidth

From fig. 2 (b), it is evident that the transmission bandwidth of the VSB modulated wave is given by :

bandwidth of VSB

Where  f= Message bandwidth

fv  = Width of the vestigial sideband

Advantages of VSB

  1. The main advantage of VSB modulation is the reduction in bandwidth. It is almost as efficient as the SSB .
  2. Due to allowance of transmitting a part of lower sideband, the constraint on the filter have been relaxed . So practically, easy to design filters can be used .
  3. It possesses good phase characteristics and makes the transmission of low frequency components possible .

Application of VSB

VSB modulation has become standard for the transmission of television signal . Because the video signal need a large transmission bandwidth if transmitted using DSB-FC or DSB-SC techniques .

Generation of VSB Modulated Wave

The block diagram of a VSB modulator is shown in fig.3 .

generation of VSB Signal

Fig.3 : Generation of VSB Signal

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 . The output of the product modulator is then given by :

m(t) = x(t) . c(t)

= x(t) .  Vc  cos(2π fct)

This represents a DSB-SC modulated wave .

This DSB-SC signal is then applied to a sideband shaping filter . The ddesign of this filter depends on the desired spectrum of the VSB modulated signal.

This filter will pass the wanted sideband and the vestige of the unwanted sideband .

Let the transfer function of the filter be H(f) .

Hence, the spectrum of the VSB modulated signal is given by :

VSB modulated signal

Demodulation of VSB Wave

The block diagram of the VSB demodulator is shown in fig.4 .

synchronous demodulation of VSB signal

Fig.4 : VSB demodulator

Working Operation

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

Hence, the output of the product modulator is given by :

demodulation of VSB EQ1

Taking the Fourier transform of both sides, we get

VSB EQ2

But

VSB modulated signal

Hence, we have

vsb eq3

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 .

Therefore ,

vsb eq4

This spectrum is shown in fig.5 .

spectrum of VSB DEMODULATOR

fig 5: Spectrum of 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 :

VSB EQ5

Where H( fc) is constant .