Vestigial Sideband Transmission
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.
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 .
Fig.1 : VSB Transmitter
Frequency Domain Description
The spectrum of VSB is as shown in fig. 2 .
(a) Spectrum of message signal
(b) Spectrum of VSB Signal
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 .
From fig. 2 (b), it is evident that the transmission bandwidth of the VSB modulated wave is given by :
Where fm = Message bandwidth
fv = Width of the vestigial sideband
Advantages of VSB
- The main advantage of VSB modulation is the reduction in bandwidth. It is almost as efficient as the SSB .
- 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 .
- 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 .
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 :
Demodulation of VSB Wave
The block diagram of the VSB demodulator is shown in fig.4 .
Fig.4 : VSB demodulator
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 :
Taking the Fourier transform of both sides, we get
Hence, we have
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 .
This spectrum is shown in fig.5 .
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 :
Where H( fc) is constant .