VSB Modulation The Stringent (Very Strict Conditions) frequency response requirements on the sideband filter in SSB – SC modulation can be relaxed by allowing a part of the unwanted sideband (called as vestige) to appear in the o/p of the modulator. Due to this, the design of the sidebard filter is Simplified to a great extent. But the bandwith of the system is increased slightly. Explain VSS modulation? Mention the advantages and applications of VSB modulation. In VSB, one sideband & a part of the other Sideband called as vestige is transmitted. So the bandwith required for VSB transmission is somewhat higher than that of SSB modulation. Modulating Signal M(t) Balanced Modulator Sideband Fitter VSB Signal Carrier oscillator To generate a VSB signal, 1st we have to generate a OISB – SC Signal & then pass it through a sideband filter as shown in figure. This filter, will pass the wanted sideband as it is along with a part of unwanted sideband. Frequency – domain description of VSB waves :- Fig (b) Spectrum of VSB modulated wave. Fig shows the spectrum of a VSB modulated wave S(t) along with the message signal m(t).
Transmission bandwidth is given by B = (W+fv) Hz Where W is message bandwith Fv is the width of the vestigial sideband. Advantage of VSB :The main advantage of VSB modulation is: 1. The reduction in bandwidth. It is almost as efficient as H SSB 2. Easy to design the filter. (Due to allowance of transmitting a part of lower Sideband the Constraint on the filters have been relaxed.) Applications of VSB :VSB modulation has become standard for the transmission of TV Signals. Because the video Signals need a larger transmission bandwidth if transmitted using DSB – FC or DSB – SC techniques. What is meant by VSB? Explain how VSB signal can be obtained from a modulating signal m(t) using a carrier ACOS(2 fct) and later demodulated. Explain the scheme for generation and demodulation of VSB modulated wave, with relevant spectrum of signals in the demodulation scheme. Give relevant mathematical expressions. In VSB, one sideband & a part of the other sideband called as vestige is transmitted. So the bandwidth required for VSB transmission is Somewhat higher than that of SSB – modulation. Generation of VSB modulated wave :m(t) Product modulator DSBSC Sideband shaping filter The o/p of the product modulator is the OSB – SC ware & is – given by :
This DSB – SC Signal is then applied to a Sideband Shaping fitter. The filter will post the wanted sideband as it is & the vestige of the unwanted sideband. Let the transfer function of the filter be H(f). hence the spectrum of the VSB modulated ware is given by: (1) Demodulation OR detection of VSB modulated wave : Explain the coherent detection of VSB – SC wave. V(t) VSB wave s(t) Product modulator Low pass filter The demodulation of VSB modulated wave can be achieved by passing VSB wave S(t) through a coherent detector. Thus, multiplying S(t) by a locally generation carrier wave the carrier wave Which is synchronous with in both frequency & phase as shown in fig, we get 2 Taking Fourier transform on both side of eq 2, we get 3 W.K.T H(f + H(f + 4 5 Substituting eq 4 & 5 in eq 3, we get
(6) VSB Wave Unwanted Term eq (6) is passed through a LPF, which eliminates unwanted term & passes only wanted term i.e. VSB wave & is given by : (7) The spectrum is as shown in below figure : Fig (b) Spectrum of V0(f) To obtain the undistorted massage signal m(±) at the o/P of the demodulator, the transfer function H(f) should satisfy the condition as follows: Where H(fc) is constant