VSB
- 1. VSB Vestigial Sideband UNIT 1 Presented By Mrs.M.P.Sasirekha
- 2. Vestigial Sideband Transmission • In case of SSB modulation, when a sideband is passed through the filters, the band pass filter may not work perfectly in practice. As a result of which, some of the information may get lost. • Hence to avoid this loss, a technique is chosen, which is a compromise between DSB-SC and SSB, called as Vestigial Sideband (VSB) technique. The word vestige which means “a part” from which the name is derived.
- 4. Vestigial Sideband • Both of the sidebands are not required for the transmission, as it is a waste. But a single band if transmitted, leads to loss of information. Hence, this technique has evolved. • Vestigial Sideband Modulation or VSB Modulation is the process where a part of the signal called as vestige is modulated, along with one sideband. A VSB signal can be plotted as shown in the following figure.
- 6. Frequency Domain Description Frequency Spectrum 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 .
- 7. Transmission Bandwidth • the transmission bandwidth of the VSB modulated wave is given by : • Where fm = Message bandwidth • fv = Width of the vestigial sideband
- 8. Generation of VSB Modulated Wave
- 9. Generation of VSB Modulated Wave • 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)
- 10. • This represents a DSB-SC modulated wave . • This DSB-SC signal is then applied to a sideband shaping filter . The design 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
- 11. Demodulation of VSB Wave
- 12. Demodulation of VSB Wave • 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
- 13. Demodulation of VSB Wave • But • Hence, we have
- 14. Demodulation of VSB Wave • 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
- 15. 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 .
- 16. 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.
- 17. VSB Modulation − Disadvantages • Bandwidth when compared to SSB is greater. • Demodulation is complex.
- 18. 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.
- 19. Thank you