In the previous chapters, we have mentioned about SSBSC modulation and demodulation. As we know that SSBSC modulated signal has only one sideband frequency, with this theoretically, we can prove one sideband frequency component completely by using an ideal band pass filter. However, practically it is not possible to get the entire sideband frequency component. Due to this, it may possible to lose some information.
In order to avoid this loss of information, a technique is chosen, which is known as Vestigial Side Band Suppressed Carrier (VSBSC) technique. The word “vestige” means “a certain part” from which, the name is derived.
VSBSC Modulation is an amplitude modulation technique, where one of the side band is removed to form a vestige modulation hence, the name vestige modulation. In other words a part of the signal is modulated along with one sideband. The frequency spectrum of VSBSC wave is shown in the following figure.
With the two sidebands, the upper sideband and also a part of the lower sideband is also being transmitted in this technique. It is quite opposite with the lower sideband, it transmits the lower sideband along with a part of the upper sideband. To avoid the interference between the bands a small width of guard band is laid on either side of VSB the. VSB modulation is mostly used in television transmissions.
We know that the bandwidth of SSBSC modulated wave is fmfm. Since the VSBSC modulated wave contains the frequency components of one side band along with the vestige of other sideband, the bandwidth of it will be the sum of the bandwidth of SSBSC modulated wave and vestige frequency fv.
Following are the advantages of VSBSC modulation.
Following are the disadvantages of VSBSC modulation.
The most prominent and standard application of VSBSC is for the transmission of television signals. Also, this is the most convenient and efficient technique in the case of bandwidth usage.
Now, let us discuss about the modulator which generates VSBSC wave and the demodulator which demodulates VSBSC wave one by one.
Generation of VSBSC wave is similar to the generation of SSBSC wave. The VSBSC modulator is shown in the following figure.
In this method, first we will generate DSBSC wave with the help of the product modulator. Then, apply this DSBSC wave as an input of sideband shaping filter. This filter produces an output, which is VSBSC wave.
The modulating signal m(t)) and carrier signal Accos(2πfct)Accos(2πfct) are applied as inputs to the product modulator. Hence, the product modulator produces an output, which is the product of these two inputs.
Therefore, the output of the product modulator is
The above equation represents the equation of VSBSC frequency spectrum.
Demodulation of VSBSC wave is similar to the demodulation of SSBSC wave. Here, the same carrier signal (which is used for producing VSBSC wave) is used to detect the message signal. Hence, this process of detection is called as coherent or synchronous detection. The VSBSC demodulator is shown in the following figure.
In this process, you can get the message signal from VSBSC wave by extraction when it is multiplied it with a carrier, which is having the same frequency and the carrier phase used in VSBSC modulation. The resulting signal is then passed through a Low Pass Filter. The output of this filter is the desired message signal.
From the figure, we can write the output of the product modulator as
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