VESTIGIAL SIDEBAND TRANSMISSION

• In the video signal very low frequency modulating components exist along with the rest of the signal. These components give rise to sidebands very close to the carrier frequency which are difficult to remove by physically realizable filters.
•  Thus it is not possible to go to the extreme and fully suppress one complete sideband in the case of television signals. 
• The low video frequencies contain the most important information of the picture and any effort to completely suppress the lower sideband would result in objectionable phase distortion at these frequencies. 
• This distortion will be seen by the eye as ‘smear’ in the reproduced picture. 
• Therefore, as a compromise, only a part of the lower sideband, is suppressed, and the radiated signal then consists of a full upper sideband together with the carrier, and the vestige (remaining part) of the partially suppressed lower sideband. 
• This pattern of transmission of the modulated signal is known as vestigial sideband or A5C transmission. In the 625 line system, frequencies up to 0.75 MHz in the lower sideband are fully radiated.
• The net result is a normal double sideband transmission for the lower video frequencies corresponding to the main body of picture information.
•  As stated earlier, because of filter design difficulties it is not possible to terminate the bandwidth of a signal abruptly at the edges of the sidebands.
•  Therefore, an attenuation slope covering approximately 0.5 MHz is allowed at either end. Any distortion at the higher frequency end, if attenuation slope were not allowed, would mean a serious loss in horizontal detail, since the high frequency components of the video modulation determine the amount of horizontal detail in the picture.
•  Fig. 4.3 illustrates the saving of band space which results from vestigial sideband transmission. 
• The picture signal is seen to occupy a bandwidth of 6.75 MHz instead to 11 MHz.