Vestigial Side Band:

• 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.
• 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. In the 625 line system, frequencies up to 0.75 MHz in the lower sideband are fully radiated.
• The picture signal is seen to occupy a bandwidth of 6.75 MHz instead to 11 MHz.

Bandwidth Diagram:

• Though the total power that is developed and radiated at the transmitter. This can be demonstrated by considering the power relations in the modulated wave. The total power Pt, in the modulated wave is the sum of the carrier power Pc, and the power in the two sidebands.

  Mathematically,

where, Ec is the r.m.s. value of the sinusoidal carrier wave, and R is the resistance in which the power is dissipated. Note from the above expression that Pc remains constant but Pt depends on the value of the modulation index m.