Why we use vestigial side band (VSB-C3F) transmission for
picture?

Answer Posted / alan james

The video baseband signal used in TV in countries that use
NTSC or ATSC has a bandwidth of 6 MHz. To conserve
bandwidth, SSB would be desirable, but the video signal has
significant low frequency content (average brightness) and
has rectangular synchronising pulses. The engineering
compromise is vestigial sideband modulation. In vestigial
sideband the full upper sideband of bandwidth W2 = 4 MHz is
transmitted, but only W1 = 1.25 MHz of the lower sideband is
transmitted, along with a carrier. This effectively makes
the system AM at low modulation frequencies and SSB at high
modulation frequencies. The absence of the lower sideband
components at high frequencies must be compensated for, and
this is done by the RF and IF filters.

courtesy
http://en.wikipedia.org/wiki/Vestigial_side_band#Vestigial_sideband_.28VSB.29

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