A ves­ti­gial side­band (in radio com­mu­ni­ca­tion) is a side­band that has been only part­ly cut off or sup­pressed. Tele­vi­sion broad­casts (in ana­log video for­mats) use this method if the video is trans­mit­ted in AM, due to the large band­width used. It may also be used in dig­i­tal trans­mis­sion, such as the ATSC stan­dard­ized 8-VSB. The Mil­go 440048 modem (cir­ca 1967) used ves­ti­gial side­band and phase-shift key­ing to pro­vide 4800-bit/s trans­mis­sion over a 1600 Hz chan­nel.

 

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.


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