Auto clamp for modulator for suppressed sync signals

摘要

Suppressed sync pulses in a scrambled video signal are restored to their proper level in another video signal that is applied to a clamp circuit for controlling the DC level of an amplifier coupled between the head end encoder and the modulator in a CATV system. This prevents the clamp from drifting and changing the DC signal level in the scrambled video signal supplied to the modulator. The encoder supplies a suppression signal that indicates when a sync pulse is suppressed. This signal is used to trigger sample-and-hold circuits to measure the level of the first suppressed sync pulse after a vertical interval and the level of a sync pulse during the vertical interval when sync pulses are not suppressed. The levels are compared and their difference is used to restore the suppressed sync signals in the video signal applied to the clamp.