US 4,790,011 A · Auto clamp for modulator for suppressed sync signals

Abstract

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.

Claims4

Dependent claims

In combination:

a source of first video signal including a vertical interval portion having normal sync pulses and other portions having suppressed sync pulses;
an amplifier having an input for receiving said first video signal and an output for supplying an output signal to a modulator;
clamp means for maintaining the correct DC level in said output signal of said amplifier;
correction means for developing a second video signal with restored sync pulses for application to said clamp means for automatically compensating operation of said clamp means for suppressed ones of said sync pulses, said correction means including;
means for sampling the level of said sync pulses during said vertical interval;
means for sampling the level of the first suppressed sync pulse occurring after said vertical interval;
means for comparing said levels to develop a signal representative of the amount of said suppression;
and
means for restoring said sync pulse level in said second video signal.

6 more elements

Dependencies are inferred from the claim text.

Auto clamp for modulator for suppressed sync signals

Life of this patent

Auto clamp for modulator for suppressed sync signals

Life of this patent

Abstract

Claims4