Circuit and method for producing accurate dc restored video waveform, horizontal sync pulses, and vertical sync pulses

摘要

A composite video signal is input to a DC restore circuit and a sync separator circuit, the output of which is connected to a sync decoder circuit. The DC restore circuit includes a negative sensing peak sample and hold circuit that generates a first reference voltage equal to the sync tip voltage. A first differential amplifier substracts the composite video signal from the reference voltage and produces an inverted, level shifted replica, the sync tip voltage of which is zero. A second differential amplifier having an output clamping circuit, in conjunction with a comparator, produces a composite sync voltage that is used to strobe a sample and hold circuit to produce a second reference voltage. The replica is inverted by a second differential amplifier that subtracts the replica from the second reference voltage to produce the DC restored video signal, a "back porch" portion of which is accurately referenced to zero volts.