There are times when CCTV sync signals must maintain temporal fidelity when derived from composite video. One such case involves applications in which several video sources are polled by an instrument which then extracts photometric data from video scenes.
The circuit described here replicates the incoming sync pulse width exactly. This allows accurate clamping at dark level, which succeeds the horizontal sync level.
The composite video input is terminated at resistor R1 and ac-coupled by capacitor C1 to the inverting input of op amp IC1 (see the figure). A negative voltage is inverted by IC1 with a gain of approximately −1 because of the conduction of diode D1. This clamps the negative sync tip at the amplifier reference which, in this case, is the circuit ground. When D1 isn’t conducting (the video input is positive), the amplifier input pin presents a high impedance and the clamping action ceases.
The voltage at the negative input of IC1 is shared at IC2 (positive input in this circuit, but an inverted output may be obtained by using the negative input) to create the Composite Sync Out signal. Potentiometer R8 is used to adjust the output for best symmetry.
Reprints
