Abstract: An analog-to-digital conversion circuit for converting an input analog video signal to a digital video signal includes a sync separator for deriving from the analog video signal a horizontal synchronizing signal, and a voltage controlled oscillator (VCO) for producing a clock signal. An A/D converter converts the input analog signal to digital video based on the timing of the clock signal. The VCO, which produces the clock signal, is controlled by a circuit which compares the phase of the horizontal synchronizing signal extracted from the digital video signal to the phase of the clock signal. The clock signal is therefore highly stable and free of any adverse effects generated by drifts of the synchronizing signal separator and adverse effects produced by a lowpass filter employed in the circuit.

Abstract: A feedback clamping circuit for producing a drift error free digital signal from an input analog video signal. The circuit includes a differential amplifier which receives the input analog signal and an error signal. At its output, the differential amplifier delivers a drift free analog video output. To produce the error signal, the output of the differential amplifier is digitized and supplied to a switching circuit. A reference signal is also supplied to the switching circuit. Alternatively, the switching circuit supplies the two signals it receives to a D/A and from there to first and second sample and hold circuits. A comparator compares the level of the two signals that are supplied by the switching circuit and produces the error signal from them. The video signal is sampled during its pedestal phase and the reference signal shortly thereafter. The difference amplifier adjusts its output to minimize the level of the error signal.