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Video baseline drift in CCD cameras with variable frame formats is often difficult to correct. Conventional baseline restorers generally do not work well at very high frame rates, since frequently only a short, few nanosecond wide point in each video line is available for reference. A.C. coupled video level drifts as the signal average changes with illumination. Also, low frequency noise (mainly from power supplies) modulates the video signal, especially when high video amplifier gains are required. On the other hand, although D.C. coupled video amplifier output does not drift due to the change in CCD illumination, it does drift as a function of temperature. Also, externally generated noise is directly superimposed on the video signal, reducing the image quality. Either case requires some kind of baseline restoration in order to keep the signal aligned to the desired reference level. This requirement is imperative during the process of testing and evaluating state of the art multiport CCD sensors. We describe a simple method for baseline restoration of A.C. coupled video signals, using a fast voltage comparator and a gated latch. An external strobe pulse must be provided and timed precisely at the point in video line selected as a reference. This reference is compared with a preset external D.C. level. The comparator output would therefore go to its high state when the reference signal is below that level. The comparator output status is latched into a D-type flip-flop by strobe pulses, provided externally, and timed to be coincident with the video line reference points. The inverted output of the latch is used to either charge or discharge the video signal coupling capacitor with a constant current. It tries to shift the video signal reference point, after each strobing pulse, closer to the comparator D.C. bias voltage, in a characteristic zigzag pattern. Best results are achieved when the rate of coupling capacitor voltage change, done by the restorer, matches the baseline drift rate.
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