Automatic dark current control

Claim 1

In a television camera having a vidicon with a photosensitive surface and a mask on said surface having an opening defining the imaging area of the surface, said vidicon having a scanning electron beam progressively and repeatedly traversing said surface to define successive fields and producing an input video signal comprising horizontal blanking pulses and video information with a dark current component, an automatic dark current control circuit for producing a corrected output video signal comprising means for clamping said input video Signal to a reference level, a signal splitter having an input connected to the output of said clamping means and having two output lines, means for automatically sampling said clamped video signal during traverse of said beam across a portion of said photosensitive surface behind said mask, one of said output lines being connected to the input of said sampling means, means for storing a signal proportional to the magnitude of the actual dark current in the sampled video signal, a source of a reference signal, means for comparing said stored signal with said reference signal and producing a control signal proportional to the difference, and means responsive to said control signal and to said clamped video signal for clipping said blanking pulses whereby to reduce the magnitude of the dark current component in said output video signal, said clipping means comprising a differential amplifier and a diode, said amplifier having two inputs and an output, said amplifier inputs being connected to the other output of said splitter and the output of said comparing means, respectively, said diode being connected across said other output of the splitter and said output of said amplifier.