Circuit for modifying the gradation of pictures produced by video signals

A circuit for modifying the gradation of pictures produced with video signals, comprising:

• a first resistance (2) interposed between a source (1) of said video signals and an output terminal (4) for supplying video signals to picture producing apparatus;
• an inverting keyable amplifier (5) having its input connected to said output terminal (4) and its keying input (12) connected so as to enable said amplifier only during blanking intervals of said video signals, said amplifier having a connection to a fixed reference potential and being constituted so as to provide a current at its output (6) of a polarity dependent on whether said potential of said output terminal (4) exceeds or is less than said reference potential;
• a capacitor (7) connected to said output (6) of said amplifier (5) for charging or discharging by output current of said amplifier (5) and for storing charge while said amplifier is disabled,
• a diode (3) and an impedance converter (8,9,10,11) connected between said output terminal (4) and said output (6) of said amplifier in such a way that said diode is interposed between said output terminal (4) and said impedance converter and is poled so as to be conductive while the portion of said video signal between blanking intervals is provided by said video signal source (1), said impedance converter serving to cause the charging or discharging of said capacitor (7) to continue during video blanking intervals until the potential of said output terminal is substantially the same as said reference potential, and
• means (15) for assuring that said diode remains conducting even when said output terminal is at said reference potential,
• whereby substantially a voltage fraction of said video signals is made available at said output terminal (4) which fraction is determined by the ratio of the forward resistance of said diode to the sum of said diode forward resistance and said first resistance, while the temperature dependence of said diode forward resistance is compensated, at least for the black level of said video signal, during each blanking interval.