Automatic brightness and contrast control circuit

Claim 1

IN A TELEVISION RECEIVER HAVING MEANS FOR RECEIVING A TRANSMITTED SIGNAL, A PICTURE TUBE HAVING A VIEWING FACE AND FIRST AND SECOND INPUT ELECTRODES INCLUDING A CATHODE ELECTRODE, MEANS FOR AUTOMATICALLY VARYING PICTURE CONTRAST AND BRIGHTNESS COMPRISING: AN AUTOMATIC GAIN CONTROL CIRCUIT FOR MAINTAINING THE VOLTAGE LEVEL OF A VIDEO SIGNAL COUPLED TO AN INPUT ELECTRODE OF SAID PICTURE TUBE AT A PRESELECTED CONSTANT LEVEL WHEN VARIATIONS IN THE INTENSITY OF THE RECEIVED SIGNAL OCCUR; SAID AUTOMATIC GAIN CONTROL CIRCUIT INCLUDING AN AMPLIFYING DEVICE HAVING A CONTROL ELECTRODE; MEANS FOR PROVIDING A CONTRAST AND BRIGHTNESS CONTROL VOLTAGE WHICH VARIES IN ACCORDANCE WITH THE INTENSITY OF LIGHT STRIKING SAID VIEWING FACE; AND CONTROL VOLTAGE MEANS INCLUDING A LIGHT-DEPENDENT ELEMENT POSITIONED RELATIVE TO SAID TELEVISION RECEIVER IN A MANNER FOR BEING STRUCK BY LIGHT OF SUBSTANTIALLY THE SAME INTENSITY AS AMBIENT LIGHT WHICH STRIKES THE FACE OF SAID PICTURE TUBE; SAID LIGHT-DEPENDENT ELEMENT HAVNG AN ELECTRICAL CHARACTERISTIC WHICH IS PROPORTIONAL TO THE INTENSITY OF LIGHT STRIKING THE ELEMENT; MEANS DIRECT-CURRENT COUPLING SAID CONTROL VOLTAGE TO SAID CATHODE OF SAID PICTURE TUBE FOR VARYING THE BRIGHTNESS THEREOF; AND MEANS CONNECTING SAID CONTROL VOLTAGE TO SAID CONTROL ELECTRODE OF SAID AMPLIFYING DEVICE IN SAID AUTOMATIC GAIN CONTROL CIRCUIT.