Black level stabilization circuit

Claim 1

AN AUTOMATIC GAIN CONTROL CIRCUIT FOR CONTROLLING THE GAIN OF A TELEVISION SIGNAL AMPLIFIER IN ORDER TO STABILIZE THE BLACK LEVEL OF A COMPOSITE TELEVISION WAVEFORM CONTAINED IN A MODULATED CARRIER WAVE, COMPRISING MEANS FOR FEEDING THE TELEVISION WAVEFORM TO SAID AMPLIFIER, MEANS FOR DETECTING THE TELEVISION WAVEFORM OBTAINED FROM THE OUTPUT OF SAID AMPLIFIER, A GATING DEVICE, MEANS FOR D.C. COUPLING THE DETECTED WAVEFORM TO THE GATING DEVICE, A SOURCE OF GATING PULSES, MEANS FOR FEEDING THE GATING PULSES TO SAID GATING DEVICE TO GATE THE TELEVISION WAVEFORM AT LEAST DURING A PORTION OF THE TELEVISION WAVEFORM WHICH IS CAPABLE OF PROVIDING A REFERENCE POTENTIAL, SO AS TO PRODUCE FROM THE GATING DEVICE A PULSE OUTPUT WAVEFORM WHICH EXTENDS BETWEEN THE GATING PERIODS AND WHICH HAS AN AMPLITUDE DEPENDING UPON THE D.C. LEVEL OF THE REFERENCE PORTION OF THE TELEVISION WAVEFORM, A CLAMP CIRCUIT, MEANS FOR APPLYING A D.C. POTENTIAL TO THE CLAMP CIRCUIT, MEANS FOR A.C. COUPLING THE PULSE OUTPUT WAVEFORM FROM THE GATING DEVICE TO THE CLAMP CIRCUIT, A CLAMP PULSE SOURCE FOR CONTROLLING SAID CLAMP CIRCUIT TO CLAMP SAID PULSE OUTPUT WAVEFORM FROM THE GATING DEVICE TO THE D.C. POTENTIAL APPLIED TO SAID CLAMP CIRCUIT DURING A PART OF SAID REFERENCE PORTION OF THE TELEVISION WAVEFORM, A FILTER CIRCUIT, MEANS FOR D.C. CONNECTING THE CLAMPED WAVEFORM TO SAID FILTER CIRCUIT TO PRODUCE A D.C. CONTROL VOLTAGE AND MEANS FOR FEEDING SAID D.C. CONTROL VOLTAGE TO CONTROL THE GAIN OF SAID AMPLIFIER.