GB581018A - Signal clipping system particularly for television - Google Patents

Abstract

581,018. Television; valve circuits. HASELTINE CORPORATION. March 29, 1944, No. 5845. Convention date, May 14, 1943. [Classes 40 (iii) and 40 (v)] A system for asymmetrically clipping an applied signal having recurrent peaks comprises a valve whose conductance is increased by the peaks of the signal fed with appropriate polarity and an anode impedance connected by passive coupling elements-i.e. elements which do not include any power amplifying devices-to the input of a second valve whose conductance is increased by the peaks of the signal together with means for causing the peaks to be stabilized in the input to the second valve, and the amplitude of the signal applied to the second valve is so related to its characteristic that anode current clipping is effected by the second valve. In the television system shown, the composite signal derived from an amplifier 16 is applied to a valve 30 having a coupling condenser 32 and a grid-leak resistance 33 with a time constant greater than the period of the synchronizing signals, thus stabilising the input to the valve 30. The anode of the valve 30 is connected to an impedance 38 and the signal output of the valve is applied to the cathode of the valve 35 having a load resistance 39 connected to its anode and a grid-leak resistance 36 and a condenser 37 in its grid circuit providing a time constant greater than the period of the synchronizing impulses. The amplitude of the signals applied to the second valve is so related to its grid voltage-plate current characteristic that anode-current clipping is effective to produce an output signal in which picture components are absent. The resistance 38 is high relative to the resistance 39 or to the sum of the resistance 39 and the minimum anode-cathode resistance of the valve 35. The use of the high resistance 38 increases the gain of the valve 30 during the interval between synchronizing impulses and since the tips of the synchronizing impulses are stabilized at the zero grid-voltage level of the valve 35, the picture frequency portions of the signal are shifted well below the cut-off point. The control electrode or grid of the valve 35 reduces the capacitative coupling between the anode and cathode and prevents the passage of video-frequency signals from resistance 38 to the output terminal 40 when the valve 35 is non-conductive.