GB653381A - Improvements in and relating to interference limiting arrangements for electric carrier-wave signalling systems - Google Patents

Abstract

653,381. Television receivers. BUSH RADIO, Ltd., and WRIGHT, K. M. B. Nov. 11, 1946, No. 33436. [Class 40 (iii)] [Also in Group XL (c)] For a television receiver or like apparatus employing a cathode-ray tube for picture reproduction an interference-limiting device comprises a diode valve connected in the tube input circuit and arranged effectively to offer a low impedance shunt to said circuit only in the presence of a signal, the amplitude of which exceeds approximately the peak value of the video signal. The arrangement may include a diode in series with the control electrode of the tube in combination with the diode operating as the low-impedance shunt. The picture signals are applied directly from the anode 10 of the final video amplifier valve V1 to the grid 11 of the tube CRT and to the anode 15 of a diode V2 whose cathode 16 is biassed at a voltage slightly higher than that of the anode by connection to a variable tap 14 on a resistor R4 in series with a resistor R5 and connected to a variable tap 13 on a potentiometer R2, R3 connected between a source HT and earth. Under normal conditions the diode V2 is non-conducting and offers a very high impedance path to the picture signals. An interference signal in excess of the peak picture voltage makes V2 conducting to provide a shunt path for the grid 11 so that the voltage applied to the grid is limited to a value slightly in excess of the peak white signal. Biassing voltage for the first anode 17 of the tube CRT is derived from a potentiometer R6, R7, of which the resistor R7 is connected to the tapping point 13 on R2. The flow of current through the diode V2 causes the voltage at the grid 11 and cathode 12 to be increased slightly relative to earth and to that of the first anode 17, thereby reducing to some extent the beam current in the tube. During normal operation the variation of the bias voltage applied by the tap 14 is nearly independent of the variation of the setting of the tap 13 so that the biassing tap 14 can be preset whilst the tap 13 which is effectively a brilliance control, can be varied as desired. In a modification, Fig. 2 (not shown), a second diode is inserted between the anode of the valve V1 and the grid of the tube CRT, the diode V2 being connected in a shunt path around the second diode which is biassed to be conducting for all video signals, but is cut off by interference signals. The first diode is biassed, as in the arrangement in Fig. 1, to become conducting in response to interference signals exceeding peak white and is effective to apply a negative voltage to the cathode of the tube CRT so that the beam current is much reduced or cut off. In a further arrangement, Fig. 3, modified for cathode modulation, and in which interference signals will be more negative than the negative peak white signals, the diodes V4, V5 are biassed to be conducting and non-conducting, respectively, in response to the video signals. The strong negative interference signals render V4 non-conducting and V5 becomes conducting so that a negative-going voltage is applied via a condenser C5 to the grid 11 of the tube CRT. The excess signal voltage is coupled by condenser C6 to the first anode 17. In this arrangement the normal fixed bias voltage derived from the tap 14 on the resistance R4 is applied to the grid 11, the normal bias on the cathode 12 being derived from the tap 13 on resistance R2 and resistance R1. Specification 446,663 is referred to.