GB656603A - Improvements relating to circuit arrangements for cathode ray tubes - Google Patents

Abstract

656,603. Radio interference suppression. EDISON SWAN ELECTRIC CO., Ltd. Dec. 31, 1947, No. 35160. [Class 40 (v)] [Also in Group XL (b)] The effect of transient interference signals in input signals applied to a modulating electrode of a cathode-ray tube is eliminated by also applying the input signals above a predetermined level which is varied automatically in accordance with variations in the peak value of the signals to a further modulating electrode so as to oppose the modulation produced by the first modulating electrode. In one arrangement according to the invention Fig. 4 (Comp.), the input signals (television signals including transient interference signals) are fed to the modulating grid of a cathode-ray tube CRT from the junction point of load resistors R1, R2 in the anode circuit of a valve V1 (video amplifier stage in a television receiver) and signals are also fed direct from the anode of V1 to the cathode of CRT through a diode D1, the conduction level of which is determined by the biassing potential applied to its cathode from condenser C3 which is charged through a second diode D2 during the peak periods of the signals. The circuit constants R1, R2 are so selected that the signals fed to the cathode of CRT during the occurrence of the transient interference signals are just sufficient to oppose the effect of these signals of the grid of CRT. Other embodiments in which the compensating signals are fed to the modulator grid instead of to the cathode of the cathode-ray tube and in which the biassing potential for the diode D2 is derived from the anode of V1 via a cathodefollower stage so as to reduce the loading on V1 are described with reference to Fig. 3 (Prov.) and Figs. 5 and 6 (Comp.) (not shown). In a further embodiment, Fig. 7 (Comp.), a diode D3 supplies D.C. restoration both to the cathode-follower stage V<SP>1</SP> 2 and to the grid of the cathode-ray tube and a switch AB is provided for selecting one of two different timeconstants for the load circuit of the diode D2. In order to prevent the D.C. voltage supplied by D2 from following sustained interference signals when a high time-constant is employed (to prevent clipping of white peaks), the load circuit is shunted by a valve which is rendered conducting periodically (e.g. by means of pulses at line or frame frequency) so that the reduced resistance of the valve reduces the time-constant by a desired amount and prevents a voltage build-up (Fig. 8, not shown).