GB1249497A - Apparatus for automatically eliminating an influence of the earth's magnetic field on colour television receivers - Google Patents

Abstract

1,249,497. Cathode-ray tube circuits. SONY CORP. 30 Dec., 1968 [29 Dec., 1967; 2 March, 1968], No. 61670/68. Heading H4T. In an arrangement for automatically eliminating the influence of the earth's magnetic field on the cathode-ray tube in colour television receivers means are provided for detecting such field and for producing an output voltage which is utilized to control a proportional current in a coil positioned in relation to the tube such that the magnetic field produced thereby neutralizes that due to the earth's field. The means for detecting the earth's magnetic field comprises a magneto-resistance element formed from semi-conductor material SCM (Fig. 1) and consists of a high resistance region I, P and N regions and a recombination region F. This element has a voltage current characteristic A2 (Fig. 2) in the presence of a magnetic field (+ ) H acting in the direction shown by the arrow and at right angles to the current paths in the element and a characteristic A3 when the direction of the field is reversed (- H), so that the magnetic field resistance characteristic is as shown in Fig. 3. In carrying out the invention two such elements connected in series, in the forward direction, may be employed (Figs. 5A and 5B, not shown) but, preferably, and as shown in Figs. 7A and 7B two such series connected pairs SCM 1 ... SCM 4 formed as a bridge circuit and operating differentially are employed. In the arrangement of Fig. 7A, a D.C. bias source E I is utilized and the output of the bridge is amplified in D.C. amplifiers DA 1 , DA 2 feeding opposite ends of the neutralizing coil LT. In the arrangement of Fig. 7B an A.C. bias source comprising, e.g. a sine wave or the line or frame pulse voltages available in the television receiver, is employed and the bridge output is amplified in A.C. amplifiers 12 1 , 12 2 , rectified by diodes 17 1 , 17 2 and further amplified by D.C. amplifiers 18 1 , 18 3 feeding the coil LT. In practice three such bridge-connected elements are employed. These are positioned with the angular relationship shown in Fig. 8A, the output of the bridge ST controlling the current in a coil LT surrounding the screen end of the tube CT whilst the outputs of the bridges SL and SR control the current in coils LL 1 , LL 2 and LR 1 , LR 2 positioned at the top and bottom of the left and right-hand sides of the screen. With this arrangement currents having relative phases -45‹, 0‹, +45‹ are produced in the coils LL, LT and LR respectively and when once set up, with suitable adjustment of the gains of the D.C. amplifiers, neutralization of the earth's magnetic field is produced irrespective of changes in the orientation of the receiver. The sensitivity of the arrangement may be enhanced by the use of flux collectors on both sides of the elements of the bridges (Fig. 11, not shown).