GB399469A - Improvements in or relating to television systems - Google Patents

Abstract

399,469. Converting. BULLIMORE, W. R. and BEDFORD, L. H., Cossor Works, Highbury Grove, London. Feb. 27, 1932, No. 5929. [Class 38 (ii).] In a television system in which a scanning cathode ray moves over a picture at a rate dependent upon the occurrence of light and shade in the picture and is controlled in its movement by a varying potential produced by alternately charging and discharging a condenser, the charging current for the condenser is derived from a thermionic valve, and the discharge of the condenser is also effected through a thermionic valve. The cathode ray oscillograph CRO produces a scanning beam which passes through a film to a photoelectric cell the output from which is fed to an amplifier PCA and to the grid of a screen-grid valve V1 with its screen maintained at a potential such that the plate current charging the condenser C1 and the voltage impressed on the oscillograph plate Py1 are proportional to the output from the photoelectric cell. The condenser C1 is arranged to discharge under control of a valve V3 which may be biased to pass current when a predetermined voltage is reached. Alternatively the valve V3, which may be a power pentode valve, is normally biased to be inoperative but is operated to pass current discharging the condenser C1 by control impulse of the line frequency and produced by a timing circuit N2, R2, R4, C7. The movement of the scanning cathode beam along a line may take place in its own time or a given maximum time mav be allowed for each line. In the latter case the beam is locked just off the edge of the film by the incorporation of a locking valve V2, to the anode of which is applied a predetermined voltage such that when the plate voltage of the valve V1 falls below that of the valve V2, current passes in the valve V2 'and the charging of the condenser C1 ceases. 'At each discharge of the condenser C1, the grid of the valve V5 is biased so that current incrementalty charges the condenser C5 at the end of each line for the line-by-line movement of the scanning beam. At the end of each picture, a pulse is applied from a neon tube timing circuit N1, R1, R3, C6 to the valve V2 to neutralize its locking action, and a larger discharge pulse from the condenser C1 is fed through a small condenser C4 to the grid of a valve V6 which now passes current effecting the discharge of the condenser C5, by which the beam is returned to the first line of the picture. The variations of the voltage applied to the Py1 plate, Fig. 2, are transmitted over a wireless or similar channel and at the receiving station, Fig. 7, are applied to the corresponding plate Py1 of a cathode ray oscillograph. At the end of each line the grid of the valve V5 is biased so that it passes a charging current to the condenser C5, and the large impulse at the end of a picture causes the valve V6 to become operative, effecting the discharge of the condenser C5 and control of the Px plate to return the scanning beam, as at the transmitting station. When the film is moved continuously and is scanned in lines parallel to its direction of motion, the scanning beam is given a displacement in the direction of motion of the film by applying to the normally idle and grounded plate Py2 a voltage at each change of line, and by reducing this voltage to its normal value at the end of each picture. This voltage may be obtained from the voltage supply to the condenser- C5 by throwing the switch K1 to the position shown in dotted lines. The system may be modified to effect scanning in each direction of movement of the cathode beam by incorporating an arrangement as shown in Fig. 8. The grids of the valves V1, V1<1> are controlled by the photo-cell amplifier PCA in antiphase and the screens of the valves are similarly controlled by a multi-vibrator so that only one valve is operative at any time. A screen-grid valve V8 is arranged to pass a constant current for the voltage range of the condenser C1 and in the absence of signals the condenser C1 is alternately charged and discharged by equal currents. When, at any time, two voltages of opposite signs are applied to the valves V1, V1<1> and, for example, a positive voltage to V1, there will be an increment in the V1 current if the V1 valve is operative, or in the current of the valve V1<1> if operating, thus reducing the charging or discharging current, as the case may be, of the condenser C1 and the speed of the scanning spot. The cathode ray tube may be of the form described in Specification 396,422, [Group XL].