Patent GB-483935-A (GB)
Title: Method of and means for electric transmission of pictures and for television
IPC: H04B1/66
Filed: 1937-01-15
Abstract: 483,935. Television. BRIZA, J. Jan. 15, 1937, No. 1237. [Class 40 (iii)] - The velocity of exploration and reconstitution is controlled according to the differential of the photoelectric current with respect to time. The velocity decreases linearly with increase of the differential. Amplified photo - electric currents are applied to the controlling grid of a pentode L1, to the screening grid of which is connected the primary of a transformer Tr1, and both halves of the voltages induced in the secondary are rectified in valves L2, L3. Alternatively resistance-capacity coupling is used, Fig. 6 (not shown). The output of the rectifiers controls the rate of charge of the line-scanning condenser C4, and hence the velocity of exploration. At the receiver, Fig. 5, the influence of corresponding changes of the velocity of reconstitution on the apparent brightness of the luminous spot is compensated by a proportional change in the bias, and hence in the steepness of the static characteristic, of a hexode L16, which amplifies the received picture currents. When transmitting a film, Fig. 1, the movement of the exploring spot at rightangles to the picture lines is controlled by applying to plates ky, which deflect the electronic beam, the voltages across two condensers C6, C10. The chasing condenser C10 is charged with a constant current corresponding to the constant velocity VF of the film, and the condenser C6 is charged with a current proportional to the charging current of the line-scanning condenser to compensate the effect of changes in the velocity of scanning. The receiver is kept in synchronism by controlling the velocity of reconstitution by the received picture currents in the same way as the velocity of exploration is controlled by the amplified photo-electric currents. Special synchronizing signals are also transmitted. A framing signal is produced at the completion of each scanning by blocking the photo-electric amplifier Z over a lead b by the charging of a condenser C13 caused by the discharge of the condenser C6 through a gas-discharge tube T2 at the end of each picture. At the end of the time fixed for each picture, the condenser C13 is discharged by the action of a current applied to a potentiometer R16. If the scanning would take longer than the prescribed time, a corrective framing signal is produced by blocking the amplifier Z over a lead c, when the chasing condenser C10 is discharged by the pulse applied to the potentiometer R16. The duration of the corrective framing signal is given by the time constant of a circuit R20, C11 in the anode circuit of the tube T3, which discharges the condenser C10, and is of sufficient length to cause the discharge of the condensers C4, C6. According to a modification, Figs. 10-13 (not shown), the differential of the photo-electric currents controls the frequency of an auxiliary current, which controls the velocity of exploration and reconstitution and is transmitted simultaneously with the picture currents. The line and framing signals change the frequency of the auxiliary current in the opposite sense to the change for controlling the velocity of exploration.