Patent GB-449242-A (GB)
Title: Improvements in apparatus for amplifying electrical variations
IPC: H04N5/16
Filed: 1934-09-18
Abstract: 449,242. Television. BROWNE, C. O., 29, Monk's Drive, West Acton, London, BLYTHEN, F., Heathcote, Keith Road, Hayes, Middlesex, and BLUMLEIN, A. D., 32, Audley Road, Ealing, London. Sept. 18, 1934, No. 26848. [Class 40 (iii)] In an amplifier for reinserting low-frequency components and preventing zero wander in television or like signals, the input and output voltages or currents are compelled to return to a datlun level periodically. On the output side a condenser 18 is periodically charged to a level fixed in relation to earth, to the black parts of the picture, or to the peaks of the blacker-than-black synchronizing impulses, and this is effected through a stabilizing-device such as a valve 24, Fig. 2, which periodically brings about connection of the condenser with a source of standard potential. The stabilizing- device may be operative during the synchronizing intervals only, or alternatively (when the stabilization relates to the black parts of the picture) it may be operative during scanning and inoperative during the synchronizing intervals. The principle of the invention is illustrated with reference to equivalently acting mechanical switches 12, 14, Fig. 1. These are closed intermittently so as momentarily to impart the potentials derived from batteries 11, 13, to the input grid 6 of the amplifier and the output plate of the condenser 18. The switches are so timed that switch 14 is never closed when switch 12 is open. If the voltages of batteries 11, 13 bear to one another a ratio proportional to the amplification, the outgoing signals have the correct mean value in relation to earth-potential; but in order to prevent distortion, the battery 13 may have a voltage such that outgoing potentials are always negative. In an equivalent arrangement, Fig. 2, an iconoscope 15 feeds the amplifier 17 which emits signals through a valve 19 arranged as a cathode - follower valve, the anode-potential being fixed while the cathode follows the changes of grid-potential. Positive impulses are applied by the source 25 at the end of each scanning line to the control grid 19g so that the potential of condenser 18 is equalized with that of the cathode 19c, the grid-cathode space being analogous to the switch 14. Simultaneously at the input end, a negative pulse from the source 25 cuts off the cathode ray and so causes the input potential to assume a definite value. It is necessary to ensure that the grid 19g shall be insulated when the cathode ray is cut off, and for this purpose a single impulse travelling along a delay-action network, Fig. 3 (not shown), may be applied through suitably chosen tappings to the control grids of valve 24 and a parallel pair of valves feeding the control electrode 22 of the iconoscope. In an alternative arrangement the stabilization of potential may take place with reference to the black parts of the picture as a datum level, the valve 24 being automatically switched on during the scanning of those parts, but rendered non-conducting during picture-elements other than black and also during the intervals between lines. In the modification shown in Fig. 4, two-way-conducting switch devices are used. Picture signals are fed through the amplifier 17 to the cathode-follower output valve 19 ; negative switching pulses are sent to the iconoscope over lead 35 ; and positive switching-pulses are sent to two valves 36, 37, periodically rendering them conductive. Valve 36 behaves as a cathode-follower during the intervals when valve 37 is conductive and has that valve as its cathode - earth impedance. The potential on condenser 18, which follows the cathode 36c, is thus stabilized with respect to battery 42, the valve being rendered conductive by each switching-pulse but rendered non-conductive by the action of diode 43 at other times. The action of valves 36, 37 may be co-ordinated by control of their bias or of the amplitudes of the impulses applied to them. The datum level with respect to which the potential on condenser 18 is stabilized may, Figs. 5, 6, 7 (not shown), be that of the peaks of blacker-than-black positive synchronizing-impulses which pass through an A.C. amplifier 17 together with the negative picture signals. The positive synchronizing impulses render conductive a tetrode valve which allows a fixed positive potential to be applied to the right-hand plate of condenser 18. The tetrode valve is such that no leakage occurs from the condenser during intervals between the stabilizing impulses. The arrangement shown in Fig. 4 may be modified for the purpose of stabilizing with reference to the peaks of the synchronizing-impulses, Fig. 8. The synchronizing-impulses passing through amplifier 17 are applied through a valve 51 to a multivibrator 52, 53. Whenever the multivibrator, which vibrates at line-scanning frequency, is triggered by such an impulse, the anode of valve 52 becomes more positive for a time slightly shorter than the synchronizing impulse, the time-constants of the circuits being such that valve 53 is in action for a longer period than valve 52. Positive pulses are fed by valve 52 to the stabilizing valves 54, 55, which stabilize the potential of condenser 18 in the same manner as do valves 36, 37, Fig. 4; but in Fig. 8 it is the battery 56 which determines the potential acquired. The valves 54, 55 may both be controlled by the switching pulses, or valve 55 may be permanently conductive. In a further modification, the datum with reference to which stabilization takes place is the fullblack interval which immediately follows each synchronizing-impulse, an additional valve 63, Fig. 10 (not shown), being suitably coupled after the valve 51 for that purpose. Instead of the potential of a condenser 18, the current in a coil of high inductance may be stabilized, Fig. 11 (not shown). Specification 422,906 is referred to.