Patent GB-799949-A (GB)
Title: Improvements in or relating to pulse generating circuits
Inventor: URRY DENIS EDWARD
IPC: H04N5/18
Filed: 1954-04-08
Abstract: 799,949. Television. RANK CINTEL, Ltd., [formerly CINEMA-TELEVISION, Ltd.]. April 7, 1955 [April 8, 1954], No. 10353/54. Class 40 (3). [Also in Group XL (c)] Simultaneously occurring pulses of opposite polarity the sum of the amplitudes of which is constant but the relative amplitudes of which vary in accordance with the difference between the actual black level in a television signal supplied to terminal 1 and a control potential determined by the setting of potentiometer 23 and derived respectively from the cathode and, via an anode follower stage 30, from the anode of valve 13, are supplied across one diagonal of a diode bridge 41 ... 44 from the other diagonal of which (at terminal 46) a potential is derived which is dependent on the setting of potentiometer 49 and on the departure of the actual black level present in the signal from the desired level which is determined by potentiometer 23. The potential at 46 is then employed in known manner to restore the desired black level in the signal, e.g. by introducing it into the input of an amplifier, the output of which is connected to terminal 1. The operation of the pulse generating arrangement is as follows. The grid of valve 3, to which the television signals (with the synchronizing signals extending in the positive direction) are supplied, is pulsed positively during the occurrence of the picture information by means of a valve 6 which is cut off during the occurrence of the blanking periods by means of negatively-going pulses from terminal 7 so that together with the action of a diode 10 the negatively-going picture information is substantially suppressed. The output of valve 3 during the blanking periods then consists of a signal proportional to the black level actually present in the television signal and this is supplied to the grid of a second valve 13. The latter has equal anode and cathode load resistors 14, 15, a further resistor 18 being common to the cathode circuit of both valve 13 and a (always conducting) valve 19, the low impedance of which is effectively in shunt with 18 which does not therefore materially increase that of the load resistor 15. The component values are so chosen that when potentiometer 23 is adjusted to apply to the grid of valve 19 a potential equal to that which true black level should produce at terminal 1, valve 13 is cut-off whilst valve 19 is operative. In this condition the anode of valve 13 is at the potential of the H.T. line whilst its cathode is at the same potential as the cathode of valve 19. During the occurrence of the blanking periods the negatively-going pulses at terminal 7 cause the cathode potential of valve 19 to fall so that the cathode potential of valve 13 goes negatively until it reaches that of the grid (determined by the level of the blanking interval) at which point valve 13 conducts, passing a current determined by its grid potential, so that the potential of its cathode now remains fixed for the duration of the blanking period. A negatively-going pulse is also produced at the anode of valve 13, the amplitude of which is determined by This is a constant less a value proportional to the anode current of valve 13 and therefore to its grid potential (i.e. the blanking level) and by a suitable choice of circuit parameters the sum of the amplitudes of the (negatively-going) pulses produced at the anode and cathode of valve 13 during the blanking periods can be made constant. To prevent variations of the working conditions of valves 6 and 19 due to changes in the mark/space ratio of the pulses at terminal 7 D.C. restoration diodes 6 and 27 are provided.