Patent GB-732907-A (GB)
Title: Improvements in or relating to levelling circuits
Inventor: CLAYDEN RONALD THOMAS
IPC: H04N5/217
Filed: 1951-12-05
Abstract: 732,907. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Nov. 21, 1952 [Dec. 5, 1951], No. 28567/51. Class 40 (3). In a circuit for levelling television signals comprising image signals during line-scanning intervals and blanking signals during line and frame blanking intervals, portions of the blanking signals being more susceptible to interference than other portions of the blanking signals, means are provided for levelling the signals in dependence upon the blanking signals during intervals which include said first portions, and further means are provided responsive to portions of the blanking signals not including said first portions for modifying the levelling of the signals which would otherwise be effective so as to reduce errors in the levelling due to spurious signals. In Fig. 2 the output of a television pick-up tube 10 of the low-velocity type is passed through an amplifier 11 and levelled by means of a clamp circuit 12-14 which may take the form described in Specification 449,242. The circuit comprises a fourdiode bridge circuit 13 as described in Specification 512,109 and is switched into operation during the line blanking intervals by a pulse generator 14 so as to clamp the signal to a reference potential established at point 15. Curve 30, Fig. 3, shows the nature of the pickup tube signals, the line blanking intervals 2 including spurious damped oscillations which interfere with the operation of the clamp circuit. Curve 31 shows the switching pulses from generator 14., The time constant of the clamping circuit is selected to be of the order of 10 microseconds which largely ensure the removal of noise, power, supply ripple and microphony signals but is insufficient to eliminate completely variations in the level of the aligned signals due to the spurious signals. In accordance with the invention, these variations are reduced by means of a second circuit 22 similar to circuit 13 which is connected to the signal channel following a cathode follower 20 and is switched into operation by pulses 32, Fig. 3, from a generator 24 so as to develop a potential or a condenser 23 representative of the signal level during the line intervals 4, Fig. 3, occurring during the frame blanking interval. The signal level during such intervals corresponds to black and is free from spurious signals. The potential across condenser 23 is applied to a D.C. amplifier 25 which invents the sense of the variations and the output is then applied via resistor 26 so as to modify the reference potential established at point 15. In a modified arrangement, Fig. 4 (not shown), a circuit following cathode follower 20 superimposes the spurious signal on a pulse in the white direction, the amplitude of the pulse being such that the most extreme portion of the spurious signal in the negative direction does not extend to black. With this arrangement it is possible to arrange circuit 22 as a single diode functioning as a negative peak rectifier and to switch it into operation throughout the whole of the frame blanking interval by a pulse such as that shown at curve 33, Fig. 3. In a variation of this arrangement, the superposed pulse is of such amplitude that no part of the spurious signal extends below white level and a circuit is arranged to remove the spurious signal by clipping at white level. In a circuit based on this variation, Fig. 6 (not shown), an amplifier and/or gamma correction circuit is included in the channel following the cathode follower and has couplings such as to remove the D.C. component. A further clamp circuit is then provided at the output of the amplifier and operates on the superposed pulses to align the clipped (white) level with a reference potential. Specifications 688,195 and 729,621 also are referred to.