GB732677A - Improvements in or relating to interference reduction in television - Google Patents

Abstract

732,677. Correcting inductive disturbances. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 11, 1952 [April 17, 1952], No. 9724/52. Class 40 (4). [Also in Group XL (b)] A circuit arrangement for reducing or eliminating hum in a television signal circuit, for example the hum picked up in the outer screens of coaxial cables, comprises means for sampling the television signal at recurrent periods when picture information signals do not occur, i.e. the line blanking intervals, means for integrating the sampled signals to provide an integrated hum-correcting signal and means for utilizing said signal for opposing the hum interference in the circuit. In Fig. 5 the television signal passes through a channel comprising three amplifiers GA, V1, V2 and the D.C. component at the output is restored by a conventional clamp-type circuit D1, D2. In order to eliminate hum superimposed on the signal the output from the final stage F1 of amplifier V1 is applied through a buffer stage BF to a sampling stage 2 in the form of a gate which is opened under the control of pulses (a) applied to the suppressor. The pulses occur at line frequency and their timing is arranged so that gating occurs either during the " front porch " or the " back porch " or during the line synchronizing period. The signals at the input to stage 2 are arranged with the synchronizing signals in a positive direction and D.C. restoration is effected by current flow at the grid. Alternatively D.C. restoration may be effected to the "black" " level. Since the gating is effected when picture component signals are not present and the input is held at a fixed level the anode pulses across resistor 6 are proportional to the amplitude of the superposed hum. A linear relationship between the anode pulses and the input signal is ensured by a negative feed-back connection between the screen grid and control grid. The anode pulses are integrated by condenser 7 and the gating frequency component removed by filter 8, 81. The resulting hum-correcting signal is then applied through an adjustable phase-shifting circuit 9 to amplifying stage CV1-CV4. The output from CV2 is applied to oppose the hum at the input to amplifier V1 and a further reduction is effected by a connection from CV4 into the cathode circuit of the first stage in amplifier V2.