Patent GB-767508-A (GB)
Title: Improvements in or relating to pulse separating circuits
Inventor: INGHAM WILLIAM ELLIS
IPC: H04N5/10
Filed: 1953-10-17
Abstract: 767,508. Transistor and discharge tube pulse discrimminating circuits. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Oct. 1, 1954 [Oct. 17, 1953], No. 28694/53. Class 40(6) In a circuit for separating long and short pulses, the input pulse train is applied directly to one point and over an integrating circuit to a second point of a circuit having at least one stable state, such that at the termination of the long pulse the circuit is caused to change to a different state. In the embodiments described, point contact transistors are used in a circuit for separating line and frame pulses in a television receiver, but a valve circuit operating as a flip-flop or multivibrator with long time constant couplings may be employed. Positive-going pulses are applied over terminal 8 and rectifier 7 to non-conducting transistor 1, the base of which is near positive supply 3 in potential. The base is carried more positive for the duration of the pulse and then returns to its previous level. The emitter is held near earth due to rectifier 4 conducting and the collector near negative supply 6. During a short pulse, capacitor 12 is charged slightly over rectifier 9 and resistor 10 but discharges over resistor 11 before the next pulse. During a long pulse the emitter potential is carried above the non-conducting potential of the base so that when, at the end of the pulse the base tends to return to that potential, the transistor conducts and the collector potential becomes more positive, all the electrodes being near earth. The circuit is restored to its non-conducting condition by the next pulse, the leading edge of which carries the base positive with respect to the emitter. In a modification, Fig. 2 positive pulses and negative going picture signals are applied at terminal 8 frame and line synchronizing pulses being derived from resistor 2 and frame from resistor 5. In its non-conducting state the base is held near or slightly above earth the emitter being biased off by the potential divider 14, 15 and the collector assume a potential near the supply 6. Short pulses are ineffective as the emitter bias is hardly changed, the pulses passing out over terminal 17. Long pulses cause the capacitor 12 to charge sufficiently over resistor 13 that when the base potential falls at the end of the pulse, the transistor conducts to feed out a pulse over terminal 16. The electrodes all assume a potential near earth with rectifier 4 conducting. This pulse is terminated when capacitor 12 discharges sufficiently to cut off the transistor the electrodes returning to their normal potentials. The circuit may be fed from a clipper amplifier 18. Fig. 3, a complete picture signal with positive-going synchronizing pulses being fed in at terminal 8. The pulses cause transistor 18 to conduct to establish positive output pulses across resistor 22, the picture component carrying the emitter below cut off potential. Current passes through resistor 20 between pulses to restore the charge on capacitor 21 which is removed during synchronizing pulses by emitter current. In a further modification, Fig. 4 (not shown) resistors 2, 22 and rectifier 7 are omitted, the collector of transistor 18 being connected directly to the base of transistor 1.