Patent GB-972873-A (GB)
Title: Apparatus for changing-over from one television signal to another of which the synchronizing component is derived from a different source
IPC: H04N5/268
Filed: 1962-08-14
Abstract: 972,873. Television; transistor pulse circuits; synchronizing oscillators. FERNSEH G.m.b.H. Aug. 14, 1962 [Aug. 16, 1961], No. 31100/62. Headings H3F, H3T and H4F. Television; synchronizing oscillators, Fig. 1.- In a system for changing from one television synchronizing source to another (e.g. from an external source to a local source and vice versa) and utilizing the horizontal synchronizing pulses from each source which are of substantially equal frequency, it is arranged that at the initiation of the change-over process, the phase of the horizontal synchronizing pulses of the locally generated source is swept over a range of at least one period of the horizontal synchronizing signal frequency and the change-over is effected at the instant of phase coincidence between the two sources. As shown, the locally generated horizontal synchronizing pulses are generated at twice the required frequency in the generator 4 (a crystal oscillator) and signals of the required frequency are extracted by the divider circuit 12 and applied to a phase-shift network 13 controlled by the function generator 22 (see " Transistor circuits " below). The externally generated horizontal synchronizing pulse are separated from a composite television signal by separator stages 6, 7 and 8. At each actuation of the manually operated change-over switch 15, which is ganged to the switch 23, the function generator 22 generates a control signal e in the form of a slowly rising and then slowly decaying pulse which causes the phase shift circuit 13 to produce horizontal synchronizing pulses k where instantaneous phase displacement corresponds to the amplitude of the control signal and which vary over a range corresponding to at least the horizontal synchronizing period. These phase-displaced horizontal synchronizing pulses and the horizontal synchronizing pulses separated from the composite T.V. signal are applied to the coincidence circuit 9 which when actuated switches the bi-stable circuit 16 to open either gate 11 or 14 depending on the position of the changeover switch 15. The selected horizontal synchronizing pulses are used to drive a master oscillator 2 controlling a synchronizing signal generator 1 by way of a retroactive loop ineluding a phase discriminator 3 and sawtooth generator 18. Transistor circuits, Figs. 2-4. Operation of switch 23, Fig. 2, applies a negative step, Fig. 3a, to capacitor 28 or 29 and causes a corresponding negative step, Fig. 3b, to be applied to the base of transistor 29 whereby the transistor cuts off until the capacitor discharges back to conduction level. The resulting collector pulse, Fig. 3c, is integrated in a circuit 35-43 to produce a bell-shape signal, Fig. 3d, which appears as the output control signal, Fig. 3e, via emitter follower stage 44. The phase shifters comprises two monostable trigger circuit 51, 52 and 66, 67 which trigger in cascade and the first of which is triggered by horizontal synchronizing pulse, Fig. 4f, applied at 82. The control signal, Fig. 3e, varies the bias applied to the second stage of each trigger circuit whereby the trigger times d, Figs. 4g and 4h, vary correspondingly and likewise the phase of output synchronizing pulses, Fig. 4k.