GB798773A - Improvements in or relating to synchronising circuits for television receivers - Google Patents

Abstract

798,773. Automatic synchronizing systems. TELEFUNKEN GES. Feb. 18, 1955 [Feb. 20, 1954], No. 4859/55. Class 38 (4). [Also in Group XL (c)] In a flywheel synchronising system for television receivers, voltage pulses derived by differentiating the rear edges of received linesynchronizing pulses are used for phase comparison with the short flanks of the sawtooth deflection waves. Differentiated pulses may be obtained from the anode circuit of a valve having a C-R or L-R differentiating circuit, the time constant of which is such that the leading differentiated pulse has no effect on the trailing differentiated pulse, or the leading pulse may be removed by a shunt diode or a triode threshold circuit. As shown, Fig. 19, differentiated line synchronizing pulses are derived from a transformer 91 and applied via capacitors 105, 106 to the grid of a phase comparison triode 107 which is gated and whose mutual conductance is periodically increased by flyback pulses applied to its anode from a blocking oscillator timebase generator 119 and to the junction of the capacitors 105, 106, a sawtooth voltage being applied from the line deflection generator 16. A control voltage is thereby derived from the cathode circuit of the valve 107 dependent on the phase relation between the positive-going differentiated pulses applied to the valve 107 and the sawtooth flyback voltage, which after smoothing is applied to the grid of the blocking oscillator 119 to phase lock it. A compensating voltage applied to the cathode circuit of valve 107 from the cathode circuit of valve. 102 allows for the greater duration of conduction during picture flyback. In fig, 4, capacitor 93 and resistor 94 differentiate the input negative-going line synchronizing pulses to a biased. triode 95 so that only pulses due to the rear edges are derived across a transformer 150 in the anode circuit of the valve. Flyback pulses 151 from the deflection generator 16 are also differentiated by the capacitor-resistor circuit 152, 153 to derive the waveform 154 applied to the centre-tap of the transformer secondary. A differential voltage from the two series of superimposed pulses (one shown at 161) is therefore derived from the output of the rectifiers 155, 156 and, after smoothing, applied to phase-lock the blocking oscillator or multi-vibrator time-base generator 14. It is shown that the use of the rear edge of the synchronizing pulse avoids the positional change, due to circuit time constants, of the derived line synchronizing pulses when lines, for some length of time, change from ending in white to ending in black. Specification 753,871 is referred to.