Patent GB-856823-A (GB)
Title: Improvements in or relating to apparatus for generating television synchronizing signals
IPC: H04N5/06
Filed: 1957-06-28
Abstract: 856,823. Television. FERNSEH G.m.b.H. June 28, 1957 [June 28, 1956], No. 20463/57. Class 40(3). [Also in Group XL(c)] Television frame and line synchronizing pulses for transmission to receivers are maintained in correct timing relative to each other even if the line pulses sequentially vary in timing relative to the frame pulses as in interlaced scanning, or the source, e.g. mains frequency, from which the frame pulses are generated varies in frequency or phase, by retroactively controlling the phase of a sinusoidal oscillator, 86, from which the line pulses H are derived, by a control voltage from a phase discriminator 76 to which the frame pulses and oscillations are applied, the latter through a gate or switch 65, 66 sequentially altering the phase of the oscillations. As shown mains frequency may be applied at terminal 51, through a phase shift network 52, 53, to the centre tap of a transformer winding 54<SP>1</SP> receiving pulses from a blocking oscillator 55 so that rectifier circuits 56, 57 differentially control and stabilise the blocking oscillator in frequency and phase. The trailing edges of blocking pulses derived from the screen grid of the blocking oscillator are then applied through capacitors 71a, 71d to control a multivibrator 65, 66 so that alternately valves 65 and 66 are conducting. To the grids of the valves 65, 66 also are applied oscillations 180 degrees out of phase from the sinusoidal oscillator 86 from which line synchronizing pulses at H are obtained and which is to be locked in correct phase relative to the frame pulses. From the common cathode impedance 73-75 of the multivibrator are thus derived alternately in opposite phase, oscillations applied to the phase discriminator 77, 78 at a centre tap of the transformer winding 76<SP>1</SP>. The oscillations are phase compared with the leading edges of frame pulses derived from the anode circuit of the blocking oscillator 55, and applied in push-pull to the transformer winding 76<SP>1</SP>. The phase discriminator output controls, through a reactance tube 84 energized through a winding 72<SP>111</SP> ,  the frequency and phase of the oscillator 86 so that its frequency is (n + ¢) times that of the frame pulses where n is an integer and the line pulses generated at H are equidistant in timing on opposite sides of the frame pulses, as required for interlaced scanning. Since the frame pulses applied to the discriminator occur just before the. multivibrator switching pulses any transients due to phase reversal of the oscillations have the duration of a frame period in which to die out. The oscillator frequency may be adjusted, for giving a different odd number of lines per set of frames forming one picture, by tappings on the tuned circuit inductance 72<SP>1111</SP>. In a modification the blocking oscillator is dispensed with and the multivibrator switching pulses derived by applying the mains frequency to two shunt rectifier circuits in parallel which eliminate the negative half cycles and square the positive half cycles so that the negative going leading edges synchronise the multivibrator. The frame pulses are derived by slightly phase retarding the mains frequency, squaring and phase inverting it in a valve and differentiating the leading edges. The multivibrator valves are arranged not to pass any grid current by self biasing the cathode current and connecting the grids to earth through clamping diodes.