Patent GB-720877-A (GB)
Title: Improvements in or relating to circuit arrangements for use in television receivers
IPC: H03B21/02
Filed: 1953-04-07
Abstract: 720,877. Automatic frequency and phasecontrol systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. April 7, 1953 [April 10, 1952], No. 9397/53. Class 38 (4). [Also in Group XL (b)] Relates to frame synchronizing arrangements for television receivers of the kind in which the timing signal for the frame scan source is derived by frequency division from an oscillation set up by the received line sync. pulses; and consists in maintaining correct phasing of the frame scan source by comparing its phase with that of the received frame sync. pulses to derive a control voltage which operates on a frequency-dividing stage. Fig. 1 is a block diagram of the arrangement, and Fig. 2 a suitable circuit. The incoming television signal is applied to a sync. separator 2. The line sync. pulses control the line scan source 3, and are also applied to a frequency doubler 4, followed by frequency dividers 5, 6 which reduce the frequency to about 50 c/s. to control the frame scan source 7. The output frequency of the divider stage 6 is kept at the desired sub-multiple of the output of the stage 5 by a phase comparer 9 feeding a control voltage at 8 to the stage 6, with the result that the desired frame scan frequency is maintained at the source 7. In order to ensure correct phasing of this saw-tooth source, the fly-back pulses are fed to a further phasecomparer 10 to which are also fed the received frame sync. pulses. Any error of phase gives rise to a control voltage which temporarily adjusts the output frequency of the divider 6, e.g. by closing a switch 12 to connect an impedance 13 in the divider circuit. As soon as phase agreement is produced, the control voltage vanishes and the frequency of the scan source reverts to its correct value. In Fig. 2, the frequency-divider 6 of Fig. 1 comprises a cross-coupled multivibrator 14, 15 whose output is differentiated by the reactances 29, 30 and applied to a control grid 26 of a pentode valve 25 functioning as the phase comparer 9 of Fig. 1, input pulses from the preceding stage 5 being fed to the suppressor grid. Current passes in the valve 25 only when both its input grids are positive, and its anode voltage, smoothed by a condenser 40 shunting the anode resistor 24, is applied through resistors 23, 41 to the grid of the valve. 15 to maintain the multivibrator output at a desired sub-multiple of the frequency supplied to it. The phasecomparer 10 of Fig. 1 also takes the form of a pentode valve 34 with its control grids fed respectively by the fly-back pulses of the frame scanning source 7, and by the received frame sync. pulses from the separator 2. When these two inputs are in phase, the valve passes current, and its anode voltage falls to a value lower than the anode voltage of the valve 25. In this condition, the diode 39 is blocked and the valve 34 produces no effect on the multivibrator 14, 15. If, however, there is a phase error, the valve 34 is blocked, its anode voltage rises, and through the diode 39 temporarily adjusts the frequency of the multivibrator, until the fly-back pulses from the scan source 7 are in phase with the received frame sync. pulses, whereupon the valve 34 again passes current and the multivibrator re-assumes its synchronous frequency.