GB1238684A - - Google Patents

Abstract

1,238,684. Television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 27 Nov., 1968 [30 Nov., 1967], No. 56276/68. Heading H4F. In a television synchronization separator of the type in which the high frequency pulse noise in the video signal is selected by a filter and applied via a threshold circuit and an envelope detector to an interference suppressing stage, the threshold level is raised during the sync pulses. As shown, Fig. 1, the video signal from demodulator 11 is applied to a filter 9, 10 which selects the high frequency pulse noise which is fed to a triode 13 to invert and rectify the noise which is then smoothed by capacitor 14. The resulting noise envelope signal is applied to the control grid 17 of a valve 1 together with a positive voltage V b which determines the threshold above which the noise signal reduces the anode current of the valve. The composite video signal 4 is fed via video output stage 3 and network 5 to the second control grid 2 of the valve 1 which operates as an anode bend detector to separate out the sync pulses 6. The noise pulses in the composite signal are partially cancelled by the signal applied to the first control grid 17 but if a large noise pulse occurs during a sync pulse the valve may be cut off with the result that the separated sync pulse is subdivided into two or more pulses. To reduce this effect positive going line frequency pulses 19 derived from the line time base generator 20 are applied to the grid 17 of valve 1 to increase the threshold voltage for the pulse noise applied during the occurrence of the sync pulses. In an alternative embodiment, Fig. 2 (not shown), negative going line frequency pulses are applied across a resistor inserted between the lower end of inductor 10 and ground to increase the threshold during the sync pulses.