Patent GB-1422453-A (GB)
Title: Circuit arrangement for inserting pulses having a defined pulse edge duration in a television image signal
IPC: H04N3/24
Filed: 1973-08-15
Abstract: 1422453 Television; transistor pulse shaping and gating circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 15 Aug 1973 [18 Aug 1972] 38524/73 Headings H4F and H3T In an arrangement for inserting pulses having a pre-determined edge duration, specifically blanking and/or synchronizing pulses, in a television image signal, the latter is supplied to a gain controllable amplifier the output of which is fed to a comparator having the pulses as a second input and providing an output which acts to reduce the gain of the amplifier when its output exceeds the amplitude of the pulses. Thus, by preventing the amplitude of the television (video) signal from exceeding that of the pulses the production of transient signals at the leading and trailing edges of the pulses especially when peak-white signals occur at these positions is prevented. Referring to Fig. 1, the various waveforms occurring in which are shown in Fig. 2, line blanking pulses A, of standard duration but slightly displaced in time, are supplied to input 1 of a waveform converter circuit 6 to produce the waveform B by controlling the elements 20- 27 acting as a current source for the discharge and charge of capacitor 28. A pulse generator 8 produces the waveform D from waveform B and waveform D is employed via "switch" 4 to produce a blanking interval in the video signal E which is supplied via input 2, "switch" 4 and as waveform F, via inverting amplifier 5 to the inverting input of amplifier 44 the output G of which forms one input to comparator 11 having as second input an attenuated version B<SP>1</SP> of waveform B. When the G input to the comparator reaches the level of waveform B<SP>1</SP> an output 13 of the comparator operates via the gate electrode of a transistor 42 to reduce its effective resistance in the feed-back of amplifier 44 so that the gain of the amplifier is reduced until the G input to the comparator is reduced to the instantaneous level of the B<SP>1</SP> input. The level +V B1  of the latter during trace periods is slightly higher than the peak white level +aVw so that, during such periods the comparator 11 operates as a "white" clipper and removes video signals exceeding peak white. The actual edge duration of the "inserted" blanking interval (G, Fig. 2) is determined by the duration Td which is itself determined by the discharge of capacitor 28. In one standard (C.C.1.R.) the edge duration is recommended to be between 200 and 400ns and 300ns is chosen for Td. Since in this system the duration of the blanking pulse is determined at the 50% signal level the instant To in the waveform A must be approximately 150 ns earlier than that prescribed in the standard. In another standard (R.T.M.A.) 200 ns is chosen for the duration Td. For the insertion of line synchronizing pulses the "blocks" 3 and 8 are not utilized, the line synchronizing pulses are supplied at input 1 and an inverted version of waveform G is supplied to input 17. Alternatively the line synchronizing pulses may be incorporated in the signal B<SP>1</SP>.