Patent GB-1040578-A (GB)
Title: Improvements in or relating to television blanking pulse generating circuit arrangements
Inventor: ILES GORDON DAVID
IPC: H04N5/073
Filed: 1962-08-28
Abstract: 1,040,578. Television; transistor monostable circuits. MARCONI CO. Ltd. June 28, 1963 [Aug. 28, 1962], No. 32978/62. Headings H3T and H4F. Television.-The invention relates to a circuit for generating a composite (line and frame) blanking pulse train in response to a composite synchronizing pulse train, the circuit being of particular application where direct access to the system blanking and synchronizing generator unit is not possible, e.g. at a remote point or where signals are obtained from a magnetic tape recorder. The invention is stated to be applicable to the British 405-line system, but is described only as operating with the 525- and 625-line type of synchronizing waveform, Fig. 2 (line IN), where normal line synchronizing pulses SP occur but the frame synchronizing pulses (not shown) are preceded by equalizing pulses EP. The circuit, Fig. 1, consists of four pulse generators A, B, C, and D and a three-input OR gate E which produces an output when any one or more of generators B, C and D produces an output of appropriate polarity. Generator A responds to the composite synchronizing pulse train, and produces an output, A2, in response to only the equalizing pulses. The generator comprises a special transistor monostable circuit (see below) and an AND gate A2 which combines the output of A1 of the transistor circuit with the original synchronizing pulse train to produce the difference A2. The transistor circuit triggers " ON " and " OFF " in response to the front and back edges respectively of the line synchronizing pulses SP, but only triggers on in response to the equalizing pulses EP and reverts to off in its own time. Generator C is a long pulse generator in the form of a monostable multivibrator which triggers in response to waveform A2 to produce waveform C which determines the frame blanking pulses. Generator B is a monostable multivibrator which responds to each pulse of the composite synchronizing pulse train to produce a pulse train B, the trailing edges of which correspond in time with the trailing edges of the desired line blanking pulses. Generator D is a further monostable multivibrator which responds to the trailing edges of pulse train B and produces pulse D which determine the leading edges of desired blanking pulses. Waveform E comprises the desired composite blanking waveform formed at the output of the OR gate in response to wave forms B, C and D. Transistor monostable circuit.-Transistors 4 and 5, Fig. 1, form a monostable circuit with D.C. coupling in one direction and RC coupling 6, 7 in the other direction, the time constant of which determines the trigger time. The circuit is triggered by the composite synchronizing waveform IN, Fig. 2, which is differentiated in CR circuit 1, 2 to produce waveform 12. The negative-going transients are transmitted by diode 3 to the base of transistor 4 to trigger the circuit " ON," whilst the positive-going transients are by-passed via capacitor 8 so as to be available to trigger the circuit " OFF." The waveform at the base of transistor 4 is as shown at 4. With the line synchronizing pulses SP (waveform IN) the transients are effective to trigger the circuit, but with the shorter equalizing pulses EP the transients appear too early on the exponential discharge curve and are ineffective. The circuit then reverts to " OFF " in its own time determined by time-constant 6, 7.