GB517170A - Improvements in or relating to television receiving systems - Google Patents

Abstract

517,170. Television. WHITE, E. L. C., and CHASMAR, R. P. July 20, 1938, No. 21506. [Class 40 (iii)] In order to reduce the interfering effects of extraneous signals in television transmission, means are provided for suppressing all signals between consecutive line and consecutive frame signals and for reforming correct interline and interframe signals and re-inserting these into the signal train. A circuit for this purpose is shown in Fig. 1. An initial signal train is shown in Fig. 2a and comprises picture signals separated by synchronizing and black-level signals and also interfering signals marked with a X. This signal train, after being treated to make the picture signals negative and the synchronizing pulses positive, is fed to grid 11 of valve 10 anode 12 of which is coupled to grid 14 of valve 13 by resistance 16 and condenser 15 as described in Specification 456,450 so as to pass direct and very low frequency components. Cathode 18 of valve 13 is connected through a 4-diode or 6-diode device 23 as described in Specification 515,362, to a condenser 24 and grid of valve 28 for black-level stabilization. Specification 458,585 is also mentioned in this respect. The same cathode 18 is also connected through a delay network 22 of 1 microsecond to valve 32 and through resistance 29 to grid 31 of valve 30. Valve 32 is arranged as described in Specification 446,663 to reduce the interfering signals x from the amplitude shown in Fig. 2a to that shown in Fig. 2b. Valve 30 has its cathode 38 coupled by resistance 39 to cathode 41 of valve 40, grid 43 of which is fed with pulses lasting 15 microseconds and'occupying the synchronizing and black-level period, whereby output at anode 44 of valve 30 becomes as shown in Fig. 2c, where the synchronizing pulse and the interfering signals between the picture signals have been eliminated. Anode 44 of valve 30 is connected through resistance 95 and condenser 96 to grid 99 of valve 98, which merely reverses the phase of the signals, and anode 100 of valve 98 is connected directly to anode 46 of valve 45 and through resistance 47 and condenser 48 to grid 50 of valve 49. Positive pulses lasting 10 microseconds and occupying the period of the synchronizing pulses are fed to grid 56 of valve 45, whereby correct synchronizing pulses are reinserted in the signal train and the output from cathode 52 of valve 49 is then as shown in Fig. 2d. All interfering signals between picture signals have been eliminated and interfering signals during the period of picture signals have been reduced in amplitude. The circuits described in Specification 450,675 may be used to supply the suppression pulses and the negative synchronizing pulses. Derivation and production of pulses. Anode 57 of valve 13 is connected by resistance 58 and condenser 60 to grid 62 of valve 61, which removes the picture signals by anode bend limiting. Cathode 63 of valve 61 is connected to grid 70 of valve 69 by resistance 65 and also by a delay network 66 of 1 microsecond, whereby interfering signals, which are of short duration, are eliminated from the synchronizing pulses. These devices are similar to those described in Specifications 373,509, and 515,210. The output pulses at anode 72 of valve 71 coupled to valve 69 last 10 microseconds and are fed through resistance 73 and condenser 74 to grid 56 of valve 45 for the purpose described above. These pulses are delayed 1 microsecond ; that is why the delay network 22 described above is needed. These pulses are also fed directly to anode 83 of valve 78 and through a 5 microsecond delay network 76 to anode 84 of valve 79. Cathodes 80, 81 of valves 78, 79 are connected together and yield a 15 microsecond pulse, which is fed to grid 43 of valve 40 for the purpose described above. The pulses from anode 72 of valve 71 are also fed directly to grid 89 of valve 85 and with a 4 microsecond delay by a tapping on the delay network 76 to grid 90 of valve 86. The valves 85, 86 are arranged in push-pull as described in Specification 514,065, to generate short duration pulses, which are conveyed to the points 26, 27 of the device 23 producing blacklevel stabilization. To prevent condenser 24 being charged by positive interference, the negatively conducting portion of the device 23 may alone be connected to cathode of valve 13, the positively conducting portion being connected to a source of steady potential equal to or slightly more positive than the correct blacklevel.