GB515364A - Improvements in or relating to the transmission of electrical signals - Google Patents

Abstract

515,364. Television ; automatic volume control. BLUMLEIN, A. D. May 30, 1938, No. 16084. [Classes 40 (iii) and 40 (v)] Variations in a datum level due to loss of direct-current components or varying attenuation, are reduced by developing a plurality of correcting signals from the datum levels, the variation due to leakage of one correcting signal being compensated by the variation due to leakage of another. Direct-current re-insertion. As shown in Fig. 1, valve 16 and condenser 13 produce re-insertion at the grid of valve 12 as described in Specification 449,242, but leakage occurs over reststance 14 ; this leakage is compensated by a second re-insertion by valve 23 and condenser 19, as described' in Specification 422,906, since the leakage over resistance 20 is in an opposite sense to that over resistance 14. In an alternative, an amplifier 26, Fig. 2, e.g. as described in Specification 482,740, provides two outputs in phase opposition and valves 29, 30 and condensers 27, 28 provide for re-insertion, the valves 29, 30 being rendered conducting only during datum level periods by pulses applied at 42 ; leakage occurs over resistances 31, 32, but this effect is eliminated by applying the leads 33, 34 to an amplifier 39 which is responsive only to push-pull potentials to the exclusion of pushpush potentials. Automatic volume control. An amplifier 45, Fig. 3, provides two outputs in opposite phase, each associated with a voltage-equalizing device, e.g. as described in Specification 512,109, so that condensers 56, 57 are charged by the datum potentials occurring on lines 47, 46 respectively; the datum potentials at 47, 46 should be zero, but if this condition does not obtain, condensers 56, 57 are charged in opposite senses to produce a potential difference at 56a, 57a ; this potential difference will remain unaffected by leakage over resistances 60, 61, and is applied at 77, 78, Fig. 4, to vary the proportion of current taken by each of the valves 71, 72, and so vary the output of 71 amplifying the incoming signal from 70. Since resistance 73 is taken to a large negative potential, equal changes at 77, 78 due to leakage at 60, 61 will have no effect. The effect of the automatic control is to reduce the datum levels in 46, 47 to zero. Specifications 458,585, 515,360, 515,361 and 515,362 also are referred to.