GB490396A - Improvements in or relating to television transmitting or receiving systems - Google Patents

Abstract

490,396. Television. WHITE, E. L. C., and RATSEY, O. L. Feb. 18, 1937, No. 4856. [Class 40 (iii)] The amplitude of picture signals is varied while the amplitude of synchronizing signals is maintained constant by a circuit including a unilaterally conducting device. In one embodiment, the mixed signals are applied to 3, 4, Fig. 1, bridged by a resistance 5 in series with a diode 6 biassed to conduct on application of synchronizing signals so that the resistance 5 is in series for picture signals and the diode is in shunt for synchronizing signals ; the output is taken from 8, 9, the pixture signals varying according to the slider on resistance 5, while the synchronizing signals are of constant amplitude. This circuit may be used for fading out one picture while holding it in synchronism and changing the transmitter for fading in another picture. Alternatively, two such circuits each associated with a different transmitter may be ganged so that when one picture is faded out, another may be faded in. Owing to the finite resistance of the diode 6, it is impossible to reduce the picture signals to zero ; the circuit may therefore be modified so that the tapping 30, Fig. 3, is taken to the grid of a valve 31 whose cathode is biassed from a resistance 34 across the input 3, 4, so that when the tapping 30 is at its lowest point, the potentials on the grid and cathode of the valve are equal and no output occurs at 35, 36. In this circuit, however, the resistance 34 cannot be made too high as it must be tapped for biassing the valve 31, and the resistance therefore is a load on the input. A further modification employs two valves 37, 38, Fig. 4, the anode current of valve 38 providing a potential across resistance 39 for biassing valve 37 so that the grid and cathode of valve 37 are at the same potential when the tapping 30 is at its lowest point. In another embodiment, the mixed signals are amplified in one channel and the synchronizing signals are isolated and amplified in another channel, the two channels feeding a single channel, Fig. 2, (not shown) ; if necessary, the mixed signals may be stabilized as described in Specification 449,242 and limited to provide a fixed amplitude and level of synchronizing signal. The invention is also applicable to the multiplex signalling system disclosed in Specification 470,495. Specification 419,441 also is referred to.