Patent GB-648304-A (GB)
Title: Improvements in television systems, more particularly for colour television
IPC: H04N9/11
Filed: 1946-10-11
Abstract: 648,304. Television. COLUMBIA BROADCASTING SYSTEM, Inc. Oct. 11, 1946, No. 30447. Convention date, Sept. 17, 1940. [Class 40 (iii)] [Also in Group XL (c)] In a television system employing an initially produced sequential composite video signal comprising a plurality of series of signal waves, the waves of said plurality of series alternating in regular sequence, the balance of said series is altered by apparatus comprising a plurality of signal amplifying channels to which the sequential composite video signal is applied, means associated with at least one of said channels for blanking out at least one of said series of signal waves, means for combining the outputs of said signal channels to yield a sequential composite output video signal and means associated with said channels for altering the balance of said series in the said output video signal from that of the initial input video signal. In the arrangement shown, the composite video signal such as 1 (a) corresponding to successive field scansions in red, green and blue is passed to three channels on which are impressed blanking signals of the form indicated in 1 (b), 1 (c), 1 (d) so that signals representing solely one colour are fed from the outlet of a channel. In the circuit arrangement, Fig. 4, the composite signal is fed from an input 21 to an amplifier 24 and can be adjusted by a resistance 22. The amplified signal is fed from adjustable input resistances 25R, 25G, 25B to three separate channels including D.C. reinserting circuits comprising diodes 27R, 27G, 27B with associated resistances 28R, 28G, 28B and thence to the control grids of valves 29R, 29G, 29B having second control grids 32R, 32G, 32B to which blanking waves as indicated in Fig. 1 are applied at terminals 33R, 33G, 33B and input resistances 34R, 34G, 34B. The grids of the valves are provided with screens to which voltage is applied from the +B plate source and is maintained constant by a voltage regulator 39. The gain of the valves can be controlled by adjustable rheostats 36R, 36G, 36B. A circuit arrangement (Fig. 5), for generating the blanking waves comprises three pairs of valves 50R and 50<SP>1</SP>R, 50G and 50<SP>1</SP>G, 50B and 50<SP>1</SP>B connected in a ring. Impulses corresponding to blanking impulses 11 at the end of each field scan are applied simultaneously over line 51, condensers 56 and resistances 57 to the grids of the valves 50R, 50G, 50B. If, for example, the valve 50<SP>1</SP>R is passing current, a positive impulse is derived at cathode resistance 53 and the circuit is arranged so that the valve 50R cannot operate by normal pulses applied to its grid. When valve 50<SP>1</SP>R is cut off, the valve 50R passes current under control of its grid. On the assumption that the valve 50<SP>1</SP>G is passing current due to the cutting-off of the valve 50G by a control pulse, the drop in its anode resistance R<SP>1</SP> will allow the subsequent negative pulse to cut off the current in the valve 50B, whereupon the valve 50<SP>1</SP>B conducts and produces a positive pulse at resistance 53. The operation of the valve 56<SP>1</SP>B provides a cut-off bias for the screen-grid SG of the valve 50B and the same negative bias is also applied over cutoff resistance 59 to the grid G<SP>1</SP> of the valve 50<SP>1</SP>G which thereupon stops conduction. The drop in the anode resistance R<SP>1</SP> of the valve 50<SP>1</SP>B allows the valve 50R to be cut off by the next negative pulse so that the valve 50<SP>1</SP>R becomes conducting and produces the positive pulse of the blanking wave to be applied over conductor 52R. The operation of valve 50<SP>1</SP>R biasses the valve 50R to cut-off and also renders the valve 50<SP>1</SP>B non-conducting. The composite signal may result from the scanning of consecutive fields in a double-interlaced system with the adjustments provided to compensate for any unbalance due to the operation of the optical elements in the successive field scans. In lieu of channels in parallel, successive pairs of channels may be employed, the red, green and blue signals being adjusted in one channel of the respective pairs. Specifications 647,714 and 647,715, and U.S.A. Specification 2,306,386 are referred to.