Patent GB-713764-A (GB)
Title: Improvements relating to cathode ray tube circuits
Inventor: LEE DAVID FREDERICK
IPC: H04N5/44
Filed: 1951-07-25
Abstract: 713,764. Television receivers. ELECTRIC & MUSICAL INDUSTRIES, Ltd. July 2, 1952 [July 25, 1951], No. 17573/51. Class 40 (3) In a cathode-ray tube arrangement for a television receiver circuit means are provided for modifying the intensity of the beam at the termination of the signal transmission. Where the transmission is such that full white modulation corresponds to zero amplitude carrier and the beam intensity becomes large on the termination of transmission, the circuit means operates to reduce the beam intensity, Where the transmission is such that full white modulation corresponds to maximum amplitude carrier and the cathode-ray beam is normally blacked out on the termination of transmission, the circuit means prevents the intensity from falling to zero so that a visible raster remains on the screen. In Fig. 3 the video signals from detector 31 are directly coupled to the grid of valve 3 and applied directly in reverse phase from the anode to the cathode 13 of the cathode-ray tube 8. The transmission is such that full white corresponds to zero amplitude carrier so that on the termination of transmission, due to the D.C. couplings, the cathode-ray beam assumes a maximum intensity. The synchronizing pulses at cathode 13 are positive going and a connection extends through condenser 15 to a separator valve 17 which functions in a normal manner due to grid circuit limitation to separate the synchronizing pulses from the image signals, the pulses being derived from the anode on lead 22. A connection also extends from the anode through resistor 24 to the grid circuit of valve 3 whereby the bias of this valve is determined by the anode potential of valve 17. Condenser 25 prevents the synchronizing pulses from being applied to valve 3. On the termination of transmission, the grid bias on the separator valve 17 due to the charge on condenser 15 disappears and the valve conducts heavily lowering its anode potential. This results in a corresponding lowering of the grid potential of valve 3 with an increase in the potential applied to the cathode 13 of the cathode-ray tube whereby the beam intensity is reduced. By reversing the connections of diode 31 the circuit may operate with a transmission in which full white corresponds to maximum carrier, since, however, this results in the cathoderay tube beam being cut off when transmission ceases it is necessary to insert a phase-reversing stage between resistor 24 and the grid circuit of valve 3 so that cathode 13 potential is here reduced. In a modification, Fig. 1 (not shown), the video signal is' applied to the grid of the cathode-ray tube and the connection through resistor 24 from the anode of the separator valve extends to the grid circuit of the tube. In a further modification, Fig. 2 (not shown), the video signal is applied to the cathode-ray tube cathode and the connection from the separator anode extends to the tube grid. Reference is also made to modifying the cathode-ray tube beam intensity by the D.C. component in the output circuit of the sound detector or by the line or frame synchronizing impulses alone.