Patent GB-817762-A (GB)
Title: Cathode ray scanning systems
IPC: H04N9/24
Filed: 1955-12-23
Abstract: 817,762. Automatic control systems for cathode-ray tubes. SYLVANIA ELECTRIC PRODUCTS Inc. Dec. 23, 1955 [Dec. 23, 1954], No. 36993/55. Class 38 (4). [Also in Groups XL (a) and XL (c)] The invention relates to a cathode-ray tube having a screen bearing a regular pattern of index and picture reproducing strips, the index strips generating an index signal when bombarded by a cathode-ray beam and in the embodiment shown in Fig. 1 the invention is applied to a colour television receiver tube 14 having a gun system of four separate cathodes 18, 20, 22, 24 generating an indexing beam 181, Fig. 2, and three equally spaced beam 20<SP>1</SP>, 22<SP>1</SP>, 241 generating colour signals on the associated phosphor strips 40, 42, 44, the usual control grid 26 and anodes 30, 32 being provided and all four beams being commonly deflected by electrostatic means, the vertical deflection plates 14 being shown. The cathodes may be mounted in a grooved insulating block and provided with a common heater or a single cathode may be provided with a control grid having insulated apertured segments dividing the main beam into the index and three colour beams. Each group of three colour strips 40, 42, 44 is separated from the adjacent group by an insulating strip 48 and the whole structure backed by an aluminium layer 50. Indexing strips 52, of high secondary emission half as wide as the strips 48 are superimposed as shown. The screen 16 is set at a slant relative to the horizontal scan direction so that vertical deflection errors result in changes in the secondary emission generated by the varying impact of the beam 18<SP>1</SP> on the strips 52. These changes appear as error signal voltages on an electrode 53 capacitatively coupled to the wall anode 52 and are fed into an error signal amplifier 58 whose output controls the variable gain vertical deflection amplifier 60 in such a manner as to correct the deflection error. In the embodiment shown in Fig. 5 an additional secondary emission strip 96, wider than the indexing strips 98, is provided to assist in starting the vertical scan and the beams are deflected by a coil arrangement which is normally biased to position the index beam on the strip 96. On removal of the blanking signal at the start of a picture the index beam impacts the strip 96 and starts the flow of secondary emission current to anode 100. This results in a voltage drop at grid 110 of amplifier tube 103, a voltage rise at anode 114 and a voltage rise at grid 118 of the vertical scan control valve 104. The increased current flow in this valve charges the condenser 124 raising the voltage of grid 130 of amplifier tube 106 permitting an increased current flow in scan transformer primary 138. The resulting flow in the transformer secondary 142 reduces the permanent bias in the vertical deflection coil and the pilot beam is downwardly deflected. The secondary emission from starrting strip 96 is thereby reduced and the vertical deflection remains steady until the index beam impinges, by horizontal scanning, an index strip 158, thereafter the control maintains the beam on the index strips, line by line, until the end of the frame. As shown the strips are arranged for interlaced scanning and the beam follows alternate strips. At the end of a frame the beams are blanked and the receiver 88 applies a synchronizing pulse to grid 130 of tube 106 thereby reducing the current in the scan transformer permitting the permanent bias to commence the return of the beams. Another winding 146 also applies a positive pulse to grid 148 of return trace control tube 108, turning the tube 108 on to discharge the condenser 124 and to reduce the potential still further on grid 130 thereby augmenting the restoration of the bias position.