Patent GB-528734-A (GB)
Title: Improvements in or relating to light modulating devices for use in television receivers
IPC: H01J31/08
Filed: 1939-02-23
Abstract: 528,734. Television; light modulating; cathode-ray tubes. I.M.K. SYNDICATE, Ltd., NAGY, P., and GODDARD, M. J. Feb. 23, 1939, Nos. 6030 and 12162. [Classes 39 (i) and 40 (iii)] Relates to light modulating devices comprising a medium capable of carrying mechanical oscillations, means for producing such oscillations under the control of a cathode-ray tube including a series of contact elements adapted to be affected in turn by said beam, for example, as described in Specification 526,875. According to the invention, each contact element comprises two or more sections of different characteristics such that an alternating current component is imparted to said element when the cathode-ray beam contacts alternately with said sections. Contact elements comprising two or more sections of different characteristics. In one form, Fig. 1, each contact element comprises two sections 14, 15 transverse to the scanning line. These sections may be constituted by metal layers deposited on a single foundation. If this foundation is not metallic, the layers are made to overlap slightly to ensure the necessary conductivity between the sections. Alternatively, one section is constituted by a metal foundation, half of which is covered with a layer of another metal to constitute the other section. In another form, each contact element is divided into a number of sections in the direction of the scanning line. In another arrangement, the two sections 14, 15 of a contact element are given different characteristics by the arrangement of electrodes 22, 24, Fig. 3, The electrode 24 extends over the section 14 only and is given a slight positive voltage of about 100 volts, and the electrode 22 extends not only over the other section 15 but also over the electrode 24, the arrangement being such that the electrode 22 causes secondary emission from the section 15, and screens the other section 14 which is charged directly by the electron beam. To enhance this charging action, an additional electrode 25 is inserted between the electrode 24 and the section 14 and is made of strips coated with a material having a high secondary emission so that the primary electron beam striking the strips of this additional electrode obliquely, secondary electrons are emitted and fall on the section 14. A screening electrode 26 may be arranged between the sections and kept at a low potential. The electrodes 24, 25 may be replaced by a single electrode. Modulation. The electron beam is oscillated at a desired carrier frequency and modulated in various ways. In one arrangement, the carrier signal is applied by deflecting plates 21, Fig. 1, and modulation is superimposed on the same plates to cause the electron beam to pass from one section to the other of a contact element 14, 15 about a mean position partly on one section and partly on the other section ; or, at zero modulation the beam is on one section only, and a " rectified " modulation oscillates the beam more or less on the other section and back to the first section. Alternatively, the modulation may be performed by an additional pair of deflecting plates 23. In another arrangement, the modulation is performed by varying the intensity of the electron beam by potentials applied to the grid 17. When the contact element is divided into sections in the direction of the line scan, the carrier frequency is caused by the scanning movement of the beam controlled by the deflecting plates 20. For example, the contact element is divided into a number of strips of alternate characteristics constituting two sets or sections, and the electron beam is split into strips spaced to fall on one set of strips at a time, so that as the beam moves in the scanning direction it falls on the sets or sections alternately at the desired frequency which is determined by the number of strips and the scanning speed. The cathode-ray tube may contain two separate cathodes, a main cathode 30, Figs. 3 and 5, for producing a main electron beam capable of covering the whole line of contact elements 13, and an auxiliary cathode 36 for modulating the main beam in sections corresponding to each contact element 13. The main beam as a whole is oscillated at carrier frequency by plates 31. The individual sections of the main electrode beam are independently modulated by independent grids 27, Fig. 3, or independent deflecting plates 33, Fig. 5, close to the electrode system 30, 27, 28 connected to auxiliary contact elements 32 scanned by the beam from the auxiliary cathode 36. This beam is deflected for scanning by saw-tooth voltage applied to plates 45 and is modulated in lateral direction by plates 46 or in intensity by grid 37. Each auxiliary contact element 32 may be in one piece as in Fig. 3 with an adjacent electrode 40 to absorb secondary emission from the element 32 which is connected through a resistance 44 to an electrode 47 maintained at a potential corresponding to zero modulation. Or each auxiliary contact element 32 may be divided into two sections 34. 35, Fig. 5, associated with a system of electrodes 40, 41, 42, 43 acting like the electrodes 22, 24, 25, 26 described above, the two sections 34, 35 being connected by a resistance 44 the mid point of which is connected to the anode 28.