Patent GB-694175-A (GB)
Title: Improvements in or relating to television apparatus
IPC: H04N7/16
Filed: 1950-03-22
Abstract: 694,175. Television. PRODUCTS & LICENSING CORPORATION. March 22, 1950 [March 22, 1949], No. 7218/50. Class 40 (iii). In order to achieve secrecy in a television system employing cathode-ray scanning at the transmitter and receiver, means are provided to superimpose a predetermined sequence of control signals on the deflecting means at the transmitter, while at the receiver there is provided a record of, or means set in accordance with, the predetermined sequence and means responsive thereto for  generating control signals corresponding to those at the transmitter so that by applying these signals to the deflecting means at the receiver, the scanning at the receiver is effected in synchronism with that at the transmitter. In one embodiment, the scanning rasters at the transmitter and receiver are similarly displaced, both horizontally and vertically, by different amounts in successive frames, the deflection remaining constant during each frame. The receiver comprises a drum driven by a synchronous motor at 5 revolutions per second and having a plurality of conductive tracks thereon. These are connected to respective deflection plates of the cathode-ray tube and also, as the drum rotates, periodically make contact with differentlyvalues potential sources. The tracks result in ten different voltage combinations being applied to the tube during each revolution of the drum, each combination thus being applied for the duration of a frame, and the tracks being so chosen that the voltage combinations correspond with the simultaneous combinations being employed at the transmitter. Means are provided for rotating the drum by successive one-tenths of a revolution until synchronism between the transmitter and receiver is achieved. This is indicated by arranging a bright spot to appear in the corner of the reproduced picture once in every ten frames; this bright spot appears through a hole 26, Fig. 3, in a projecting flange to the rotating drum, the flange being arranged as shown to overlap the edge of the cathode-ray tube screen. In an alternative arrangement, the receiving screen is divided up into a large number of " group "areas, each group comprising a predetermined number of elemental areas. Fig. 5D shows a group comprising 16 elemental areas. During each frame a predetermined number of elements from each group is scanned; thus one element of each group may be scanned in each frame in which case 16 frames are required to complete a picture, or four elements from each group may be scanned in each frame in which case four frames are required to complete a picture. The scanning sequence represented by the numbers shown in Fig. 5D corresponds to the " knight's-move " scanning described in U.S.A. Specification 2,479,880; in the present invention this sequence may be altered to achieve secrecy by a set of control potentials applied equally at the transmitter and at the receiver. Fig. 5 shows such a set of control potentials comprising four sets of interlaced waveforms 44, 45, 46 and 47 generated by four separate pulse generators and repeating with a period equal to the time taken in the scanning of each group. A further set of control potentials is required for the perpendicular component of scanning. Further generators are required to develop the potentials required, for the second, third and fourth frames of each picture. Fig. 10 shows an embodiment of the invention. Oscillators 204, 205 generate sinusoidal oscillations at frequencies respectively equal to twice and three times the line-scanning frequency multiplied by the number of groups per line. By combining these potentials short pulses may be developed as shown in Fig. 7 (not shown). This is effected at the outputs of the circuits shown within the dotted rectangles which comprise phase-shifting devices as described in French Specification 964,768.. For a group as showh in Fig. 5D, 16 such pairs of phase-shifting devices are required, though for convenience the Figure only shows three pairs. Thus 16 phase-displaced sets of pulses are developed. By means of circuits such as the two rectifiers 209, 216 and the switch 213 selecting a predetermined tapping on the battery 212, the relative amplitudes of these output pulses are controlled and the pulses are then applied in sequence through resistors 226, 227, &c., to the vertical deflecting plates of the cathode-ray tube 83. A further similar set of pulse generators is required to produce deflecting potentials to be applied to the horizontal deflecting plates of the cathode-ray tube. In this arrangement each element of the group is scanned before passing on to the next group; thus the picture is completely scanned once in each frame. Instead of the control potentials being chosen by switches 213 &c. a device as shown in Fig. 11 may be employed. A card 250 which is supplied to authorized users is placed within a holder 251 having springs 257 &c. which are connected to the rectifiers 216 &c. and contact bars 253 &c. which are connected to respective tappings on the battery 212. When the folder is closed, the springs press through holes in the card 250 to make contact with appropriate ones of the contact bars. In a further embodiment, Fig. 12, the scanning pattern of Fig. 5D is modified by control potentials derived by commutators driven by synchronous motor 91. In this arrangement only one element per group is scanned in each frame; thus the frame frequency is 16 times the picture frequency. The video signal applied to the control grid of the cathode-ray tube 83 is modulated by a high-frequency pulse signal developed at 85 to ensure that only one element in each group is excited during each frame. The commutators rotate at the picture frequency and since each contains 16 contacting elements a fresh tapping from voltage source 93 or 94 is applied to the deflecting plates in each suc. cessive frame. Fig. 13 shows an embodiment applied to colour television, the scanning sequence being illustrated in Fig. 5E, wherein the line-scanning is shown as vertical. The scanning is double interlaced and the electron beam is pulse-modulated so that during the first frame red elements 1 and 2 from each group are excited, the electron beam being subjected to a deflection of one unit in the line direction in alternate lines. During the second frame, the electron beam is subjected to additional deflection of two units in the line direction, so that red elements 3 and 4 are excited in alternate lines. During further pairs of frames, green, blue and green elements are excited, thus completing the scanning sequence, though means to achieve this are not described. Fig. 13 shows how the scanning of the first two frames is effected. Pulse modulation of the beam in the tube 83 is achieved by frequency multiplier 85 controlled from the line-scanning generator 84. Two commutators are provided; one is driven by motor 91 at the frame frequency and applies a deflecting voltage of two units to the cathode-ray tube from the battery 99 in alternate frames and the other is driven by motor 103 at the line frequency and applies a deflecting voltage of one unit to the cathode-ray tube from battery 110 in alternate lines.