Patent GB-556331-A (GB)
Title: Facsimile system
IPC: H04N1/024
Filed: 1942-03-28
Abstract: 556,331. Facsimile telegraphy ; cathode-ray tubes. STANDARD TELEPHONES & CABLES, Ltd. March 28, 1942, No. 4155. Convention date, Oct. 31, 1940. [Classes 39 (i) and 40 (iii)] A facsimile telegraph transmitter is provided with an auxiliary scanning device operating in advance of the main scanning device and arranged to cause blank spaces to be skipped over by the main scanning device to speed up the transmitting operation. As shown in Fig. 1, the copy 1 is driven by a belt 2 extending over rollers 3, 4, 8. The roller 3 is driven through worm gear 24, 25 by a ratchet wheel 23 adapted to be rotated the space of one tooth at a time by a pawl 22 and the space of two teeth at a time by a pawl 21, so that the copy is correspondingly moved the space of one or two lines according to the pawl in operation. The belt 2 comprises a fabric base with a thick rubber surface and passes between a fixed upper guide plate 11 and a lower guide plate 12 spring urged to press the belt and copy up against the upper guide plate. This plate has a transverse slot 40 through which at least two lines of the copy are exposed to illumination by lamps 41. Light from these lines are reflected by a rocking mirror 42 to a reciprocating lens 43 followed by a fixed lens 44, and a fixed mirror 45, which reflects the light to a shield 46 formed with two apertures 47, 48 situated over prisms 62, 61 reflecting the light to photo-electric cells 63, 64. The rocking of the mirror 42 causes two adjacent lines of the copy to be scanned respectively by the apertures 47, 48, so that when the copy moves normally in line by line steps, each line is scanned first by the aperture 47 and then by the aperture 48. The reciprocation of the lens 43 serves to adjust the focus of the light to the varying length from the mirror 42 to the various points along the slot 40. The mirror 42 and the lens 43 are operated by cams 55, 59 on a sleeve 56<SP>1</SP> on a spindle 56. The sleeve 56<SP>1</SP> can be shifted along the spindle to bring a second pair of cams 551, 591 in the place of the cams 55, 59 to vary the length of the scanned line on the copy 1. The photo-electric cell 64 affected by the aperture 48 is connected to an amplifier 65 for producing signals to be sent out at 69 for transmission to a receiver. The photo-electric cell 63 associated with the preliminary aperture 47 is adapted to trigger a gas-discharge valve 71 when a mark occurs in the line being scanned by the aperture 47. The valve remains triggered until the end of the scanning of the line ; but, before the scanning of the next line begins, the valve is restored to inoperative condition by the temporary opening of its anode circuit at contacts 78 controlled by a cam 79 on the shaft 56 operating the scanning mirror 42. When the preliminary scanning by the aperture 47 detects one or more marks on a line and triggers the valve 71, a relay 77 is operated so that at the end of the scanning of that line the closing of contacts 84 by the cam 79 causes a magnet 29 to operate the pawl 22 to feed the copy one step to bring the line preliminarily scanned by the aperture 47 in position to be scanned by the aperture 48 to transmit signals corresponding to the marks on that line. When the preliminary scanning by the aperture 47 detects no marks on the line, the valve 71 is not triggered and the relay 77 is not energized ; hence when the contacts 84 are closed, the magnet 28 is operated instead of the magnet 29, and the pawl 21 feeds the copy 1 forward the space of two lines, so that the blank line is skipped over. Two devices 91, 94 producing signals of different frequencies are associated respectively with the magnets 28, 29 and serve to send out signals to the receiver to enable the copy at the receiver to be stepped in the same way as the copy at the transmitter. In a modification, Fig. 2 (not shown), the light devices are arranged in reverse directions. Lamps take the place of the photo-electric cells 63, 64 (or a single lamp is used there and its beam split in two), and photo-electric cells take the place of the lamps 41 ; in addition the two scanning beams are differently coloured by the use of sources of different colours or by suitable filters, and the two cells are made sensitive to different colours by being provided with filters or with anodes sensitive to different colours. The scanning mechanism may also be modified, Fig. 3 (not shown), the cam shaft being arranged to be rotated gradually in one direction for the scanning operation and then declutched and snapped back by a spring between successive scannings. The scanning stroke of this arrangement may be made adjustable. The receiver, Fig. 4, comprises three separate circuits 201, 202, 203 accepting respectively the scanning signals from the signal transmitter 65 of Fig. 1, and the stepping signals from the signal generators 91, 94 of Fig. 1. The scanning signals are fed to a copying apparatus comprising a cathode-ray tube having a cathode 206, a control grid 207, a focusing anode 208, an accelerating anode 210, a pair of beam deflecting elements 211, an array 212 of discrete insulated conductors, an outside anode 221, and a conductive lining 213 connected directly to the accelerating anode 210 and through a resistance 216 to the external anode 221. The copy 220 to be marked is passed between the array 212 and the external anode 221, and the marking is produced by the passage of currents or sparks between these two parts depending on the voltages used. The saw-tooth voltages required for line scanning are produced simply by the gradual charging of a conductor 224 by a voltage source 222 through a resistance 223 or a pentode valve. The sudden discharge of the condenser is produced by short circuiting by contacts 225 or 226 in parallel. These contacts are controlled by relays 237, 238 influenced by the stepping signals accepted by the circuits 202, 203. The operation of these contacts is accompanied by the operation of pawls 235, 236 to produce the desired feeding steps of the copy 220 through ratchet wheel 234 and worm gear 230.