Patent GB-550005-A (GB)
Title: Facsimile telegraph signalling system
IPC: H03G5/26
Filed: 1941-08-01
Abstract: 550,005. Copying telegraphy; cathode-ray tubes. STANDARD TELEPHONES & CABLES, Ltd. Aug. 1, 1941, No. 9799. Convention date, Aug. 10, 1940. [Classes 39 (i) and 40 (iii)] In a facsimile telegraph system in which the rate of scanning at both transmitter and receiver depends upon whether a black or white portion of the copy is being scanned, an auxiliary scanner sights a number of record elements simultaneously in advance of a principal scanner sighting a single element, with the result that the scanning of a line proceeds at a higher speed when a length about to be scanned is wholly white and proceeds at a lower speed when any part of that length is black. In the system described, white portions are represented by a frequency of 2,000 and black by a frequency of 200. As in Specification 549,010 the potential applied to the scanning electrodes is derived from a large condenser which is charged by the repeated discharge of a small condenser. which is charged and discharged by successive half-waves of the 200 or 2,000 frequency current, so that as the picture density and consequently the frequency, varies, the rate at which the large condenser is charged varies and consequently the rate of scanning. In one system currents determining the rate of scanning are transmitted to the receiving station but in a modified system only currents from the principal scanner are transmitted, these being recorded at the receiving station in some form of storage device having main and auxiliary pick-up devices, the latter determining the scanning rate at the receiver. Transmitter. -The tape 2, Fig. 1, bearing the copy to be transmitted, passes over rollers 3, 4, is fed line by line by a ratchet 5, and is illuminated by a line filament 7, the reflected light being focused as a line image 10 on a narrow line of photoelectric material on the cathode 11 of a scanning tube 12 of the image dissector type. The tube 12 is roughly oval in section to reduce stresses due to atmospheric pressure. The tube 12 is provided with a plate 14 having a small aperture 20 co-operating with the main anode 22 and an elongated aperture 21 co-operating with an auxiliary anode 23. The anodes 22, 23 are connected through amplifiers J, K to the second and third grids of valves 42, 62, and to the second grid of valve 82, as shown. Valve 42 connects a 2,000-cycle generator 41 through filter 52 to line 1 and transformer 101, a 200-cycle generator 61 being connected by valve 62 and filter 72 to line 1 and transformer 101 and by valve 82 to line 2. The grids of valves J, K and 42, 62, 82 are so biased and connected that valve 42 is blocked by a negative signal on either the second or third grid, whereas valve 62 is blocked only when a negative signal is applied to both the second and third grids. The second grid of valve 82 is in parallel with that of valve 62. Thus either generator 41 supplies 2,000-cycle current to line 1, or generator 61 supplies 200-cycle current to line 1 alone or to both line 1 and line 2. The current passing to line 1 also passes through transformer 101 to valves 104, 105, so connected that during one half cycle valve 104 conducts and charges a small condenser 106, whilst during the next half cycle valve 105 conducts and discharges condenser 106 into a large condenser 110. The condenser 110 is thus stepped up in potential by uniform amounts at a rate dependent on the modulation frequency (200 or 2,000), so that the scanning beam is moved by coils 18 at a corresponding speed. After a predetermined number of pulses, the beam reaches the end of its movement and the condenser 110 discharges through a gas-discharge valve 117 and magnet 121, thus stepping on the tape 2. Operation of transmitter.-Assuming first that the copy sighted by both apertures 20, 21 is wholly white. Both anodes 22, 23 carry full current, with the result that valves 62, 82 are both blocked, but valve 42 conducts and supplies 2,000-cycle current to line 1 and transformer 101, so that scanning proceeds at high speed at both stations. As soon as the auxiliary aperture sights a black spot, the current from anode 23 falls sufficiently to block valve 42 and make valve 62 conductive, valve 82 remaining blocked. 200- cycle current is thus connected to line 1 and transformer 101 so that scanning proceeds at low speed at both stations. As soon as the main aperture sights the black spot, valve 82 is unblocked, valves 42, 62 not being affected, so that 200-cycle current is applied to line L1 &c. as before and also to line L2, thereby causing the receiver to print a black mark as described below. If a black mark is sighted by the main aperture but not by the auxiliary aperture, the conditions remain as described in the preceding sentence. Receiver, Fig. 3.-Currents of frequency 200 or 2,000 received over line 1 control a cycle counter 201 ... 210, similar to 101 ... 110, Fig. 1, to determine the scanning potential applied to electrodes 260 of a cathode ray recorder, the modulating grid 253 of which is controlled by the 200-cycle currents over line 2 through the rectifier 263 A line of conductors 251, engaged in turn by the beam, passes through the end of the tube and engages the recording tape 270 opposite a knife-edge 266 connected to battery 259. The beam sweeps over the ends of the conductors 251 at a speed determined by potential of condenser 210, and makes a black mark on the paper 270 when the grid 253 is raised above cutoff potential by the signals on line 2, the paper being fed at the end of each transverse line by magnet 221 energized by the discharge of condenser 210. Modified receiver. Fig. 4, using a single channel.-This is used with the transmitter shown in Fig. 1, but no currents are transmitted over line 1, the currents controlled by the principal scanner transmitted over line 2 being recorded at 307 on a magnetized tape 301 or other storage device, the signals being picked up at two spaced points, the main pick-up 310 (corresponding to the main anode 22, Fig. 1) being influenced by a single record element, whereas the auxiliary pick-up 309 is influenced by, say, ten elements. The coils 311, 312 of the pick-ups are connected through rectifiers 313, 314, to the second and third grids of valves 342, 362 which control the current from 2,000-cycle and 200-cycle generators to the cycle counter 401 ... 410 (similar to 101 ... 110, Fig. 1) which, as before, determines the speed of scanning in the tube 450. The valve 362 also controls the modulating grid 453, so as to make a black mark on the paper 470 when valve 362 conducts. The arrangement is such that when either pick-up sights a black signal on the magnetizable tape, the 2,000-cycle tone will be blocked and the 200-cycle tone will control the speed of scanning. Only when both pickups see that the tape is clear of signals is the 200-cycle tone blocked and the 2,000-cycle tone ensure high-speed scanning. When the principal pick-up 310 sees a black signal, the cathode beam records a black mark. The magnetic recorder may be replaced by other storage devices, such as a strip of fluorescent material, and the recorder and reproducer may be optical and photo-electric units. Or photographic or phonographic recorders using a wax record may be used.