Patent GB-549010-A (GB)
Title: Facsimile telegraph system
IPC: H04N1/17
Filed: 1941-04-25
Abstract: 549,010. Copying telegraphy; cathode-ray tubes. STANDARD TELEPHONES & CABLES, Ltd. April 25, 1941, No. 5355. Convention date, April 26, 1940. [Classes 39 (i), 40 (iii) and 40 (v)] In a facsimile telegraph system using frequency modulation, the scanning at both transmitter and receiver is effected in discontinuous steps at a rate which varies with the instantaneous frequency of the modulated current, i.e. with the density of the picture. The potential applied to the scanning electrodes is derived from a large condenser which is charged (or discharged) by the repeated discharge (or charge) of a small condenser, which is charged and discharged by successive halfwaves of the frequency-modulated current. As the picture varies in density, the frequency of modulation varies and this in turn determines the rate at which the large condenser is charged and consequently the rate of scanning. At the end of each line, the large condenser is discharged to return the scanning beam to its normal position. In the cathode-ray tube, the beam moves in one direction only and scans a single row of insulated conductors 5, Fig. 2, which pass through the wall of the tube and are connected outside the tube to a strip of selenium at the transmitter or to an electrolytic or thermal recorder at the receiver. Transmitter.-The picture to be transmitted is carried by a belt 21 fed line by line by magnet 92. Light from a single line of the picture influences a selenium strip 11, Fig. 2, which is connected to the conductors 5 scanned by the cathode-ray beam. The beam current passes in parallel through the selenium cell and a resistance 17, Fig. 1, so that the voltage applied to the grid of valve 38 varies with the density of the picture element scanned. The anode current of valve 38 varies a capacity 33 so as to determine the frequency generated by an oscillator 32, the oscillations from which beat with those from a fixed oscillator 31 so as to produce beat currents varying in frequency from say 200 per second for black to 2,000 for white. Part of the beat currents pass through transformer windings 46, 47 to valves 48, 49 so arranged that during one half cycle valve 48 conducts and charges a small condenser 51 whilst during the next half cycle valve 49 conducts and discharges the condenser 51 into a large condenser 54. The condenser 54, connected to the scanning plates 3, 4, is thus stepped up in potential by uniform amounts at a rate dependent on the modulation frequency, so that the beam is moved at a corresponding speed. After a predetermined number of pulses, the beam reaches the end of the line and causes current to pass through a resistance 81, thereby causing valve 83 to de-energize relay 84 which short-circuits the primary of transformer 39, thereby stopping the transmission of beat currents over transformer J. Energy no longer passes through rectifier 45 and valve 91 to relay 92, which de-energizes to short-circuit the large condenser 54, and feed the picture one line. For black and white pictures, two frequencies only are used. Receiver.-The cathode-ray tube at the receiver is similar to that at the transmitter except that the row of conductors 105 engages a sheet of paper for electrolytic or thermal recording. The scanning potential is obtained in the same way as at the transmitter from the beat currents received over line 44. When these cease at the end of a picture line, valve 191 de-energizes relay 192 which feeds the recording sheet one step and short-circuits the large condenser 154. The thermal recorder may use an arc discharge passing through dry white paper in contact with carbon or dyed wax paper. Several copies may be made simultaneously in this way. If desired, intensity, modulation may be used in addition to frequency modulation to obtain greater differences in density.