Patent GB-555307-A (GB)
Title: Improvements in or relating to facsimilie telegraph systems and apparatus
IPC: H04N1/00
Filed: 1941-06-06
Abstract: 555,307. Facsimile telegraphy. CREED & CO., Ltd., and THOMPSON, A. E. June 6, 1941, No. 7174. [Class 40 (iii)] In a facsimile telegraph transmitter, a signal modulated light beam and an unmodulated light beam both chopped by common means are made to impinge alternatively upon a common photoelectric device under the control of an electromagnetic shutter. The chopping of the two light beams is made at two different frequencies. The beam not modulated by the signal services for control purposes. Drum drive and photo-electric scanning chariot. The clutch device 38, Fig. 1, for driving the drum 12 from the motor 65 is geared to that motor through wheels 66, 67, 70, 71, 72, 73. The feed screw 74 for moving the scanning device along the drum, when engaged by a nut 98 under control of a magnet,97, is geared to the wheel 73 by wheels 75, 76, 77, 78. The chopper disc 69 of the scanning device is feathered on the shaft 68 of pinion 70 to be rotated thereby. The photo-electric scanning device comprises a lamp 79 at one focus of an ellipsoidal reflector 80 which forms an intense luminous image on a small white screen 81 at its other focus. A small hole in the screen allows the illumination of a small area of the message just behind the screen. The light from the screen is directed by an optical system 82 through the holes in the chopper disc 69 and through holes in a box 83 to a photo-electric cell 84 in the box. The box has three holes 85, 86, 87 controlled by two shutters 88 and 89, and the chopper disc has three series of holes 94, 93, 92 optically aligned respectively with the three holes 85, 86, 87. While a line of the message is being scanned, hole 86 is uncovered, and the light from the scanning spot on the message passes to the cell 84 through the central set of holes 93 of the chopping disc which gives the light a certain carrier frequency. When the longitudinal joint in the paper on the drum passes behind the screen 81 at each revolution of the drum, a cam 95 on the drum spindle 23 closes a switch 96 which energizes a magnet 91 causing the shutter 89 to close the hole 86 and open the hole 87. Light then reaches the cell from the screen 81 through the inner series of holes 92 of the chopping disc which gives the light a different carrier frequency. This series of holes 92 occupies only a part of a circumference of the disc and comprises a long section a and a shorter section b, the second section being displaced half a pitch relatively to the first section. The line " start " signal produced by this arrangement comprises a first section caused by the section a followed by an out of phase section caused by the section b. This out of phase section is intended to suppress the oscillations of the corresponding resonant circuit of the receiver. At the completion of the scanning operation, a switch is closed and energizes an electromagnet 90 causing shutter 88 to cover hole 86 and open hole 85. Light from the screen 81 then reaches the cell through the outer series of holes 94 of the chopper disc. The carrier frequency produced by these holes is the same as that produced by the inner holes 92, but the holes occupy the whole circumference and produce a signal of long duration. Also at the end of the scanning operation, magnet 97 is de-energized and the nut 98 withdrawn from the feed-screw, whereupon the scanning chariot is returned to the starting position by a spring. Transmitter circuit system and general operation.-The circuits are switched on by a main switch 101, Fig. 2, which supplies heating current to the amplifier valves and to a thermal relay 102. After a time sufficient to allow the valves to be heated up, the thermal relay 102 closes its contacts 103, 104. The closing of contacts 104 lights a lamp 105 to indicate that the transmitter is ready for operation. The contacts 103 are in series with switches 58 and 56. When the message sheet is inserted by the operation of a handle 33, switch 58 is closed; and when the pushbutton 56A is pressed, switch 56 is closed and the magnets 57, 97 in series with each other are energized. Magnet 57 closes contacts 64 which set driving motor 65 in operation, and a generator 108 driven by the motor 65 generates plate current for the amplifier valves and the photo-electric cell. Magnet 57 also closes contacts 59, which close circuit of scanning lamp 79 through an adjustable resistance 106, a voltmeter 107 being provided to indicate the voltage on the lamp. Contacts 59 also serve to hold these circuits closed despite the release of push-button 56A. Magnet 97 in series with magnet 57 connects the scanning chariot to the feed screw. The scanning operation begins. Continuous signal carrier frequency is produced while the initial blank of the sheet is being scanned, but no line " start signal is produced. This is to enable the receiver motor to start up and the amplifier volume control to become effective. After the scanning chariot has advanced several millimetres, a switch 109 closes and the contacts 96 operated by the cam 95 on the drum spindle become operative to send the line " start signals by the energization of magnet 91. Transmission then proceeds until the message is completed, when a switch 110 is closed by the scanning chariot. Then, when next the contacts 96 are closed, magnet 90 is energized to send the " stop " signal. Magnet 90 locks up by contacts 111 and opens contacts 112 which break the circuit of chariot feed magnet 97. Magnet 57 remains closed through resistance 113 until switch 109 is opened by the return of the chariot. During that period, therefore, scanning lamp 79 remains alight and motor 65 continues its drive, so that emission of the " stop " signal continues. When switch 109 is opened, magnet 57 releases and lamp 79 goes out, but motor 65 still runs until contacts 116 are opened by a cam on the drum spindle. If the operator wishes to stop transmission at any moment, he presses a stop button to close switch 117, and this produces the same results as the closing of the switch 110. To prevent the out-ofphase part b, Fig. 1, of the holes 92 of the chopping disc interfering with the action of the "stop " signal, contacts 115 in series with the contacts 96 and switch 110 are opened by the cam 95 while this out-of-phase part b is operative, so that if the switch 110 closes during that period, no stop signal is produced until the next closing of the contacts 96. Transmitter amplifier.-The photo-electric cell 84, Fig. 3, is fed by potentiometer R1, R2 and is connected to the grid of a pentode P1 with a leak R3. The pentode is coupled to a triode T2 by a resistance-capacity circuit R4, C1, R5 for transforming pulsating current to alternating current to reduce transients. The triode T2 is coupled to two triodes T4, T5 in push-pull through two transformers Tr1, Tr2 and a double diode D3, the whole circuit being designed to suppress signals below a certain peak voltage and to drive the push-pull triodes to saturation. The amplifier thus delivers a fixed voltage for the white spaces in the message, and this voltage is cancelled completely by the black marks of the message. Receiving amplifier.-This amplifier comprises a volume-control and motor-starting circuit, Fig. 5, a start signal circuit, Fig. 6, and a printing signal circuit, Fig. 7. The output currents from a radio receiver are fed by a potentiometer PM1, Fig. 5, to a pentode P1, which has a load circuit comprising a transformer Trl coupled to a polarized double diode D1 adapted to limit peak voltage. The plate of the pentode is resistancecapacity coupled to a triode T1 having a load circuit CR1 flatly tuned to the message carrier frequency and coupled to another triode T2 by a highly resonant circuit CR2. The triode T2 is coupled by a transformer TR2 having two separate secondaries to two separate double diodes D2, D3. The double diode D2 is biased by an adjustable potentiometer PM2 to determine the level at which volume control of pentode P1 commences through network R5, R6, R1, C2, C3, C4 which is such that lowering of potential takes place rapidly as soon as carrier signal is received but rises slowly so as to be unaffected by the rapid modulations representing the message. The second double diode D3 has a detector circuit CD1 coupled to a triode 118, Fig. 4, in the receiver control system. The start signal circuit comprises a pentode P2, Fig. 6, which has an input circuit CR3 tuned to the start signal carrier frequency and fed through a resistance R7 and a condenser from the anode of pentode P1, Fig. 5. The pentode P2 is connected through a transformer Tr3 to a double diode D4 with a detector circuit CD2, and this detector is coupled to the pentode 123, Fig. 4, of the control system. One action of this pentode is to operate a relay 126 for various purposes and one effect of this is to deaden the tuned circuit CR3 by the introduction of a resistance R8 in parallel with the circuit, a resistance R9 being at the same time inserted in parallel with the resistance R7 to avoid too much reduction in voltage applied to the grid of pentode P2. The reason for this deadening is to reduce the time taken for the oscillations to cease after the cessation of the start signal to make the moment of operation of relay 124, Fig. 4, more precise. The signal from B, Fig. 5, obtained from the anode of triode T1 is fed to the printing signal circuit, Fig. 7, at B, and passed through a transformer TR4 adjustably biased by a potentiometer PM3, a double diode D5 and another transformer Tr5 with a byepass condenser C5 to a triode T4. Only peak voltages are passed and these are smoothed. The triode T4 is coupled by a transformer Tr6 having two separate secondaries to two separate double diodes D6, D7 having detector circuits CD3, CD4. These are coupled to two power pentodes P4, P5. The pentode P4 is connected to the printing electromagnet and serves to operate th