Patent GB-739594-A (GB)
Title: Facsimile telegraph apparatus
IPC: H04N1/00
Filed: 1952-12-05
Abstract: 739,594. Copying-telegraphy. CREED & CO., Ltd. Dec. 5, 1952 [Dec. 13, 1951], No. 30916/52. Class 40 (3). In facsimile transmission apparatus in which a blank is scanned to produce signals corresponding to the matter to be transmitted, a first pulse having a first time duration is generated and controls the generation of a second pulse having a second time duration, and means responsive to the first pulse controls the generation of a blanking pulse having a time duration equal to the sum of the first and second time durations and employed to block, for the duration of the blanking pulse, a normally unblocked modulator connected between the scanning means and the output circuit. In the circuit arrangement of Fig. 4, pertaining to the alternate use of two transmitters, the output of amplifier 40 or 41 selectively under control of a relay circuit 42, is fed via amplifier 43 and a blanking modulator 44 to an amplifier 45 also under control of the relay circuit 42, and to an output transformer 46. A gap in the transmitting blank in combination with a light source and a photo-cell produces a square negative pulse A, of duration d, amplified and inverted at 47 to form B, differentiated by elements 49, 50, 51 form C, and produced from amplifier 52 wave form D comprising a negative pulse followed at an interval d, by a small positive pulse. The negative impulse triggers a flip-flop circuit 53 to initiate the production of a square pulse E which is applied to the blanking modulator 44. The pulse B is also applied to network 49, 54, 55 to produce a pulse wave form F which is reversed in amplifier 56, and the positive pulse of wave G triggers a one-shot multivibrator 57 producing a pulse H of duration d1 which is differentiated at 58, 59, 60 and reversed in amplifier 61 to produce a peaked wave J with a small positive pulse followed by a larger negative pulse which triggers the flip-flop 53 to terminate the pulse E which therefore has a duration d + d1. A positive pulse K from the multivibrator 57 is amplifier in a cathode follower 62 and applied to a phasing modulator 63 permitting oscillations of 25 kc/s. from a generator 64 to pass to a phasing amplifier 65 under control of circuit 42. Stand-by or end-of-message signals are transmitted from generator 66, operating at 12.5 kc/s., and amplifier 67 under control of the relay circuit 42. Transmission of picture signals. The output of the operative photo-cell is applied to valve 401 or 41<SP>1</SP>, one of the valves being cut off by positive bias applied over conductor 72 or 73 by the relay circuit 42. The output of the valve 401, 41<SP>1</SP> is applied over potentiometer 74 or 75 to a valve 43<SP>1</SP> and to the grid of a valve 78 operating as a cathode follower and via modulator 441 to a valve 45<SP>1</SP> whose output is fed to transformer 46. The blanking impulse E is fed to the grid of a phase-splitting valve 90 and the rectifiers 83, 84 of the modulator bridge 44<SP>1</SP> are so poled that the pulses E, E<SP>1</SP> block the modulator from passing signals to the valve 45, during the blanking period. Transmission of phasing and stand-by signals. The phasing impulse L is applied to a valve 100 having its anode and cathode connected to opposite points of a phasing modulator 63<SP>1</SP>, and applying voltages such that the rectifiers 102, 103 are normally blocked. The phasing pulse varies these voltages to unblock the rectifiers 102, 103 and oscillations at 25 kc/s. are passed from generator 64, valve 110, modulator 631 to the valve 65<SP>1</SP> whose output is fed to the transformer 46. For the transmission of the standby signal, the valve 671, which is normally cut off, is rendered conducting over conductor 118 by operation of relay circuit 42 to pass oscillations from the generator 66 to the output transformer 46. The valve 45<SP>1</SP> is cut off during the phasing and stand-by signals by operation of relay circuit 42 to open the cathode biasing circuit over conductor 99. The operating circuits of the flip-flop 53 and the one-shot multivibrator 57 are described in connection with Fig. 8 (not shown). Circuits for photo-electric modulation: In Fig. 9, a carrier frequency from one oscillator 147 is applied to opposite corners 148, 148<SP>1</SP> of a bridge, two adjacent arms incorporating the cell 34 and capacitor 142, and the other two arms including a capacitor 143 and a circuit comprising an adjustable resistor 144 and an adjustable capacitor 145. The other corners of the bridge are connected to ground and the control grid of valve 401 or 41<SP>1</SP>. The bridge is arranged to be balanced when the background portion of the copy sheet is being scanned. An alternative arrangement, shown in Fig. 10, comprises an electron multiplier photo-tube 34<SP>1</SP> with its dynodes connected to a potentiometer 151 and anode connected to the grid of a valve 152 whose anode is connected to positive potential by the relay arrangement 42. The picture signals are taken from the cathode of the valve 152 and applied to the grid of a valve 40<SP>11</SP> or 41<SP>11</SP>, Fig. 11, which shows a modulator circuit alternative to that shown in Fig. 6. Valves 40<SP>11</SP>, 41<SP>11</SP> are operated as cathode followers and are connected to ground through a voltage regulator tube 162 which also serves as a source of D.C. bias for the associated valves, such as 152 of Fig. 10. Facsimile signals are passed via capacitor 168 and resistor 169 to a centre tap on the secondary of a transformer 165 and a rectifier 171, connected to the junction of elements 168, 169 is poled so as to pass signals corresponding to white or the background level of the transmitted copy, and therefore forms a D.C. restoration circuit. The secondary of transformer 165 is connected to the primary of a transformer 166 through resistors 172, 174 and rectifiers 173, 175, and the secondary winding of transformer 166 is connected to the grid of valve 45<SP>11</SP>, the anode of which is connected by conductor 96 to the output transformer 46, Fig. 7. The blanking signal E from the cathode of the valve 90<SP>1</SP> is applied to the centre of the primary winding of transformer 166 and biases rectifiers 173, 175 so that they are non-conductive during the period d + d<SP>1</SP> of the blanking impulses. Rectifier 179 together with resistor 178 and a capacitor of Fig. 8 (not shown), form a D.C. restorer circuit for clamping the blanking pulse to ground potential, and potentiometer 181 in combination with rectifier 182 and capacitor 180 clamp the zero level of the output blanking wave to a fixed potential above ground. Potentiometer 181 may be set so that facsimile signals below the predetermined level will have no effect on modulator 164. The circuit arrangements, constituting the relay circuit 42, for the control of the various functions of a transmitter for the complete operation of the transmission of a copy sheet of material, and the switching to a second transmitter, or the shutting-down of the system if there is nothing further to be transmitted, are described in connection with Figs. 12 to 15 (not shown). The operation of the respective transmitters is dependent on relays controlled by switch members sperated by a gate member forming a support for one end of the cylinder carrying the blank to be transmitted, as described in Specification 739,600. Specification 717,726, [Group XL (c)], also is referred to.