Patent GB-551087-A (GB)
Title: Improvements in picture telegraphy
IPC: H04N5/073
Filed: 1941-08-01
Abstract: 551,087. Picture telegraphy. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 1, 1941, No. 9861. Convention date, Aug. 3, 1940. [Classes 40 (iii) and 40 (v)] Relates to picture telegraph systems in which a synchronous motor drives a scanner at the transmitter and the frequency of the supply is transmitted to the receiver to drive a similar motor there. According to the invention, a higher frequency is produced, for example by multiplication of the basic supply frequency, and this higher or multiplied frequency is interrupted or modulated at the basic frequency to produce a train, for example as shown in Fig. 3, which is then transmitted to the receiver and there con- . verted into basic frequency for driving the motor there. The object of this is to reduce transmission errors which are represented by phaseshifts of the transmitted frequency and are therefore smaller for the multiplied frequency of the transmitted train. Transmitting system, Fig. 1.-A picture on a drum 2 driven by a synchronous motor 3 is scanned by a photo-electric device 5, 8. The motor 3 is driven from a 60 cycle supply, which is also connected to a transformer 43, the secondary 45 if which is connected to a rectifier 46 producing D.C. plate voltages between the point 49 and earth across resistance 50 and regulator valves 51, 52. Another secondary 44 of the transformer supplies low lamp and heater voltages. Tn the filter choke 47 of the circuit of the rectifier 46, there is a 120 cycle ripple. This ripple is successively multiplied to 480 cycle by the valve circuits 58a, 60, 64, 66 and amplified in push-pull by valves 69, 70. The grid bias of these valves is controlled by the connection of the cathodes to the point between resistances 72, 73 connecting the 90 volt supply to earth, and by the connection of the mid-point of secondary of input transformer 68 to the load resistance 75 of a diode 76 supplied with a 60 cycle frequency from the secondary 44 ; and this grid bias control is so designed that the valves 69, 70 are operative during one half wave only of the 60 cycle supply. The output of the valves 69, 70 is therefore a synchronizing signal of the shape shown in Fig. 3 and is passed by transformer 71, to an amplitude-modulation radio or other transmitter 42. The secondary of the transformer 71 is in series with the secondary of a transformer 41 which passes picture signals and line pulses to the transmitter. The D.C. picture signals from the photoelectric cell 10 are amplified by valves 12, 16. The valve 16 acts as a variable resistance in series with a tuning condenser 20 of an oscillator 23 connected to beat with another oscillator 30. The fundamental frequency of the oscillators is set at 300 kilocycles and the picture signals are made to produce beat frequencies ranging from 3000 to 4000 cycles. The oscillator 30 is adjusted to produce the beat above or below the fundamental according to whether the picture signal is to be sent with or without reversal as desired. This frequency modulation is passed to a detector valve 32 followed by a filter 36 which removes the high frequencies and leaves the frequencies varying from 3000 to 4000 cycles representing the varying picture intensities. These are amplified by valve 38 and passed by transformer 41 to transmitter 42. This picture channel also serves for the line pulses. The pulses are produced in the usual way between each line of the picture by a commutator 6 closing contacts at each revolution of the drum 2, causing a voltage pulse to be applied to the grid of the valve 16. This pulse is made to produce a beat frequency of 2000 cycles in the oscillators 23, 30, and the 2000 cycle frequency is detected and amplified by the valves 32, 38 and passed on to the transmitter 42. Receiving system, Fig. 2.-The signals sent by transmitter 42, Fig. 1, are received and detected in the usual way and produce voltage variations across a resistance 82, which is connected to two filters 83, 115. The filter 83 is a low-pass filter cutting-off above 1000 cycles, which passes only the 480 cycle train interrupted at 60 cycles. This is fed to a full-wave rectifier 89 giving out a 60 cycle square-shaped wave which is limited, amplified and cleared from harmonics by valves 93, 97 and fed to oscillator 99. The output of the oscillator is amplified by push-pull amplifier 100 to feed the synchronous motor 102 driving the picture drum 103. The other filter 115 is a high-class filter cutting-off below 2000 cycles, which only passes the 2000 frequency representing the line pulses and the 3000 to 4000 frequency representing the picture signals. These frequencies are limited by amplifier valves 118, 119, 122 and fed to a demodulating filter 124, which in conjunction with other parts of the circuit has the overall characteristic curve shown in Fig. 9. The output of the filter is fed in push-pull to the grids of valves 132, 133 biased by resistance 134 ; and the valves are connected in parallel, the arrangement giving full-wave rectification with a cut-off below " black " level of Fig. 9. The output of the valves 132, 133 is fed through resistances 146, 142 to a valve 147 connected to a valve 151 and so arranged that valve 151 passes only line pulses corresponding to the 2000 cycles. If the receiver drum 103 is in step with the transmitter drum, the pulses passed by the valve 151 are short circuited by the commutator 112. If the receiver drum falls out of step, the pulses from the valve 151 pass through a relay coil 106 which opens contacts 105 in the circuit of motor 102, causing the motor to miss one or more half waves of supply; and this is repeated at every pulse until the drum gets into step again. To ensure that the contacts 105 are quickly snapped open and remain open for a sufficient time to slow the motor down one half cycle, a condenser 108 with a leak 109 is arranged in series with the relay through contacts 107 which close when the relay is energized. The condenser thereupon produces a powerful surge of current through the relay, and this holds the relay on for a sufficient time. The output of the valves 132, 133 is also fed to a valve 138 associated with a valve 141 for controlling currents through the coils 143, 144 of a recorder. Fig. 13 (not shown). A volume control 130 in the input circuit of the valves 132, 133 is so set that when a " black signal occurs, no blocking voltage is applied to valve 138 and the anode current of this valve passes through coil 143 of recorder, causing a black mark to be produced ; at this time, the nositive grid bias on valve 141 from resistance 142 common to the interconnected cathbleeder 137 is balanced by voltage drop in odes of valves 138, 141, and valve 141 is therefore cut off and no current passes through recorder coil 144. When a " grey " signal occurs, a voltage drop occurs on grid of valve 138 causing a reduction of anode current through coil 143 ; at. the same time, voltage drop in resistance 142 decreases and valve 141 becomes operative causing current to pass through recorder coil 144 opposing coil 143, this together with the reduction of current through coil 143, reduces the pressure on the master. When a " white " signal occurs, valve 138 cuts off and valve 141 becomes fully operative, and current in coil 144 lifts marker completely off. When a line-pulse occurs, a still greater negative bias is produced in resistance 139 more effectively cutting off valve 138. Various modified or accessory circuit details are also described. In the transmitter circuit of Fig. 10 (not shown), the picture signal amplitudemodulator includes a diode for contrast amplification and a commutator device for cutting off the picture signals during the transmission of the frame signal. In a modification, Fig. 14 (not shown), the synchronizing signals are amplitude modulations of the subcarrier which is frequency modulated by the framing and picture signals. In another modification, the synchronizing signals are sent out as frequency variations by the transmitter, Fig. 16 (not shown), and the receiver, Fig. 18 (not shown) includes a demodulating filter.