Patent GB-786589-A (GB)
Title: Improvements in or relating to frequency-shift telegraph transmitters
Inventor: HEATON-ARMSTRONG LOUIS JOHN
IPC: H04L5/08
Filed: 1955-10-04
Abstract: 786,589. Diplex telegraphy. STANDARD TELEPHONES & CABLES, Ltd. Oct. 4, 1955, No. 28195/55. Class 40 (3). [Also in Group XL (c)] A combining circuit for producing an output control voltage having one of four different specified values corresponding respectively to the four possible combinations of marking and spacing conditions of two telegraph channels, comprises means for producing four separate direct current components and means controlled by the marking or spacing potentials present in both channels at any instant for supplying to a resistive device, one or more of said components in such manner as to produce in said resistive device a difference of potential proportional to that one of the specified values which corresponds to the combinations of marking and spacing conditions of the two channels at the said instant. In the arrangement described, the four different voltages are used to control an oscillator to produce or transmit four discrete frequencies representative of the four possible different combinations of marking and spacing conditions which the two channels can assume at any instant. When the marking condition, assumed to be a negative voltage, is applied to the terminals 1, 2 of channels A and B, the valves 9, 10 and 11 will be blocked and the only current through resistors 6, 7 will be that from source 17, and the resistors are chosen so that the voltage drop is a little greater than mV so that the potentiometer 8 can be adjusted to give mV at output terminals 4, 5, the latter being negative to the former. For condition 2, with channel B spacing, and a positive voltage applied at terminal 2, the rectifier 20 is unblocked so that valve 10 conducts and a portion of the cathode current from source 14 flows upward in resistors 6, 7 in opposition to the current from source 17 which is also reduced since these resistors are shunted by the valve 10. The resistor 16 is adjusted until the voltage at terminals 4, 5 is - V. For condition 4, the valve 9 is unblocked by the positive voltage applied to rectifier 19, and the upward current in resistors 6, 7 derived from the valve 9 is adjusted by a resistor 15 so that it exceeds the downward component and is such that terminal 4 has a voltage +mV relative to terminal 5. In the final code denoted by SS, the valves 9, 10, 11 are all unblocked, and rectifiers 26, 27 are blocked so that the control grid of valve 11 is positive to its cathode. The upward components contributed by the valves 9, 10 will be too great, but the valve 11 applies a downward component in resistors 6, 7 and the cathode resistor 23 is adjusted so that the potential of the terminal 5 is +V relative to the terminal 4. To produce the voltages for the code 2, the switches 22, 24 are moved to the positions opposite to those shown, thus reversing the direction of the current supplied to resistors 6, 7 through the valve 11. Adjustments of resistors 8, 16 for the conditions 1, 2 are as in the previous case, and for condition 3 with the valve 9 unblocked the resistor 15 is adjusted to provide an upward current to produce a potential +V at terminal 5 relative to terminal 4. For condition 4, with all the valves unblocked, the valves 9, 10 produce an upward current which is insufficient and the deficit is provided by the adjustment of resistor 23 so that the potential of terminal 5 with respect to terminal 4 is +mV.