GB637473A - Improvements in and relating to carrier telegraph systems - Google Patents

Abstract

637,473. Frequency and phase modulation. STANDARD TELEPHONES & CABLES, Ltd., and HARGREAVES, T. F. S. Sept. 23, 1947, No. 25817. [Class 40 (v)] [Also in Group XL (b)] In a carrier telegraph system of the kind in which marks and spaces are transmitted by changing the frequency or phase of the carrier, distortion introduced, for example, by inserting a D.C. blocking condenser in the receiver circuit, is neutralized by combining with the main rectified signals a second set of signals bearing an opposed distortion. Fig. 2 shows a receiving circuit in which the received waves are applied to a limiter valve 1 having two anode circuits 3, 5 tuned respectively to the marking and spacing frequencies and coupled to full-wave rectifiers producing signals of opposite polarity across their load resistors 6, 7. These signals are applied through the series condenser 8 and across shunt resistor 10 to the grid of a valve 9 which is cut off when the grid is negative and conductive when the grid is positive. The relay 11 has two opposed windings, one in the anode circuit and the other carrying a steady direct current, so that positive and negative grid volts throw the armature to its respective contacts. Fig. 3 (a) shows the signal voltage across the resistors 6, 7, the zero line o indicating that the carrier waves have drifted from their correct values. Fig. 3 (b) shows the effect of inserting the network 8, 10 which removes the D.C. component, but distorts the signal voltage. The invention consists in inserting across a series condenser 18 the counter-distorting voltage shown in Fig. 3 (c), so that the voltage at the grid of the relay valve 9 has the correct form shown in Fig. 3 (d). To produce the voltage of Fig. 3 (c), current is fed, irrespective of frequency, from a coil 12 in the anode circuit of the valve 1 to two separate full-wave rectifiers 15, 16 of opposed polarity, with their load resistors connected across the condenser 18 in series with each other and with a resistor 17. The circuits of the rectifiers 15, 16 include bridge switches 13, 14 which are opened and closed in the rhythm of the signal by connecting them across the leads from the H.T. supply to the respective coils of the relay 11. As the time constant of the network 17, 18 is made equal to that of the network 8, 10, the voltage across the condenser 18 rises as the main signal voltage falls and vice versa. When signalling is effected by phase shift instead of frequency shift, the receiver discriminator gives an impulse at each shift, and these impulses are passed to an integrating circuit, the time constant of which has a distorting effect similar to that shown in Fig. 3 (b), and which may be neutralized in the manner described above.