Patent GB-683612-A (GB)
Title: Improvements in or relating to carrier telegraph systems
Inventor: TERRY VICTOR JOHN
IPC: H04L27/06
Filed: 1948-12-03
Abstract: 683,612. Code telegraphy. STANDARD TELEPHONES & CABLES, Ltd. Dec. 2, 1949 [Dec. 3, 1948], No. 31359/48. Class 40 (iii). An arrangement for receiving electric carrier current telegraph signals comprises means for reproducing the signals in single current form in each of two separate paths, means in the first path for shaping the signal wave in such manner that the times when the signal amplitude passes through a value equal to half the maximum amplitude are spaced in the same way as the vertical edges of the original rectangular wave prior to transmission, means in the second path for differently shaping the signal wave in such manner as to produce a modified wave having a series of amplitude maxima all of the same value, means in the second path for deriving from the modified wave a bias voltage wave, and means for combining the shaped signal wave in the first path with the bias voltage wave in the second path to produce a double current signal wave, the amplitude of which passes through zero at times which are spaced in the same way as the vertical edges of the original rectangular wave before transmission. In the diagrammatic arrangement shown in Fig. 1 the carrier received at 3 is demodulated and passed to the first path comprising a signal network 5 which produces a wave of the form shown in Fig. 2 (B) varying in voltage between O and +V. In the second path comprising the bias network 6 to produce a voltage - V/2, the path includes, Fig. 7, a demodulator 23, filter 24 and a resistance 25, connected at its centre tap via a rectifier 30 to a resistance 29 having in parallel a condenser 28 charged to a voltage - V/2 by voltages V appearing across resistance 25. The output voltage is also fed to a shaping circuit 26 which, as shown in Fig. 5, modifies the long signal elements as indicated at 15, 17 (marking) and 16, 18 (spacing) and the short elements as at 19, 20. In the case of the short impulses the bias V/2 is provided by the peaks 19, 20 and the rectifier 30 blocks charging from the reduced amplitude pulses appearing at the resistance 25. When the long pulses are received the bias is initially produced by the over-shoots, such as 16 or 16. Owing to the short time constant of the shaping circuit, the voltage falls off and is blocked by rectifier 27, but the full amplitude of the long pulse is then effective via rectifier 30 to charge the condenser 28. A cathode follower valve may be interposed between the rectifier 27 and the shaping circuit 26 if its output impedance is too high, Fig. 8 (not shown). In a modification, Fig. 9, in which the signal is demodulated before application to the signal network 35 and the bias network 26, the received waveform indicated at J, Fig. 10, is differentiated by the primary 38 of a transformer 37 and condenser 39 and the differentiated waveform K is fed via a full-wave circuit comprising rectifiers 42, 43 producing a waveform L which is then applied to a condenser 44 to produce a modified waveform M having voltage minima at - V/2. The combination of this biasing waveform M with the output of the signal network 35 produces a waveform N which has its signal instants at the correct times in spite of the distorted waveform J. A small bias battery 46 having a potential v prevents the curve M from falling to the zero line so that during spacing intervals, as indicated at 62 in curve N, the negative voltage level ( - v) prevents small noise pulses from causing false actuation of the signal receiver 4. In a modification of Fig. 9, Fig. 12 (not shown), the signal inputs to the signal network and the bias network are applied in series instead of in parallel.