Patent GB-624560-A (GB)
Title: Improvements in or relating to double-current audio-frequency telegraphic transmission systems
IPC: H04L27/20
Filed: 1945-07-03
Abstract: 624,560. Electric telegraphs. COMPAGNIE GENERALE D'ELECTRICITE. July 3, 1945, No. 16957. Convention date, March 12, 1943. [Class 40 (iii)] In a phase-reversal telegraph system oscillations from a generator GA are phase-modulated at MA by the transmitter of the local telegraph apparatus LOC<1>, the resulting signals being sent over a line LAB from station A to station B where they are translated into D.C. signals by the circuit described in Specification 449,999, modified as described in Specification 618,589. For transmission from B to A the output from the modulator XM at station B is amplified at V2 and applied to a ring-type modulator MB where it is phasemodulated by the local transmitter, and at station A the received signals are translated as above described. If transmission accidentally ceases (e.g. due to line failure) and is later resumed, the phase of the output from modulator XM may then be incorrect since the first signal-element it receives may be different from that invariably received at the commencement of each message. At station A, therefore, a ring demodulator # is fed with the output of amplifier V<1>2 and also with the phase-retarded output of the generator GA. So long as these inputs are, correctly, in phase, the output is of a predetermined polarity and amplitude, and neither relay 1 nor relay 2 is operated. If transmission ceases in the line LBA, the amplifier V<1>2 sends no input to the demodulator # and its output falls to zero, operating the relay 2, which operates relay 7 at contacts 2.1 and in turn operates relay 8 at contacts 7.1 and starts a vibrator 9, 10 over contacts 7.2. At contacts 8.1 and 8.2 the local telegraph circuits LOC<1> are isolated from the demodulator DM<1> and the modulator MA and the latter, in the absence of control current, sends an oscillation of spacing phase to line LAB. The vibrator 9, 10 keys this oscillation at contact 9.1 and the first interruption reaching station B releases relay 3 which has been held operated in the position shown by the rectified and smoothed output of the demodulator DM. Relay 3 operates self-locking relay 4, whose contacts .4.2 operate relay 5, which isolates the local telegraph circuits LOC and at contacts 5.3 prepares a circuit for relay 6 which, however, is short-circuited by contacts 3.2. When transmission resumes, the demodulator DM operates relay 3 and contacts 3.2 open allowing relay 6 to operate after a suitable delay due to the circuit PI, K. This releases relay 4 and relay 5 which at 5.3 releases relay 6. All circuits at station B are now normal and the inputs to the demodulator # cause relay 2 to release, restoring station A to normal. If, however, these inputs are out of phase, the demodulator output is reversed in polarity and operates relay 1 which acts in the same way as relay 2. Transmission cannot continue, therefore, until the inputs to the demodulator # are in phase, and neither relay 1 nor 2 operated. If transmission in the direction LAB ceases, then relay 3 releases and initiates the operations above described at station B. Since the output from the modulator MB is then zero, the demodulator # operates relay 2 and the same conditions are set up as when channel LBA was interrupted.