Patent GB-388424-A (GB)
Title: Improvements in alternating current signalling systems, particularly multi-channel carrier telegraph systems
IPC: H04L5/06
Filed: 1931-08-25
Abstract: 388,424. Multiplex carrier telegraphy. RYALL, L. E., 52, Crescent Gardens, Eastcote, Ruislip, Middlesex. Aug. 25, 1931, Nos. 23871 and 28705. [Classes 40 (iii) and 40 (v).] In a multiplex telegraph system employing alternating currents of different frequencies for different channels, the signals repeated in each channel are corrected for envelope-distortion due to filtering and for variations in amplitude, and are freed from the interference of transients set up by keying in other channels. The sending-key 2 admits alternating current from generator 1 to the line 13, 14 through two or more filter sections 4, 8 and an amplifier 12. At the receiving station the signals control a relay 41 by means of the valve system shown, to which thev are fed through two filter sections 18, 20. The filter sections 4, 8, of the transmitter are tuned to frequencies a1, a2, Fig. 3, bracketing the carrier frequency F, while the receiver filter - sections 18, 20 are tuned to bracketing frequencies b1, b2 (curve B), which are closer together than a1, a2 (curve A). C is the response curve for the combined effect of filters 4, 8 and 18, 20. The sections contain resistance. They are loosely coupled by tapped inductances as shown and are terminated by coupling resistances. The series-tuned circuits may be replaced by parallel-tuned circuits. Several transmitters are connected to the leads 10, 11 and corresponding receivers to leads 56, 57. A.C. impulses are amplified by valve 23, further filtered by a circuit 33 tuned to the carrier frequency, and rectified by rectifier 35 and grid leak circuit 36. They thus control an output relay 41, which is stabilized by a biasing winding in series with a resistance 49. The grid of amplifier 23 has normally a positive bias derived from potentiometer 29, but when signals are received it acquires a negative bias owing to the action of rectifier 28 and grid condenser 26 (4ÁF), with grid leak 27 (0À5 M#). The gain of this amplifier is thus reduced by heavy incoming oscillations and persists during the subsequent reception of weak oscillations ; this has the effect of shortening marks and lengthening spaces, and thus compensates for the opposite action which is exerted, particularly on signals having a sinusoidal envelope, by the current-limiting device 32. Grid current, which would cause a variation of gain with variation in length of signals, is checked by a resistance 24 of 5M# shunted by a condenser 25 of 0À001ÁF. The output oscillations from amplifier 23 are limited by a coupled neon tube 32, as described in Specification 375,346, [Group XXXIX], The output from amplifier 23 being strongly filtered while its amplitude is limited by the current-limiter 32, the operation of relay 41 is determined by the frequency of the received signals independently of their amplitude. The shape of the signal envelopes passed on by circuit 33 can be regarded as due to the presence of side-band frequencies ; it can therefore be varied, with consequent variation in the length of marks, by adjustment of the selectivity-broadening resistances 45 in the filter circuit 33. The valve 40 has normally a large negative bias and is readily saturated ; consequently its D.C. output is of square wave-form. Grid current is checked by resistance 38. Instead of controlling a D.C. relay 41, the valve 40 may control the output passing from an oscillator 52, Fig. 2, into a line 55, so that received oscillations at the frequency of generator 1 are relayed at the frequency of generator 52.