Patent GB-228649-A (GB)
Title: Improvements in multiplex carrier telegraph systems
IPC: H04L5/06
Filed: 1923-11-14
Abstract: 228,649. Western Electric Co., Ltd., (Western Electric Co., Inc.). Nov. 14, 1923. Multiplex signalling; keying systems and apparatus; receiving systems and apparatus; tone frequence tuning-arrangements.-A multiplex telegraph system, particularly for use on existing telephone lines with telephone repeaters &c., utilizes a number of audio-frequencies separately modulated by the transmitting keys, the separate channels each including a filter and being connected jointly to the main transmission line. The keys are associated with shunt and series impedances arranged to limit crossmodulation. The receiving rectifiers are arranged to reject weak interfering signals, and to shape up the desired signals. The mixed audio-frequency currents may be used to modulate a high-frequency wave, and thus transmitted by radiation or over lines. As shown in Fig. 1, a number of audiofrequency generators G1 ... G3 are connected respectively to transmitting channels TL, ... TL3, each of which includes a keying apparatus M1 and a band filter TBF1. The three channels are combined in an amplifier TTA. and the outgoing currents are sent into a common line EL. Incoming currents in the line WL pass through an amplifier RTA, and are selected for the receiving channels RL1..RL3 by means of filters RBF1, the selected currents being further amplified by valves A1, rectified in valves R1 and operating relays 11. The generators G1 .. G3 may also supply channels TL<1>1... TL<1>3 belonging to a separate system. Between the generator G, and the filter TBF1 are connected two series impedances 1, 2 and a shunt impedance 3, all preferably being non-inductive resistances. The signal key 7 when depressed energizes a relay 6 and moves a switch 4 from its spacing-contact S to its marking contact M. It thus opens a short-circuit across the line TL1 and causes current to be sent to the line EL. The impedances 1, 2, 3 and the surge impedance of the filter TBF1 are so proportioned that the voltage variation of the generator G1 when the key is operated is less than ten per cent of its open circuit voltage, it being found that this position ensures the absence of undue cross-modulation between the channel TL1 and the channels TL<1>1 supplied from the same generator G1. In order to prevent signals in the channel TL2 from affecting signals simultaneously transmitted at a different frequency in the channel TL1, the combined impedances 2 and 3 are approximately equal to the surge impedanc e of the filter. If the frequency spacing of the two channels is increased, the impedance 2 is reduced, and impedance 3 increased. When the key 7 is depressed to send a signal, current is sent to the line immediately the switch 4 leaves the contact S. When, however, the key is released, current still goes to line until the switch 4 has completed its passage from M to S. This results in the duration of the received signal being extended at the expense of the spacing duration. To assist in remedying this, the marking contact M may be connected to a point beyond the impedance 2, so that this impedance is in the line circuit TL, as the switch 4 is moving between the contacts and is cut out when the switch 4 is on the marking contact, the transmitted current thus rising and falling in two stages. At the receiving end, the received currents after selection, amplification, and rectification are passed through one winding of a three-coil relay 11. A second winding is in a circuit inductively coupled at 12 to the rectifier circuit, the current through this winding being proportional to the rate of change of the rectified current. The third winding serves to bias the relay armature. The second winding receives a current impulse at the beginning and end of each signal element, which serves to shape up the received signals. A resistance or impedance 13 is connected in the grid circuit of the rectifier R1 between a biassing-battery 14 and the lower end of the input coil 10. This serves to enhance the discrimination between normal signals and weak interfering signals. Fig. 7 shows means for combining the audio-frequency telegraph system with a wired wireless system. The transmitting line EL, Fig. 1, is connected to the line T,L, Fig. 7, and the audio-frequency currents modulate a high-frequency carrier supplied by a generator CS, the modulated output passing to the line ML through filters T,BF, HGF. Incoming currents on the line ML are of a different range of carrier frequencies and pass through a filter LGF and are selected by filters R1BF. They are then detected or demodulated at DM1 and pass through a low pass filter LPF, to line R1L which is connected to the line WL of Fig. 1. The line ML may also serve as a low-frequency line, for which purpose a high-pass filter HPF, and a lowpass filter LPF may be provided. An ordinary telephone line L may also be associated with the main line ML. by the balance transformer 20, modulator M2 and de-modulator DM2. For wireless transmission, each modulated carrier current is passed to a correspondingly-tuned branch of a multiple-tuned transmitting-aerial. The receiving-aerial is also multiple-tuned, each branch being loosely coupled to a secondary circuit from which connections are made to de-modulators, and thence to the respective receiving-channels.