Patent GB-222558-A (GB)
Title: Improvements in or relating to high frequency signalling systems
IPC: H03C1/52
Filed: 1923-07-02
Abstract: 222,558. Western Electric Co., Ltd., (Western Electric Co., Inc.). July 2, 1923. Keying and modulating systems; multiplex signalling; wired wireless systems.-Modulated carrier currents for signalling are produced by commutating the signal currents at an appropriate rate and are received by means of a commutator running synchronously with the sending commutator. A single commutator may perform both functions at any one station. . Multiplex telephone system, Fig. 4. The commutators 17, 18 are on the same. shaft but commutate at different rates and commutators 17, 19 and 18, 20 are in synchronism. Telephone and signal currents from station III pass via a filter F' for suppressing frequencies higher than those essential to speech, to a commutator 17 which commutates them at a rate higher than the highest essential speech frequency. The resulting currents are of the form shown at 5, Fig. 2, and comprise both side-bands but have no unmodulated component. The filter F<3> is designed to pass either or both side-bands to the line ML<2>. Currents from station V are commutated at a different frequency and ordinary telephone currents may be fed to the line ML<2> from line 15. The various channels are separated by the filters F<4>, F<1>2, F<6> and the commutated currents are restored by the synchronous commutators 19, 20 to the form of curve 4, Fig. 2. If the switch 21 is thrown to include oattery 22 the signal wave takes the form shown at 7, Fig. 1, and the commutated wave 2 comprises an unmodulated carrier component as well as the two side bands. The filter F<3> may be designed to pass this also. If the commutation frequency is about 2,000, i.e. the same as the highest essential speech-frequency, the filter F<3> may be a duplicate of filter F<7>. In this case only the lower side band goes to line and the line currents are of audible frequency but are so inverted as to be unintelligible even when rectified, and can only be received by commutation at a suitable rate or by a rectifier which is being fed with a current of the same frequency as the carrier current. Ringing currents are transmitted in the same way as voice currents. Multiplex telegraph systems, Figs. 5 - - 7. Printing telegraph equipment is connected to a line ML over a low pass filter LP and operates in known manner. The synchronizing-apparatus 22 is also used for controlling commutators 30 - - 33 which are provided to allow the line ML to be used for duplex telegraphy between the lines LL<1>, LL<2>, and two similar distant lines. Direct current signals from the line LL<1> control relay 36 which repeats the signals to commutator 30 over the filter LF<1> which suppresses frequencies higher than (say) 100. The commutator 30 operates at a frequency of (say) 150 and produces two side bands having frequency range 50-150 and 150-250 respectively. The filter TF<1> passes only the latter. Signals from line LL<2> are similarly commutated at a different frequency (say) 300. The resulting currents are fed to the line through the usual three coil transformer and a high-pass filter HP. Currents incoming over line ML are first sorted by the filters HP, LP so that the primary telegraph impulses and the carrier-wave signals take their proper channels. The carrier wave signals are sorted by the band filters RF<1>, RF<2> which resemble the filter TF<1> and are then commutated at suitable rates to restore them to their direct current form. Lowpass filters LF<3>, LF<4> suppress undesired high frequencies due to imperfections in the commutators and relays 39, 40 are actuated to send the signals to their respective lines. In a modification, Fig. 6, the same commutator 45 is used both to modulate and demodulate. Receiving and transmitting circuits L<1>, L<2> which may be separate or associated in any known duplex arrangement, carry signal impulses differentiated from spaces by polarity. Impulses from circuit L' control a polarized relay 50 to connect positive or negative battery to line. The relay 46 remains unaffected the line to the right of the line ab being balanced by the network N. The filters are arranged as in the other system. Pulses of either sign produce alternating current in the line ML<2> but at each change over there is a phase inversion as shown at 5, Fig. 2. This inversion is interpreted for received currents by commutator 45 which restores the signals to their original form in which they actuate relay 46. The polarized sounder 51 may be used for monitoring purposes. Fig. 7 shows a simplex circuit with a key 52 for switching from receiving to transmitting. Normally the polarized receiving relay 53 follows the movement of the sending key 55 but when the distant operator wants to break in he throws his key 52 to the upper sending position and relay 53 then ceases to follow key 55. The system shown in Fig. 5 may be modified by replacing relay 36 by a relay such as 56 and making relay 39 a polarized relay such as 53. Specification 142,115 is referred to.