Patent GB-203711-A (GB)
Title: Improvements in multiplex electrical signalling
IPC: H04J9/00
Filed: 1923-09-08
Abstract: 203,711. Day, A. van T. Sept J.1. 1922, [Convention date]. Itlultiplex signalling; secret and selective signalling; thermionic amplifiers and modulating systems.-Signalling or distant control is effected by radiation or along wire conductors by a method in which a carrier wave is modulated by a sub-wave, which is itself modulated by a second subwave, and so on, the last sub-wave being modulated by a voice current or other signal. By selecting combinations of two or more of the sub-waves, a large number of channels may be provided for sending independent messages over the same aerial or conductor. The modulating-apparatus is of the kind in which the pure input waves are balanced out in the output circuit, and only the side-bands, or the sum and difference frequencies, are permitted to pass. At the receiver, the waves are passed through separate and successive de-modulators or detectors, each of which is supplied locally with a wave of the same frequency as was supplied at the corresponding stage at the transmitter, and suitably adjusted in phase. These de-modulating waves are sent out by the transmitter and picked up by the receiver so as to ensure complete synchronism. The various waves may each be split into two waves in phase quadrature, each phase functioning as an independent wave. In this way, further independent signals may be sent over the same conductor, or same pair of aerials. Fig. 1 shows the circuits of a twin modulator for modulating the two phases of current supplied by the source Ee in accordance with currents flowing in the separate circuits FF, ff. The current in the circuits 67, 68 is adjusted into phase quadrature by condensers and inductances and the circuit 67 carrying the leading phase is coupled to the grid circuits of a pair of duplex valves 41, 42. The mid-points of the grid coils 49, 50 are connected through a coil 53, which has its mid-point connected through a potentiometer 69 to the filaments. The valve plates are connected through coils 25, 26 which have their mid-points connected to the plate battery 29 and are coupled to the coils 21, 22 connected in series in the output circuit GG. The modulating circuit FF is coupled directly or indirectly to the coil 53. As a result of the double balance effected by the splitting of the coils 25, 26 and by their joint coupling to the output circuit. GG, neither of the input circuits have any individual effect on the output circuit, but the -side bands or sum and difference frequencies are produced therein. The modulator for the lagging phase is shown on the right of Fig. 1 and is similar to that described above, ff being the input circuit and gg the output. Fig. 3 shows the arrangement of Fig. 1 in a simplified diagram, the modulators E, e for the two phases being denoted by circles enclosing arrows for indicating the direction of transmission. The thermionic amplifiers employed in the system are preferably of the duplex type with balanced input and output circuits. The circuits and apparatus used for de-modulating or detecting at the receiving station are identical with those shown in Figs. 1 and 3, except that when modulating, the input circuits FF, ff are separate and the output circuits GG, gg connected in series, whilst in demodulating the input circuits are connected in series and the output circuits separate. Figs. 15a and 15b show respectively the transmitter and receiver of a multiplex system, using the symbols of Fig. 3 for the modulators and de-modulators, and showing an amplifier as a circle and arrow, with one input circuit and one output circuit. Waves from two sources Aa, Bb are employed, each being split into two phases A, a, B, b, each phase carrying a separate message from the microphones W, w, X, x. For example, the microphone W is connected through line 123, transformer 126, and amplifier 128 to the modulator A, the output side of which is in the/ line 113. An artificial line 125 may be used to balance the line 123, so that the transformer 126 is unaffected by currents in the line 121. Further channels are provided for the microphones Y, y, Z, z by combinations of the A and B frequencies. For example, the microphone Y modulates the A phase of the Aa wave, and this modulated current modulates the B phase of the Bb wave, the resulting current being transmitted to the line 113 through the coupling 176. By employing further frequencies, the number of channels may be further increased. The composite current in the line 113 is amplified in the amplifier 114 and in the case of a wired wireless system is transmitted over the distant line 116 to the receiving line 117. The waves of the Aa frequency are transmitted through the tuned amplifier 148 and tuning-out bridges 190<a>, 190<b> to the input side of the de-modulators A, a. The waves from the source aA are split into two phases as at the transmitting-station, each phase serving to detect the current arising from the corresponding transmitter, thus rendering in the telephones W<1>, w<1> the messages spoken into the microphones W, w. The received waves of Bb frequency are passed through a tuned amplifier 147 and tuning- cut bridges to de-modulators B, b. The output circuits 284<1>, 285<1> carry telephone currents (which are heard in the phones X<1>, x<1>), and also modulated currents of the Aa frequency. The latter currents are passed through tuned amplifiers and are subjected to a second de-modulation by local currents of the Aa frequency, thereby separating and detecting the telephone currents for the phones Y<1>, y<1>, Z<1>, z<1>. In order that de- . modulating waves may be provided at thereceiver in strict synchronsm with those used at the transmitter, weak unmodulated waves of Aa and Bb frequencies are supplied to the line 116 through couplings 143, 144. These are selected by tuned amplifiers 145, 146 at the receiver and passed through filter circuits 196, 197. Each wave is then split into two phases in quadrature which energize a number of rotary fields. Coils. 162, 163 adjustably placed in these fields enable currents of correct phase and frequency to be supplied to the various de-modulators. The tuning-out bridges connected to the amplifiers 147, 148 have equal resistance in all the arms, whilst the opposite arms also have capacity and inductance tuned to the frequency to be suppressed. The tuning of the bridge 190<b> is to the B frequency and is broad enough to suppress both the pure B wave and its side bands. The bridge 190<a> is tuned sharply so as to suppress the pure A wave but to transmit its side-bands, the sharp tuning being affected by employing elements of large reactance. When the system is employed' for radio signalling, the composite current in the line 116 is employed to modulate a carrier wave, and at the receiver, this carrier wave is received and de-modulated and the resulting composite current is subject to the same series of separations and de-modulations as the current in the receiving-line 117. The carrier wave may itself be split into two phases, thereby doubling the number of transmission channels. The Specification describes a more complicated system in which a carrier wave and four sub-waves are used,. all being split into two phases, enabling 162 separate channels to be provided.