Patent GB-830408-A (GB)
Title: Improvements in or relating to telecommunication systems
IPC: H04J3/06
Filed: 1956-05-09
Abstract: 830,408. Multiplex transmission systems. SIEMENS & HALSKE A.G. May 9, 1956 [May 9, 1955], No. 14471/56. Class 40 (4). [Also in Group XL (b)] In a telecommunication system having synchronizing signals transmitted in addition to signals carrying the information, the synchronizing signals are transmitted as the displacements of the phase of a carrier through a predetermined angle different from 180 degrees. The predetermined angle is preferably 90 degrees, and the system may be used for printing or facsimile, telegraphy, telephony or radio communication. To facilitate the detection and separation of the synchronizing signals, information signals are not transmitted immediately prior to each synchronizing signal. In the telegraphy system described, marks and spaces are transmitted as carrier phase displacements of 180 degrees and the phase of the carrier is displaced by 90 degrees to transmit the synchronizing signal. In the diagrammatic transmitter arrangement, Fig. 1, a fixed frequency oscillator 2 controls an impulse generator to operate a distributer to apply the signals for a number of channels and also operates a generator 3 of saw-tooth waves amplified at 4 and applied to a modulator 5. The output of a generator 1 of the carrier frequency comprises two voltages U1, U2 with a phase difference of 90 degrees and the output U1 is modulated by the saw-tooth voltage so that the output of modulator 5 is in phase with U1 during positive portions of the saw-tooth wave and is 180 degrees out of phase during the negative portions. At the times when the saw-tooth voltage is zero the output of the circuit arrangement 6 is in phase with the input voltage U2 of the generator 1. During the gradual increase of the saw-tooth voltage the carrier which is subject to a slow phase displacement may be modulated at 8 with signals applied over inputs R1 and amplifier 9 and the 90 degrees phase shift of the carrier takes place when the saw-tooth voltage drops suddenly to its initial value. Detailed transmitter arrangement, Fig. 6. The carrier frequency generated by a valve V4 and a tuned circuit L1, C1 is applied to the modulator 5 and to the transformer T1 which also receives the 90 degrees phase-displaced carrier frequency from a small resistor W1. The generator 2 comprises the valve V1 back coupled through a transformer T2 and tuned circuit L2, C2. The pulses from the winding I are fed to terminals K1 connected to a distributer and from winding II to a frequencydividing circuit comprising a valve V2 with a tuned circuit L3, C3 in its anode connection and a resistor W2 in the input circuit of the grid which receives a large positive pulse when the maximum voltage in circuit C3, L3 coincides with a maximum voltage from the winding II. The flow of grid current causes a voltage drop across resistor W2 and charges the capacitor which applies to the grid an additional negative bias which is reduced as the capacitor discharges and is such that it prevents the grid from becoming positive again until the next two maxima coincide. The pulse at the cathode of V2 is applied over line L3 to terminal K2 to synchronize the distributer with the saw-tooth voltage. A capacitor C4 charged through W4 is rapidly discharged when the valve V2 becomes conductive and a saw-tooth voltage is applied over conductor L5 to the grid of valve V3 from which a saw-tooth wave symmetrical with regard to a zero line, is received at the junction of resistors W5, W6 and applied to the modulator 5. The resultant sum voltage is applied to the modulator 8 to the terminals K3, K4 of which the information to be transmitted is applied. Diagrammatic receiver arrangement, Fig. 4. The incoming signals at K12 are fed over filter 11, limiter 12 and amplifier 13 to a demodulator 14 which at terminal K6 receives the transmitted carrier. The output of the demodulator 14, which is reduced to zero when the phase of the carrier is displaced through 90 degrees is fed to channels 15 by a receiving distributer, and also through low-pass filter 16, rectifier 17, filter 18 and amplifier 19 which generates a pulse when the output of 14 is reduced to zero. A relaxation oscillator 21, controlled from the output of an oscillator 20 driving the distributer generates rectangular pulses of a period equal to that of a cycle of the distributer, and if the receiver is in synchronism, the pulse from 19 is located symmetrically with respect to the positive to negative transition of the rectangular voltage generated by oscillator 21. If the symmetrical relation does not exist, a comparator circuit 22 generates at capacitor 23 a positive or negative voltage to control the frequency of the oscillator 20 to produce the symmetrical synchronous condition. The detailed circuit arrangement, Figs. 5, 7 (not shown), is generally similar to that described in Specification 832,632. Modified circuit arrangement for obtaining 90 degrees phase-shifted carrier, Fig. 8. The carrier voltage from a generator 82 is fed to the primary winding of a transformer T7 and with a phase shift of 90 degrees by network 81 to the primary winding of a transformer T8. The transformers have secondary windings I, II, III and IV and if solely the rectifiers R1, R2 are conductive, the carrier without phase displacement is fed to winding 1 of transformer T9 and thence to terminals a, b. If solely rectifiers R5, R6 are conductive, the carrier displaced through 90 degrees is fed to winding 2 and terminals a, b. By means of rectifiers R3, R4 and R7, R8 together with the crossover connections, the carrier with phase displacement of 180 and 270 degrees is fed to windings 3, 4 respectively of the transformer T9. The pairs of rectifiers R1, R2 ; R5, R6 ; R3, R4 and R7, R8 are made conductive in sequence by a chain of cold-cathode tubes V14 ... V17 in conjunction with timing pulses applied at terminal K16. With the tube V14 conducting the remaining tubes V15 ... V17 are cut off and forward bias for rectifiers R1, R2 is provided by current flow through resistors W7, W8. A reverse bias is produced over resistor W9, which, as in the case of resistors W10, W11 and W12, is of much higher value than W8, so that the rectifiers R1, R2 conduct and pass the non- displaced carrier until a further pulse at K16 makes V15 conducting so that rectifiers R5, R6 become conductive and the carrier with a phase displacement of 90 degrees is passed to the terminals a, b.