Patent GB-887948-A (GB)
Title: Multifrequency telegraph system including means for obviating the adverse influence of signal element prolongation
Inventor: DUUREN HENDRIK CORNELIS ANTHON
IPC: H04L1/00
Filed: 1959-07-17
Abstract: 887,948. Multiplex radio signalling. DE STAAT DER NEDER-LANDEN TEN DEZE VERTEGENWOORDIGD DOOR DE DIRECTEUR-GENERAAL DER POSTERIJEN, TELEGRAFIE EN TELEFONIE. July 17, 1959 (July 18, 1958], No. 24608/59. Class 40(5). In an electrical telegraph system in which signals are transmitted in the form of equal length signal elements, each comprising one of a plurality of different signal frequencies, no two successive signal elements are transmitted by means of one and the same frequency, and in order to reduce the adverse effects of signal-element prolongation; the receiver in response to reception of any one signal frequency is disabled from responding again to that frequency for a predetermined period which is at least equal to the duration of a signal element or the maximum expected element prolongation; whichever is the greater. The transmitter shown in Fig. 1 forms part of a diplex system in which the four possible mark-space combinations of the two channels are represented by frequencies f 1 -f 4  or an auxiliary frequency f 5 . The signals on channels TORI and TORII are respectively supplied to triggers X, Y which control static relays 121-124 to select one of frequencies f 1 -f 4 . Coincidence circuit 141 detects the occurrence of two successive signal elements in which the states of X,Y remain unaltered, and selects auxiliary frequency f 5 . If the states of X, Y are the same for more than two successive elements the sequence of transmitted signals would be, for example, f 2 , f 5 , f 2 . Alternatively more than one auxiliary frequency may be employed, in which case the sequence may be of the form f 2 , f 5 , f 6 . At the receiver the incoming signals are supplied. in parallel to band-stop filters 611-615 corresponding to frequencies f 1 -f 5  respectively, and to attenuator 616. Relays' 621-626 connect filters 611-615 and attenuator 616 to a common limiter 607 supplying narrow band-pass filters 631-635 corresponding respectively to f 1 -f 5 . The output of each filter 631-635 is rectified and fed to a bridge circuit 641-645 where the rectified output is compared with a combined noise signal derived from the other four signal paths. The outputs of circuits 641-645 control respective triggers T 1 -T 5 , only one of which can be "open" at a time. Assuming T 2  to be open, frequency f 1 , for example, can pass through filter 612 and relay 622, which is controlled by T 2 , and reach circuit 641 through limiter 607 and filter 631. A control pulse on line 510 reverses trigger T 1  to close the transmission path for f 1  since possible paths through 612-615 are now closed. If a signal element fails to arrive at the receiver, T 1 -T 5  are closed by the controlpulse on line 510 and T 6  is opened so that a path is provided through attenuator 616 for the next element to arrive. A further control pulse on line 511 operates that one of triggers A-E which is receiving an output from the corresponding circuit 641-645, and the appropriate triggers WI, RI, WII, RII are set to represent the corresponding mark-space combination. When trigger E is operated the states of the output triggers remain the same as in the preceding signal element. Each trigger A-E has a connection controlling the corresponding filter 631-635 to damp away any energy stored in that filter as soon as the signal element is registered at the output. Specifications 519,633, 632,774, 778,621 and 869,566 are referred to.