Patent GB-637289-A (GB)
Title: Improvements in signal transmission systems
IPC: H04Q9/08
Filed: 1948-05-31
Abstract: 637,289. Electric selective signalling. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd., WALMSLEY, T., and LEIGH, R. C. May 31,1948, No. 14686. [Class 40 (i)] A cyclic succession of trains of different frequencies is fed to an indicator or recorder directly over a first path, the recorder or indicator being connected to the source of signals over a second path which includes a series of switches actuated by condition responsive elements and each associated with one of the different frequencies, so that the signals over the second path depend on the position of the switches and by re-inforcing the effect of the signals over the first path indicate the position of the switches. The recorder may be actuated only when one of the condition responsive switches is actuated. The switches may be actuated by temperature-responsive bi-metallic strips or by pressure-actuated snap diaphragms. As shown in Fig. 1, SCG develops a succession of trains of different frequencies which are fed over attenuators At1, At2 and a network N favouring the individual frequencies to a detector D1 and a milliameter SID; the generator SCG, also feeds a line OP connected to line IP through filters F1, F2 ... Fn tuned to the different frequencies by associated switches S1, S2 ... Sn; line IP thus feeds amplifier AMP, detector D2 and the milliameter SID with such frequencies as correspond to actuated switches ; the milliameter deflection is thus increased in accordance with the closed switches as shown in Fig. 8. As shown in Fig. 2, the oscillations from SCG are successively modulated on to a carrier at M and applied over line SL to switches S1, S2 ... Sn; if, e.g. S1 is closed, the oscillations are rectified at CD1 and applied over a filter F1, selecting the frequency associated with S1, for feeding back to line SL, passing through low-pass filter LP to the recorder or indicator SID. The carrier may have its frequency varied between each complete cycle of modulation, each of the switches S1, S2 ... Sn being fed through a filter selecting the appropriate carrier ; in this way, the number of switches can be increased without extending or crowding the range of audio-frequencies, Fig. 11 (not shown). The generator SCG may comprise a plurality of generators cyclically switched into circuit, Fig. 3 (not shown), or a single generator whose tuning element-is continuously rotated, Fig. 4 (not shown). The existence of a signal over the second path may actuate a relay A, Fig. 7, to energize relay,B which releases relay A and locks itself and closes switches for an alarm bell AB, a signal lamp AL and the recording device DM, RMA. The recorder will operate until switch RSS is opened. In Fig. 9 a signal from detector D2 actuates relay K which energizes relay U which operates relay V before being released by the release of relay K. Relay V locks and actuates relay J which closes contact j<SP>1</SP> in the circuit of a two-coil recorder RMAI. As signals come in from SL, relays K and U are actuated by each signal to open contacts u2 and close contacts u3 in the recorder circuit and thus reverse the direction of recording to produce, e.g. a record as shown in Fig. 10 ; an alarm lamp AL is lit by relay V, and recording continues until switch RRS is opened.