Patent GB-657776-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: DAVISON ALAN
IPC: H04Q1/446
Filed: 1948-07-13
Abstract: 657,776. Automatic exchange systems; current supply equipment. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. July 13, 1948, Nos. 18847/48 and 23615/49. [Class 40 (iv)] [Also in Group XXXVIII] Oscillation generators used for transmitting signals over one or more signal paths are connected to the signal path(s) and operate continuously but the flow of alternating current to the path(s) is prevented by the use of at least one symmetrical non-linear resistor, the operation of contact means serving to bias the non-linear resistor(s) so as to reduce the impedance and render it (them) highly conductive to alternating current. The invention may be applied either to the transmission of currents of a number of different frequencies over a single signal path or to the transmission of currents of a single frequency over any of a plurality of signal paths.  As shown in Fig. 1, alternating currents of a plurality of frequencies are transmitted over a plurality of signal paths, the circuit being particularly applicable to a telephone system having separate signal paths and speech circuit, one signal path being provided for 20 speech circuits and carrying 20 frequencies. The output of each oscillator is fed via a D.C. blocking condenser C to the parallel-connected variable resistors R1, R2, R3, each in series with a symmetrical non-linear resistor NLR1, NLR2, NLR3.. The side of NLR1 remote from C is connected over a front contact a1 of the relevant signalling relay to a potential divider P and thence to the signal path SP. NLR2, NLR3 are connected via leads 10, 11 and contacts of signalling relays of speech circuits which use the same signalling frequency on different signal paths, and connections are made over leads 12, 13 from contacts of signalling relays of speech circuits which use different signalling frequencies on the signal path SP, the tappings on P being adjusted in accordance with the attenuation at the various signalling frequencies. Bias is obtained from the battery connected to lead OUT through the choke L. Thus, with a1 open NLR1 is unbiassed and its impedance is sufficiently high to effect virtual disconnection of the oscillator, but when a1 is closed the direct current flow through NLR1 reduces its impedance and the signalling frequency is applied to the signal path. To obviate current surges (when the signalling contacts are operated) and pick-up between the leads the circuit is modified as shown in Fig. 2. The resistances R4, R5 are substantially equal as are R7, R8 so that the transmission circuit is balanced and on closure of a1 no direct current appears across TR1 or TR2. As no signalling current flows over a1 pick-up between the leads cannot take place. Common leads 21, 22 extend to other oscillators feeding the same signal path. Resistors R4, R5, R7, R8 may be dispensed with if contact a1 and earth are connected to centre-tapped windings of TR1 and TR2 respectively. A further modification for code signalling using six frequencies alone or in combination, particularly applicable to signalling between ticket issuing machines and a totalizator, is described with reference to Fig. 3 (not shown) (see Group XXXVIII).