GB806086A - Improvements in or relating to electric signalling circuits - Google Patents

Abstract

806,086. Automatic - exchange systems. STANDARD TELEPHONES & CABLES Ltd. Dec. 14, 1956, No. 38289/56. Class 40 (4) [Also in Group XL (c)] Signalling over a pulse channel is effected by causing signalling equipment (subscribers' dial contacts) to effect impedance changes at the remote end of the channel whereby power from the near end is reflected back to operate a detector (in a register) at said near end. As shown, Fig. 1, a voice frequency generator 6 feeds power to a modulator/demodulator 1 via hybrid transformer 8 which is normally balanced so that power from the generator 6 does not reach the detector 11. The signal then passes via a pulse channel over the path 3 to the demodulator/modulator 2 and thence to the line 4 via transformer 5. When the impedance of the line 4 is changed due to the opening of the subscriber's dial contacts, the consequent mismatch reflecas power back over the path 3 to the hybrid 8 and thence to the detector 11. The modulators/demodulators 1, 2 may be of the type described in Specification 753,645, [Group XL (c)]. In one method for effecting the mismatch, Fig. 2, a shunt rectifier 15 is normally back-biased by potential over the subscriber's loop, but when the loop 4 is opened by the dial contacts, the rectifier becomes conducting on alternate half-cycles to reflect a short circuit across the transformer 5. In a further method, Fig. 3, operation of the dial contacts is caused to increase the loop current to saturate the transformer 5 to produce the required mismatch. A small steady current may be supplied, by a battery 16, to the primary winding to balance out the effect of the loop current present in the secondary winding under normal speaking conditions. In a still further method, Fig. 4, the transformer 5 is shunted by the emitter-collector path of a transistor 18, so biased as to be non-conducting when the loop 4 is closed, but to be conducting when the loop 4 is open. In an automatic-exchange system embodying the invention, a calling subscriber 4a causes an allotter 22 to allot a pulse position to the modulator/demodulator units 2a, 1 to effect communication over a pulse channel on the path 3. Energy from the source 6 reflected due to the dialling impulses received, operates the detector 20 to set a register 21 with the identity of the wanted subscriber (say 4n). The allotted pulse is then fed to the modulator/demodulator unit 2n to effect communication between subscribers 4a, 4n and the link over path 3 to the register is released.