GB961205A - Improvements relating to automatic telephone systems - Google Patents

Abstract

961,205. Automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 4, 1961 [June 5, 1960], No. 19839/60 Heading H4K. A substation circuit includes a frequencyshift transistor oscillator VT1 powered by D.C. over the line for sending V.F. impulses under the control of dial springs DL, one frequency corresponding to " makes " and another to " breaks," and the particular permutation of frequencies represents the identity of the substation for party-line working or the class of service to which the subscriber is entitled or both. For this purpose a lock switch LS is provided having change-over contacts LSA, LSB operable by a key and terminals for crossconnecting to tappings on the primary winding of the output transformer TR1 for determining the frequencies in conjunction with a capacitor C12. The secondary of the transformer TR1 is connected to the line by the closure of dial offnormal springs DLA, and the insertion of a series resistor R3 reduces the line current sufficiently to release relay LX, Fig. 2, thus connecting in the V.F. receiving equipment. The latter comprises a 2-stage transistor amplifier VFA feeding tuned transformers TR3 ... TR6 which controls relays A ... D through rectifiers RB and transistors VT4, VT5. Relays A ... D each control a pair of relays, Fig. 3 (not shown), responding respectively to "make" and " break " impulses and feeding into a register which records the number of digital impulses and also the frequency permutation. The substation dial is preferably such that the first and last " make " impulses are lengthened. Instead of relays electronic bi-stable devices may be employed in the receiving equipment.