Patent GB-803977-A (GB)
Title: Improvements in or relating to a party-line telephone system
IPC: H04M11/04
Filed: 1956-03-16
Abstract: 803,977. Party-line systems. SKOGSBERG, E. L. March 16, 1956 [April 2, 1955], No. 8393/56. Class 40 (4). In a party-line system each substation comprises a transistor oscillator for generating a frequency characteristic of the substation whereby the latter may be identified at the exchange. As shown, Fig. 2, two transistor oscillators TR1, TR2 are provided (synchronized over a common resistor r22), each operable to battery applied at the exchange between the respective legs of the line and earth, thereby ensuring transmission even if one of the legs goes faulty. The oscillators have rectifiers in their emitter circuits so that they operate to only one direction of applied battery. When a calling subscriber's hook-switch Kl is operated, the consequent operation of the subscriber's line relay CR1, Fig. 3, initiates the operation of the line-finder LF and when this switches through, R3 operates, followed by R2 and R4. The former reverses the polarity of the battery applied to the line conductors to cause the transistor oscillator to oscillate. The resulting oscillation is selected in the exchange by the filter unit BP and caused to operate an associated relay F1. This in conjunction with a relay LR1 indicative of the party line brings up a lamp IL11 (say This lamp remains lit over holding means, not shown. Relay R4 causes R5 to operate slowly, after which R2 and R4 release, the latter connecting up talking equipment T. The operator now throws a key LFG to connect up a frequency associated with the calling substation to operate therein a tuned relay SR1 which connects polarized relay SR2 across the line. The latter operates, locks and connects up the substation speech equipment T. The call is then completed in the usual manner. In the case of a call to the subscriber, the operator operates a key P1 or P2 to effect operation of the line-finder and also bringing up R1. A frequency is applied from key LFG as before to operate SR1 which connects up the bell which is rung over the phantom. When P1 or P2 is released R1 releases to disconnect the ringing current. When the called party answers connection is set up as previously described. In the case of an automatic exchange, when the calling party effects the operation of a relay such as F1, this causes the transmission of the corresponding frequency for operating SR1 in the substation. The combination of relays F1 and LRI may also be used to switch in the corresponding subscriber's meter. In a modification, Fig. 1a, only a single reversal of polarity may be effected by the exchange. Thus in the normal state rectifier L3 is biased off and condenser CA3 is charged via rectifier L2. When the burst of identifying frequency is required the polarity is changed so that L3 conducts to provide the requisite collector bias, and L2 cuts off, emitter bias being provided from condenser CA3 until the condenser is discharged. In a fire-alarm system, Fig. 1 (part), operating to an exchange of the latter type, the release of a fire-alarm relay AR, or of a fault relay FR, connects inductance I across the line to give a calling signal. When the exchange reverses the line polarity CR2 operates and during the slow release period of CR1 connects up the locally powered transistor oscillator OSC. The latter may have its frequency varied according as AR or FR is released, or one of the relays may introduce interruptions in the oscillations.