Patent GB-347280-A (GB)
Title: Improvements relating to electric signalling systems employing alternating currents
IPC: H04Q1/446
Filed: 1930-01-27
Abstract: 347,280. Exchange systems. SIEMENS BROS. & CO., Ltd., Caxton House, Tothill Street, London, and MOORE, A. T. C., 87, Boscombe Road, Southend-on- Sea. Jan. 27, 1930, No. 2718. [Class 40 (iv).] In systems using voice-frequency currents for calling or control and supervision the transmitter and receiver are coupled to the line so that the receiver is not affected by the transmitter, the same frequency being used for both directions. The invention is applicable to a purely manual system but is described with reference to a two-way junction between a manual office, Fig. 1, and an automatic office, Fig. 2. The transmitters, G, GB and receivers AV, RV, AVB, RVB are coupled to the lines with the aid of hybrid transformers T, TB and balancing networks BN, BNB. Rejector circuits RV, RVB keep the signal currents out of the operator's currents and provide a low impedance across the lines, so that apparatus beyond them can be ignored in designing the balancing networks. During conversation, the signal receiving circuit, Fig. 1, is disabled by the speech currents passing through a bridge rejector SJ, amplified at SAV and rectified at DP. Signal currents are unable to pass the apparatus SV as described in Specification 335,035, [Class 40 (iv), Telephones &c.]. Other arrangements as described in that Specification in which the bridge rejector is in different positions relatively to the receiver may be employed. At the automatic office there is provided in the path to the signal receiver a filter BP for blocking the usual tone signal received from the switches. Otherwise the receiver is the same as that at the other end. During sending, the line transformer may be disconnected from the switchboard As an alternative to the arrangement shown, the signal circuit may be connected to the outer ends of the transformer. Manual-automatic call. When the junction is seized, CO cuts off the incoming jack and Y disconnects 9 which is normally sending out 300-cycle current. Consequently ARB at the automatic end falls and ARAB pulls up to prepare the incoming selector. During dialling, Q at the manual and DB at the automatic end disconnect the D.C. impulsing currents from the junction to prevent interference with the A.C. impulses repeated by Y to ARB. The reply signal energizes YB whereupon XB, XAB pull up to complete the talking circuit and cut out the rejector RVB, and the tone generator GB is cut off. The consequent relapse of AR is followed by energization of ARA, X, AX to complete the talking circuit and cut off the rejector RV. If the line is busy YB repeats the flash signals over the junction and XA repeats them to the operator. When the called party clears, YB falls and then ARA, X, XA to signal the operator. When the operator clears, all relays at both ends resume their normal state. Automatic-manual control. The operator plugs into OGV<1> and COB, YB pull up, cutting off GB. At the manual office AR falls and ARA lights the calling and busy lamps. When the answering operator presses her speaking key, Y cuts off G and X, XA pull up, cutting off the rejector and completing the talking circuit. At the automatic office ARB falls and ARAB, XB, XAB pull up to complete the talking circuit at this end and signal the originating operator. When the answering operator restores her speaking key Y falls back and the calling operator receives a signal and when the called subscriber replies Y pulls up again. When the calling operator clears, COB, YB fall back and on the reapplication of the signal current the other operator receives a signal owing to the relapse of XA.