Patent GB-652867-A (GB)
Title: Improvements in telephone transmission systems
IPC: H04Q1/453
Filed: 1947-05-05
Abstract: 652,867. Voice-frequency signalling. PUNCH ENGINEERING PTY., Ltd. May 5, 1947, No. 12081. Convention date, May 6, 1946. [Class 40 (iv)] In a telephone transmission system which may be a carrier system and in which an alternating current signalling circuit is common to a plurality of telephone circuits and conveys simultaneously the necessary signals for establishing and supervising calls over the associated telephone circuits, a continuous pilot signal of specified frequency and independent of any other signal is transmitted in each direction over the signalling circuit. Failure of either pilot signal resulting from a fault in the signalling circuit causes one of the associated telephone circuits to be seized and substituted for the signalling circuit. On a call occurring in an auto-manual system from the manual exchange, the operator plugs into, e.g., jack 4 connected through terminating equipment 7 to a four-wire telephone circuit 1. Sleeve relay CA pulls up and connects a generator 15, generating a voice-frequency characteristic of the circuit 1, through a filter 20 to the signalling line 44. At the far end this frequency passes through a filter to a detector to operate a corresponding relay CAX, thus closing a loop out to the automatic equipment to prepare for reception of the dialling impulses. When the operator dials, relay CA impulses the output from generator 15 and causes relay CAX to vibrate to set the automatic equipment. When the called subscriber replies the operation of relay SAX connects a voice-frequency generator to the far end of the signalling line 44, and this voice-frequency passes through an amplifier 25, and a filter 26 to a detector 31 and operates relay SA, which short-circuits a resistance 10 and causes a marginal relay in the operator's cord circuit to pull up and control a supervisory lamp. Ringing. The operator connects a 16-cycle generator to the tip and ring of the cord circuit to vibrate a polarized relay RA which interrupts the output of the generator 15 to cause relay CAX to vibrate. Relay RAX is maintained operated during ringing since its earth and battery windings are alternately connected by a contact of relay .CAX to a circuit comprising an inductance 41 and a condenser 42 tuned to 16 cycles/second. Operation of RAX connects a 16-cycle generator 40 to the outgoing loop. Trunk offering. Battery placed simultaneously on the tip and ring of the operator's cord circuit operates relay TOA to release sluggish relay TOT. Relay TOS pulls up for the release period of TOT, thus causing an interruption in the signalling current from the generator 15 of a predetermined length during which relay TOX connects battery to one of the line wires to control the automatic equipment to enable the operator to break in on the existing connection. Pilot signals. A voice-frequency pilot signal is continuously transmitted over line 44 from a generator 14 through a filter 19 and maintains relay PX operated. Similarly an incoming voice-frequency pilot signal passes through a filter 30 to a detector 35 and maintains relay P operated. The detectors for the pilot signals fail to respond with smaller variation in line attenuation than is acceptable to the other detectors. On a fault occurring, the release of relay P operates relay PF which holds over its own make contact and a reset key 36, and which actuates an alarm bell 37 and a lamp 38, and completes circuits for relays CO connected in parallel. Operation of the latter connects the near end of circuit 3 in place of the line 44, and so breaks the transmission of the outgoing pilot signal. Relay PX releases, PFX operates and holds over a reset key, and COX pulls up to complete the substitution of the circuit 3 for the signalling line 44. Operation of CO also connects earth to the sleeve of jack 6 and thus busies the circuit 3. Voice-frequency telegraphy. Impulses of voice-frequency current from a generator 18 are sent out over line 44 by the intermittent actuation of relay TAS. Incoming impulses are received through a filter 29 by a detector 34 which operates relay TAR.