Patent GB-836994-A (GB)
Title: Signalling arrangements
IPC: H04Q1/442
Filed: 1956-06-07
Abstract: 836,994. Voice frequency signalling. SKILLMAN & CO. PTY Ltd., T. S. June 7, 1956 [June 8, 1955; Aug. 19, 1955], No. 17637/56. Class 40 (4). In a telephone transmission system forward signals are transmitted by a transistion from a steady level of A.C. on the line to another steady state and backward signals are transmitted by A.C. pulses. Alternating current from oscillator OF1, Fig. 1, is applied to the line over contacts A1, A2 when the line is idle and its removal in response to the operation of relay A releases relay B at the incoming end, Fig. 2, to signal the exchange thereat and operate relay Y. The subsequent impulsing of relay A in response to dialling is repeated by relay B to set up the call and a called party answer signal is returned over contacts 2FS from oscillator OF2 under the control of relay F. Relay 2FS is operated over the contacts of relay 2E which is slow-to-release and relay 3E is energized. The pulse transmitted, shown at Q, Fig. 4, is terminated when relay 2E releases. The operation of relay 3E prepares for the transmission of a backward clear signal NR, Fig. 4, by oscillator OF3 when relay F is released. Operation of relay 2FR at the outgoing end by the called party answer pulse energizes self-locking relay D to signal the exchange and slow-to-release relay C is released. Relay C which re-operates at the conclusion of the pulse is released again by relay 3FR on the receipt of the backward clear pulse and returns the system to normal by the release of relay D. Forward clearing is effected by relay B on the return of A.C. from OF1 to the line. A four-frequency receiver for such a system with provision for guarding against false operation by the inclusion of a limiter in a common amplifier and multi-winding signalling relays connected not to respond to the presence of more than one frequency is described with reference to Fig. 3 (not shown). Receiver, Fig. 5, which includes a limiter LT has a relay S operated by signalling current in tuned circuit TC1 and a guard relay G which prevents false actuation when the presence of other frequencies is detected by circuit TC2. In the relay circuit, Fig. 6, for the incoming end, relay S performs the function of relay B and a slowto-release relay TD which is operated by relay F provides a further safeguard against false release. The receiver, Fig. 5, can also be used with relay circuit, Fig. 7, at the outgoing end, strappings being provided for changing to frequency #2. Relay U changes the tuned circuits to receive the backward clear signal #3 when the called party answer signal is received. In a further arrangement, Figs. 9, 10, forward signalling is effected with low-level currents, -10 and -20 d.b.m., and backward signalling with pulses of higher level 0 d.b.m. The system described with reference to a two-wire line may be modified for four-wire working. The idle condition in which frequency #4 is transmitted over resistor R2, Fig. 9, and relay B, Fig. 10, is operated, is interrupted by relay A at the initiation of a call, and the release of relay B applies earth to the lead into exchange XB. Relay A also operates slow-to-release relay BA which prepares circuits for relay FA, FC and CA and increases the level of signalling current from OF4. At the distant end slowto release relay BB is operated by the release of relay B to prepare circuits for relay CB and a safeguarding earth. Interruption of relay A on the first dialling impulse operates relay CA to terminate the transmission path, and dialling impulses corresponding to the impulsing of relay A are transmitted over resistor R1 at -10 d.b.m. Relay B re-operates and energizes slow-to-release relay, CB to terminate the transmission path at that end. Called party answer relay F energizes slow-to-operate, slowto-release relay FFB, completes the earth-safeguarding circuit prepared by relay B and sends a signal from oscillator OF2 to the outgoing end. The pulse is terminated by the operation of relay FFB. At the outgoing end relay FF operates releasing relay FB and operating relay FC over a circuit prepared by relay BA. The circuit is guarded against false operation by a relay GA. The relay FC prepares a circuit for relay FA and a locking circuit for itself and changes over contacts to connect relay FF with the output of rectifiers #3 and the output of rectifiers #2 with relay GA. Relay FA operates and locks connecting earth to the exchange supervisory. Relays CA, CB, have by this time removed the line terminations and the connection is established. For backward clearance a pulse of current from oscillator OF3 is sent by the separate release of relays F and FFB, and relay FF is re-operated to release relay FC. Relay FA is released by the clearance of the calling subscriber and the retransmission of signalling current from oscillator OF4 clears exchange B. The system includes provision for an "end of selection " signal from exchange XB. The application of the invention to carrier telephony is also described with reference to Figs. 11-14 (not shown). In the first of the two arrangements described, channel edge signalling is used and in the second which uses the 'same carrier frequency for both directions low-level carrier is used for forward signalling and higher level pulses for backward signalling. Continuous low-level carrier is applied to the line when it is taken into use.