Patent GB-395289-A (GB)
Title: Improvements in or relating to carrier current signalling and distant control systems
IPC: H04J1/14
Filed: 1932-01-29
Abstract: 395,289. Wired wireless systems. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, SMART, N. C. and WESTELL, E. P. L., c/o General Electric Co., Ltd., Stoke, Coventry. Jan. 29, 1932, No. 4898/33. Divided on 395,053. [Class 40 (v).] In a carrier current supervisory and/or control system, which may have telephone facilities, comprising a plurality of stations connected in parallel by line wires, certain classes of station apparatus are given priority of engaging the channel and may break down an existing connection of minor importance. The figure shows the selective equipment at one station. The banks of selector 1DM have access to a plurality of groups of terminals 1 .. 13, each group being connected to extension circuits, e.g. telephone, supervisory or control circuits. Moreover a group of terminals may be employed for apparatus for coupling two carrier systems as described in Specification 395,045. The apparatus shown in connected at its left-hand side to a carrier receiving and transmitting set of known kind and comprising a transmitting oscillator adapted to transmit either of two frequencies depending upon whether the station initiates or receives a signal, a modulator, and a demodulator tunable to either of the two frequencies and means for coupling the equipment to the line wires which may be of the type described in Specification 380,539 wherein a coupling network comprises a capacity in series with an inductance connected in series with a parallel connection of capacity and inductance. Initiating a telephone call. A telephone call is initiated upon removal of the receiver 20 from the hook, the hook switch closing a circuit for relay ST via the dial springs 22. Relay ST operates vibrating relay V which operates the stepping magnet of switch 2DM which steps and applies a test to the line in the manner described in Specification 361,558, [Group XXXVIII]. Assuming no calling carrier frequency from another station exists on the line and therefore a free line, relay T in the demodulator circuit is de-energized and' consequently relay A remains de-energized. Operation of the switch 2DM therefore energizes relays FC, TS which operate relays over leads 32 .. 36 which starts up the transmitter oscillator and tunes it to the calling frequency, switches the carrier current on to the line, and changes over the modulator and demodulator to the calling and receiving frequencies respectively. When the switch arrives at a contact characteristic of the station, the carrier calling signal is terminated by the release of relays FC, TS. The particular contact selected determines the priority of the station, different contacts at the different stations being employed for the purpose of preventing more than one station seizing the line and transmitting signals simultaneously. The release of the relays FC, TS restores the tuning of the demodulator to the calling frequency. If at this time another station is transmitting, the relay T is operated and relay A prevents further operation of relays FC, TS. If however the line is free, these relays reoperate upon continued operation of the switch 2DM to prepare the oscillator and modulator in the manner previously described. The switch 2DM closes a circuit of the stepping magnet 1DM over wiper 2DM3 and the switch 1DM steps by self-interruption up to a contact from which marking is removed by the operation of the telephone hook. Relay C which is operated during stepping, releases to operate relays M, O, LO, 2EX over the banks of the switch. Relays M, O, effect the locking of relays FC, TS, the relay D also operates DT which interrupts its own circuit at a rate of 33 cycles per second to produce " dial " tone in the receiver 20 at contacts dt<2>, dt<3>. The relay 2EX connects the receiver to the incoming leads 40, 41, prevents operation of the " engaged " signal 29, and applies +ve battery over 1DM6 to terminals 13. The digits of the wanted station are now dialled, operation of the dial springs interrupting the circuit over wiper 1DM3, relay D, lead 36 to transmit impulses of calling frequency to operate selectors at the other stations and effect calling of the required station. When the correct station is selected, this station transmits carrier current of answering frequency back to the calling station to operate relay T. Relay A then operates B which releases OT to terminate the " dialling " tone. Conversation may now take place, the calling station speaking to the distant station by means of the calling carrier frequency and the distant station answering over the answering carrier channel. If an extension connected to another group of terminals 1 .. 13 initiates a call during the progress of conversation, operation of the telephone hook energizes relay ET via contact ex4 of that extension, and the relay vibrates to send an " engaged " tone via condenser 27 to the second subscriber. Receiving a call. When a station is in normal condition, the demodulator circuit is tuned to the calling frequency. Upon reception of such frequency from a distant station, relay T operates A which energizes B. During the break in transmission following the linetesting period, T and A release causing operation of stepping magnet 1DM. Relay B is slow releasing and prevents operation of TS, FC by a caller at the receiving station. After the testing period, calling frequency transmission is resumed and relays T, A again pull up and magnet 1DM is released to step the switch and thereby operate H. Relay H disconnects relay ST and prepares ET, disconnects OT and at contact h<4> operates all the " engaged " indicators in the extension circuits via contacts 1ex1, 2ex1, &c. Upon receipt of the dialled impulses, relays T, A are impulsed to step up the switch 1DM. At the end of the first train C releases and provided the impulse train agrees with the switch marking, +ve is picked up in bank 1DM1 to maintain the relay H operated during the inter-train pause. The second impulse train operates the switch in a similar manner to position the wipers in accordance with the incoming signal. At the end of impulsing, C releases to allow LO to pull up and disconnect the stepping circuit and to operate the extension relay, e.g. 2EX. Relay M also is operated to energize relay TS which causes the carrier transmitter to transmit the answering frequency. Relay LO in addition to disconnecting the stepping magnet, switches over the impulsing circuit so that the system may be adapted for supervisory control wherein further signals may be directed directly to the extension without affecting the switch. The extension relay, e.g. 2EX, disconnects the " engaged " indicator, connects the receiver 20 to the incoming speech leads 40, 41, energizes relay W which effects stepping of switch 2DM under the control of the vibrator V. Wiper 2DM4 impulses the trembler bell on the called extension, the bell causing interruption of the current through resistance 30 at audible frequency. The voltage developed across the resistance modulates the outgoing answering frequency to inform the calling party that the bell at the called extension is ringing. This condition is maintained until the called party lifts the receiver to short-circuit the resistance 30. The switch hook also energizes relay D which operates DA to open the circuit of relay W and trembler bell. Release of W ensures that the switch 2DM stops in home position. Conversation may now take place between the calling and called parties. Release. The system is arranged for calling party release. When the calling party hangs up his receiver, calling current is switched from the line causing release of T at the called station. In consequence, A, B falls back to cause switch 1DM to drive to normal. During hunting, C operates to effect release of 2EX and M, the latter disconnecting TS, D and restoring the equipment at the called station to normal. At the calling station relays T, A, B are released if the answering carrier current from the called station is switched off the line but FC will remain operated. When the calling party hangs up, stepping of the switch 1DM is initiated over the bank 1DM2 from +ve picked up from the extension. Relay C also is operated to effect release of FC which switches the stepping magnet over to the homing bank 1DM1. If answering frequency is still on the line when the calling party hangs up relays T, A are held operated but after a short time D releases FC which changes the tuning of the carrier frequency to the calling frequency thus releasng T, A, B, and completing the homing circuit of the switch over 1DM1. Priority of important signals. The Figure shows a telephone extension provided with a priority key 42 facilitating the breaking in on an existing connection. Depression of key 42 operates relay CA which connects vibrator P of 400 cycles per second to the line to transmit a modulated answering frequency which is received by the calling party of the engaged pair who then releases the line. A similar priority is afforded to supervisory apparatus. If when line is engaged a supervisory signal is initiated, +ve potential is applied to terminal 11 and relay CA is operated to connect the vibrator P to line. To prevent the breakdown of an existing important signal on the line by a similar signal, the signal initiating potential is derived over the commoned contacts in the bank 1DM6. Commoning is omitted from the contacts corresponding to apparatus of prior importance either at the local station or other stations so that as the 1DM switches at all stations are positioned on similar contacts relay P can only be energized in the case of an existing connection being of subsidiary nature. In a modification, the current emitted by the relay P operates a relay at the calling station of an existing connection to effect release of relay FC and remove current of calling frequency from the line. Removal of this curr