788,929. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Nov. 26,1954 [Dec. 3, 1953], No. 33617/53. Class 40 (4). In an automatic exchange the supervisory signals are transmitted over a phantom circuit on the two-wire speech circuit, each such signal consisting of an electrical pulse which occupies a time position in a repetitive cycle of time positions which is individual to that particular signal. General description. The invention is described as applied to the system of Specification 788,592 in which the selectors and line finders are cold cathode gas-filled discharge-tubes of the type described in Specification 734,345, [Group XL (a)]. Such a tube has two closely adjacent anodes and a single cathode equidistant therefrom and is such that when it is conducting in both anode-cathode gaps, substantially noisefree speech transmission can occur between the two anodes. This arises because the current flowing in the cathode circuit remains substantially constant, so that if the current flowing in one anode circuit is altered, as by a speech input, a corresponding alteration in the opposite direction takes place in the current flowing in the other anode circuit. The tubes used are preferably of the multiple type described in Specification 788,591, and have one anode of each pair commoned, and the cathodes commoned. In the normal state, the tubes of a selector have a voltage between anodes and cathodes insufficient to cause breakdown; on seizure the common cathode potential is depressed and then, in a particular time position in a repetitive cycle which is individual to the individual anode to which connection is desired, the potential of that individual anode is raised. The tubes then fire across the relevant gaps. First embodiment-signals repeated at each selection stage. In the idle condition of a subscriber's line, rectifier MR1 in its line circuit (Fig. 1) conducts and the base of transistor G1 goes negative so that the gate formed by G1 is open. Negative pulses PI and P2 (see Fig. 9) applied to the collector of Glare therefore induced into the phantom circuit of the speech path via transformer TF1. These pulses are, however, ineffective as no line finder or final selector tube is ionized to the line. When the subscriber lifts his handset, current flows in the loop and the right-hand end of R1 (Fig. 1) goes positive, thus blocking MR1 and closing the gate G1. At the same time the negative potential developed at the right-hand end of R4 gates a negative pulse PL (whose time position characterizes the calling line among those served by the group of line finders) to the call detector gate shown in Fig. 2. This consists of the transistor CD which is normally non- conducting, but conducts when a negative PL pulse occurs on its base and gates the pulse to TF3 which phase inverts it and returns it to the line circuit as a positive pulse. There it is gated by a positive PL pulse to the phantom circuit and hence to the anodes of the line finders and final selectors individual to the caller. The line finders are primed singly and successively by negative pulses PF applied via R5 (Fig. 3) to the common cathode, and the line-finder having access to the calling line fires at the appropriate anode gaps as described in Specification 788,592. Rectifiers MR5A (Fig. 1) and MR5 (Fig. 3) conduct and busy the line and linefinder respectively. A free register is selected as in the last-mentioned Specification and returns dial tone via winding PW1 of TF4 (Fig. 3). The first " break " impulse of the caller's dial re-opens gate G1 (Fig. 1) and pulses P1 and P2 are sent over the phantom to the line-finder where they may be used to control timed release, if the loop is opened for an excessive period, in known manner. In the line finder the base of transistor G2 is normally positive and G2 non-conducting. When the line-finder is seized, gaps in LFA and LFB fire, the base goes to about - 15 v. due to current flow in R6, but G2 remains non-conducting. When the caller dials, the P1 and P2 pulses appear as negative pulses across R6 and each depresses the base of G2 to about - 30 v., whereupon the gate G2 is opened for the duration of each P1 and P2 pulse. As only P1 pulses are applied to its collector, G2 passes only the P1 pulses which are phase-inverted and sent forward over the phantom by TF6 (Fig. 3). To repeat the dialled digits to the register, use is made of the bi-stable pair of transistors T1-T2. In the idle condi. tion, T2 is conducting, and on seizure of the line finder, coincidence of the negative potential across R6 with the first locally produced pulse P1 causes T1 to conduct, T2 becoming non- conductive due to common collector resistance R9. During the first dialled impulse the P1 pulses are gated by G2 to the primary of TF5, so that positive P1 pulses are applied to the T1 gate; whereas the first negative PI pulse gated to lead L1, in combination with a local P1 pulse, triggers T2 into the conducting state. T2 remains conducting as long as G2 passes PI pulses and the positive potential developed across R10 is applied to the register, thus repeating the dialled impulse thereto. In every case, the register seizes the appropriate outlet from FSA-FSB by a pulse applied over R11 (Fig. 5) to the relevant individual anodes. Relay H pulls up and primes GSA, GSB. The negative potential developed across relay H by each P1 pulse sent forward renders G3 conducting so that P1 pulses are then applied via TF7 forward over the phantom. Local call. Seizure of group selectors continues until the final selector (Figs. 6, 7) is seized and the called line tested and seized, if free, as described in Specification 788,592. With the called line idle, T6 of the bi-stable pair T5-T6 (Fig. 6) is conducting and the positive potential on its emitter is applied to MR 15. Ring control interrupter pulses of a pulse repetition rate of 20 c.p.s. are applied to MR16 so that once per 50 m.secs. the gate G5 opens and passes P4 pulses via TF10 and TF11 to the called party's line circuit (Fig. 1) which includes the bi-stable pair T7-T8, of which T8 is normally conducting. The received P4 pulses are applied to MR17 and in combination with local P4 pulses on MR18 render T7 conducting. Relay RG pulls up and connects the ringing generator to the called party's line. The P1 and P2 pulses normally passing via gate G1 (Fig. 1) over the phantom are developed across R17 (Fig. 6) in the final selector so that gate G6 reverts P2 pulses via TF12 over the phantom, which are repeated in the group selectors by gates such as G4. In due course P2 pulses reach the line finder (Fig. 4) and, with relay RL (not shown) operated from the register when selection commences, as described in Specification 788,592, cause T13 to deliver an output thus changing over the pair T10-T11 so that T11 conducts. Gate G8, which has been reverting P3 pulses over the phantom, is thus closed and the cessation of these pulses indicates that the called party is being rung. On the reply of the called party, coincidence of the negative potential developed across R19 (Fig. 1), a negative peak of the ring- ing cycle, and any P pulse renders T8 conducting, whereupon RG releases and the connection is established. The gate G1 is closed (called line looped) and the supply of P2 pulses to G6 (Fig. 6) and to the cord circuit (Fig. 4) ceases. Tn the final selector, gate G6 is closed, and coincidence of a local P2 pulse, absence of an. inverted pulse from TF13, and negative potential across R17 causes T5 to conduct, thus closing gate G5, and stopping the supply of P4 pulses to the called line circuit. Outgoing call. In this case the junction circuit (Fig. 8) is seized and HO pulls up, whereupon coincidence of a local P1 pulse, negative potential over front HO, and absence of an incoming inverted PI pulse, causes T3-T4 to change over, so that T3 conducts and brings up relay A which closes the forward loop. HO also extinguishes tube GT1. The " break " of the next impulse dialled causes P1 pulses to be sent forward over the phantom from the calling line circuit, the first of which is applied to MR 12 and coinciding with a local P1 pulse changes over T3-T4, so that T4 conducts and A is released. A therefore repeats the digits as loop impulses over the junction. Pulses P2 are normally returned. over the phantom, but operation of D, when the called party answers, cuts off the supply. The relapse of HO when the caller hangs up, renders T4 again conducting, whereupon the release of A opens the forward loop, and completes a charging circuit for Cl. After about 300 m.secs., during which the junction is maintained busy, GT1 fires and the junction is available for another call. Second embodiment-end-to-end signalling. As the attenuation per stage is low, pulses can be passed through as many as six stages without appreciable distortion. The circuit of Fig. 10 is seized from a line-finder (as in Specification 788,592), selector FSA-FSB is primed by operation of relay H (not shown), and dial tone is reverted from the register (not shown). An outlet in the wanted direction is seized, HA (Fig. 11) operates and primes the next selector, and the connection is set up as before. It is assumed that three conditions have to be signalled forward, and three back. Considering the forward signals, a set of gate transistors G15, G16, G17 (Fig. 10) is provided for each cord circuit, each gate having negative pulses occurring in one time position. applied to its collector viz. P1, P3, P5 respectively. Sending is controlled by applying a negative potential to the relevant base to open the gate and send pulses forward over the phantom. At each stage the phantom passes the speech path transformer by a transformer such as TF26 (Fig. 11) and the pulses reach the receive end, e.g. a circuit (Fig. 12) associated with a final selector. The received pulses are applied in parallel to three bi-sta