Patent GB-657069-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: DAVISON ALAN
IPC: H04M19/02
Filed: 1948-10-05
Abstract: 657,069. Current supply equipment. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Oct. 5, 1948, No. 25927. [Class 40 (iv)] The normal ringing machine of a telephone exchange is replaced by equipment wherein the ringing current and other signals are generated by or under the control of electronic oscillators, and the cam-operated spring-sets which normally determine the lengths and spacings of the various signals are replaced by a relay set. Ringing current, tone and meter pulse group for small automatic exchange (Figs. 1-4). This is intended for an exchange serving up to a few hundred subscribers. The X inserted in certain circuit leads extending to earth indicates a normal disconnection point which is closed when the exchange equipment is taken into use by a subscriber. Relay set controlling connection of tones and signals to the signal leads. This relay set operates under the control of two inter-acting relays PA and PB (Fig. 1), which are shunted by QA and YC, and QB and YD respectively to render them slow to release. These relays are arranged so that the contacts of PA are alternately operated and released for periods of substantially 200 m.secs. and it is assumed that PB functions in a complementary manner to PA so that earth pulses of 200 m.secs. duration are applied alternately to the flicker earth leads FE1 and FE2 (Fig. 4). The successive operations and releases of PA control the conditions of relays CA, CB, CBR, CC, EA, EB, EC, ED and SP which perform a cycle of operations during each 3.6 seconds period. On the first operation of PA, CA pulls up; and on the first release CB operates in series with CA and brings up CBR, followed by EA. The second operation short-circuits CA, CB holding over its right-hand winding; and on the second release CB, CBR fall away and CC, CA and SP (Fig. 2) pull up. On the third operation CB and CBR re-operate; the third release bringing down CA by short-circuit. When PA comes up for the fourth time CB, CBR fall away, whereupon CC releases (short-circuit), EB pulls up, and CA is re-operated. The next release of PA again operates CB and CBR, EC pulls up and releases EA. The fifth operation short-circuits CA which releases; and on the next release of PA, CB and CBR fall away, CC re-operates, SP is released and CA again pulls up. The cycle continues, CA and CB being repeatedly operated for 400 m.secs. and released for 200 m.secs., CB being delayed by 200 m.secs with respect to CA. Relay CC is alternately operated and released for periods of 600 m. secs. Apart from these three relays, the relays controlled by PA remain in the same condition after the fifth release of PA until the seventh operation thereof. The release of CB and CBR which then occurs brings down CC (short-circuit) whereupon CA re-operates and EB falls away. The re-operation of CB and CBR on the seventh release brings up ED which releases EC. Relay ED remains up until the commencement of the next cycle when the tenth operation of PA brings down CB, CBR and CC, and ED follows. Production of S, Z and meter pulses. These pulses are controlled by two. so-called multirelay units MRA and MRB (Fig. 2) which may be of the type described in Specification No. 591,655, [Group XXXVII], each comprising four operating coils controlling separate contact springs. The S pulse is produced by the operation of AW during that 400 m.secs. period towards the end of each cycle when CA, CC and ED are up together. The periods during which each relay of the control relay set is operated and during which certain signals are applied to the corresponding leads are shown in Figure 6. The Z pulse of 2.4 seconds duration is produced while AX is operated by any of the relays EA, EC and SP being up. Meter pulses for from one to six units are applied to leads 1U to 6U under control of the relay set. Distribution of ringing current and tone potentials to the signal leads. It is assumed at this stage that continuous ringing potential is provided by transformer TRA (Fig. 4) and is extended directly to the continuous ringing lead CR. This potential is extended to interrupted ringing lead IR1 in the form of two 400 m.secs. bursts separated by an interval of 200 m.secs. during each 3.6 seconds cycle, the bursts occurring when CB and EA are up with ED and EB down. Similar bursts are applied to leads IR2 and IR3, the pairs of bursts on the three leads being equidistantly displaced throughout the cycle as indicated in Fig. 6. Interrupted ringing tone and test line OK tone, which may be assumed to be derived from a tone signal provided by transformer TRD, are extended to leads IRT and OKT under control of CB and ED, and CA, respectively. N.U. tone obtained from transformer TRB (Fig. 3) is applied to lead NUT (Fig. 4) continuously except for one period of 400 m.secs. in each cycle when EC and SP are up together. Busy tone distribution. While the previously described cycle is being performed by the relay set under control of PA (Fig. 1), a further sequence is set in operation by PB. The first operation of PB brings up A and C ; B following on the first release. The second release brings down A whereupon D operates, and B falls away on the second release. On the next operation A pulls up again and C releases, B re-operating when PB releases. The fourth operation of PB brings down A and D, and B falls away on the fourth release to complete the cycle. Thus A and B are each repeatedly up for 400 m.secs. and down for 400 m.secs., with B delayed by 200 m.secs with respect to A, whereas C and D are each repeatedly up for 800 m.secs. and down for 800 m.secs. with D delayed by 400 m.secs. with respect to C. The operation of C extends earth pulses to lead IE (Fig. 4) and continuous tone from transformer TRC (Fig. 3) to lead IBT (Fig. 4) for recurrent periods of 800 m.secs. Generation of ringing current. The symmetrical multivibrator MV (Fig. 3) substantially similar to that described in Specification 657,063, [Group XL (c)], has the coils of two high-speed relays RA, RB in the anode circuits of valves VA, VB, respectively while the cathode, control grid and screen grid of each valve function as a triode. Operation of MV, which takes place at a frequency just below 25 c.p.s., results in relays RA, RB being operated for substantially equal and complementary periods in each cycle. Contacts RA and RB (Fig. 4) alternately energize the upper and lower halves of the primary winding of step-up transformer TRA. The alternating potential of approximately 75 volts R.M.S. value at 25 c.p.s. developed in the secondary winding of TRA is extended to leads CR, IR1, IR2, and IR3, as previously described. The secondary winding of TRA is shunted by smoothing capacitor QQ, the circuit thus formed being tuned to 25 c.p.s. This circuit is coupled by capacitor QR (Fig. 3) to the grid of valve VB in the multi-vibrator MV so that a positivegoing pulse is applied thereto at the appropriate instant in each cycle so as to lock the multivibrator into a frequency of 25 c.p.s. and in suitable phase with the tuned circuit. In an alternative arrangement of the multi-vibrator (Fig. 3A) polarized relays RA, RB each having two balanced windings, are used. The anode circuit of each valve VA and VB includes one winding of each relay, the windings being of the opposite sense. When VA is conducting the relays are thus urged into opposite states of stable equilibrium with RA up and RB down, these conditions being transposed when VB is conducting. The action of the relays is positive in each case and not as in the original arrangement dependent in one direction upon the tension of the contact springs. Generation of dialling tone. Dialling tone of 50 c.p.s. is produced in the secondary winding of TRE (Fig. 4) and is extended directly to lead DT. Generation of continuous tone and NU tone. These are produced by the power pentode VC (Fig. 3) operating as an electron-coupled oscillator at 400 c.p.s., the tuned circuit comprising the balanced-winding retard coil IA and capacitor QF. The anode load is provided by the primary winding of TRB, the secondary winding of which is shunted by capacitor QH to maintain a resonably constant output under varying load conditions. This secondary is extended to lead CT directly and to lead NUT under control of EC and SP. Generation of busy tone. Part of the output of the continuous tone generator is applied to the control grid of the amplifier valve VD (Fig. 3) and the amplified output appears across the primary winding of TRC, the secondary winding of which, shunted by QG, is extended to the lead IBT as already described. The provision of this separate output path for busy tone prevents the varying drain on the busy tone output from affecting the level of the continuous and NU tones.  Generation of ringing tone and test line O.K. tone. Another oscillator including pentode VE (Fig. 3) with a tuned circuit comprising retard coil IB and capacitor QK, operates at 133 c.p.s. and its output is applied to the grid of amplifier VF, in the anode circuit of which is connected the primary winding of TRD (Fig. 4). The 25 c.p.s. potential applied by transformer TRA (Fig. 4) to lead CR is also fed via capacitor QP (Fig. 3) to the grid of VF, so that the signal appearing across the primary winding of TRD is a 133 c.p.s. tone modulated by a 25 c.p.s. tone. The signal is extended from the secondary winding to leads IRT and OKT as already described. Modified ringing current generating arrangements for large exchange. The arrangements already described are not suitable for use in large exchanges as the single output transformer might be subject to overload. For larger exchanges of, say, 10,000 lines the arrangement of Fig. 7 is used. For the purpose of interrupted ringing current supply, the final selectors are divided into groups each of, say, 1000 subscribers' lines, while for other purposes the circuits already described remain common to the whole exchange. The contacts R