Patent GB-945385-A (GB)
Title: Ringing arrangements for automatic telephone systems
IPC: H04M19/04
Filed: 1960-06-27
Abstract: 945,385. Automatic exchange systems; transistor circuits. AUTOMATIC ELECTRIC LABORATORIES Inc. June 27, 1960 [Oct. 12, 1959], No. 22511/60. Headings H3T and H4K. Ringing is effected from the line circuit by reversing battery connected to the substation line. The arrangement is applied to a timedivision multiplex system in which reversal of battery at the line circuit connects up a transistor oscillator feeding the receiver at the substation. The oscillator is interrupted at 12 cycles per second from the line circuit so as to be switched on for periods of two seconds with rests of four seconds. The exchange is the same as that described in Specification 945,386. As shown, in Figs. 4 and 5, the substation circuit of Fig. 4 has a transmitter 401 connected in the input of two-stage transistor amplifier 463, 465, having negative feed-back and the load of which is equally split between the windings 413, 455, of the line transformer anti-sidetone circuit. With the substation idle and the hook-switch operated the doublebased diode 441 is connected to line over diode 431 and, provided the line has the correct polarity, will produce an 800 cycle sawtooth waveform which drives receiver 449. The relevant part of the line circuit is shown in Fig. 5, where ringing AND gate 238 is enabled in a particular time slot by tens and units pulses T1 and U1 and a ringing pulse RG. With gate 238 enabled the one shot multivibrator using transistors 503, 505, is triggered to charge capacitor 511 over diode clamp 507. The multivibrator is switched over gate 238 every 80Á  secs. and has a period of 50 Á secs. The potential on capacitor 511 switches transistor 515 to conduction which causes transistors 517, 519 to conduct and reverses the polarity normally connected to the line from resistors 243, 247. This reversal of polarity causes diode 431 to conduct at the substation and doublebased diode 441 oscillates to generate calling tone in the receiver. Interrupter tone is received at the line circuit on lead RDA and comprises 12 pulses a second on for 2 seconds and off for 4 seconds. This tone discharges capacitor 511 and reverses polarity on the line to interrupt the oscillator 441 at the substation. Supervision is effected over AND gate 236 which is scanned over wire E from a register controller when enabled by the tens and units pulses T1, U1. Wire 253 enables the gate when the loop is closed to signal hook switch and dialling signals. When ringing is taking place wire 254, which normally enables the gate, is raised in potential rapidly to close the gate since the closed loop during ringing would otherwise simulate a hook-switch signal on lead 253. Capacitor 242 between the windings 539, 541 of transformer 210 charges and discharges with each reversal of battery and during discharge at the end of a twelve pulses per second pulse provides a transient sufficient to open gate 236. To prevent such operation of gate 236 an inhibition is provided from transistor 523 which in response to interrupter tone charges capacitor 525 to inhibit the gate over diode 526 beyond the end of each interrupter pulse. When the subscriber replies the hook-switch condition is detected over lead E during a 4-second rest period of interrupter tone and ringing pulses on lead RG are discontinued. A second embodiment is shown in Figs. 6 and 7 and differs from the first embodiment in that the substation uses a Colpitts transistor oscillator which is self-quenching at a rate of 12 cycles per second and is switched on by battery reversal over diode 631 from the line circuit which is controlled by audiofrequency transmitted over the multiplex highway from a common ringing control circuit. The audio-frequency is supplied to the line circuit over transformer 729 from the highway to winding 751 and is received by a tuned circuit 705, 707, and a rectifier bridge to switch transistor 723 to conduction. The polarity connected to the line is normally negative over transistor 735 which conducts. When transistor 723 switches in response to a 2 seconds period of audio tone, 735 is cut-off and transistor 743 conducts to switch the polarity on the line to positive. In the substation, diode 631 conducts and transistor 633 oscillates at 2,300 cycles per second with a quench period of 12 cycles per second determined by inductor 634 and capacitor 636. Line supervision over AND gate 236 is possible with the single lead 253 since capacitor 757, providing a speech path between windings 733 and 755 of transformer 729, discharges immediately through diode 753 during battery reversals and, when the subscriber replies, gate 236 is enabled by the potential on resistor 248 to signal the register controller to end ringing. A third embodiment is realized by using the substation circuit of Fig. 4 with the line circuit of Fig. 7, it being necessary to modify the common ringing control circuit to supply audio-frequency interrupted at 12 cycles per second since the double-based diode of Fig. 4 is not self-quenching. Alternatively, the 12 cycles per second interruptions may be generated by a switching arrangement in the line circuit as is done in Fig. 5. Transistor circuits.-In a line circuit of a telephone exchange transistors 517 and 519, Fig. 5, are switched together by transistor 515 to reverse the polarity normally connected to lines 502, 504. With the transistors cut off, resistors 243, 244, connect negative polarity to line 502 and resistors 247, 248, connect earth to line 504. Leakage of transistor 519 is reduced by diode 521. In the substation circuit, Fig. 4, with hookswitch contacts down, reversal of polarity on the lines 502, 504 causes diode 431 to conduct and double-based diode 441 oscillates with capacitor 439 repeatedly charging over resistor 435 and base 447 and being repeatedly discharged through base 445 to supply ringing tone to telephone receiver 449.