Patent GB-792419-A (GB)
Abstract: 792,419. Party-line systems. WESTERN ELECTRIC CO., Inc. Sept. 6, 1955 [Sept. 10, 1954], No. 25529/55. Class 40 (4). [Also in Group XL (c)] A party-line system comprises a central carrier terminal and a plurality of satellite carrier terminals distributed along the line, each satellite carrier terminal being provided with means for selectively ringing one or a sub-group of subscribers out of a group of subscribers connected to an audio line connected to the satellite. In a typical layout, three carrier terminals 11, 12, 13 and one audio terminal 15 are connected at a central office to a transmission line 14. Each of these four channels serves four subscribers. Thus the audio terminal 15 serves subscribers 16 ... 20 which are connected to the transmission line 14 by low-pass filters 21 ... 24. The transmission line contains a carrier frequency repeaters 25 bypassed by a low-pass filter 26. The first outlying terminal 27 comprises modulating and demodulating means 27 co-operating with the office terminal 11 to connect up the audio transmission line 28 serving subscribers 29 ... 32. The second outlying terminal 33 c o - operates with office terminal 12. The audio channel connected to the former however is re-inserted via the low-pass filter 35, into the transmission line 14, whence it is connected via low-pass filters 39 ... 41 to subscribers 36 ... 38. Highpass filter 34 takes the remaining carrier channel to outlying terminal 42 which feeds the audio line 43. The repeater and outlying terminals are transistorized and polemounted and utilize local sources of power. Different frequencies are used for the inward and outward directions of each carrier channel. These may be grouped so that all the lower frequencies pertain to the outward direction and the upper frequencies to the inward direction; or they may be arranged so that inward and outward frequencies of any channel are adjacent. Office terminal, Fig. 2. The audio line 51 is connected through transformer 52 to a bridge circuit 85 ... 87 via a circuit comprising condensers 56, 59 for eliminating low and highfrequency transients respectively and a highvoltage protection circuit comprising pad 57, 58, 60 and oppositely poled silicon diodes 61, 62. The bridge circuit couples the audio line to the outgoing carrier circuit comprising a compressor 55, modulator 65, amplifier 67 and band-pass filter 68, and to the incoming carrier circuit comprising band-pass filter 81, amplifier 82, demodulator 83 and expander 84. The band-pass filters are connected to the carrier transmission line 14 which comprises condensers 70, 73, for eliminating high and lowfrequency transients, and high voltage protective devices comprising silicon diodes 77, 78 and carbon block protectors 75, 76. Outgoing ringing signals consist of 20 c.p.s. superimposed on positive or negative D.C. and applied either to the tip (T) or ring (R) conductors. The alternating component is used to operate a switch 102 which serves to switch an audio frequency f on and off at 20 c.p.s. Switch 98 operates if the ringing current is superposed on D.C. of either polarity on the T-wire, to transmit continuous bursts of audio frequency f2. Switch 104 operates whenever the R-wire is positive with respect to the T-wire to transmit audio frequency/3. Thus the four ringing codes are represented by neither, either or both of frequencies f2, f3 which together with interrupted f1 are injected into the outgoing modulator 65. Off-hook supervision and dialled impulses are provided by carrier received from the outlying terminal which after amplification and detection operates a relay 96 which controls the D.C. loop over line 51. Outlying terminal, Fig. 3. This comprises the usual protective and transient eliminating devices connected to incoming and outgoing band-pass filters 119, 150. The former feeds an amplifier 124, demodulator 125 and expander 126 and the latter is fed by compressor 146, modulator 147 and amplifier 149. During conversation the expander and compressor are connected to the voice frequency line 145 by a bridge circuit 131 ... 133. Incoming ringing signals are separated by band-pass filters 158, 161, 164 to control relays 160, 163, 166 which are operated by f1, f2, f3 respectively. The intermittent operation of 160 produces 20 c.p.s. pulses in the transformer winding 173, the output winding 169 then feeding oppositelypoled rectifiers 171, 172, whose output thus consists of 20 c.p.s. current superimposed on positive or negative D.C. which rectifier is in circuit depends upon the condition of relay 166, i.e. upon the presence or absence of f3. This ringing current is switched to either the tip or ring conductor dependent upon the condition of relay 163, i.e. upon the presence or absence of f2. The intermittent operation of 160 also brings up slow-release relay 144 which connects the audio line to the ringing circuit. When the called subscriber answers, relay 179 operates during a pause in the ringing signal and operates slow-release relay 127. The latter disconnects the expandor from the ringing code receiver; connects the former to the speech circuit bridge 131 ... 133; connects the amplifier 149 to the outgoing band-pass filter and enables the compressor 146 and the oscillator 148 feeding the modulator 147 so that carrier is transmitted as an off-hook signal. In the case of a call originated from line 145 relays 179 and 127 operate as before to transmit the carrier and in the case of automatic systems dialled impulses are transmitted by the intermittent release of relay 179 which causes corresponding interruptions of the carrier by short-circuiting the input to the amplifier 149. A modified system of ringing is described in which ringing current is derived from a source capable of providing both positive and negative D.C. either of which can be interrupted by receipt of frequency f1 and one or other of these is applied directly to the T or R conductors as determined by the receipt or non- receipt off f2 and f3, Fig. 7 (not shown). Automatic gain control is applied to the output of the receive channel filters as shown in Fig. 6 and is effected by amplifying (281) and rectifying (285... 288) the output from the receive amplifier 272, the resulting D.C. output being used to control a variable attenuator, comprising shunt connected silicon diodes 266, 277, across the input circuit to the amplifier. A permanent bias applied by battery 293 brings the diodes on to the linear portion of their characteristics. Compressor and expandor. The former, Fig. 4A, comprises an amplifier 198, connected by a hybrid transformer 200 to an audio amplifier 204, whose output is rectified and smoothed and applied to a variable attenuator comprising shunt diodes 195, 196 across the input to the amplifier 198. The input to amplifier 204 and the input to the modulator are connected to conjugate arms of the hybrid to eliminate undesirable influence of the modulator impedance variations upon the input to the amplifier 204. The expandor, Fig. 4B, (not shown), operates similarly with the exception that the expandor control amplifier is fed over a hybrid from the input side of the main amplifier and the variable attenuator comprises series connected rectifiers. The amplifiers used may be of the type described in Specification 777,627, [Group XL (c)]. Specifications 792,418 and 792,420, [both in Group XL (c)], also are referred to.