527,035. Automatic exchange systems. ASSOCIATED ELECTRIC LABORATORIES, Inc. March 28, 1939, No. 9737. Convention date, April 4, 1938. [Class 40 (iv)] [Also in Group XL] Carrier frequencies, different for the two directions of transmission and superimposed on power lines, are used as inter-exchange channels or in place of subscribers' lines, signalling and impulsing being effected by interruption of a normally transmitted carrier. Figs. 1, 2 and 3 show a manual exchange to the left of the line AB, Fig. 1 connected over one line 100 of a three-phase power line to a two-way repeater 320, Figs. 2, 3, and an automatic exchange Fig. 3. At the ends of the power line section, low frequency is converted into high frequency at the transmitters 140, 240 and high frequency is converted to. low frequency at the receivers 145, 245. Different carrier frequencies are normally transmitted, but the transmission from 140, 240 can be suppressed by operation of relays 150, 200 respectively. The power line terminal arrangements are substantially the same. An A.C. relay 160 is operated by the receiver 145 when no carrier is received. The coupling networks 175, 180 and 185 are of known form, 175 being non-resonant and 180, 185 being tuned to the carrier frequencies of transmitters 140, 240 respectively. The line 109 from the manual exchange is linked to the transmitter 140 and receiver 145 through an anti-side-tone arrangement including the transformer windings 132<SP>1</SP>, 127<SP>1</SP> of the transmitter 140 and receiver 145 respectively and a balancing network 142. The network can be adjusted by the variable resistance 172. Automatic to manual call.-When a subscriber A connected to the automatic exchange dials the manual exchange, the line 321 is selected through a line circuit 305, a finder switch 310 and a final selector 315, and 235 is operated causing the transmission of 240 to be interrupted and an alarm to be given at the manual. exchange. When operated, 235 is followed by relays 230, 220, 210 'and slow-to-release relay 225 is short-circuited. Ground is applied to line 324 to mark 315 busy and during the interval between the operation of 210 and the release of 225, 200 operates to stop transmission from 240. The receiver 145 then receives no carrier and 160 operates to apply ringing current to the manual exchange line. The signal device 106 is energized to warn the operator that a call requires attention. The signal device 106 may be operated repeatedly by A dialling further impulses. The operator inserts the answer plug in the jack 105 and can then converse with A and extend the connection in the usual manner. Release of the connection is entirely controlled by the calling party A. When A replaces his receiver, 235 releases and 200 operates through contacts until 220 and 210 release, an H.F. impulse is thus sent to the operator to give an indication that the call is completed. Release of 235 also removes ground from the line 324. Manual to automatic call.-The operator inserts the plug into the calling, jack 104, to operate 110. Relay 120 follows and slow-torelease relay 125 is short circuited. Before 125 releases, 150 operates over a simplex circuit to send an H.F. pulse from 140. This pulse prepares the repeater 320. Relay 260 operates and breaks the circuit of normally operated relay 380. Relay 340 prepares a circuit for 350 and when the pulse ends, as 125 releases, 260 releases and 350 operates followed by 360 and 370, completing holding circuits for 340, 370. Impulses transmitted by the operator are repeated by 350 over the loop, the connection being extended in the normal manner. On release of 125 prior to dialling at the manual exchange the impulsing circuit is prepared by operation of 131, 135 and movement of the key 108 off-normal. The usual arrangements are made for provision of dial-tone, ringing current, ring-back tone and busy tone. The release of the connection is controlled by the calling subscriber, in this case the operator. On removal of the plug from 104, 110 releases, 125 operates and is followed by the slow release of 131 and 135. The carrier transmission from 140 is interrupted as 150 operates during this release period. Receiver 245 operates 260 to release 350, which opens the loop and allows 380 to operate again in preparation for other calls. Automatic to automatic call.-When two automatic exchanges are interconnected, the arrangement shown in Figs. 2, 3 is provided at both ends of the power line. The initial pulse transmitted by 240 as the line is seized and 235 operates prepares the pulsing relay 350 at the other exchange. Each further impulse releases 235 and operates 200 to send H.F. impulses to the other exchange. When the calling subscriber hangs up, an H.F. pulse is transmitted to cause the release at the other exchange, both operations being as described above. Power lines used as subscribers' lines.-When an automatic exchange is directly connected over a power line to a subscriber it is necessary to transmit an H.F. impulse over the line to the subscriber only for the purpose of ringing him. The subscriber's equipment is similar to that at the manual exchange. This arrangement, Figs. 8 to 10 (not shown), is substantially the arrangement shown in Figs. 1 to 3 with additional relays to control the transmission and receipt of ringing signals. Carrier frequency transmitter, Fig. 4.-The carrier frequency is generated in oscillator 401 and after amplification by valves 414, 415 is modulated by the output of an audiofrequency amplifier 420. The grid bias of the valves 401, 414, 415 is derived from the voltage drop across the resistances 411, 412 connected to a battery 413. This bias is sufficient to reduce the output of the oscillator to zero. If 411 is short circuited by joining the conductors 407<SP>1</SP>, 406<SP>1</SP> as by the contact of relay 150, Fig. 1, the output is normal. The receivers 145, 245 may be arranged as shown in Fig. 5. A pentode H.F. amplifier 501 is followed by a detector and automatic gain control stage 510 which in turn is followed by an audio frequency amplifier comprising two series connected pentodes 521, 522. The detector stage comprises a diode 511. The voltage drop across the resistance 513 in the diode circuit, which varies directly in accordance with the strength of the incoming carrier, is used as a bias voltage for the valves 501, 521 to provide automatic gain control. This voltage drop is also used to control a valve 540 and a relay 560 which corresponds to 160, Fig. 1. Relay 560 operates only when no carrier is received.