US2484211A

US2484211A - Power line carrier frequency telephone system

Info

Publication number: US2484211A
Application number: US702693A
Authority: US
Inventors: John W Emling
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-10-11
Filing date: 1946-10-11
Publication date: 1949-10-11
Anticipated expiration: 1966-10-11

Description

Oct. 11, 1949.

J. w. EMLING POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM y Filed oct. 11, 194e 2 Shee'cs-Shee'cl l N Q R.

Nb MA Puuh@ .norma u .niv

INVENTOR' J. W. 5ML/Na BV @d @da ATTORNEY Oct. 1l, 1949. .1. w.1EM1 |NG POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed oct. 11, 1946 2 Sheets-Sheet 2 Arron/vn atented Oct. 11', 1949 POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM John W. Emling, Morristown, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 11, 1946, Serial No. 702,693

6 Claims.

This invention relates to communication systems and particularly to telephone systems including subscriber stations arranged for carrier current operation over a ypower transmission line.

Objects of the invention are the provision of carrier telephone service over a power transmission line including means for holding the associated line to the telephone central office on calls between telephone stations connected to the power line, the elimination of manual control means at these stations heretofore provided for use only on such calls, and in general the simpliiication and improvement of power line carrier telephone systems.

This invention is a telephone system comprising subscriber stations arranged for carrier current operation over a power transmission line to which they are connected, carrier terminal equipment being provided for connecting the power line to a telephone line terminating in a telephone central oflice. Carrier current of one frequency is used for transmitting over the power line, voice and signal currents to any one of the stations and carrier current of another frequency is used for transmitting voice and signal currents from any one of these stations. On a call between stations connected to the power line and utilizing the same carrier channel over the power line, the carrier transmitting and receiving frequencies are automatically interchanged at the calling station and a reverting call signal is transmitted to the carrier terminal to terminate the transmission of carrier current therefrom, this signal resulting from a beat between the carrier current transmitted from the carrier terminal equipment and the carrier current transmitted from the calling subscriber station.

A clear and complete description of the invention will be facilitated by considering a system in which the invention and its features are embodied, such a system being represented schematically in the drawing. The invention is, however, not limited in its application to the system shown but is generally applicable to telephone systems. The invention is also applicable to radio transmission and carrier transmission systems over media other than power transmission lines.

The drawing, which consists of two gures, represents an automatic telephone exchange serving subscriber stations some of which are arranged for carrier operation over a power transmission line.

Fig. 1 indicates a conventional automatic telephone exchange comprising a subscriber station l0, subscriber lines Il and 2| and line circuits 2 l2 and 22, line nder switches LFI and LFZ, selector switches SI and S2, connector switches Ci and C2, a carrier terminal equipment CT individual to the line 2l, and a power transmission line PL to which the carrier terminal equipment CT is coupled by coupling means 44.

Fig. 2 shows one of a plurality of subscriber stations connected to, and arranged for carrier current operation over, the power line PL.

The subscriber station I9 is of the usual type provided for use on lines terminating in an automatic central oilice and includes a dial or impulse sender (not shown) for use in selectively controlling the switches through which desired connections are established. The line finder, selector and connector switches are of the two motion step-by-step type, each of the line ilnder and selector switches being represented in the drawings by a set of brushes and a single set of terminals and each of the connector switches being represented by a set of brushes and two sets of terminals. The relays and other apparatus associated with the line finder, selector and connector switches are represented by broken lines. Reference may be had to Automatic Telephony by Smith and Campbell, 2nd Edition, pages 53 to 65 for a detailed description of the structure of two-motion step-by-step switches and their operation when used as selectors and connectors. Reference may be had to the patent to R. L. Stokely 1,799,654, granted April 7, 1931, for disclosure of a line circuit and a line finder of the step-by-step type. The ringing code generator 2l comprises a source of ringing current and means for interrupting the ringing supply conductors as required to produce a plurality of different ringing codes for signaling subscriber stations. Reference may be had to the patent to Duguid No. 2,225,907, granted December 24, 1940, for disclosure of such a ringing code generator and to the patent to Koechling No. 2,289,503, granted July 14, 1942, for a detailed disclosure of a connector switch arranged for ten party code ringing.

The carrier terminal equipment CT, power line PL and the carrier telephone subscriber stations of Fig. 2 constitute a power line carrier telephone system similar generally to the system disclosed in the copending application of R. C. Edson and J. W. Emling, Serial No. 653,254, filed March 9, 1946, now U. S. Patent No. 2,481,915, granted Sept. 13, 1949, to which application reference may be had for discussion of the carrier frequencies suitable for use in such systems and for a more complete disclosure of the coupling means between the power line and the carrier receivers and transmitters and details of the carrier equipment CT and carrier telephone stations. The power line PL is one phase of a power transmission line and serves a plurality of power and light customers, some of which are equipped for carrier telephone service as shown in Fig. 2.

The carrier terminal station or equipment unit CT comprises a hybrid coil or group of transformer windings 39 arranged to transmit signal and voice currents to and from line 2|. The resistor 3| and condenser 32 constitute a network connected to the middle right pair of windings to balance the line 2|. The upper pair of wind` ings receive voice frequency currents from a carrier receiver comprising the ampliiier-demodulator 50; and the lowest pair of windings transmit voice frequency currents from line 2| to the v input circuit of a carrier transmitter comprising the oscillator-modulator-amplier 49. The power line PL is connected through coupling means 44 and Vconductors '43 to the carrier receiver coinprising receiving band lter 49 and the associated amplier-demodulator 50, to another carrier receiver comprising receiving band filter 59 and ampliiier-demodulator VV(i0, and to the carrier transmitter comprising transmitting band filter 4| and oscillator-modulator-amplier 40. The amplier-demodulators 59 and and the oscillator-modulator-amplier 40 may be of any known suitable type. The oscillator-modulator 49 is tuned to transmit through filter 4| carrier current of a frequency F! as modulated by either signaling or voice currents incoming over line 2|. rThe amplier-demodulator 56 receives, through filter 49 from-the power line PL, carrier current of a frequency F2 modulated by voice and signal currents from any one of the carrier telephone stations connected to the power line PL. The amplier-demodulator 69 is tuned by band lter 59 to receive carrier current of frequency FI transmitted from oscillator-modulator-amplifier 40 and to receive carrier current of a frequency (FI-i-a) where a is a low frequency, for instance, 200 cycles per second, (Fl -i-a) being the frequency of the carrier which is transmitted from the calling one of the carrier tions.

The carrier terminal equipment CT further comprises relay controlled by ringing current incoming over line 2| to energize the oscillatormodulator-amplifier and thereby transmit impulses of carrier current corresponding to the ringing code received by relay 35 from line 2|. The carrier terminal equipment CT further comprises relays 5! and 52 controlled by amplifierdemodulator 59 responsive -to signals transmitted over the power line by carrier current of frequency F2 from any of the carrier stations. The functions of each of the relays is hereinafter described in detail. The sources of energy for operating the relays and for energizing the ampliers, modulators and demodulators is represented yas being batteries Bl and B2 but may be supplied by rectifying the output of a transformer which receives its energy from the power line PL.

Each of the carrier telephone subscriber stations is similar to the one shown in Fig. 2 and is connected through individual coupling means 59 to the power line PL. Each station comprises a carrier receiver, a carrier transmitter, a telephone receiver 84 and transmitter 85 combined a handset which normally rests on the receiver hook 83, a dial 86 for controlling automatic switches in the telephone central oiiioe to establish desired connections, a relay 79 and a ringer 19. The carrier receiver comprises a receiving 'band filter 'H tuned to pass the carrier frequencies Fl and (FH-a), a receiving band filter "i2 tuned to pass the carrier frequency F2 and amplier-demodulator apparatus i3. The carrier transmitter comprises oscillator-modulatoramplier apparatus 88, a transmitting band filter 99 for transmitting voice modulated carrier current of frequency (FI-l-a) and also current of frequency F2, a frequency control relay 99, oscillator crystals 9| and 92, and a transmitter control relay 99 for connecting the output conductors of filter A89 to the power line. The transmitting and receiving filters transmit the voice frequency sidebands in addition to the carrier frequency. The oscillator-modulator-amplier apparatus, conventionally illustrated `as .a rectangle 88, includes an oscillator whose frequency is controlled by the one or .the .other of the crystals 9|, 92 depending upon the `operated or unoperated condition of relay 90 together with suitable amplifyingV and modulating means Vwhereby the carrier waves are modulated -at'times in accordance with voice waves applied through transformer 81. The oscillator may be any `suitable and well-known crystal controlled oscillator such as found shown in Cadys book on Piezoel ectricity, first edition,

1946, published by McGraw-Hill Book Company, Incorporated, pages 495 and 499, Taylor Patent 1,639,817, dated August 23, 1927 or Nicholson Patent 2,212,845, dated August 27, 1940. The one or the other of the two crystals 9| or 92 is switched into circuit to establish the carrier frequency at-one value or another at different times as herein described. Modulation may be accomplished in any suitable well-known manner by means of any one of many well-known modulating circuits of which the following examples are given: Van der Bijl Patent 1,350,752, August 24, 1920 and Helsing Patents 1,343,562, June 15, 1920 and 1,560,054, November 11, 1925. Lack Patent 2,218,209, October 10, 1940 discloses a suitable combined oscillator and modulator which could readily be arranged for shift from one crystal to another Aby relay .9.0. kDevi-ces of this kind are considered to be so well known in the art that they need `no further description. The power supply for the amplifier, modulator and demodulator apparatus includes a rectifier 61 rand a transformer 55, connected to the power line either directly or through an intermediate transformer. Other transformer windings may provide the energy for heating the laments of vacuum tubes and supply whatever other operating potentials are required. No reverting call key is provided since the transmission of a reverting call signal is automatically effected.

Cdl! from station 10 to a carrier telephone station Assume that a call is initiated at station I9, that the line l is connected by a line finder 'LFI to a selector Si, that thefselector Slis selectively controlled by dial impulses from station I 0 to select a group of trunks and an idle trunk therein associated with a connector Cl, and that the connector C| is selectively controlled by dia1 impulses from station I9 to select the line 2|. Assume further that the line 2| is idle when tested by the connector CI, that a ringing code is selected responsive to dial impulses from station l0, and that the selected ringing code is transmitted through the brushes of connector CI over the conductors of line 2| through condenser 33, rectifier 34, and a back contact of relay 52 to the winding of relay 35. Relay 35 is thereby operatively energized during each ringing interval of the ringing code transmitted over line 2|. The operation of relay 35 connects the positive lpole of battery B2 tc conductor 39 to energize the oscillator-modulator-amplifier 49, disconnects the lower pair of windings of hybrid coil 39 from the input conductors of oscillator-modulator-amplier 4U, and connects these conductors through condensers 3G to the conductors of line 2|; whereby the ringing current incoming over line 2| modulates the carrier current transmitted through lter 4|, conductors 43 and coupler 44 over the power line PL to each of the carrier telephone stations connected to the power line.

At each of the carrier telephone subscriber stations the ringing current modulated carrier of frequency Fl incoming over power line PL is transmitted through a coupler 69, conductors 1Q, and a normally closed contact of the springs controlled by receiver hook 83, through receiving band lter '1| to the input circuit of amplifier-demodulator 13. The output circuit of amplifier-demodulator i3 includes resistor 'I4 and the winding of relay and relay 'i6 is operated responsive to carrier current incoming over the power line. A condenser i5, connected in parallel with the winding of relay it, renders this relay somewhat slow in operating so as to tend to prevent its operation responsive to interfering currents such as static. The output conductor of modulator i3 is further connected te condenser 8| thence through the offnormal contacts of dial 85 and telephone receiver 84 in parallel with resistor 82. While the telephone handset is in normal position on hook 93, receiver 84 and resistor 82 are short-circuited at a back contact of the springs controlled by hook 83. The operation of relay 15 closes a circuit from the lower winding of transformer 66 through rectier t?, conductor 68, resister 8i), front contact of relay it, condenser 1l, and through the windings of ringer i9 to ground at a back contact of the springs controlled by receiver hook 83. The windings of ringer i9 are thereby energized while condenser il is charging. Since the incoming carrier current of frequency Fl is modulated by the frequency of the ringing current transmitted over line 2| from the central oflice, relay 'i6 is alternately operated and released at the ringing current frequency (for instance 16 cycles per second) during each ringing interval of the ringing co-de. Each time relay 'i9 releases, condenser 'il discharges through resistor 18; and each time relay i6 reoperates, condenser 'Vi is charged through ringer i9. Thus, the ringer 19 at each of the carrier telephone subscriber stations associated with power line PL is actuated in accordance with the ringing code of the particular station which is being called.

When the handset is removed from the hook 83 at the called station, the circuit through ringer I9 is opened, the short circuit across the receiver 84 is opened, and the positive potential supply conductor 68 is connected to the winding of. relay 93 and to the oscillator-modulator-amplifier 88, operating relay 93 and energizing the amplifiermodulator 38. Relay 93 is somewhat slow in operating to allow time for relay 98 to operate if relay T6 is operated at the time the handset is removed, as is the case at the calling station on a reverting call as hereinafter described. However, on the call now being described, relay 'i6 is not operated at the called station when the handset is removed because at this time there is no carrier current of frequency FZ being transmitted over the power line, so that relay 99 is not operated; and the operation of relay 93 disconnects the winding of relay 99 from the armature of relay 16, thereby to prevent the operation of relay 90 on this call from station l. Since relay 99 is normal, the oscillator crystal 92 is included in the oscillating circuit of oscillator-modulator 88, whereby carrier current of frequency F2 is generated and transmitted through filter B9, a front contact of relay 93, conductors 'i9 and coupler 59, over Ipower line PL to each 0f the other carrier stations and to the carrier terminal equipment CT. The carrier current of frequency F2 does not reach the demodulator i3 at each of the other carrier subscriber stations because, at each of these stations, the input circuit of filter i2 is open as long as the handset is in normal position on hook 83. But at the carrier terminal equipment CT, the carrier current of frequency F2 passes through coupler 44, conductors 49 and receiving band lter 49 to the input circuit of arnpiier-demodulator 59 causing the operative energization of the winding of relay 5i. The operation of relay 5| closes a circuit for operating slowto-release relay 52 and closes a bridge, consisting of the left middle pair of windings of hybrid coil 3D, a cross line 2| to tri-p the ringing in the connector Cl The opening of the ringing circuit and closing of the talking connection through connector CI is effected in usual and well-known manner. The operation of relay 52 opens the circuit connection between the winding of relax7 35 and line 2|, and connects the positive pole of battery B2 to the amplifier-demodulator 59 and to the oscillator-modulator-amplier 4d. The energization of the amplifier-demodulator @d performs no useful function on this call; but the energization of oscillator-modulator-amplifier 49 effects the transmission of carrier current of frequency Fi through filter 4| over the power line PL to each of the carrier stations, causing the energization of relay TE at each of these stations. The voice currents incoming over line 2| from the calling station |9 are transmitted through hybrid coil 3! to the input circuit 0f modulator-- amplifier 40 to modulate the carrier current of frequency F| transmitted over the power line to the carrier stations. At the called station, the voice current output of ampliiier-demodulator 'i3 is transmitted through the receiver 84, but at the other carrier stations, the receiver is short-circuited at a contact of the receiver hook springs. The transmitter of the handset of the called station generates voice currents which are transmitted through coil 8'! to the input circuit of modulator-ampliiier 88 to modulate the carrier current of frequency F2 which is being transmitted over the power line. The Voice modulated carrier current of frequency F2 passes through filter 49 to demodulator 50; and the voice currents are transmitted through hybrid coil 3|! over line 2|l through the` connector; selector and; line;r findenI switches,. over line ll.- tothe calling..V stationz lef, Two-way conversationf now takes..place,=. over. the? established connection: including powerf line PLy between' the calling and called station.

When the: handset is. replaced` on hoolr-l 83;' of.' the called station, receiver 84` isV short-circuited,v relay Sfis released-,and the oscillator-modulator@ amplifier is deenergized;Y terminating.;- the" transmission' of carrier current. of' frequency: E over.- the power line and-thereby causing the `.suc-- cessive release of relays 51|y and 52of=lthe carrierf terminal equipment CT. The=releaseofirelaysil opensfthe bridge across line-2l thereby'releasingS thez answering. supervisory relay: (not shown) .y in; the. connector CI'- Therelease of relay fdisconnects battery BZ'from oscillator-modulatoramplier il@ to terminate the transmission-.ofthe carrier current of frequency FIV over the power line, thereby releasingY relay 'Hi at. each` ofvthe carrier stations. YWhen the subscriber at ther callingfstation la releases the connection; the Iconnector.V C1, selector Sl` and line iinderLll-A are restored to. normall in usual and' Welllknown.

manner.

Call from carrier. telephone station Assume next thatv the handsety is removed'from; hook Bti-at' one of: the carri-er telephonevstations connected to power` line PL to,y initiate a, calL. f-

Relay 93` isthereby operated;. relay remains;

Vnormal andthe oscillator-n'lodulator-ampiiier Sais-energized@ transmit carrier current of frequency F2.- over power line PL causingthe-sucscessiveoperation of relays 5l and52zof carriers` Y terminal` equipment CT. At eachV of the; other carrier subscriber stations, the carrier'currentofi frequency F2 transmitted over line.; PL from the: calling carrier station is.v ineffective. to. operate: relay 'F6 because the input conductorsf'leare nor--` many-'connected across thefinput'v conductors. of; lter 1 l' and are not normally vconnected across the..- input conductors of ilter l2. The operationA of; relay 5l -of-.carrier equipment CT closes .the bridge'i across line 2l whereby the line relayl (notshown)v of line circuit 22 is operated to startlan idle line. finder such as LFZftoconnect with the callingiline; and line 2| isthus connected. through the brushes of line finder LFE' to the associated sel'eetorSZ'.l The operation ofI relay 52 opens thecircuitconnecton` between relay 35-and line.2l1andconnects. battery B2` to oscillatoremodulator-amplifier' 60S and to-ampliier-demcdulator The energizaftion ofoscillator-modulaton-amplier: 40:' effects the transmissiony of: carrier currentiof'. frequency FIy over power line; PL. Dial tone isz transmitted from selector S2 :over line. 2 l throughhybridlcoil :3m totheinmit of.- modulator All to modulate-'the can rier currentV ofl frequency FI being t1'.a1'1s1nitted:v over. theepowerfline; to; the?carriertelephonesta-A tions. The carriercurrent of frequency Flfeifectsv the operatonfofrelay 'l at eaoh ofthefcarrier sta-y tions; and, at'. the callingr carrier sta-tion., thefdiali tonele/transmitted through'the receiver M: toindicate that the dialing of the number of; the telephone station with whichconrrection: is desired. maybe starte-d..r The calling party theres upon. dials. the directory number of' the*v called: station. Whenthe dial is moved off normal, the;

receiver 84 is disconnectedffrom amplier.demod ulator 1'3`; and, as the dial returnsto..normal, the.. irnpulsecont'act's ofthe dial modulatethe carrierl current` of frequency F2 being transmitted from. the*oscillator-m'o'dulatonamplier 8B: Each dial'.

. amplifier Lll.

i over'A the: powcn1 line causes.: thee releases and; rf operation of'. relay 5l.: Each. release; and'.y reoperu ation ofirelaysl; responsive-to idialimpulses, opens.Y andv closes the? bridge` across line 2 I thereby ref*- peating: the-dial impulses to selectively' control:

the operation; ofL selector.vr SZvand arconnect'or-GL to@ extend. thei connectionl to the called line;` and t0; select the; ringing; code required#V for'signaling-1l the`l called station:- Since-relay 52iss1ow"t0.2re-

. lease, itf remains operatedif during. the responseroft;

relay: toi dial impulses.

Assume first: thatzthe called` station is'not'; onef of: the carrier stations connected? to thef power.- lineiPL. Whenctlie oallfis-v answered, the'oonneef-v tor C2 opensstherv ringing; circuit andi closesthei talking circuit inusual andxwell-known manners; and :talking currentspzare transmitted throughhy.-y brid coil.: 3l=andoverthe power linef inibothzdirecf tionsa. The; connection is undert the control: off thecalling carrier telephone station; and; Wheni the handset. ofl 'thisfstation isrestored` to. normal?. conditionon the; hookf: 63', relay 93., is releasedand; the oscil1:ttor-modulator-arnpliiier- 88- is'- deenergized; terminating:thetransmission of carrier our;A

. rent offrequencyF-Z andgtl'ierebyrjcausing the suc;

cessi-veerelease; of relays: 5l'V and 52 offth'emarreit'Y terminal equipmentv CT. The release of` rela-w51: opensfithebridge acrosszline-Zl and the releaseeof relays 52: deenengizes the oscillator-modulator The opening of the bridge across; line 2.12 causesthe release; of the.V line relay (not. shown) of the connector C2- and'returncof'. the: selector; S2.- and line. indeifLFZ to, normal in usuali manner. The connector C2v is also restoredto:`

f normalin usual manner as sooncas both the calleA ing and called;- stations have:` released. the conf-- nection.,

Assum'elnext. that the. call initiated` at azcarriei: telephone..V subscriber stationv as above described; is. a. revertingcall;` that is a; callto anothersubscriber; stationv connected. to; the power'V line: PIs.; Invsuch: a. case; theconnecter GZ'nds the called:Y line. 2l'. busy' wherritrnakes.;the.l busy test',Y and; a busy. tonef is.: thereupon` transmitted" from. the? connecton throughr selector. S2, and" line LE2,xover line 2|, through hybri'd'tcoil 30;.modu-- lator-amplierr; ltei. lgpower line PL, filter* Tlf, ampli'eredenrodulator '131. and condenser.` 8-12 til-telephone.receiverrBllatithecalling.station; The? connector G2 is arranged to make ay reverting. calltest and; ifithe; call. is alreverting call, the?. line: inden LEZA and selector.K S2. are; restored; tou normal when the calling: subscriber-hangs; ups., but: the connector G remains connected'. to` the'.Y

` line 2lbto.ringi'thefcalledlstationz. Referencefmay."

behad: to the' patent to I-I. Hovland. 1,849,0{l7`,` granted March I5, 1932; for; disclosure ot a. conm neotorrarrangedtoxmake;` theI aforementioned; reil verting call test; Upon. hearing: thefbusy tone;- transmittedifro'm connector C2, the callingsubscriberhangs the-handseti omthe'hook. Relay e3` releases andi the:v oscillator-modulatoreampligf er- 88 eis: deenergized to terminate the; transmisesion.` of; carrier current offrequency-F2 over-'ther l power line and thus release relay 5I; of. thevcar' rierl terminal equipmentzCT. 'Ihe'release'rofy rea lay 5l. opensv the bridge across the conductorsvof" linefl-J; theeline nderf LFZ and selector- S2 are.: restored to .normaL and the connector G2 is operi-A ated., to. closey thefringng.v cirouit.- and apply ther; ringing code of the called station.- tothe conclue--n tors.of.1ine' 2l... Tfherelease of relayl alsocauses the release of relay 5.22, but.` the energizationf of. the ringing; relay responsive to, ringing, our-- impulse thus transmitted fromthecalling station rent received over line '2l' effects the energi'zation.

of the oscillator-modulator-amplier 4D to transmit ringing current modulated carrier current of frequency FI over the power line PL. At each of the carrier telephone subscriber stations the relay 'i6 responds to the ringing current and causes actuation of the ringer 19 according to the ringing code of the called station. When the called subscriber answers by removing the handset from hook 83, relay 93 is operated and the amplifiermodulator 88 is energized to transmit carrier current over the power line. Since at this time there is no carrier current of frequency F2 being received over the power line, relay 16 is not energized at the called station and relay 90 does not operate. With relay 90 normal, oscillator crystal 92 is connected in the oscillator circuit of modulator 88 so that the carrier current transmitted therefrom is of frequency F2; and with relay 93 operated and relay 90 normal, the input circuit of lter ll is closed and the input circuit of filter 'l2 is open, so that the amplier-demodulator 'i3 is tuned to receive carrier current of frequency IFl and frequency (F I-l-a). The transmission of carrier current of frequency F2 over the power line PL from the called station eifects the reoperation of relays and 52 of the carrier terminal equipment CT. The operation of relay 5l closes the bridge across the conductors of line 2l to trip ringing and to operate the answering supervisory relay (not shown) of the connector C2 in usual manner. The operation of relay 52 connects the positive pole of battery B2 to energize the oscillator-modulator-amplifier All and also to energize the amplier-demodulator 60. The energization of the oscillator-modulator-amplifier 4B eects the transmission of carrier current of frequency Fl over the power line to energize relay 'le at each of the carrier telephone stations.

When the operation of the ringers at the carrier telephone stations ceases due to answer of the call at the called station, the calling party removes the handset from receiver hook 83 thereby opening the connection between input conductors 'lli and the input conductors of filter 'H and connecting input conductors 1D to the input v.

conductors of lter 12. Since, at this time, carrier current of frequency F2 is being transmitted over power line PL from the called station, relay 15 is operated at the calling station and removal of the receiver closes a circuit through the winding of relay eil; which circuit includes the outer right back contact of relay 93, a normally open contact of the springs controlled by hook 83, front contact of relay i6, resistor 85, conductor .68, rectifier 5l and the lower winding of transformer 55. Relay 9G is thus operatively energized before the slow-to-operate relay 93 operates, and relay 99 locks independent of relay 93. The operation of relay 913 includes the oscillator crystal Si instead of crystal 92 in the oscillating circuit of modulator 88 whereby carrier current of frequency (Fl-l-a) is transmitted from the calling station instead of carrier current of frequency F2. With both of relays 90 and 93 operated at the calline station, the input circuit of lter l2 is closed and the input circuit of filter 'il is open; so that voice modulated carrier current of frequency F2 incoming from the called station is transmitted to the receiver 84. The carrier current of frequency (FH-a) transmitted over the power line from the calling station passes through filter 59 at the carrier terminal to amplifierdemodulator il() thus causing the operation of relay 6 l. The carrier current of frequency (Fl -l-a) incoming over the power line from' the calling Cil station and the carrier current of frequency Fl transmitted over the power line from oscillatormodulator-amplier 40 produces a beat of a cycles per second-for instance, 200 cycles per second. Relay 62 is tuned to operatively respond to energization of its right winding by current of the beat frequency. When operated, relay 62 is held operated by its left winding under the control of relay 5i The operation of relay 62 disconnects battery B2 from oscillator-modulator-amplifier Ml to terminate the transmission of carrier current of frequency FI therefrom while conversation is taking place between the calling and called stations. It is to be noted that this arrangement has the advantage that if the receiver is removed at some carrier station other than the calling or called carrier station while a call other than a reverting call is in progress, the transmission of carrier current of frequency Fl from the carrier terminal equipment will be resumed as soon as the receiver is replaced at the interfering station.

If the calling subscriber hangs up rst, relay @t releases and oscillator-modulator-amplifier 88 is deenergized, whereby the transmission of carrier current of frequency (Fl-l-a) from the calling station is terminated, causing the release of relay 6i and 52 of carrier terminal equipment CT and the release of relay 'I6 at each of the carrier stations not involved in the call described. The release of relay 6| connects battery B2 to oscillator-modulator-amplier il whereby carrier current of frequency Fi is now transmitted from the carrier terminal equipment CT over the power line causing the reoperation of relay 'i6 at each of the carrier stations. Relay 5i is held operated by the carrier current of frequency F2 being transmitted over the power line from the called subscriber station. When the called subscriber disconnects, the oscillatormodulator-amplifier 88 thereat is deenergized terminating the transmission carrier current of frequency F2 over the power line, releasing relays 5I and 52 of the carrier terminal equipment CT. The release of relay 5i opens the bridge across the conductors of line 2l thereby causing the release of the connector C2 and the release of relay 52 again connects the winding of relay 35 across line 2i for response to ringing current on a succeeding call.

If the called subscriber hangs up rst, the transmission of carrier current of frequency F2 therefrom is terminated causing the release of relay 5l of the carrier terminal equipment CT and causing the release of relays 'l5 and Se at the calling subscribers station. The release of relay 90 reconnects the oscillating crystal 92 in the oscillating circuit of modulator 88 in place of the crystal 9i whereby the transmission of carrier current of frequency (Fl-l-a.) is terminated and the transmission of carrier current of frequency F2 is initiated at the calling subscribers station. Relay 5i of the carrier terminal equipment CT is thus reoperated before the slow-torelease relay 52 has released so as to hold the central office connection until both of the subscribers have disconnected. When the calling subscriber hangs up, relay 90 releases and the oscillator-modulator-amplifier 33 is deenergized at the calling station.

If the called subscriber fails to answer on a reverting call, the ringer at each of the carrier stations is actuated by the ringing current until the receiver is removed from the switch-hook at some one of the stations. When the receiver is replaced at the subscriber station, the carrier l-ll iterminal-.equipmentfl` is restored to normal as `Wellzas tlfiecarrier equipment at-the answering sstation. f

.1.;In a telephonefsystemaftelephone line, :a

vpower utransmission .line, sa carrier :terminal sta .tion interconnecting :said lines, a plurality =oi :telephone vsubscriber stations A connected to said powerd-ine,- :said .carri-er t'ern'linall station compris- .ing a-narrierf currenttransmitter.;for-:transmitting .over-.the pewerflinefea-rrierseurrent Oia fre- -guencyf a :first carrier freceiver for receiving over thegpowerfline carrierfcurrent of assecond fre ...quency a-ndfasecondearrier receiver for receivfing over the'` power -lineiearrier current of said iirstf-requencyand carrier current of a third ifreguency, each of fsaidfsubscriber stations Vcornprising :carrier current transmitting and yreceiv- :ingfmeansarranged to selectivelytransmit over` fthelpovverrlineearrier currentccf :said second or said third frequencies and ,-to .receivevover the :poweriline `carrier.current of=a1l offsaid .carrier frequencies .means .at `said carrier terminal sta- .tion forl energizing l the .transmitter there- .atresponsive'tocarrier currentof said second ire- 'gquency .incoming-over said power,line,.andmeans fat vsaid carrier -terrninal .station =operatively reesponsive `,to the beatbetwseen Nthe carriere. current of said irst frequency ,transmittedxby the .carr-ier transmitter at said ..carrier .terminal station rand Pthe Icarriercurrent .of said .third frequency .received-rover `tl1e.,povver llinefrom the calling stationon a .reverting-.call for disabling the car- .rientransmitter at .the carrier terminal station.

`In .a telephone system, a telephone line, a lnewer transmission .linea .carrier .ternnnal Station,interconnectingisaidlines,,aplurality of tele- .plionesubscriber stationsonnected to said power iine,:said carrier `terminal.statien 4comprislng a normally deenergized .carrier transmitter tuned to 'transmit .carrier .current of .a .rst i'irequency, .eacliof said,subscriberstationsicomprising a carrier transmitter andmeansior tuning the transmitter to transmit .carrier current of a .second frecuency .on .calls rover Asaid Ltelephone ..1ine, .to 'transmit .carrier .current .of said second .frequency Aon .incoming reverting, calls -and to i transmit. carr-iencurrent. of a .third .frequency on outgoing .reverting calls, each of said subscriberfstations.fur- .tner comprising acarrienreceiver .tunedat times to receivecarrier current of..said .rst and .third 'frequencies and .tuned attimes to i receive .carrier 'currentioffsaid second ,.freguencv. saidicarrier .ter-

minal fstation .further `comprising .a `i'lrst carrier Areceiver .tuned to `receivecarriercurrentof said second frequency, means `operated .responsive .to the ,receipt .of carrier `.current .of said; second frequency by saidfirstcarrierreceiverifor Aenergizing the carrier wtra-nsmiter .atsaidarrier Vternfiin-al statien, a second.carrier.L receiver. tuned to receive carrier current of saidifirst:and-thirdifrequencies, means .tuned teoperatively 4respend tc .the .beat between said-.first and thirdfrequencies when .carrier i current .of both fsaid and vtlnird frequencies being received-*bv said. seconde. rier receiver. the .operation ef. .said lastenamed means being-effe tive yto deenergize.tne..carrier transmitter: -atfsaid carrier terminal station .to prevent the energization of said transmitter as :long `4as carrier current of Asaid-.tl'iird vfreguency'is being lreceived fby vsaid-second:carrier receiver.

3. In sa telephone system, :a rtelep'honelina a :newer transmission line, .a;carrier terminal station interconnectingisaidlinesforztelephone communication, and fa A'plurality of telephone Ysuie- ..sc-ritienstationsF conneetedfto saidtpowerilineseaeh fof f said subscriber stations .comprising za carrier acurrent .transmitter to ,selectively transmit over ,said power =line voi'cesfrequency .modulated carrier Vcurrent --ef either -one of two -.earrier fre- ..quencies-said'terminal station. comprising a .-carer currentetransmitteretunedtto transmit carrier veurrentf'ofia third-carrier frequency, arst-car- .rierfcurrentreceiver V.tuned to .receive voice ,fre-

.quencyfmodulatedlcarrierxxurrent of a rst.one tof :said itu/.o carrier frequencies, .-.a :second-carrier .receiver :fatesaid f terminal :station tuned *.to -.rec.eive .carrier .current 'of Ltherotl-ier of .said :two :gire- Vtqiiencies :and ncarrier current :of .said vthird :frerguency, means controlled by fsaid 'rst .carrier .receiver for :energizingitheyz carrier .current-transnnitter-fat-said fterminalstatien, and means con- :trol-led by said fsecpnd carrier receiver vat fsaid `:terminal station treesponsive to ,simultaneously -ceivi-ng .carrier .current tof saidother of saidtwo frequencies and carrier currentfef said-.thirdcarfrierefrequencyffon deenergizingfthe. carrier .transamitterna-t.:saidterminal station fandifor'maintain- Iingsaidideenergizatien:aslong .asf'ca-rrier current of .sa-id other of .ssaid :two `.frequencies -is fbeing ransmittedover fsai'd power eline, veach 'of :said subscriber stations further comprising -fa :carrier lcurrent .receiver selectively tunable ato :receive .carrier` cur-rent .of i .said firstone fof :said :two frequencies er lto receive carrier scuri-ent fof -said etnerfof said f two .frequencies and carrier current of :said third frequency, and means feiect-ve '-at ltimes Anesponsive to -earr-ier :current .of :sa-id rst .one cf v"said t two .freqnencies ineoming Iover ysai-d ypervver :line i-for Atuning .the :carrier transmitter rtnereat ,to transmitfcarrierfeurrentfof saidf'other l ffsaiditvro frequencies and fortuningfsaidcarrier ireceiver :to :receive :carrier :current of said rst onefzofsaiditwmfrequencies.

4. .Ine vtelephone :system zcomprising Ca dial subscriber itelennone fiine, .fa power transm-ission .line,fa @carrier current terminalzeguipmentzinterconnecting said zlines, .and a :.pluralityfef fcarrier .telcel-iene ,subscribe-r stations leonnectei to fsaid ver line.,4 a carrier :current ftransmitter :in Asaid .rier iterminaliequipment fcritransmitting -carfiere current of a frstffreguency :over -rsai-d ,fpower line, carrierecurrent=transmittingameans k'at each .ef carrier stations for :transmitting carrier currentlnf n :seeondrfreguencylor carrier'xurrent ci e.itnirdifrequencyfever said :power yline, a first .carrier freeeiving means :in :said-:carrier `terminal equipment tun-edi tereceive .carrier current nf fs aid .frequency :from esaidzpewer line, iarsecond .carrier receiving .means in :said carrier terminal equipment ztuned 'to zreceive fc-arrier cnrrmt fof said instand third ffreguenc-iesemeans vresponsive .te current `ofy said vsecond frequency ringever-saidipowerilineeto saidrst #carrier receiving means fornenergizing theccarrier .current .transmitter of `said `carrier 1terminal 'equipment and :se-id l:second carrier `:receiving rmeans, 'control means-tuned to the beatsfrecluenc-ybetween said rstiend -it-hird frequencies for :deenerg'izing fthe .carrier -current `.transmitter said -equipment and-.means for :maintaining the tdeenergizationf of .the carrierv :currenttransmitter;ofzisaidfequipment .as -iongf'a's carrierzcurrent LOfzSadIthrd 'frequency is fleeing-,received byfsaid iseconrlscarri'er receiving means.

f5. 'In La =.telephone `system :comprising a dial subscriber telephone line, La #power transmission line, carrier lcurrentiterminalequipment interconnecting said inea-antifa fpluralityof carrier iteiephone fsubscriber stations Jconnected Lto '-said 13 power line, a carrier current transmitter in said carrier terminal equipment for transmitting carrier current of a rst frequency over said power line, carrier current transmitting means at each of said carrier stations for transmitting carrier current of a second frequency or carrier current of a third frequency over said power line, a rst carrier receiving means in said carrier terminal equipment tuned to receive carrier current of said second frequency from said power line, a second carrier receiving means in said carrier terminal equipment tuned to receive carrier current of said first and third frequencies, means responsive to carrier current of said second frequency incoming over said power line to said first carrier receiving means for energizing the carrier current transmitter of said carrier terminal equipment and said second carrier receiving means, control means tuned to the beat frequency between said rst and third frequencies fr deener- H gizing the carrier current transmitter of said equipment, means for maintaining the deenergization of the carrier current transmitter of said equipment as long as carrier current of said third frequency is being received by said second carrier receiving means, and carrier current receivingr means at each of said carrier subscriber stations normally tuned to receive carrier current of said first and third frequencies and tunable to receive carrier current of said second frequency.

6. In a telephone system comprising a dial subscriber telephone line, a power transmission line, a carrier current terminal equipment inter connecting said lines, and a plurality of carrier telephone subscriber stations connected to said power line, a carrier current transmitter in said carrier terminal equipment for transmitting carrier current of a rst frequency over said power line, carrier current transmitting means at each of said carrier stations for transmitting carrier current of a second frequency or carrier current of a third frequency over said power line, a rst carrier receiving means in said carrier terminal equipment tuned to receive carrier current of said second frequency from said power line, a second carrier receiving means in said carrier terminal equipment tuned to receive carrier current of said rst and third frequencies, means responsive to carrier current of said second frequency incoming over said power line to said first carrier receiving means for energizing the carrier current transmitter of said carrier terminal equipment and said second carrier receiving means, control means tuned to the beat frequency between said rst and third frequencies for deenergizing the carrier current transmitter of said equipment, means for maintaining the deenergization of the carrier current transmitter of Said equipment as long as carrier current of said third frequency is being received by said second carrier receiving means, carrier current receiving means at each of said carrier subscriber stations normally tuned to receive carrier current of said first and third frequencies and tunable to receive carrier current of said second frequency, and means at each of said carrier subscriber stations effective at times responsive to carrier current of said second frequency incoming over, said power line for tuning the carrier current transmitting means thereat to transmit carrier current of said third frequency and for tuning the carrier current receiving means thereat to receive carrier current of said second frequency incoming over said power line.

JOHN W. EMLING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,773,613 Clark Aug. 19, 1930 2,289,048 Sandalls, Jr July 7, 1942 2,294,129 Purrington Aug. 25, 1942 2,294,905 Honaman Sept. 8, 1942