US1568194A - Carrier-wave transmission and communicating system - Google Patents

Description

Jan. 5 1926.

E. H. SMYTHE CARRIER WAVE TRANSMISSXON AND commumcmme SYSTEM Filed May's, 1921 4 Sheets-Sheet l Jan. 5 1926. 1,563,194 E. H. SMYTHE CARRIER WAVE TRANSMISSION AND COMMUNICATING SYSTEM Filed May 5, 1921 4 Sheets-Sheet 2 TJHJUL JUMLW I i 1 q Jan. 5 1926.

E. H. SMYTHE CARRIER WAVE TRANSMISSION AND COMMUNICATING SYSTEM Filed May-5, 1921 Sheets Sheei Lian. s ,1926. 1,568,194

E. H- $MYTHE CARRIER WAVE TRANSMISSION AND COMMUNICATING SYSTEM Filed May 5, 1921 4 Sheets-Sheet 1 I w b L R K {M mm P F K) E V R I I g.\ I.

//1\/@ x7 for: Edw/n f7. Gmyfhe Patented Jeni. 5, i926.

"E" P AT- N nnwxn n. sMYTnE. on nvenswon, ILLINOIS. Assxonon: TO WESTERN 'ELnc'mIo COMPANY, INCORPORATED, on new Yonx, in; Y., A conPonA'rIoNoF new YORK.

' seamen-wave mnansmssron "im lication filed May 5,

f0 all whom it may concern:

Be it known thatI, Enwm H. SMYTHEf a citizen of the United States, residing: at Evanst'on. in the couutyof Cookand State of Illinois. have invented certain new and useful liiiproveuieuts 1n Carrier-Wave 7 Transmission and Cou'imunicating Systems,

of whieh the following is a full, clear,-concise. and exact description. 7 ,7

This invention relates to an improved communicating system, and more particle larly to a conu'i'iunicating system of the typeiu-which carrier waves or oscillations, either free 'orgiuded, are employed as. the

medium for conveying energy for efle-cting the operation and control of electric signal ling, controlling and comnnmicating ap- 1 paraius located atseparated points in the system.

The invent on especially relates to a system n which so-called carrier currents, in

the form ofhigh frequency waves guided upon wires, are used as the medium of electrical connection between the electrical devices and apparatuses located at the separated points in the system. These carrier currents are assigned such frequencies that they are non-interfering one with another. so that each carrier serves as a separate and distinct connecting medium superposed upon the line or conductor over which the carrier is transmitted. Each 'or' the carriers is modificrl or modulated in accordance with the voice currents or other signalling or controlling currents that are to he carried. The carriers thus modulated are transmittcdover the toll or long distance line and are passed through individual receiving" arrangements each of which is selective of one carrier frequency to the exclusion-of the others, and each of which serves to reproduce from the modulated carrier-the voice or signal currents with which the ear The apparatus which is used to transmit and receive by means of any one high frequency wave as distinguished from that which is used fox-transmitting and receiving by means of a wave of adiflerent frequency, is con'unonly knownas a channel, or a high frequency communication channel. I

It is the object of the invention to provide a system in which a frequency selective communication channel associated with a line is Ann coMMUnIoATINo SYSTEM; 1921 semi in. 467,643.

frequencyselective channel. whenin use'is guardedt against intrusion; in which any one of a "plufality of frequency selective channels is made 'availablefor inter-communication oetwe'en any twoof awplurality which the-high frequency, channels are associated; ina which means are provided for enabling any one of the stations simultaneouslyto employ for use/all of the'ch'am nels,' or as manyas are not in use for comstations connected with the line with munication between other stations; in'which each; station has means. for applying to the line {a plurality of dilierin'g frequency waves to eachone of which a diiierent distant station of the line is'responsive, so that each station may select any other station on the line; in which the selection of acalled station bye calling station is accompanied 'by the production or'display of a signal; in which ineans are provided in the initiation ot-a call for automatically testing the frequency selective communicating ehannels; appropriating one thatis not in use; in which the automatic channel testing andselecting operation is an automatic switch that may he located at the called station and maybe made operativehythe select on of that station; in which a special calling high or other frequency channel is employed for effecting the selection of a particular called station by a calling station; in which the selection of the difl'erent stations through the medium of the special-calling channel is accomplished hy' the 'lnodulation ot the carrier frequency of. the channel with difierent lowerfrequeucies to each one of which a different one of-the stations is selectively responsive; in whichtheselec'tion of an idle communicating channel by'the automatic testingand selecting switch at the called station is accompanied loyi -the identifica tion of the selected communicating channel at the calling station; in which the comple tion of connection with the identified channel at the calling station causes the identification of the said channel at the called station;-in which the completion ofconnection.

with the identified channel at the called stetion or failure to select" a channelcauses the restoration of the channel 1 testing "and seperfornied by a that station is returned.

of the invention in Eli tion or upon -tioned above,

other stations.

looting switch; in which a second selection of the called station is prevented until the automatic testing and selecting a )paratus of to norma in which all other stations are prevented from using the special calling channel while it is in use by any one of them; in which the calling channel is automatically made available for the use of other stations upon the completion of the station and channel seleciailure to select a channel; and in ivhich. the condition of use or disuse of a selected communicating channel at each of the two stations that are using. it is automatically indicated at the other station.

The foregoing are some of the principal objects that the system of the present in; vention attains. The foregoing and other features are all included in the present embodiment of the invention, but are capable ofseparate use where it may be desirable to include only part of the features that are herein disclosed. In addition, the system the preferred form in which it is disclosed. embodies a'munber of other features incidental to those menand incidental to its use as part of a comprehensive system providing for inter-communication between toll or I The manner in which the above mentioned objects and features are attained and realized will be explained in the following description otthe preferred form of the invention,- and the structures and combinations through which the objects are attained will be more particularly defined in the appended claims.

The preferred embodiment of the invention is illustrated in the accompanying drawings. in which Figs. 1 to 4 inclusive, arranged as indicated in 5, show diagrammatically as much of the circuit organization at one roll station of the system as is necessary to enable the invention to be understood. and Fig. 6 shows an alternative detail circuit. Tit-will be understood. without the necessity of full diagrammatic representation, that there are al plurality of stations connected with the toll line L, a total of four such stations being assumed in the following description to be present, andqthat each of these stations may be in all respects identical in apparatus andv circuit arrangement with the particular station illustrated in Figs. 1 to 4;,the only differences between the stations being in the use oi a harmonic producer at one of the stations and or" a harmonic reproducer at each of the other stations, and in the particular calling channel modulating frequencies whiclfcach station is adapted to trans mit and to which it is adapted to respond, as will be fully explained hereinafter.

Referring now to the preferred circuit organization as illustrated in Figs. 1 to t of? the drawing, only as much of the Wiring at a single one of the stations of the toll line is illustrated as is necessary to enable the invention to be understood; it being possible for one skilled in the art to which this invention pertains. by mere duplication and extension ol the wiring and apparatus illustrated. and without the exercise of inventiou. to organize a system of such capacity as may be desired. .\s illustrated iu the circuit diagram of Figs. 1 to l. each one of the toll'stalions or toll points is couucctcd with the toll line L through the medium ol a twoway repeating coil or conjugate coil 100. this coil terminating in an arlilicial line or network N that is constructed and arranged as is well. understood in the art. to balance and match in its'charactcristics the toll line with which connection is made. Extending from the windingsof this twoway repeating coil arc two pairs of conductors, T and ll. the pair 'T serving as the transmitting path to the toll line of the locally generated currents. and the pair R serving as the receiving path from the toll line of the currents generated at and transmitted from the other stations of the line. The transmitting and receiving paths each have connection wi h the local apparatu oi the four communicating channels (1 C. C and C" and with the local apparatus oi the calling channel .\t each ofthc toll stations there is also a connection oi either the transmitting. path T or the receiving path Rwith the base frequency channel BC The connection of the transmitting path T is with the transmitting branches TC, TBC. TC TC T i and TC of the various chan nels respectively: and the connection of the receiving path R at each station is with the receiving branches RC. RFC. RC. RC. RC and RC ot' the various channels rcspcctively. The transmitting branch apparatus of the base frequency (lumnvl BC is provided at only one of the toll stations. At the other three stations the transmitting branch apparatus of this channel is omitted and the apparatus indicated in comicc' tion with the receiving branch Rilfi is providedin its stead. At the station where the transmitting branch apparatus of the, base frequency channel, BC is provided. the circuit is closed at the points X and is opened at the points X At the other stations where the receiving branch apparatus of the base frequency channel is used. the circuits are closed at the points X and are opened at the points X. Obviously, in each case the opening of the circuits at lbc points X or X ,.as the case may be, in omitting one or the other of the groups of apparatus, is accompanied by a closing of the transmitting path pa'h B at the points X to make these paths continuous for the passage o'fcurreut to and T at the points X or the receiving branch TC of n) leg carrier frequency lower niodulliting from the other channel branches associeted with these paths. I

The organization of the transmitting and receivingbranches of the various channels 5 will be specifically described hereinafter. As a general description of-the organize t'ieny it may he suid that the transmitting ganized as to apply to the toll line the call quency by the V 7 keys KB, KC and Kl) respectwely Each of these keys in heme operated causes the application to the calling carrier frequency of the particular adapted to bring about the response of the creceivingapparatus oi the particulzir one of the distant toll stations that, lS llQlllQ called.

. The three station selecting keys KB, KC and KD have associated ivith them a pilot ongiierd lamp 79that is illuminated to in form the operator that the selecting apparutus is in use, either locally or at a distant station.

The receiving hranch'RC of the calling channel is so organizedthat the unmodulisted component of the calling carrier frequency received from the toll line brings shout th operetion'ot the relay 66, while the modulated component, if the modulating frequency is that to ivhich this particular station is adapted to respond, brings about i the operation of the relay 67. The relay 66 acts through the medium of the relay 58 to render ineffective the station selecting keys of? the comn'umicnting channel transmitting uuton'uitic selection of the ealled toll station of ad .itlle onelof the communicating channels.

in the oscillator OP. and its harmonics are trihuting circuit comprising the conductors 47, 48,'fro1n which separated out and applied ting path T byway of the branch TBG, I while the harmonies are selectively passed to the transmitting apparatus of the comthe calling channel C lSvSO orsupplied by theeoseih tortion of a base frequency SllpPllOQbY the.

Each of the niodulating MC and MD is macleopera 9 tire to apply its -particular modulating fife-y pressing of corresponding;

1mg conductors- 85 l'1'l()( ulatiugfrequency Kill. KC and KD, end to light the key guard E lziuip 79 to maintain the existing condition pr g i ksL apparatus. The relay (l7 acts through the nu-diiun of the relay 53 to bring about the c inform the calling tolloperator The Apparatus in the transmitting branch THC of the base frequency channel BC is l 4 so organized as to distort in the harmonic producer HPH e louse frequency generated The base frequency conducted to a disthe base frequency" is to the transmitto notify theoperator at th n unicating channels C C (Pjand G to supply the earrier frequencies for conveying the speech and signalling modulations over the respective channels.

The apparatus in thejroceiving branch REC of the base freqi iency channel BC is so orgenized lthat the base frequency received fromthe particular one of the four toll stations herelthehase frequency producing a'pparatusis located, is separated, out from the other-carrier frequencies passing over the receiving path R, is amplified, passed through a harmonic reproducer HRP, and the harmonics of the base frequencyfthus produced are supplied to the distributing conductors 4T,;4t8,to supply the carrier frequencies required by p the eo'mmumcatmg channels C, C C and G.

The organization of each of the eommunicatingchannelsfCl, C C and C is such that th voice currents received from the local telephone circuit by way of the talkand 86ers passed'through the two-Weylrepeating coil- 33, caused to modulate the carrier frequency received from'the distributing conductors 47, 48, and the modulated-carrier frequency is" conducted tothe transmitting pat-l1 T. The

organization of eachof -the communicatingchannelsis alsolsuchthat the characteristic carrier frequency to whiehzthechannel is adapted to respond is separated out from the e quency components are'conveyed by way of the two-way repeating coil 33 to the local other frequencies passingfover the receiv- 19 acts through the medium of the relays l6 and '10 to control'the signallingend channel selecting operations. I Each communicating channel w th its lowjfrequency extension constitutes a, trunk for estdblishing communication withanother' station."

nch of theeommunica-tin "ehannels'C,

path R, is detected, and the voicefretalking conductors '85- and 86, While the carrier frequency components are divert-ed and caused to operatethe'relay 19. The relay C; C 'and'C is terminate locally upon I 33nd4'res"peotively. 'With these four spr ng jacks are essociated the assigning lamps 11, 21; 31 11nd 41 and the canin lamps 12; 22, 32 andl tQ, respectively. The- Emctionof the assigning lamp isfto of the particularchannel that has been selected for. use; and the function of is to inform the operator the called toll station of the receipt of a, call upon the corresponding channel spring the spring 'jeckterminals of the communi- I I jack, Asso elated Wlllll the channelterininals and their corresponding assigningand calling lamps,- there is acall pilot lamp 27 the calling lano'pl,

cating channels the connections are extended .to spring 130k terminals 77 of local stations SS by means of a plurality of connecting.

cord circuits 70. of which there may be enough provided to utilize all of the chair nels simultaneously. or as many of them as are not at the time in use between other stations. Obviously, connection between the communicating channels and the local lines may be cll'e cted in any other desired way of the many that are well-known in the art; as for instance, by means of circuits involvin the use of automatic switches.

he selection of an available couu'nunicau ing channel, in the particular embodiment of the invention illustrated. is cilected by means of an automatic switch S that is set into operation by the energization of the relay 5 3 under the control ot the relay 6? which is responsive to the detected modulating component of the calling carrier frequency. The movement of this switch in its channel testing and selecting operation and the lighting of the lamp signals associated with the channel that is selected for use are under the control of lays 10, 13.

group re 14, 15 and 16 associated with the channel C and like groups of relays associated with the other communicating cnannels. As-

sociated with the channel selecting switch 5'3 are a stepping magnet 60. a release magnet 62, and the switch controlling relays 54c and 56. The channel selecting switch S with its associated magnets and relays, as Well as the relays 50, 51, 52, 55, 57, 58 and 59 that cooperate in the control of the channels, are common to all of the channels of the toll station. I

The organization and operation of that part of the system which acts to generate, modulate, transmit, receive and de-rnoclulate or detect the high frequency carrier currents employed in the systeiuis as follows: The frequency of the carrier waves selectively utilized by the communicating channels is determined, in the present embodiment of the invention, by frequency generated by .a thermionic or vacuum tube oscillator .0? located at one of the toll'stations or at some point upon the toll line. This oscillator may be of usual and Well known type, such as the aiulion type, with a teed-back connection from its output into its input circuit. The frequency of the oscillatory current produced may be regulated by an adustable piondenser, and may conveniently epsay,

ve oscillator OP supplies subst-antiall sinusoidal oscillations to the input circuit of the harmonic producer HPH. An inductance 101 is bridged across this circuit. he magnitude of the oscillations supplied is such as to overload the thermionic tube a base or control thousand oscillations per second. The

asagna H'PH; that is, to supply an input wave of greater amplitude than the tube can repeatwithout rlistortion; The repeated osci lla- ,tionsin the output circuitot the tube HPH are theretm-c of non-sinusoidal'wavc form. which is the equivalent of base frequency current of sinusoidal form and a series of currents of frequencies which are multiples or harmonics of the base frequency. The plate current supply of the overloaded tube or harmonic producer HPH is through a circuit comprising a resistance 102 and an inductance 103 which constitutes an impedance that is large as compared with the impedance between the plate and filament of the tube. This impedance relationship assists in producing distortion and in stabilizing the output circuit. put circuit of the harmonic producer HPH is connected with a distributing circuit 47, 48, by means of a transformer 104, and tho distributing circuit is thus supplied with both the'base andthe harmonic frequency oscillations. Fromthe distributing circuit 47, 48, the base and harmonic frequencies are selected by means of tuned circuits'lead ing to the transi'nitting branches of the various communicating: channels. and the transmitting branch of the base frequency channel. The inductance of the transformer 105 of the base frequency trans" mitting branch and the capacities of the variable condenser associated with it are so adjusted that only, the base frequency oscillations are selected. Similarly the constants of the transformers and condensers of the other tuned circuits leading from the distributing circuit are so adjusted that The out each of these tuned circuits selects a. ditterentone of the harmonics of the base frequency?" The harmonic frequencies selected by the transmitting branches of the communicating channels C "J", and C may be ten tl1ousand,'fifteen thousand. twenty thousand, and twenty five thousand respectively.

The base frequency oscillation selected from the distributing circuit by the tuned circuit including" the transformer passes through a potentiometerllOG, from which it is supplied at the desired potential to the input circuit of the amplifier HA. The output circuit (it the amplifier HA is inductively coupled with. the terminals of a band filter IlF through which the amplified base frequency oscillation passes to the transmitting path T. The band filter HF is of the well-known type described in Patent 1,227.113 to Camp ell. patented May 22, 19W. The characteristics at this filter, which consists of similar sections having series and shunt capacities and 'inductances is such that it transmits oscillations of the base frequency with little attenuation, and

' 2 .tive channels.

--lectedis amplified by the amplifienHRAi,

y efiecuvely diverts from this branch currents.

ofh'equencies differing more thana slight amountfrom the base frequency. a

As has been stated, the base frequency 05-. I

i cillator. ;is provided at blltm one? station or point on the toll line. At the'other toll stations-or points the base or controlling frequency thus supplied to the line passes channel BBC is provided with a tra'nsform- I "er 107 withassociated condensers to 'consti time a tuned circuit-that acts to divert the J lfl' base "frequency oscillationzfrom the receiving path to, the amplifier HRA by way of vthe potentiometer 108, while permitting the free passage of, the other carrier frequencies through the receiving; path to "The base frequency thus seand the amplifier oscillations areimpressed 'upon the input-circuit of the thermionic tube HRR' The magnitn'deof the oscillations aresuch asto overload this tube, with the result that, as in the case of the tube HPH associated 'with the base frequency; I

extendlng to the spring jack terminals of oscillator OP; the repeated oscillations are l distorted into'a series of currents the fre- :quencies ofwhich-arethe harmonics of-the base it'reque'ncycurrent, the tube HRP thus, acting as a harmonic i-egen'erator. The base freguency and its harmonics are then arm 3 pli ed by theainplificr HRAfand are impressed upon the distributing circuit -17 48 throggh lthetransformer 109. From the distributingcircuit the harmonic frequencies' of the base or controlling oscillation are selectively su plied'to the transmittizg '"tnanche'sof the c anne1s.C -C ,"C ..and

way of'the tuned circuits-ifprev iously-irestation Where the base frequency os'cillator'jOP. and the; harmonic producer HPH areF-located -the" *ferredto. Thus atfthe toll communicating channel carrier frequencies are supplied d1rectly',"whil e"-at the other sta- .tions they are suppiiedjfby a harmonic regenerator HRP controlled by thebase he l1ne.. Consequently the correspondingcarline. The organization illustrated and described for producing a plurality ofharmonic carn'ier frequencies; from a, base frequencyby distorting that frequency, and. for insuringidentical frequencies 10f the carrier oscillations employedat each of the various quencies at all points from a, single base frequency, is substantially in accordance with the OigfilliZfliZiQllwdiSOlOSGd in United Statespatentto Kendall No. 1,446,572, Feb- 1mm 12.7,. c1223, land. British aten No.

I invention.

the station into the receivinpath R. The

their' 'respec- ,85 and .86 are consequently pressed at the proper potential upon the inquency oscillations transmitted over the toll toll stations by controlling" the carrier fre- 133.,426, respectively. The method and organization described above for synchronizingcorresponding carrier waves at all stations is the preferred Way of securing this result for use in connection with the present But: it will be understood that other ways of securing substantiallyidentical frequencies of the carrier Waves at the difi'erent points at which they are applied may be used.

At each of the communicating channels 5 0 C and (3* the particular carrier frequency diverted from-the distributing circuit 4.7, i8 by ineansof the. corresponding tuned circuitisapplied to the terminals of a potentiometer 110 from which the carrier oscillationsat the desired potentialare applied to thein put circuit of an amplifier HAG. The output circuit of the amplifier is connected by Wayof the potentiometer 111 with the input circuit of'a modulator M0 The input circuit of the modulator MC also has connection byway of the inductive coupling 112 j and the potentiometer 113 with the appropriate winding of thet'woway repeating coil 33 in the low frequency or talking circuit. The talking conductors and relay contacts with a balancing netv.work, the characteristics of which are made to match the characteristics of the low frequency, lines adopted to be connected tvitli' the channel. The voice-currents. that reach the repeater 33 by rayof the conductors repeated and imput circuit of the modulator M6 and'are thcre combined with the carrier frequency derived from the distributingcircuit 47, 48

to cause correspondingly modulated current ,to 'fiow inthe output circuit of the module tor. The outputcircuitoi the .nrodulator is connected through an inductive coupling 114 with a hand filter MEOrof the type dis closed in the Oainphell patent previously referred to; The characteristics of thisparticularform of theCainpbell band filter are such that it suppressesany currents of the frequency of the voice currents inthe talk= {ingco'nductors ofthe channel whichmight be repeated as low frequency currents by 1 the modulator. ,Thecorrespondmg band filter ofeach of the channels serves to transmit currents of a range of frequencies inlow frequency ortalkin circuit. i

' 'rhe band filters Mltof n r ery? frequencies extending. up and down of the other channe s "of the base Frequency channel municating channels, the band filter HF of the base frequency" channel BC, and the band filter MFG of the calling frequency channel C are allarranged serially in the transmitting path 13. 14: and the constants of these various filters are so adjusted that each filter passes freely to the transn'iitting path the carrier frequency from its own channel while at the same time preventing the frequencies supplied through the filters trom being diverts into its ehanneh The hand filters DEC of the receivingbranches of the various Iconiinunicating channels C. C, G aud C the tuned circuit ot' the receiving branch BBC BC, and the band filter DEC of the receiving branch RC of the calling channel C, are similarly included serially in the receiving path 'H extending from the. t=:-:o-wa v repeating coil 100; and the constants of the filters and the tuned circuit are so adjusted that each diverts from the receiving path It into its own channel the particular frequency to which the channel apparatus is adapted to be responsive, while perr 'itting to pass frecly through the series circuit the frequencies for which the other channels are adjusted.

The receiving branch of each of the Conn inunicating channels is provided with a band filter DFG adjusted, as stated above, to divert? into the channel the characteristic frequency band of that channel. The oscillatorycurrent 'thus diverted is supplied through a potentituneter 115 to the low side of a shielded transformer 116, the high side of which is included in the input circuit oi. the detector DC. The C potential for the grid of this detector tube is derived from the filament battery by including a resistance 117 in the filament current supply cir" cuit beyond the point where the input circuit of the detector is connected with the filament circuit. The output circuit of the detector DC is connected to the input circuit. of the amplifier AC through the me drum of a transformer 118 which is designed to transmit eliicicntly the voice frequencies obtained t'rom the demodulation of the modulated carrier wave. as well as the umnodulated carrier component. The output circuit of the amplifier Ao is connected to the low pass filter LPG. designed to pass the. voice frequencies and exclude the carrier frequencies, by way of a. transformer 119. The other terminals of the low pass filter are connected to the l)1l( ge terminals ot' the twoavay repeating coil 33, where the voice currents are divided between the talking conductors SC) a d S and the balancing network N in such a in; as to have their eil'ects neutralized in the coil oi the' repeater that is cminected with the outgoing transmission branch of" the channel. The output circuit of the amplifier AC also has a branch pathing path R of each uncut-v t'iscillator OP. and may be su through a high pass filter HPC which acts to pass the unmodulated carrier wave component and to prevent the passage of the de-inodulated voice frequencies. The other teri-ninals of the high pass filter HPC are connected with the low side of a transformer l'ltLthe high side ot which is connected in the input circuit. of the rectifier tube HRC The output of the rectifier is connected with the winding of a relay 19 which is adjusted to res-pond and hold its contacts closed as long as the associated communicating channel is receiving the carrier frequency which it is adapted to divert from the receiving path R. The filament and plate energizing currents are supplied to the apparatus of the receiving branch of the channel under the control of a relay 1?, and arefsnpplied to the transmitting branch of t'liei channel under the control of a relay 1S. llie'snppl circuits are so arranged that'the filament and plate current is normally disconnected. and is applied to the receiving and transmitting branch apparatus only when the correspomling relays grlzed.

17 and 1S enerhas previouslv been stated, the receivtoll station is arranged to receive a calling carrier frequency by way oi? the transmitting branch TC of the calling chlinnel the calling carrier being inodulated by lower frequencies in accordance with which o i the modulating branches lii B, MC and MD is made operative. In accord unce with the present embodiment of the invention, the "ailing carrier frequency is supplied by an oscillator DC the output circuit or which is connected by way of potentiometer 121 with the input circuit of the modulator MC. 'lhecalling carrier may be of any suitable frequency that ditlers selectively from the frequencies that are used by the base channelllG and the tour communicating channels C C C and C If the base channel frequency is live thousand and the tour communicating channel frequencies are ten thousand, fifteen sand and twent vdi've thousand respectively, as has been assumed. the trequencj' to which the calling channel. is adjusted may be thirty thousand. The calling channel trequeue is illustrated as being supplied to the modulator MC by aseparate oscillator ()(l; but obviously, if desired, the calling carrier, frequency of thirty thousand may be one oi the harmonics of the base tre- 'iplied to the modulator MC by 'a cor esponuingly tuned bran-eh ot' the distributing circuit ti',

-18. This alternative arrangement dia' granunatically indicated in Fig. tl where the tuned circuit or filter CF is selective ot' the calling am. frequency and serves to derive the harmonic of this trequenc from the distributing circuit &7, l8 and to apply it. to

" is connected with} the input circuit of the,

masco the modulator MC. The rest of the circuit is as shown in Figs. 1 and 3, the oscillator HOG being, otcourse, omitted when the arrangement of Fig."6"is*used.

The modulation of the calling carrier Wave is efl'ected'by lower frequency currents produced by the oscillator O and the harmonic producer HPL. The oscillator O as in the case of oscillators OP and DC is of the wellelgnown type having a feed-back connection from its output circuit to its input circuit byivay' of the inductive coupling 121. 'The output circuit of t he oscillator harmonic producer tubeHPL. The mag- "nitude of the oscillations supplied is such as to merload the tube HPL, thus "causing the tube to distort the sinusoidal wave of the oscillator Q and produce, an its. output circuit non-sinusoidal currents that are the equivalentofthe'base frequency current of the oscillator together with theharmonics of the base frequency. This series of. currents is sup plied way of transformer 122 to the disa The various of the modulatingcircuits MB, MC and MD.

From the otentiometers the oscillations: of the selected frequencies are conducted to the input circuits of the amplifiers A A and A The output circuits of these am plifiers are connected in tuned branches of the modulating circuit M, the tuning of the branches being such as to permit the passage to the modulating circuit of the frequency of the corresponding modulating branch and to exclude irorn thebranch 1n question the frequencies of the other modulating branches that i'eedinto the modulating circuit'M.

' The modulating circuit M is connected with terminals of a transformer 126, the other terminals of which are connected With the input oircuit of the modulator MC.

The modulating. branches MB, MC and are normally;inoperative and are made operative under theycontroltof thea'ssoeiated I relays 8 and U respectivelxthese relays actingiu hen operatedzto apply current from he filament and p-late batteries F and P tohenergige the. apparatus of the correspond ing branches. The relays 7, 8 and 9 in turn are controlled respectively by the keys KB,

KC and KD, Which are the station selecting keys of the organization. 0 and harmonic producer-HPL are also normally inert, and are energized through the energization of relay 69, which operates. tdi eonnect the filament and plate batteries g input circuit of an amplifier A tothe arms-.

The oscillator,

l The oscillator 0 may be arranged to produce oscillations of any suitable fre quency as, for instance, 200 cycles. As a result of the actiouof-theharinonic producer HPL there is transmitted to the distributing circuit 93, 99the base frequency 200 of the modulator 0 together with the harmonic frequencies thereof, If three modulating paths MB, MCand MD are employed, these" paths'have their constants adjusted to divert from the distributing circuit, amplify and pass to the 'inodulating circuit M the three harmonic frequencies lOO, 6.09 and 800. 1i additional modulating branches are to be used, they maybe adjusted to employ the base, frequency 200 and other higher harmonic frequencies.

,As will subsequently be explained, each of the toll stations is adapted to have the receiving branch oi its'callin'g channel selec tively responsive to a particular one of these transmitting branch of the calling channel at each station is arranged to transmit the call ingccarrier frequency and as many lower frequency modulations thereof astliere are distanttoll 'stationsto be selectively controlled or" signalled. The transmitting branch of the calling channel at each toll SllitlJlOIrlS of course arranged to modulate the calling channel frequency only with the lower frequencies to which the distant toll st ations responsive, and not with the lowerfrequency to which the particular toll statii'in itself is responsive. Thus the par-- ticular station illustrated inthe drawing is assumed. to be responsive to the 200 1nodulat-, ing frequency, and consequently the B10631- lating branches are so arranged as not to apply'tlie 200 frequency to the modulating.

path M,zbut to apply the three harmonic frequencies 400, 600 and 800. Atthe toll eta-'- tion responsive to the lOlliirequehoy the modulating branches are arranged to em ploy the 200, 600 and'SOO frequencies; and similarly at the other stations the particular modulating frequency to which that station lower modulating frequencies; shjthat the 'r ice is adapted to respond is not arranged to be applied to the transmittingbranch.-

At each of thetoll stations thereis a con necticn from the receiving path Rhy way of a band filter DFC anda shieldedtrausformer 12710 the input circuit of a detector A IX). The BEHdiiltcr DFC is designed to 7 divert into the receiving branch of the calling channel C the loand f frequencies in eluding the calling channel 'frequeney, in this instune'eassuined to he thirty thousand, its modulations extending up and down from this frequency through a range inc determined by the modulating frequencies derived fronithe'moduleting path The output circuit of the "detector i3 5} is connectcdthrough a transformer $28 :Wl?ll H p mes-mite 5 calling channel controlling keys and cir-- cuits are held ineffective for a of time.

further period The lighting of the assigning lamp notifies the operator atthe calling station of the selectedchannelis energized and the char acterlsti'c carrier frequency is applied. at the calling station. To the application of the carriertfrequency at the calling station the receiving apparatus'of the selected channel at the called station .responds and causes ,-the, lighting of "the corresponding calling station, thus causing the supervisory lamp lampassociated'with the spring jack terminal of the selected channel at the called station. At the same time the carrier.fre-

quency isi withdrawn' from the transmitting branch of the selected channel atthe called "in the calling operators cord circuitto light to indicate to that operator that the operator atthe called station has been signalled but has not yet responded. i When the oper lamp associatedtwith the operators connect ator at the called station respondsby plugging into the s ring qjacki-of the selected channel indicate by the lighted calling lamp,

the calling lamp and the calling pilot lamp;

at the called station and the supervisory ing cord circuit at the calling station are extinguishedfandat the same time the chan- 1 nel selecting switch at the called station -is released and returned to normal. 'When v gthe connectingc-ircuit plugs areinserted in. the spring jacks. of the selected channel at the calling and called stations, the key or,

guard pilot lamps at the-respective stations are extinguished and the calling channel controlling circuits at these stations are againmadeavailahle for use. The extension of the selected'channel to-local lines at the callingand called stations is effected through the medium of the conndcting cord circuits atthese stations, and the;withdrawa1 of-the connecting circuit plugilfrom the channel spring jack at either 11 station causesthe lighting vof the corr spo'nding supervisory lamp in thecord cir uita't the other station. Ifthe channel selecting switch at the v calledstation fails to select a channel, the

calling carrier frequency is automatically withdrawnat the callingstation, and the 'chan'netselectin switch at the called stad v connection of. the plate battery P and the The operation of the system of the inven I 'cuits'yof thereceiving branch RC ot the tion is automatically released.

tion now be described in detail. When an operator atone of the toll points 01''. stav .maike the ireceiving branch of the calling "toll station tions has occasion to utilize thetoll line for estahlish'ingconnection with .anothertoll sta Ition, she first vpressesthe particular one of the three keysKB, KC- or KD corresponding to the distant station that is to be called.

is locked '50 is also operated.

If it is assumed that the distant station through whichthe connection is to be estali -l1shed .is the station D, then the operator ymomentarily depresses the key KD. On

the assumption that mother toll station is at the time calling, this closes a circuitthat of relay 50, 'upper'armature and back con; tact of relay 52, 'upperarmature and back contact ofrelayc 51, innerjcontacts of timer 132, conductor 34, winding of relay 9, mo mentarily closed contacts-of key KD,conduc extendsfrom battery through the winding tor 33, and bacjk contact and armature of relay 58 to ground. The effect of the closure of this circuit is. to energize relay 9, which thereupon locks up toground byway of its upper armature *andfro'nt contactii If it pressed some other call over the toll line at theincluded contacts of may 58, relay 51 or relay 52 as will. hereinafter he de scribed. But-"under. the present assumption thatthe callingcircuit is free for use, the circuit tracedahovejis closed, the relay 9 as described," and the relay "The relay -50 in operating closes at its contacts a an energizing ci'rcuit for relay 57; and that relay inoperating closes at its had 'happenedthat'at the time the ,keywas inner frontcont'act and; armature an energizing circuitfor the key p'iiot lamp 179. The illumination of thislamp persists until the apparatus at the' called toll station has selected a channel for useandldentified the" selected channel by causing the lighting of the associatedassignmglamp at the call 'ing tollboard, as willhedescrihed, Belay in operating also closes a circuit that extends from, battery through the outer armature and front contact of relay 57, 0021- doctor 49, and thence to ground in I the conductor 84 and the left-hand 18 associated withachannel that is at time in useat the calling toll station, This isfor the purpose of preventing any alter. ation in the condition of use "of a channel during' the testing operation that is about to take place.

- Relay '50 in operating also, at its contact.

6, applies grou'nd 'to conductor 37 that has parallelfbranches from battery extending through thewindingsoffrelay 68 and 78.

mo ulating frequency to ,whiclith'e calling tollstation is normally responsive, and thus way of locking contact c and winding of: any I of the relays the me i

to prevent any possible operation of the associated relays 66 and 67 'while this particular toll station is calling. At the same,

' of the calling channel C. The closure of the filament and plate battery circuits makes operative the oscillator 06 and the modulator MC of the transmitting branch TC, and as a result the calling carrier frequency is applied to the toll line.

The modulating frequency determined by the pressing of the key KD is not immediately applied to the calling carrier wave, but awaits the operation of the timer or slowacting relay The energizing circuit of this relay is closed at contacts (3 upon the operation of the relay 50; and when, an interval after the operation of the relay 50 deteru'iined by the time required for timer 55 to draw in its plunger, the inner contacts of timer 55 are closed, the energizing circuit is completed for the operation of the relay 69. This relay in operating connects the associated filament and plate batteries with the circuits of the oscillator O and the harmonic producer HPL, which thereupon operate to generate the pre-determined base modulating frequency and its harmonics. These modulating frequencies, as has been described, are selectively separated out by the dill'erent appropriately tuned circuits -an d are applied by way of the associated inductive couplings 95, 96 and 97 to the modulating branches MB, MC and MD of the transmitting branch TC of the calling channel 'C. The only one of the modulating branches that is at the time energized is, under the present assumption, the branch MD corresponding with the key Kl) that has been pressed. Therefore the modulating frequency which this-branch is adapted to transmit is applied to the input circuit of the modulator MC associated with the calling channel oscillator ()0. As a result the calliug carrier wave that is applied to the toll line is modulated by the particular harmonic frequency of the oscillator O that is adapted to be transmitted through the modulating branch MD. The effect of the application to the toll line of thecalling carrier wave carrying this particular frequency modulation will be described in detail hereinafter.

When relay 50 operates'as a result of,

pressing one of the station selecting keys, it applies battery at its contacts e, f, g and oi; to branches 83 extending to ground through the windings of each of the relays 1719f the four communicating Schannels. The relays 17 of all of the communicating channels are thus held operated during the period of energization' of the relay 50, and act to energize the receiving brain-hes RC, RC RC and RC of the 26. etc., in operating close energizing cir-' cuits for the associated relays 10, 20, etc., that extend from battery by way of the right-hand windings of these relays. At

the same time that the relay 50 causes the energization of the relays 17 of all the channels to cause the response of the relays 10, 20, etc., of the channels that are in use, it also applies at its contact d a ground to conductor 89 which has branches to the lefthand .armatures of the relays 10, 20, etc., of the various channels. This causes alocking circuit to be completed from battery through the left-hand winding of such of the relays 10, 20, etc., as may have become energized as a result of being associated with the channels that are in use, so that these relays arepreventedfrom changing their operated condition during'the succeeding test and as- 4 An interval after the operation and locksignme'nt of a selected channel.

ing up of the relays l0, 20, 'etc.,'assoeiated with busy channels, the timer. 55 completes the inward movement of itsplunger and applies batter Y to the conductor 39... This conductor has ranches to the outerright-hand lays 14,524, et which is associated with a relay 10;20, etc., that has not been operated,

has its energizing circuit closed as soon as the timer 55 closes its inner contact; and such of the relays 14, 24 etc., that are thus ener ized are locked up byway of the associated rent back contact of each of the relays 10, 20, etc.,

contact and armature, resistances 14, 24'.

etc., and outer lower back contact and annature of associated relays 15, 25, etc., to battery. The channels which atthetime are idle and available for selection are thus marked or identified by the fact that their associated relays 14, 24, etc., are operated and locked up; the energizing circuits of the corresponding relays. associated with the busy channels being held open at this time- 'by the energized condition of the associated relays 10, 20, etc.

Turning now to the operations that take place at the toll points orstations other than the one which is calling, the calling frequency oscillator ()C of calling branch Qmp''h he pressing of key KD, causes .tl1g /"i"es onse of the relays 66 ass he application of toll stations oi-the line.'

- lilacs sociat edj n'ith the receiving branches RC 58in operating opens atits back'contactand relay 57.

. op erat-ingalso appliesbattery to coiidnctor 1 49, which. conductor,

scrihed,has branches 84 to lockingcontacts armature the path by way of conductor 33 over which thefljinit'ial energization of the modulating branch relays 7, 8 and 9 is ef- .fected V I KB, I C*fan(l KDare'deprired of control so that the station selecting Keys over these relays and over the associated re? clay in the path common to the branches through relays? -8 and 9. \t :the same time re'layi58 in operating closes at its armature and front contact the-energizing circuitof ,Itelay' 5?" in operating appl es bat :tery t0"conductor,-3 5 to illuminate the key .pilot l'amp 7 Oonsequentlythe instant one ofthe station selecting keysis pressed atany,

of the toll points, the "station selecting keys at all of the otherltoll"points'are madeineffective, and the associated keypilot .lanip 79 is lighted to warn theoperat'orsat these sari-1; yous other points that thecalling apparatus isjunavailahle fortheirzown use. Each jot io' therelays 57 attheseIother toll. points :in

as hereinhefore de- "of each of the relays 18'of all ofthe'chann'els and C of the associated station.

.herefore the pressing of any of the sta- -tion selecting keys at a calling toll station has the effect of causingthe continued ener- I otthe relays -1-8- associate gization, tlin'ingthe calling ogeration of all with chain nels that are inuseateach of the other toll stations.

The. result'lis to keep all of ,the channels in the condition of use thatthey .were inat theinstiintthe station selecting key at the calling station was pressed, and;

to maintain the'ur in this condition until the end of thechaniiel selecting and assignv energize the oscillator Q ing operation. I

As has previoi1sly"heen' described, the timer at the call'i-ng station operates upon. the-completion ofthe inward:movement of its plunger to energize 1"elz'1y..69fa1ul thus and the harmonic producer HPL "to generate the. base and harmonic frequencies, one of which, in the present instance that controlled by the relay 9,; isapplicd to modulate. the calling carrier wave. At theparticular'toll station that is being called the modulating frequency 'is separated out and passes. by way of the filter. LPG; tuned to this particular fre qnency to the associated rectifier LRO, and is there rectified to cause the response of the 1 associated relay 67:] .The filters LPO at the other toll stations, {on the line are tuned to prevent'the pass hf this particular freconductolrfii to keepihe station selecting quenc y and therefore their relays 67 are not operated.

of stepping magnet of channel selecting switchSj, armature and back contact of this magnehlwinding of slow-release relay 54, right hand front contact and arnmtnre of rc- .lay. 53, and switch arm SK audits normal or resting contact to ground. Magnet 60 in operating interrupts at its armatureand back contact this energizing circnit, andthe armature of magnet 60 in falling hack acts through the medium of the stepping pa W128 andratchet Wheel 63 to cause all of the switch arms to take one step. As soon as the At the toll station that isheing a called, therelny 67 in operating acts through "the medium of relay 53;} to close arircuit that extends from battery through the winding armature ofrelayh't" for maintaining (luring the operation of-switch S the illumination of the select ng key pilot lamp 79", disconnects the energizing winding of relay '50 from k'eysKB, KC and KD from exercising control, over the relays 5,0,, 7, 8 and 9 (ifthe callingci ciiitaslong as the switch -S' is off of no1 n1al;qand the closure of the circuit throughsthe calling pilot lamp 2'? lights that 190 lan pto indicate to the operator at the called toll station that a call for her station is coining. j: r J lipon the ,first step taken by switch S, a

circuit is closed that extends from hatterv 9 through fnorlnally closed contacts 91 sociated' with tlileholtling pawl of the switch S, switch iii-ms", conductor 83. and-whirling lof relay lT ol' the lirstcl'nmnel (,l to ground.

If the first .coimnnnicating channel C hap- 1m pens to (he in use locally at the tlll'lt the relayv l7 is alrca'dy energized by a branch energizing circuit closcd'by' the insertion of aplug in the spring jack 1 of the channel, as will; hereinafter he described. 17 or the channel is not already energized, it will. be put in its energized condition by the advance" of the switch arm-S to the cor-v responclingcontact point, and in becoming It the relay energized it connects the filament and plate l batteries F and P with the associated apparatus o'tthechannel, thus making the receiving branch-RC of the'channel responsive to its particular carrier wave frequency.

Consequently ,if the particular channel coriresponding to 'thiscarrfer', wave frequency is in'use atany of its toll station branches,

the relay: 19 associated with the receiving branch of the'channel C at the called at];

tion is energized and closes the energiiaing circuit of relay 16 by way of conductor 81,

I the-relay 16 thereupon causing the energizaing channel are not operated, and the ener:

as long as the switch arms 'arc in engagement with contacts as-' sociated with channels that are in use either locally or at distant stations. lVhen the switch arms encounter contacts associated with a channel that is not in use, the relay 19 ot the particularchannel tested isnot operated when it is made responsive by theoperation of the associated relay 17, As a result the associated relays that correspond to relays 16 and 10 of the first communicattnzin circuit of'the ste in ma 'net 60 v D h b terminating upon switch arm S is opened and remains open. When this condition comes about, the continuing interruption of the stepping circuit permits the slow release relay 54, which may be of any desired slow releasing type, to release its armature and close a circuit that extends from batterythrough contacts 91 associated with the holding pawl 'of switch S, back contact and armature of release magnet 62, armature and back contact of slow release relay 54, switch arm S and the contact upon which it is at the time resting, the conductor of the communicating channel under test which corresponds wi'th conductor 82 of channel 0, and the winding of the relay 18 of this channel to ground. The consequent operation of relay 18 of the tested channel connects the associated filament and plate batteries with the transmitting apparatus of that channel to energize the same and cause the applia 'ation to the toll line filhe corresponding carrier wave fremicncy.

Returning now to a consideration of the operations at the calling toll station, the ap plication at the called station of the particular carrier ave frequency'of the selected channel, as described above, causes the response of the relay 19 associated with the receiving branch of this channel at tl calling station. Assuming that it is channel C that has been found idle and available, the operation of the relay 19 of this channel causes the operation of the relay 16, which in turn causes the operation of the associated relay 10. a It will be remembered that only, each of the relays 10, 20, etc., at the calling station have remained unenergizcd as were associated with idle or available channels at the time that tit e operator at the calling toll station pressed the station selecting key,

and that the only ones of the relays 14, 24, etc., that were operated and locked up by the operation of the timer. 55 were thoseiwhose, associated relays 10,, 9.0, etc., had remamed back. Therefore when the relay 10 of one of 'these channels, in this case channel G is operated as a result of the application of i the corresponding carrier frequency at the" called station, a' circuit is closed that extends from battery through the outer lower armature and back contact of relay c l5, ;re-

sislauce l tfiarinatiire and 'front contact of rclay 14, enter right-hand armature and front contact of relay 10, andwinding of relay. 13 to ground. Relay 13 in closing its outer armature andffront contact appliesparatus of the channel C energized after the relay 50 releases, which occurs a moment later. Relay 13 1n operating also closes acircuit that extends from battery by way of the inner armatureand front contact of relay'13, assigning lamp 11, and the lower winding of relay 51 to ground. The illumination of the assigning lamp 11 notifies the cahing toll operator that the channel that has been selected for use'in response to herkcall is the channel terminating in spring jac 1.

Relay 51 in operating closes: at its inner front contact and armature a path for maintaining the illumination of the selecting key pilot or guard lamp79; and at its outer armature and back contact opens the en'ergizing circuit including the windings of relays 50 and 9 closed at thetime selecting key KD was pressed. These two relays thereupon release. Relay 9 in releasing de-. energizes the associated modulating,branch Ml), thus withdrawing from the calling carrier wave the modulating frequency to which the distant station 1) hasresponded. hclay 50 in releasing opens at its contacts r, f. 1 and It the branch energizing paths by way ,ol these contacts for, the relays 17: of all of the communicating channels C, C, (-J and C". The relay 1? 0i. channel C, however, remains energized byway of the path completed through the outer front con-' tact and armature of relay 13, as previously explained: and the relays 17 of such of the other channels as may have been in use locally at the time the operator pressed the calling key remain under the control of the branch paths completed from their con ductors 83 through cont-acts closed when a plug is inserted in the corresponding channel spring jack. 1 I

Relay -50 in releasing as a result of the operation of rclayifil 11 )On the lighting of the assigning lamp 1], also opens at its contacts (I tlic cncrgizing circuit of relay 57, and

hand 1 Win incense that .relay in releasing opens conductor 4:) to permit the relays l8 0f'tl1e various channels thereafter to be under the exclusive con-' troloftheir energizing paths byway of the associated conductors 82. Likewise re,--

lay"5.0-at;,its contact d opens conductor 89 to break. lockingvpaths by way of theleftof relays 10, 20, etc, and thus leave relays under control of their right-ha11dwindings the relay 50 in releasing interrupts conductor 37, thus releasing relay 78 to withdraw the calling carrier frequency from the toll line,

and alsoreleasingrelay 68 to reapply the associated plate and filament batteries and re store the normal responsiveness of the re cciving branch RC. otthe calling channel (3. At its'contact a relay :50 opens the energizing circuit of timer 5.5 {and the timer in releasing its plunger first disconnects battery from the conductor 39, and then at the limit of the outward movement v of the plunger v connects the conductor .39 to ground. This application of ground to con i doctor 39' shortcircuits such of the relays 14, etc., as have been ;,locked up as a result of being associated [with available com:

.inunicating channels and whose associated channels have not been chosen for usein the selecting operat on; These relays tlierefore release, theirelay lhhowever remain ing locked up by reason of its associated relaylO havingbeen operated upon the se-i withdrawn as aresult of the selection of an available communicating channel, as described above, the relays 66;and 58 at all of the distant toll stations are released. The relay 5.8 at eachof the distant stationsin reieasirig applies ground to the associated conductor 33 to make the station selecting keys at that toll point, again operative, and

also breaks the energizing circuit of theassociai'ed relay 5?. Relay 57in releasing cxtinguishes thekey guard or pilot lainp T9 and also'opens conductor 49 to permit the-relays 18 of'the various communicating channels at the" associated toll point to be again solely under the control of the energizingpaths by way of their conductors 82.j

stored at the various distant toll stations,

and these stations areagain enabled to use 7 the calling channel can calling th r stacoil when it is extended by the insertion of tions.

In addition to the iib'ove describedresults due to the withdrawalof the calling carrier frequency, there is atithe calleclrtoll station aturthcr ,resultldue to the withdrawal At its contacts of the particular iuodululiug frequency that has been superposed upon the calling car ricr wave. The Witlulrawul-of this 1nodulilting frequencypermits the lc-energization of relay (i7 oi the re: civing brunch RC, and the consequent de-energization of relay 53. Relay 53 in releasing closes at its left-hand back contact and armature a short circuit of it's own winding that extends to ground by Way of the switch arm 8. The effect c his is to render it impossible for a subsequent energization of relay 67 toell'ect the operation of the relay 53 until tl1eswithLSIhas been restored to normal and has thus been placed in condition to respond properly toa subsequent call for thisstation, A further effect of the release of the rclaybtl at thecalled station is to prepare for subsequent closure the-circuit that extends] through the right-hand arma-- ture and back contact of the relay 53.

Taking up again the sequence of events at the calling toll station, when the calling oporator has been informed by the lighting of the assigning lamp 1]. ofthe particular communicating channel selected, she inserts the plug 72 of the cord circuit 7 0 into the associated spring jack 1.-

jack 1, andthence to ground by way of both windings of the relay 15. The relay thereupon operates andcloses by Way of its twolower armatures and front contacts the I energizing paths 82 and 83 of the relays l8 and 17 respectively, at the same time open- This-closes a circuit. that extends from batterythrough the cord circuit sulieryisory lamp 76, co-0perating sleeve contacts of the plug 72 and. spring ing at its outer lower back contact the energizing circuitof the relays 14 and 13, thus releasing these relays. As hereinbefore described, the relay 13 has been kept operated since the selection of this communicating channelto holdenergized the relay 17,50

that the 'relay 17a's a. result of the insertion of the plug continues energized tokeep 'the v receiving branch RC of the selected channel inlits operative condition. The relay l8 bf the transmitting branch TC of this channel however has not heretofore been operated, and this relay now operates to apply the ClltlI'ilCtEllSllO carrler frequency of the selected channel C to the toll line. Relay 15 in operating also closes at its inner upper frontfcontact and'arxnature a circuit byway [of conductor 87 for the energization of relay 88., Relay 88 in operating connects the ,two way repeating coil 33 of channel C with the artifical line or'network N designed "to match the changed characteristics of the circuit lying to the other'side of the hybrid the plug 72, p Y

When the characteristlc carrier frequency this causes the energization of relay16 which in turn causes relay to V operate;

Relay 10in operating closes a circuit that entends from battery through the winding of relay 59, upper winding of relay 51, calling lamp 12 associated with spring jack 1 of the selected channel, upper armature and back contact of; relay 15, theswitch contact associated with'the selected channel and switch arm S that is resting upon it, windingof release relay 56, right-hand back contact and armature of relay 53, switch arm S and its associated contact corresponding to the; selected channel, and inner right-hand armature and front contact of relay 10 to ground. As a result relays 59, 51 and 56 are operated, and the calling lam 12 of the selected channel is lighted. Re ay 59 in operatingextinguishes the calling pilotlgmp 27. Relay 51 in operating closes a branch energizing circuit to keep the key guard or pilot lamp 79 lighted after the energizingbranch by way of the inner front contact and armature of relay '52 has been opened by'the re; lease of relay 52 upon the operation of relay 59. Relay 51 also introduces another break into the calling relay energizing circuit by way of conductor 31 to continue the inoperativeness of the associated relays after the path has been closed by the release of relay 52. The lighting of calling lamp 12 informs the operatorat the called station that the call of which she is first notified'by thelightiug of pilot lamp 27' is to be extended over the channel terminating in spring jack from the moment of selection.

18 releases it withdraws the characteristic 1. The operation of relay 56 operates the carrier frequency of channel C from the toll line and the effect of this at the'calling station is to release relay 19 of the'receiving branch RC and the associated relay 16. lVhen rcla y 16 at the calling station is thus released it short circuits the lower half of the winding of sleeve relay 15. The resistances of thewindings'of relay 15 are so proportioned with rspect to the operating char-.

acteristics of the cord eircult supe visory lamp 76 that whenboth windings are included in series in the circuit the lamp does not receive enough current for its illumination, but. when the lower winding is short circuited the current is sufficiently increased tohght the lamp. As a result therefore of the withdrawal of the carrier frequency of channel C at the calledstatiun, the cord 7 circuit lamp 76 at the calling operators po sition is lighted to indicate to the operator that the calling lamp at the called station plug 72 andspringjack 1', and to ground by way of both windings of the sleeve relay 15, the normal short circuit of the lower winding being at this time opened as a result of the operation of relay 16 due to the application of the carrier frequency of'this channel at the calling station. Therefore when the operator at the called station inserts plug 72 into spring jack 1 the supervisory lamp76 remains dark, indicating .to her that the callingoperator has established connection'with the corresponding channel.

v When relay 15 at the called station operates as a result of the operators answering the call, as described above, it breaks at its upper back contact and armature the previously traced circuit including windings of relay 59, Til-and 56 and the calling lamp 12. As a result the calling lamp 12 is extinguished, the circuit by way of conductor 34: 1 01i the energization of relays 50, 7, 8 aud t) is again made effect-ire and the key guard lamp 7t) extinguished by the release of relay 51, and release relay 56 is de=energized to open the circuit of releaseniagnct 62. When the magnet 62 releases the engagement between the end of the arm 64: associated with the right-hand armature and the holding pawl (2). causes the lifting of the pawl to permit the release and restoration to normal of the switch arms S S S, S and S At the same time the mechanical relation that exists between the holding pawl 61 and contacts 91 causes the opening of these contacts during the release out the switch, so that switch arms S and S are disconnected from battery during the return movement of the switch, The return of switch arm to normal opens the short circuit of the winding of relay 53, so that the relay is again responsive torelay 6'1 in the receiying branch RC of the calling channel C. This completes the restorationv to normal of the station calling and channel selectingapparatus at all of the toll points or stations, and the conversation is now-free to proceed between the calling stationand the called station over the communicating channel that has been selected. I

The operators at, bothttoll points may now complete the connection with'the subsoribers station. is calling and the suhseribera I 1 incense station that'is being called. In the present instiaince this is illustrated as being-accomplished by'means of connecting cord:- cir.

cults provided with the usual plugs 71' and 72 with their associated connecting cords and intermediate apparatus, operators set 92, listening -key 73', ringing key 74,- and superv'isory lamps75 and 76. The cord circuit maybe connected with any client a number i of lines extending to subscribers stations- SS by inserting the plug .71 into- ,the spring jack-77 of the line with which connection is to he established. The suhscrib'ers lines may be providecl with the usual'cut-oif'andline' relays and line lampsh The apparatus shown inthis connection is InerQIyQiIIustratiVe; as

any desired one of alarge number of wellknown terminal and connecting ;arrangements may be employed. r I V y When the conversation between the connee-ted subscribers is completed; the subscribers disconnect signals are received in the usual manner upon the supervisory lamps 75 atthetwo toll stations. When either op orator withdraws the'plug 72 from the coininunicating channel'spring jack in which it has been. inserted, the'sle evc relay 1!) is released."' Thi's 'leleases relays 17 and 18 of the channel, tlmsiwithdrawing the character-' istic carrier frequency from the channel at V [the endwhere the plug is removed, and also rendering thcreceivingf relay 19 of the channe-l unresponsive.-' Upon'the withdrawal of the carrien frequency at either end otthe channel the relay 19 at the other end releases thus releasing the associated relay 16 and short icircuitint" the lower winding of the corresponding relay 15. This has the effect of increasing the flow of current in the sleeve circuit'so that theslq'iervisory lamp '76 associated with the plug 72 that is still inserted is lighted. Thus either ope 'ator by taking down theconncction ca'uscsthe lighting of the cord circuit supervisory lamp 76 at theo'therfend of the ehanneh an'd .gircs the disconnect signal to the distantoperator. In the foregoing description of the opera- 'iion; it has been assumed that when the switch S tests the channels as aresult of the 'ressin of the .station-selectin thex" at the- ?f Y b calling station, one of these channels is seg. lectable.

for an y reason the arms oi the switch S pass over their contacts without selecting, means are provided :tor autcnnatically terminating the calling condition on the toll line and-for restoring the selecting switchto normal. These results are accomplished "as follows:

Vs hen the slow acting device or timer operates a predetermned interval after the pressing of the key at the calling station and" i applies the station-selecting modulating frequency and starts the movement of the selecting swi'tch S-atthecalled station, it also If it should, happen that atJthiS- time all ot-the channels are in use. or that other slow acting'relay device or timer-132.

-T he timer 132 thereupon starts to draw in its plunger. o The interval re ul'red for the complete inward movement 0 theplunger ot' timer 132 isso regulatedas to' be someto test allot the channels to which it has access. Consequently if theswitch S does not nialiea selection, the relay 50 at the callingstation is not'released and therefore the timer 55 at the calling station isalso not released. The timer 55 consequently holds the energizing circuit of the timer 132 closedv long enough to permit the full inward movement of the plunger of the tinier 132. hen thisoccnrs the inner contacts of'tinier 132 are opened, and the energizing circuit including=ther elay 50 and the particular one Closes a circuit for the energization ofanwhat greater than-the interval required-tor fthe. selecting switch S at the called station of the relays/7, 8 and Qthat has been-op gerated is opened, thus releasing these relays. This effects the restoration of the calling station apparatus to normal, in the manner previously described, thus withdrawing'the calling 'arrierwaye and its modulating frequency. Upon thewvitliclrawal of the call i g carricrwayejand its 'inodnlating frequency'the relay53 at the called station is released. The switch S; having failed to contact points associated with the various "channels and has le'ft'its, switch arms rest- I ing upon thefirst contacts beyond the chan- -nel contacts. Therefore, when relayf53 at the called station releases, a circuit isicl'o'sed that extends from battery through contacts 91 associated withthe holding pawl. of switch S, contacts 98 associated with the releasing lever 6% of release magnet-62, the

nialge z 1"'selection,'has adyancedbeyond the. u

contact upon which switcharm Sis resting and that switch arm, winding'of release leases and brings about the release of the unagnet The magnet (32 in releasing lifts the holding ,pawllil and permits the arms of the switch S to return to normal. Thus.-

'the calling condition upon theline is withdrawn and theswitclixS restored to normal upon the. tailureot the switch to. select as? channel. I

opening of'thcsc contacts breaks the energiz in'g cirouitlot relay 56, which thereupon re- When the, energizingcircuit at the calling station for. the relay 50'and the particular one of the relays 7, 8 and-9 that hasbeen operatedis broken by the -complete inward? Stroke of the timer 132 as a result of the switch S at the called station failing to find and energize an idle channel, as described,

thecircuit for relay 57, and consequently the circuit for lamp 79, are also broken so that the calling operator upon observing that the 1ampQ79- is extinguished is apprised of.

the fact that no idle channel is available.

The invention is not to be construed as limited to the specific details of the foregoing description or of the drawing, but its ,sco e is defined inthe appended claims.

, hat is claimed is: 1. In a carrier .wave communicating system,.the,comblnavlon w1th a line and three or more stations connected thereto, of frequency responsive means operatively associated with said hne at each station and selectively responsive to a particular fretransmitted at any instant by said line for producing and applying-to said line said waves to which the various other stations are adapted to respond.

2. In a carrier wave communicating system, the combination with a line having a plurality of stations connected thereto, of

'wave responsive means at each station,

means at each'st'ation controlled by said wave responsive means for roducing and applying to said line a plurality of different frequency waves, and means at each of a plurality of said stations selectively responsive to aparticular one of said waves.

3. In a carrier wave communicating sys tem, the combination with a line having a plurality of stations connected thereto, of means at each station'for producing and applying to said line a plurality of difierent frequency waves, means at each other station selectively responsive to a particular one of said waves, signals at the stations at which the waves are being produced and atthe station selectively responsive to the corresponding wave, and means for actuating said signals responsive to the means at the station selectively responsive to the corresponding wave when said means is actuated.

4. In a high frequency communicating system, the combination 'with a plurality of.

high frequency-stations, of means at each stationforlproducing and transmitting a plurality ofdifiering high frequency waves, means at each station selectivel responsive to a particularone of said higi frequency waves, a high frequency communicating channel having terminals at said station, and means operated by the response of the station to its corresponding wave for preparing said channel ;;for communication.

5. In a carrier wave communicating system, the combination with a line and a plurality of stations connected therewith, of a calling channel associated with said line and having a plurality of branches at each station, meansfor energizin said calling channel and' any one of the ranches thereof at any 0 said stations, and apparatus at each.

station adapted to be selected and operated at a particular one of said stations in ac cordance with the particular calling channel branch that is energized.

' 7. In a carrier wave communicating system, the combination with a line and a phi-- rality of stations connected therewith, of a calling channel associated with said line and having a. plurality of frequency selective branches at each of said stations, means for energizing said calling channel and any of said branches at any oi said stations, apparatus at each station selectively operated in accordance with the particular calling channel branch that is energized, a plurality of frequency selective communicating chain nels connected with said line and available for use at each of said stations, and apparatus at thecalling station and the selected station. for establishing communication over one of said communicating channels.

8.1n a high frequency communicating system, the combination with a plurality oi high frequency stations, ,of means at each station for producing and utilizing a plurality of high frequency waves, means controlled at a calling one of said stations for utilizing one of said waves to select another one of said stations, means at the selected station for selecting another of said high frequency waves, and means for establishing communicating relation between said calling station and said selected station through the medium of said selected high frequency wave.-

9. In a high frequency connnnnicating system, the combination with a plurality of stations each having means for producing and utilizing a plurality of high frequenr station for selecting another of said high frequency waves for communicatingpurposes, means for identifying the selected 'wave at said calling station, and means at the calling and selected stations for establishing communicating relations therebeinn

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