US1542121A - Carrier-wave communicating system - Google Patents

Description

1,542,121 S. B. WILLIAMS, JR

CARRIER WAVE COMMUNICATING SYSTEM Filed April 1, 5 Sheets-Sheet 1 i Q A WT a Q w @Y M 158 d m W e w NN\ LET a June 16, 1925.

June 16, 1925. I 1,542,121

s. B. WILLIAMS, JR

CARRIER WAVE COMMUNICATING SYSTEM Filed April 1, 1922 5 Sheets-Sheet 2 um 111mm bx I June 16, 1925. 1,542,121

s. B. WILLIAMS, JR

CARRIER WAVE COMMUNICATING SYSTEM Filed April 1, 1922 5 Sheets-Sheet 3 //1 van/0r. Sa/m/e/ 5 VW/famsdz June 16, 1925.

S. .B. WILLIAMS. JR

CARRIER WAVE COMMUNICATING SYSTEM Filed April 1 1922 5 Sheets-Sheet 4 Aff'y.

June 16, 1925.

S. B. WILLIAMS, JR

CARRIER WAVE COMMUNICATING SYSTEM Patented June 16, 1925.

UNITED STATES PATENT OFFICE.

SAMUEL B. WILLIAMS, JR., OF BROOKLYN, NEW YORK, AS'SIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CARRIER-WAVE COMMUNICATING SYSTEM.

Application filed April 1, 1922.

To all whom it may concern:

Be it known that I, SAMUEL B. WILLIAMS, J r., a citizen of the United States, residing at Brooklyn. in the county of Kings, State of New York, have invented certain new and useful Improvements in Carrierave Communicating Systems, of which the following is a full, clear, concise, and exact description.

This invention relates to an improved communicating system, and more particularly to a communicating system of the type in which carrier waves or oscillations, either free or guided, are employed as the medium for conveying energy for effecting the operation and control of electric signaling, controlling and communicating apparatus lo' cated at separated points in the system. The invention especially relates to a system in which carrier currents, in the form of high frequency waves guided upon wires, are used as the medium of electrical connection between the electrical devices and apparatuses located at different points in the, system. These carrier currents are assigned such frequencies that they 1 are non-interfering with one 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 of the carriers is modulated in accordance with the voice currents or other signaling or controlling currents that are to be carried. The carriers thus modulated are transmitted over the toll or'long distance line and are passed through individual receiving 211- rangements, 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 oarrier was modulated at the distant station.

The apparatus which is used to transmit and receive by means of any one high frequency wave as distinguished from that which is used for transmitting and receiving by means of a wave of a different frequency, is commonly known as a high frequency channel, or frequency selective channel.

In the type of carrier communicating system to which the present invention particularly relates, a toll line or long distance line having associated with it a number of toll stations is provided at each station with Serial No. 548,681.

such high frequency channel terminal ap paratus as is necessary to give each station access for calling and communicating purposes to the high frequency channels thus superposed upon the toll line. \Vhere in this type of system there are three or more stations associated with the toll line, each station is provided with meanswhereby it may call any of the other stations, preferably through the medium of a high frequency calling channel, and may establish conversational connection with the called station through the medium of any one of the other high frequency channels that is not at thetime elsewhere in use.

One of the objects of the present invention is to organize a high frequency carrier system of this general type for the handling of toll or long distance calls between the stations in substantial accordance with present toll practice with respect to the putting up and taking down of connections, the operation of the calling, supervisory, guarding and disconnect signals, the selective signaling of any station by any other station, and the common use of the high frequency channels of the system by the different stations and by different operators at the same station; and to provide a system where the reciprocal controls between the distant stations necessary for the accomplishment of these results may be effected through the medium of the high frequencv channels. To this end each of the high frequency channels in the preferred form of the invention is provided with a plurality of connection terminals at each toll station. These terminals at each station are accessible to a number of operators, and have associated with them signals that constantly inform the operators of the condition of use or disuse of the channels, whether at the local or at a distant toll station, as well as a signal that informs an answering operator at each toll station of an incoming call over the particular channel with which the signal is associated. The operators at each toll station are provided with means whereby, upon takingan idle channel for use, they may select and cause the operation of the calling signal associated with the channel at any desired one of the distant stations, a busy indication at the same timebeing produced to guard the channel at other points. The channels are also adapted to co-operate with the connecting apparatuses at the various associated toll stations in such a way that such signals as are necessary for the guidance of the operator in the supervision and disconnection of an established connection are controlled between the stations through the medium of the high frequency channels. Furthermore, the channels and the toll station terminal and channel extending apparatus are arranged to co-operate in such a way that the selection of a channel and the selective calling of a particular station places the channel in operative relation with the called station and prevents its extension to or use by any other of the distant stations.

Another object of the present invention is to provide means for automatically regulating the transmitted and received energy in the high frequency channel of a carrier wave communicating system in such a way as to compensate for varying distance or normal energy attenuation between the particular transmitting and receiving stations and thus hold the transmission at a substantially constant value. In the application of this feature to the embodiment of the invention herein disclosed means are provided for holding the low frequency current input from the carrier channel to the low frequency hybrid coil at an approximately constant value irrespective of the degree of line attenuation between the particular two stations that are at the time connected over the channel. In a carrier communicating system where there are but the two terminal stations to be interconnected, the line attenuation as affected by the distance between the stations is constant. In the present system, however, which contemplates a plurality of stations on the toll line located at different points along the line, any two of which stations may be interconnected over any of the carrier channels that is free for use, the line attenuation is not constant, but varies with the distance between the stations that are at the time in communication. The same carrier channel may be used for establishing connection with a station that is relatively close to the calling station, or with a station that is relatively remote. Consequently the degree of amplification of the low frequency talking and signalm currents that is necessary for standar transmission between any station and the most remote of the other stations on the line, may be greater than is desirable when connection is established with a less remote station and the line attenuation factor is correspondingly smaller. Such variation in the volume of the transmitted and received energy is objectionable, not only because it leads to non-uniformity in the speech transmission but also because an unduly large low frequency input from the carrier channel may cause sustained oscillations or singing around the circuit as a result of unbalance in the low frequency h'ybrid coils. The present invention contemplates the provision of means for automatically regulating the transmission gains over the line to an amount such that, irrespective of the distance between the communicating stations, substantially uniform transmission will be had and the setting up of persistent oscillations will be prevented.

The amount of energy delivered into the input of the low frequency hybrid of each station can be held approximately constant either by automatically varying in proportion to the separation of the stations the amount of amplification to which the currents of the carrier channel are subjected, or by providing amplification in the proper amount for communication with the most distant station and automatically introducing a variable compensating attenuation that holds the transmission gains down to the proper amount for communicating with a less distant station. In the embodiment of the invention herein disclosed, the latter method is employed, and means are pro- Vided for automatically introducing artificial line sections of varying values into the transmitting and receiving branches of the chosen channel in the act of calling one or another of the less remote stations. Normally, in this embodiment of the invention the artificial line sections are excluded from the branches; and when an operator establishes connection with the channel, her actuation of the controlling mechanism that acts to bring about the display of a calling signal of this channel at a particular desired one of the less remote stations, also automatically brings about the inclusion of as many artificial line sections as are required to interpose the proper amount of attenuation.

The foregoing and other features are disclosed as co-operating'in one and the same system in the; present embodiment of the invention; but they are capable.of separate use where it may be desirable for any reason to employ only part of the features that are presented herein. In addition, the system of the invention in the preferred form in which it is disclosed embodies other features that have not been particularly referred to in the foreging general statement. The manner in which tli'ese other features, as well as those that have been mentioned, are attained and realized will be explained in the following description; and the structures and combinations through which the objects of the invention are attained will be more particularly defined in the appended claims.

The preferred embodiment of the inven tion is illustrated in the accompanying drawings, in which Figs. 1 to 5 inclusive, ar-

ranged as indicated in Fig. 6, show diagrammatically as much of the circuit organization at one toll station of the system as is necessary to enable the invention to be understood. It will be understood, without the necessity of diagrammatic representation, that there are a plurality of stations connected with the toll line L and that each of these stations may be in all respects identical in apparatus and circuit arrangement with the particular station illustrated in Figs. 1 to 5; the only differences between the stations being in the use of a harmonic producer at one of the stations and of a harmonic reproducer at each of the other stations, and in the particular calling channel modulating frequencies which each station is adapted to transmit 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 5 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 of the wiring and apparatus illustrated, and without the exercise of invention, to organize a system of such capacity as may be desired. As illustrated in the circuit diagram of Figs. 1 to 5, each one of the toll stations or toll points is connected with the toll line L throughthe medium of a two-way repeating coil or hybrid coil 100, this coil having one end of its line windings connected with the toll line and the other end connected with an artificial line or balancing network N that is constructed and arranged, as is well understood in the art, to balance and match in its characteristics the toll line with which connection is made. Extending from the bridge and series windings of this two-way repeating coil are two pairs of conductors, T'and R, 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 with the local apparatus of the four communicating channels C C C and C and with the local apparatus of the calling channel C. At each of the toll stations there is also a connection of either the transmitting path T or the receiving ath R with the base frequencychannel BC The connection of the transmitting ath T is with the transmitting branches C, TBC, TC, TC*, TC and TC of the various channels respectively; and the connection of the eceiving path R at each station is with the receiving branches RC, BBC, RC RC RC and RC of the various channels respectively. The transmitting branch apparatus of the base frequency channel 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 connection with the receiving branch BBC is provided in 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. The specific organization of the transmitting and receiving branches of the various channels will be described hereinafter. As a general description of the organization, it may be said that the transmitting branch TC of the calling channel C is so organized as to apply to the toll line the calling carrier fre uency supplied through the amplifier HA and modulated in accordance with one or another of three lower frequencies supplied b way of the associated modulating branches MB, MC and MD. These lower modulating frequencies are supplied from a harmonic producer HPL b distortion of the base frequency supplie by the oscillator 0 Each of the modulating branches MB, MC and MD is made operative to apply its particular modulating frequency by the pressing of corresponding keys 51, 52 and 53. Each of the operators positions OP, OP etc. at the 'toll board is provided with a group of these keys. Each of the keys in being operated causes the a plication to the calling carrier frequency 0 the particular modulating frequency adapted to bring about the response of the receiving apparatus of the particular one of the distant toll-stations that is being called. The groups of keys 51, 52, 53 at the different toll operators positions operate to control their respective modulating branches through the medium of a group of common relays 3, 4 and 5, these relays in turn 0 erating to energize the associated modu ating branches through the medium of the relays 7, 8 and 9 respectively. Each group of station selecting keys 51, 52 and 53 has asso- -ciated with it a pilot lamp 50, the illuminaadapted to respond, brings about the operation of the relay 67. The relay 66 acts through the medium of the relay 2 to render ineffective all the station selecting keys 51, 52 and 53 in the various operators position groups and to light the guard lamp 50 associated with each group of station selecting keys to inform the operator that the calling channel is elsewhere in use. The relay 67 acts through the medium of the relay to co-operate with the relays 13, 11, 10 and 12 in controlling the illumination of the answering lamp 40 of the selected channel at the selected station to signal the incoming call to the called station operator.

The apparatus in the transmitting branch TBC of the base frequency channel BC includes the harmonic producer HPH and a base frequency oscillator OPH. The base frequency and its harmonics are conducted to a distributing circuit comprising the conductors 47, 48, from which the base frequency is separated out and applied to the transmitting path T by way of the branch TBC, while the harmonics are selectively passed to the transmitting apparatus of the communicating channels C C C and C to supply the carrier frequencies for conveying the speech and signaling modulations over the respective channels.

The apparatus in the receiving branch RBC of the base frequency channel BC is so organized that the base frequency, received from the particular one of the four toll stations where the base frequency producing apparatus is 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 frequency thus produced are supplied to the distributing conductors 47, 48, to supply the carrier frequencies required by the communicatin channels C, C C and C of the associate station.

The organization of each of the communicating channels C C C and C is such that the voice currents received from the local telephone circuit by way of the talking conductors 79 and 80 are passed through the two-way repeating or hybrid coil 20, and caused to modulate the amplified carrier frequency received from the distributing conductors 47, 48 and the modulated carrier frequency is conducted to the transmitting path T. The organization of each of the communicating channels is also such that the characteristic carrier frequency to which the channel is adapted to respond separated out from the other frequencies passing over the receiving path R, is demodulated, and the voice component is conveyed by way of the two-way repeating 0011 20 to the local talking conductors 79 and 80, while the carrier component is diverted and and 32.. Each operator caused to operate the relay 19. The relay 19 participates, through the medium of relay 11, in controlling the guarding and si nali'ng operations incidental to the use 0 the associated channel.

Each of the communicating channels C C C and C has associated with it certain controlling and responsive elements, designated CC, CC', CC and CC, respectively; and each of the channels is terminated, in the present embodiment of the invention, upon spring jacks at a plurality of operators positions of a toll switchboard. Each channel has a multiple jack MJ on each section of the toll board, and a single answering jack AJ at a single one of the operator-s positions. Each multiple jack has associated with it a busy lamp 41 to indicate the condition of busyness or idleness of the corresponding channel; and each answering jack has an associated calling lamp 40, the illumination of which notifies the answering operator of an incoming call over that channel.

There may be as many toll sections and operators positions as traific conditions render desirable. In the present instance, two operators positions, OP and OP are indicated. Each operators position is provided with the usual equipment of connecting cord circuits and an operators telephone set 39. Each of the connecting cord circuits, only two of which are shown in detail, may comprise a pair of connecting plugsv 25, 26, with the conductors extending between them, together with a listening key 28, ringing keys 27 and 29, supervisory lamps 30 and 31 and supervisory relays 36 s position is also provided, as previously stated, with a group of station selecting keys 51, 52 and 53 and a calling channel guard or ilot lamp 50 with an associated relay 54 or preventing the lighting of the lamp under certain circumstances. A plurality of local lines, as the line extending from station S and terminating upon the answering jack 58 and multiple jack 57, are brought into toll sections for interconnection with the carrier channels through the medium of the toll operat'ors connecting cord circuits. The terminal arrangement of the carrier chan nels is such that all of the channels are available for connection at each operators position, so that each operator may be able to utilize for interconnecting purposes as many of the channels as are not in use elsewhere in; the system. The busy lamps 41 associated with each multiple jack channel extension terminal are controlled in such a way as to indicate by their illumination the unavailability of the corresponding channel, whether it be due to the use of the channel at some position of the local board or at some distant point on the toll line. The

particular sort of equipment shown for effecting interconnection between the channels and the local lines is merely illustrative, as, of course, such interconnection can be effected in any one of a variety of wellknown ways.

Specifically, the organization and operation of that part of the system which acts to generate, modulate, transmit, receive and demodulate the high frequency carrier currents employed in the system is as follows: The frequency-of the carrier waves selectively utilized by the communicating channels is determined, in the present embodiment of the invention, by a base or control frequency generated by a thermionic or vacuum tube oscillator OPH located at one of the toll stations or at some other point upon the toll line. This oscillator may be of any'suitable type but is preferably the usual and well-known audion type with a feed-back connection from its output into its input circuit. The frequency of the oscillatory current )roduced may be regulated by an adjustab e condenser, and may conveniently be, say, five thousand oscillations per second although any other appropriate base frequency and its associated harmonic multiple frequencies may be employed. The oscillator OPH supplies substantially sinusoidal oscillations to the input circuit of the harmonic producer HPH. An inductance 101 is bridged across this circuit. The magnitude of the oscillations supplied is such as to overload thethermionic tube HPH; that is, to supply an input wave of greater amplitude than the tube can repeat without distortion. The repeated oscillations in the output circuit of the tube HPH are therefore of non-sinusoidal wave form, which is the equivalent of a base frequency current of slnusoidal' form and a series of currents of frequencies which are multiples or harmonics o the base frequency. The plate current source, P, of the overloaded tube or harmonic producer HPH is connected by a circuit comprising a resistance 102 and an inductance 103 which constitutes an impedance that is of the order or magnitudeof the impedance between the plate and filament of the tube or greater. This impedance relationship assists in producing distortion and in stabilizing the output circuit. The output circuit of the harmonic producer HPH is onnectcd with a distributing circuit 47, 18, by means of a transformer 104, and the distributing circuit is thus supplied with both the base and the harmonic frequency oscillations. From the distributing circuit 47, 48 the base and harmonic frequencies are selected by means of tuned circuits leading to the transmitting branches of the calling and various communicating channels and the transmitting branch of the base frequency channel. The inductance of the transformer 105 of the base frequency transmitting branch and the capacities of the fixed and variable condensers associated with it are so adjusted that only the base frequency oscillations are selected. Similarly the constants of the transformers and condensersof the other tuned circuits leading from the distributing circuit are so adjusted that each of these tuned circuits selects a different one of the harmonics of the base frequency. The harmonic frequencies selected by the transmitting branches of the communicating channels C C C and C may be ten thousand, fifteen thousand, twenty thousand, and twenty-five thousand cycles respectively, and the harmonic frequency selected by the transmitting branch of the calling channel C may be 30,000 cycles.

The base frequency oscillation selected from the distributing circuit by the tuned circuit including the transformer 105 passes through a potentionmeter 106, from which it is supplied at the desired potential to the input circuit of the amplifier HA. The output circuit of the amplifier HA is inductively coupled with the terminals of a band filter HF 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 No. 1,227,113 to Campbell, patented May 22, 1917. The characteristics of this filter, which consists of recurrent similar sections having series and shunt capacities and inductances, is such that it transmits oscillations of the base frequency with little attenuation and effectively diverts from this branch currents of frequencies difi'ering more than a slight amount from the base frequency.

As has been stated, the base frequency oscillator is provided at but one station or point on the toll line. At the other toll stations or points the base or controlling frequency thus stipplied to the line passes from the line thru the two-way repeating coil 100 of the station into the receiving path R. The receiving branch RBC of the base frequency channel is provided with a, transformer 107. with associated condensers and inductances to constitute a tuned circuit that acts to divert the base frequency oscillation from the receiving path to the amplifier HRA by way of the potentiometer 108, while permitting the free passage of the other carrier frequencies through, the receiving path to their respective channels. The base frequency thus selected is amplified by the amplifier HRA, and the amplified oscillations are impressed upon the ln ut circuit of the thermionic tube HR The magnitude of the-oscillations are such as to overload this tube, with the result that as in the case of the tube HPH associated with the base frequency oscillator OPH,

the repeated oscillations are distorted into a series of currents the frequencies of which are the harmonics of the base frequency current, the tube HRP thus acting as a harmonic regenerator. The base frequency and its harmonics are then amplified by the amlifier HRA and are impressed upon the istributin g circuit 47, 48 through the transformer 109. From the distributingcircuit the harmonic frequencies of the base or controlling oscillation are selectively supplied to the transmitting branches of the channels 0, C C and C b way of the tuned circults previously re erred to. Thus at the toll station where the base frequency oscillator OPH and the harmonic producer HPH are located the communicating channel carrier frequencies are supplied directly, while at the other stations they are supplied by a harmonic regenerator HRP controlled by the base frequency oscillations received over. the toll line. Consequently the corresponding carrier frequency oscillations are identical in frequency at all of the stations of the toll line. I

The organization illustrated and described for producing a plurality of harmonic carrier frequencies from a base freuency 'by distorting that frequency, and

or insuring identical frequencies of the carrier oscillations employed at each of the various toll stations by controlling the carrier frequencies at all points from a single base frequency is substantially in accordance with the or nization disclosed in the British Patent 0. 131,426. The method and organization described above for synchronizing corresponding carrier waves at all stations is the preferred way of securing this result for use in connection with the resentembodiment of the invention. But 1t will be understood that other ways of securing substantially identical frequencies of the carrier waves at the different points at which they are applied may be used.

At each of the communicating channels C C, C and C the particular carrier frequency diverted from the distributing circuit 47, 48 by meansof thecorresponding tuned circuit is applied to the terminals of a potentiometer 110 from which the carrier oscillations at the desired potential are ap plied to the input circuit of an amplifier HAC. The output circuit of the amplifier is connected by way of the potentiometer 111 with the input circuit of a modulator MC. The input circuit of the modulator MC also has connection by way of the inductive coupling 112 and t e potentiometer 113 with the ap ropriate winding of the low frequency by mid coil 20 that inks the carrier channel with the low frequency or talking circuit. The talking conductors 79 and 80 extending to the spring jack terminals of this communicating channel are connected through condensers with the line side of the hybrid coil or two-way repeater,.and the. other side of the repeater is connected through condensers with a balancing network N, the characteristics of which are made to match the characteristics of the low frequency lines adapted to be connected with the channel. The voice currents that reach the repeater 20 by way of the conductors 79 and 80 are consequently repeated and impressed at the proper otential upon the input circuit of the modu ator MC and are there combined with the carrier frequency derived from the distributing circuit 47, 48 to cause correspondingly modulated carrier current to flow in the output circuit of the modulator. The output circuit of the modulator is connected through an induc tive coupling 114 with two sections of artificial line, one of which is controlled by the relay 26 and the other by the relay 27. The series windings of each section are normally short-circuited in resting contacts of the relays 26 and 27 and the shunt windings, each of which is bridged across the circuit between midpoints of the series windings, are normally open at front contacts of the relays 26 and 27. Consequently in the deenergized condition of relays 26 and 27, the artificial line sections are excluded from the circuits of the transmitting branch. The second artificial line section is connected with a band filter of the type disclosed in the Campbell patent previously referred to. The characteristics 0 this particular form of the Campbell band filter aresuch that it suppresses any currents of the frequency of the voice currents in the talking conductors of the channel which might be repeated as low frequency currents by the modulator. Thaband filter MFC of each of the channels serves to transmit currents of a range of frequencies including the carrier frequency of that channel and frequencies extending up and down from the carrier frequency by an amount depending upon the highest frequency-current that it is desired to transmit from the low frequency or talking circuit.

The band filters MFC of the four communicating channels, the band filter HF of the base frequency channel BC, and the band filter MFC of the calling frequency channel C are all arranged with their terminal shunts connected in series in the transmitting path 43, 44; and the constants of these various filters are so adjusted that each filter passes freely to the transmitting patlrthe carrier frequency from its own channel while at the same time preventing the fre-' quencies supplied through the filters of the other channels from being diverted into its channel. The band filters DFC of the receiving branches of the various communicating channels C, C, C and C, the tuned Ill till

circuit of the receiving branch RBC of the base frequency channel BC, and the band filter DFC of the receiving branch RC of the calling channel C, are similarly connected to the receiving path R extending from the high frequency hybrid coil 100; and the constants of the filters and the tuned circuit are so adjusted that each diverts from the receiving path R into its own channel the particular frequency to which the channel apparatus is adapted to be responsive, while permitting to pass freely through the series circuit the frequencies which the other channels are adiusted to divert.

The receiving branch of each of the communicating channels is provided with a band filter DFC adjus ed. as stated above, to divert intothe channel the characteristic frequency band of that channel. After passing through the band filter, the diverted oscillatorv current passes through conductors controlled at back contacts of two relays 36 and 37 each of which is adapted when opcrated to include in the receiving branch an associated section of artificial line. the arran emcnt and control of these artificial line sections being the same as that previously referred to in the description of the artificial line sections included in the transmitting branch of the circuit. The oscillatory current diverted through the band filter DFC is then carried through a potentiometer 115 and applied at the proper potential to the low side of a shielded transformer 116. the high side of which is included in the input circuit of the detector DCH The C potent al for the grid of this detector tube is derived from the filament battery by including a resistance 117 in the filament current supply circuit 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 medium of a transformer 118 which is designed to transmit efficiently the voice frequencies obtained from the demodulation of the modulated carrier wave. as well as the unmodulated carrier component. The output circuit of the'amplificr AC is connected to the low pass filter LPG- designed to pass the voice frequencies and exclude the carrier frequencies, by way of a transformer.

H9. The other terminals of the low pass filter are connected to the bridge terminals of the two-way repeating coil 20, where the voice currents are divided between the talking conductors 79 and 80 and the balancing network N in such a way as to have their ctlccts as nearly as pos ible neutralized in the coil of the repeater that is connected with the outgoing transmission branch of the channel. The ontput'circuit of the amplifier AC also has a branch path-through a high pass filter HPC which acts to pass the carrier wave component and to prevent the passage of the demodulated voice frequency component. The other terminals of the high pass filter HPC are connected with the low side of a transformer 120, the high side of 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 respond and hold its contacts closed as long as the associated communicating channel is receiving the carrier frequency components which it is adapted to divert from the receiving path R.

The filament and plate energizing currents are supplied to the apparatus of the transmitting branch of the channel under the control of a relay 24. The supply circuits are so arranged that the filament and plate current is normally disconnected and is applied to the transmitting branch apparatus only when the relay 24 is energized. 7

As has previously been stated, the transmitting path T of each toll station is arranged to receive a calling carrier frequency by way of the transmitting. branch TCof the calling channel C, the calling carrier being modulated by lower frequencies in accordance with which of the modulating branches MB, MC and MD is made dperative. In accordance with the present embodiment of the invention, the calling carrier frequency is derived from the harmonic distributing circuit 47, 48 through a tuned circuit and is applied through a potentiometer at the proper potential to the input circuit of the amplifier HAC the output circuit of which is connected by way of potentiometer 68 with the input circuit of the modulator MCC. The calling carrier may be of any suitable frequency that differs selectivcly from the frequencies that are used by the base channel BC and the four communicating channels C C C and C, but as stated, is assumed in the present description to be 30,000 cycles.

The modulation of the calling carrier wave is effected by lower frequency currents produced by the oscillator O and the harmonic producer HPL. The oscillator O is of the well-known type having a feed-back connection from its output circuit to its input circuit by way of the inductive coupling 121. The output circuit of the oscillator O is connected with the-input circuit of the harmonic producer tube HPL. The magnitude of-the oscillations supplied is such as to overload the tube HPL. thus causing the tube to distort the sinusoidal wave of the scillator O and produce in its output circuit non-sinusoidal currents that are the equivalent of the base frequency current of the oscillator together with the harmonics of the base frequency. This series of currents is supplied by way of transformer 122 to the distributing conductors 93, 94. The various frequencies that are thus supplied to the dis tributing circuit are selectively diverted by Way of a plurality of tuned circuits, including the transformers 95, 96 and 97 and their associated variable condensers, to the poten tiometers 123, 124 and 125 respectivel of the modulating circuits MB, MC and MD From the potentiometers the oscillations of the selected frequencies are conducted to the input circuits of the amplifiers A, A and A The output circuits of these amplifiers are connected in tuned branches of the circuit M leading to the modulator MCC, the tuning of the branches being such as to permit the passage to this circuit of the fre quency of the corresponding modulating current branch and to exclude from the branch in question the frequencies of the other modulating current branches that feed into the circuit M. The circuit M is connected to modulator MCC via transformer 126.

The modulating current branches MB, MC and MD are normally inoperative and are made operative under the control of the associated relays 7, 8 and 9 respectively, these relays acting when operated to apply current from the filament and plate batteries F and P to energize the apparatus of the corresponding branches. The oscillator O with its associated harmonic producer HPL, and the amplifier HAC and modulator MCC of the transmitting branch of .the calling channel are also normally inert. When any one of the relays 7, 8 and 9 is operated to energize the apparatus of the corresponding modulating branch, it also acts through the medium of the relays 69 and 7 8 to energize the above mentioned oscillator 0 harmonic producer HPL, amplifier HAG and modulator MCC to render operative the devices that furnish the calling carrier Wave and its modulating frequency.

The oscillator 0 may be arranged to produce OSCillutiOfis of any suitable frequencv as, for instance, 200 cycles. As a result of the action of the harmonic producer HPL there is transmitted to the distributing circuit 93, 94 the base frequency 200 of the modulator together with the harmonic frequencies thereof. If three modulating current paths MB, MC and MD are employed, these paths have their constants adjusted to divert from the distributing circuit, amplify and pass to the circuit M the three harmonic frequencies 400, 600 and 800. If additional modulating current branches are to be used, they may be 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 of its calling channel selectively responsive to a particular one of these lower modulating frequencies; so that the transmitting branch of the calling channel at each station is arranged to transmit the calling carrier frequency and as many lower frequency modulations thereof as there are distant toll stations to be selectively controlled or signaled. The transmitting branch of the calling channel at each toll station is of course arranged to modulate the calling channel frequency only with the lower frequencies to which the distant toll stations are responsive, and not with the lower frequency to which the particular toll station itself is responsive. Thus the particular station illustrated in the drawing is assumed to be responsive to the 200 modulating frequency, and consequently the modulating current branches are so arranged as not to apply the 200 frequency to the path M, but to apply the three harmonic frequencies 400, 600 and 800. At the toll station responsive to the 400 frequency the modulating current branches are arranged to employ the 200, 600 and 800 frequencies; and sinu iarly at the other stations the particular modulating frequency to which that station is adapted to respond is not arranged to be applied to the transmitting branch.

At each of the toll stations there is a connection from the receiving path R by way of a band filter DFC and a shielded transformer 127 to the input circuit of a detector DO. The band filter DFC is designed to divert into the receiving branch of the calling channel C the band of frequencies including the calling channel frequency, in this instance assumed to be thirty thousand, and its modulations extending up and down from this frequency through a range determined by the modulating frequencies derived from the path M. The

output circuit of the detector DC is connected through a transformer 128 with the input circuit of an amplifier AC. The output circuit of the amplifier is connected through the high pass filter HPC and the transformer 129 with the input circuit of the rectifier tube HRC. The output circuit of the rectifier tube includes the winding of the relay 66, which is thus adapted to respond to the presence in the circuit of the unmodulated component of the calling channel carrier Wave. The output circuit of the amplifier AC also has connection through the medium of the transformer 130 with a low pass filter LPC. This filter is designed to pass to the input circuit of the rectifying tube LRC, through the medium of the transformer 131, the particular modulating frequency of the calling carrier waye to which this toll station is responsive, and to exclude the other modulating frequencies to which the other toll stations respond, the demodulation of the modulated calling carrier wave into its carrier and modulating components being effected in the detector DC. The out put circuit of the rectifying tube LRC is connected with the winding of the relay 67, which relay therefore is operated only when the calling carrier wave ismodulated with the particular frequency to which the associated station is adapted to respond. At each of the other stations there will be substituted for the low-pass filter LPG a band filter designed to pass the particular modulating frequency to which that station is intended to respond and to exclude other frequencies. Thus, for example, the station which is intended to be called by means of the frequency sent out through thechannel A namely 600 cycles, will be designed to pass 600 cycle .current freely and exclude currents of 400 and 800 cycles or any other currents of frequencies differing Inaterially from 600 cycles.

In outline, the operation of the system is as follows: \Vhen any of the toll board operators has occasion to establish a connection over the toll line, she picks up the toll plug of one of her connecting cord circuits and inserts it into a multiple spring jack of a channel the idle condition of which is indicated by the unilluminated condition of the associated busy'or guard lamp. The insertion of the plug causes the lighting of all of the guard lamps associated with the multiple spring jacks of this channel on the other toll board sections of the calling station; and also, by causing the energization of the channel apparatus and the application to the toll line of the corres onding carrier wave, brings about the lighting of the guard lamps associated with the multiple spring jacks of this channel upon the switchboards of all the distant toll stations of the system;

At the same time the control over the establishment of connection with the selected channel at the distant stations is transferred from the operators at those stations to the operator at the calling station. As a result, the operators at the distant stations are prevented from gaining access to the selected channel, if they should attem t to do so in disregard of the lighted busy amp; but the calling operator has the power to bring about the extension of the channel to the spring jacks at the particular distant station t aat she may select.

In selecting the station to which the call is to be extended, the callin r o )erator first notes the condition of the pilot lamp associated with her group of station selecting keys, and if the lamp is dark, indicating that the calling channel is not in use, she presses the key corresponding to the station desired, and at the same time presses the ringing key associated with the toll end of the connecting cord circuit she is using. The depression of the station selecting key encrgizes the calling channel to appl the carrier frequency of that channel motl ulatcd with the particular lower frequency to which the selective apparatus of the desired station is adapted to respond. The energization of the calling channel deprives all of the distant toll station operatorsof control over the calling channel and also lights the calling channel pilot lamps associated with the station selecting keys of these operators positions. At the'same time, the other toll switchboard operators of the station that is calling have their calling channel pilot lamps lighted and are deprived of control over the calling channel.

The lighting of the calling lamp associated with the selected channel at each station depends upon the concurrent energization of the communicating channel,the application of ringing current to the channel, and the application to the calling channel carrier waveof the particular lower modulating frequency to which that station is responsive. The apparatus responsive to the energization of the communicating channel and to the application of ringing current operates at all of the distant toll stations; but the apparatus responsive to the particular modulating wave applied to the calling carrier wave operates only at the station that is being called, and consequently the calling lamp at that station is the only one to respond when the calling operator simultaneously presses her station selecting key and her cord circuit ringing key. The response at the called station which brings about the lighting of the calling lamp also extends the corresponding communicating channel to the spring jacks of the associated toll switchboard; so that the called station operator in inserting the toll plug of a pair of connecting cords into the answering spring jack is thereupon in communication with the callingoperator over the selected channel,the transmitting branch of the channel at the called station being energized by the insertion of the plug. In pressing-the station selecting key to signal the desired station, and simultaneously pressing the cord circuit ringing key, the calling station operator also brings about the automatic adjustment of the communicating channel energy to the (listnnrc between stations by the inclusion in the channel transmitting and receiving branches of such artificial line sections as are neccsury to introduce the required eompensating attenuation, thus holding the' volume of transmission substantially constant and preventing singing.

While the connection between the calling and called stations is established, the operators may signal each other and each con trol a supervisory lamp in the cord circuit of the other by pressing their respective ringing keys. The pressing of the ringing key implies a special ringing control modulating frequency to the carrier current at the associated end of the channel. As a result, apparatus responsive to this frequency at the other end of the channel operates and applies ordinary ringing current to the channel extension leading to the spring jacks in one of which the connecting circuit plug is inserted. This causes'the lighting of the associated connecting circuit supervisory lamp. In the application of the ringing control modulating frequency at oneend of the channel, and the resultant application of ordinary ringing current at the other end of the channel, provision is made for automatically disconnecting from the channel extension circuits at both ends normally connected responsive apparatus that might otherwise be accidentally operated; and also for short circuiting the output winding of the low frequency hybrid coil to prevent the unbalance caused by the application of ringing current from causing a coupling of the channel receiving and transmitting branches in such a way as to set up sustained oscillations. \Vhen the conversation is completed, the operators at the calling and called stations, respectively, disconnect by withdrawing the plugs of their connecting cord circuits from the channel extension spring jacks in which they have been inserted; and when disconnection has taken place at both ends of the channel, the busy or guard lamps associated with the channel spring jacks at all points in the system are ext nguished, and the channel is again available for use. The calling channel is released for use by other operators, and the pilot lamps associated with the station selecting keys at all other operators position of the system are extinguished, as soon as the calling operator has released her station selecting key.v

The operation of the system will now be described in detail. When one of the local subscribers, as the subscriber at station S, wishes to communicate with a subscriber at another one of the toll stations associated with the carrier toll line, he takes his telephone receiver from its hook, thus operatmg the associated line relay 56 and lighting the answering lamp 59 associated with the answering s )ring jack 58 before the operator at position P. The operator in responding inserts plug 26 of one of her pairs of cords into spring jack 58, thus operating the cutoff rela 55 and releasing line relay 56 to extinguish answering lamp 59, in the usual way. The insertion of plug 26 into spring jack 58 also closes the energizing circuit of supervisory lamp 31, which, however, remains dark as a result of the closure of a shunt around it due to the operation of supervisory relay 32. The operator then communicates with the calling subscriber by throwing her listening key 28, thus bringing her telephone set 39 into connection with the talking strands of the cord circuit.

7 Upon learning from the calling subscriber that he desires a connection that must be completed over the toll line, the operator observes the condition of busyness or idleness of the various channels of the carrier toll line, as indicated by the illuminated or unilluminated condition of the busy lamps 41 of the various channels, and inserts the from battery in parallel thru the busy lamps 41 associated with the multiple spring jack terminals of the corresponding channel at all the operators positions of this toll station, conductor 82 and inner right hand front contact and armature of relay 12 to ground. Thus all of the local busy lamps of this channel are lighted to indicate the busy condition of the channel immediately upon its being taken for use. Relay 12 in operating also closes a circuit that extends from battery through the winding of relay 10, inner right hand armature and back contact of relay 11, and left-hand front contact and armature of relay 12 to ground. Relay 10 thereupon operates and closes a locking path to maintain its energization by way of lower armatureand front contact of relay 10 and outer right hand armature and back contact of relay 11 to ground by way of left hand front contact and armature of relay 12. The operation of relay 10 extends the talking conductors of channel C through to the conductors T9 and 80 that are branched to the multiple jacks of this channel at the various operators positions, and thus extends the talking circuit of the operators connecting cord pair through to this'particular channel.

\Vhen relay 12 operates, as has been de' scribed, in response to the insertion of lug 25 into multiple jack MJ, it brings a out the energization of the corresponding channel C. This is accomplished by the closure of a circuit that extends from battery through the winding of relay 24 (Fig. 5). conductor 89 and outer right hand from contact and armature of relay 12 to ground. \Vhen relay 24 operates it connects the filament and plate batteries with amplifier HAC and modulator MC of channel C. As has been described, the amplifier HAC of channel C is connected with the high frequency harmonic distributing conductors 47, 48 by way of a tuned circuit that is adapted to pass the 10,000 frequency to the amplifier HAC. Consequently when relay 24 connects the filam'ent and plate batteries with the amplifier HAC and modulator MC, the 10,000 frequency is applied by way of channel C to the carrier toll line.

At each of the other toll stations, the

10,000 frequency thus applied to the toll line is transmitted by way of the two-way repeating coil 100 of each of the toll stations to the associated receiving path R; and, passing over the conductors 45, 46 of the receiving path, is diverted into the receiving branch RC of the corresponding channel C at each of the distant toll stations of the line. At each of the distant toll stations, the 10,000 frequency is diverted from the receiving path R- by way of the band filter DFC of the receiving branch RC thence by way of normally closed contacts of relays 37 and 36, potentiometer 115, shielded transformer 116, detector DC, inductive coupling 118, amplifier AC, high pass filter HPC, inductive coupling 120, and rectifier tube HRC, where the amplified high frequency wave is rectified and brings about the response of relay 19. Relay 19 in operating closes a circuit that extends from battery through the winding of relay 11 (Fig. 2), conductor 90 and front contact and armature of relay 19 to ground. Relay 11 in operating closes a circuit that extends from battery through all of the busy lamps 41 associated with the multiple jack terminals MJ of this particular channel at the various toll board positions. thence by way of conductor 82 andleft hand front contact and armature of relay 11 to ground. Thus the taking of channel C for use at the calling station and the consequent application of the 10,000 carrier frequency causes the lighting of the busy or guard lamps associated with all the channel extensions not only at the local toll station but also at all of the distant toll stations, and guards the channel against being employed by other operators.

Having appropriated an available channel. the operator now proceeds to select and signal the particular one desired of the toll stations having access to the channel,-first noting, however, the condition of the pilot lamp 50 to ascertain whether or not the calling channel is elsewhere in use. If the pilot lamp 50 is unilluminated, thus indicating that the calling channel is free, the o erator presses the ringing key 27 associated with the cord circuit that is being used, and at the same time presses a particular one of the group of station selecting keys 51, 52, 53. Assuming that it is key 53 that corresponds to the toll station wanted, the pressing of this key closes a circuit that extends from batter through the winding of relay 2 (Fig. 2 winding of relay 5, conductor 71, left hand contact and switch spring of key 53, conductor 76, and back contact and armature of relay 2 to ground. Consequently relay 2 and relay 5 are both energized. Relay 5 in operating closes an alternative energizing circuit for itself and relay 2 that extends by way of the lower front contact and armature of relay 5, conductor 70, right hand contact and switch spring of key 53, and winding of relay 54 to ground. The instant relay 2 operates,

it opens at its back contact the previously traced initial energizing circuit for itself and relay 5; but these relays are held operated over the second traced circuit by way of the lower contact and armature of relay 5. As the operating ground that gives the station selecting group of keys 51, 52 and 53 of each operators position control over the common relays 3, 4 and 5 is disconnected from the common conductor 76 the instant the common relay 2 operates, keys 51, 52 and 53 of the other operators positions are ineffective as long as any one of the operators has one of her keys 51, 52 and 53 depressed. The calling operation is further guarded from interference the instant the common relay 2 operates by the closure of a circuit that extends from battery by way of parallel branches at the various operators positions each including the pilot lamp 50 and back contact and armature of relay 54 and thence by way of the common conductor 77 and front contact and armature of relay 2 to ground. The closure of this circuit causes the lighting of the calling channel pilot lamp 50 at each of the other positions to indicate that the calling channel is in use. It is unnecessary that the pilot lamp 50 be lighted at the position of the operator who is making the call; and the prevention of the lighting is accomplished as a result of the inclusion of the winding of relay 54 of the calling operators position in the circuit previously described as being closed at the lower front contact and armature of relay 5 upon the operation of that relay. Relay-54 in operating opens the branch from conductor 77 through the pilot lamp 50 of the calling operators position, and consequently the pilot lamp at this position remains dark.

As has been stated, when the calling operator presses the key 53 of the station selecting group, the common relay 5 is operated. Relay 5 in operating closes a circuit that extends from battery through the winding of relay 9 (Fig. 1), conductor 35 and upper armature and front contact of relay 5 to ground. Relay 9 in operating closes a circuit that extends from battery in parallel branches throu h the windings of relays 69 and 78, and tience to ground by way of the upper armature and front contact of relay 9. The energization of relay 69 connects the plate and filament batteries P and F with the oscillator O and the harmonic producer HPL that produce the base frequency and harmonic frequencies used to modulate the calling channel carrier wave for station selecting purposes. The energization of relay 78 connects the plate and filament batteries with the harmonic amplifier HAG and the modulator MCC, that amplify the calling harmonic frequency derived by way of the tuned branch circuit 63, 64 from the harmonic distributing circuit 47, 48 and apply this calling frequency modulated by a particular one of the lower or station selecting frequencies to the toll line. When relay 9 operatesit also, at its lower armatures and front contacts, connects the plate and filament batteries with the amplifier A of the 800 cycle branch of the circuit M. Consequently the result of the operation of relay 9 in response to the depression of operators station selecting key 53 and the energization of the common relay 5, is to set in operation the lower or station selecting frequency oscillator O and harmonic producer HPL, to select and amplify the 800 cycle harmonic, and apply to the toll line the 30,000 or calling frequency harmonic modulated with the 800 cycle station selecting frequency.

At each of the other toll stations,,the 30,000 calling frequency modulated with the 800 station selecting frequency passes into the receiving branch R of the two-way repeater 100 of that station, and is' diverted into the receiving branch RC of the calling channel .as a result of the selective action for this particular frequency of the band filter DFC. The high frequency component of the modulated 30,000 frequency wave, after passing through the detector DC, the amplifier AC, and the high pass filter HPC, is rectified in the rectifier tube HRC and operates the relay 66. The low frequency component of the calling wave, after being detected and amplified in the detector DC and amplifier AC, passes through the low pass filter LPG to the rectifier tube LRC where it is converted into unidirectional current for the operation of relay 67. As has been described. the characteristics of the low pass filters LPG at the various stations are so adjusted that each station is selective with respect to a particular one of the low fre uency or station selecting components of t to calling wave. As a result, when the calling wave of 30.000 frequency is applied to the toll line, all of the relays 66 at the distant toll stations are operated, but the relay 67 is operated only at the particular station that has its low pass tilter LlC adjusted to the particular station selecting fI( uency with which the calling wave is modulated. Relay 66 in operating closes a circuit that extends from battery through the winding of relay 2 (Fig. 2), resistance 6, conductor 36 and front contact and armature of relay 66 to ground. The closure of this circuit operates relay 2 to make in effective the initial energizing circuit over which the keys 51, 52 and 53 at all the associated operators positions have control over the calling and station selecting operations; and also to illuminate the pilot lamps 50 associated with these keys, in the manner already described. Thus the application of the calling frequency by any of the operators signals all of the other operators at all of the toll stations of the system that the calling frequency is in use, and also positively prevents any other than the call ing operator from initiating a call.

lVhen the relay 67 operates at the particu lar toll station that is being called, it closes an energizing circuit for the associated relay and that relay in operating applies ground to the associated conductors 38, thus rendering these conductors effective to cooperate with other signal controlling apparatus in a manner that willzsubsequently be described.

The application of this station selecting frequency that has brought about the operation of relay 65 at the called toll station has been accompanied, as already stated, by the pressing of the ringing key 27 in the calling operators connecting cord circuit. The pressing of ringing key 27 applies current from ringing generator 99 to the talking conductors 79 and of the channel into the multiple jack MJ of which the plug 25 is inserted. This ringing current passes from the conductors 79 and 80'by way of the upper armatures and front contacts of relay 10, upper armatures and back contacts of relay 13, and left hand back contact and armature of relay 14 to the winding of relay 15, thus causing the energization of that relay. The energizing circuit of the slow-releasing relay 17 is normally closed by way of the *back contact and armature of relay 15, so that when relay 15 is'operated by the operators application of ringing current to the conductors 79 and 80, the energizing circuit of relay 17 is interrupted and the relay releases its armature. This closes a circuit that extends from battery in parallel branches through the Winch ings of relays 21 and 23 and back contact and switch spring of relay 17 to ground. Relay 23 in operating takes part in the performance of the attenuation controlling function in a manner that will be described hereinafter. Relay 21 in operating closes a circuit that extends from battery through the winding of relay 42 (Fig. 1), conductor 60 and upper front contact and arinature of relay 21 to ground. Relay 42 in o )erating applies current from the plate and filament batteries to the circuits of the ringing frequency oscillator and amplifier G0 lay 14 will be described hereinafter.

and GA, thus energizing these elements and causing them to generate and amplify a. ringing frequency wave that may conveniently have a frequency of 133 cycles per second. At the same time relay 21 in operating breaks the talking conductors 84, 83 of the transmitting branch of channel C from its normal connection with the series winding of the two-way repeater 20, and connects them with the conductors 61 and 62 extending to the ringing frequency oscillator. The result, therefore, is to apply the ringing frequency to the modulator MC of channel and'efl'ect a corresponding modulation of the carrier wave that this channel is; applying .to the toll line.

At each of the distant toll stations this ringing frequency component is detected and amplified in the receiving branch RC of channel C and after passing through the low pass filter LPG, causes the operation of the relay 18, which is bridged across the receiving branch conductors 91, 92 in a'circuit tuned to the 133 cycle ringing frequency. The energizing circuit of slow-releasing relay 16 is normally closed at the back contact and armature of relay 18. When relay 18 is energized as a result of the application of the ringing frequency at the calling station, the energizing circuit of relay 16 is broken and relay 16 releases its armature and closes -its back contact. This completes a circuit that extends from battery through the winding of relay 13, armature andback contact of relay 16 and armature and switch spring of relay 17 to ground, relay 17, as has been described, being normally held energized by the clo sure of its circuit at the back contact of relay 15.

Relay 13 in operating applies ringing current from generator 119 to the talking conductors 79, of the corresponding channel extending to the spring jacks of the associated toll board sections. However, at this time this application of ringing current is without'effect, as this particular channel is idle and none of the spring jacks associated with conductors 79 .and 80 have plugs insorted in them. Relay 13 in operating also closes a circuit that extends from battery through the winding of relay 14 and inner lower front contact and armature of relay 13 to ground. The particular function per formed as a result of the operation of i:-

t its outer lower front contact and armature, relay 13 in operating closes one break in a conductor that is normally also broken at one of the front contacts and armatures of relay 65, and also at a front contact and armature of relay 11, and which conductor includes the winding of relay 10. Consequently, in order to effect the energization of relay 10 as a result of the operation of relay 13, the relays 65 and 11 must also simultaneously be operated. It will be re- .membered that the relays-11 at all of the distant toll stations are operated through the medium of their associated relays 19 at the instant the carrier frequency of the corresponding channel is applieck as a result of the calling operator taking that channel for use. In like manner, as has just been described, the relays 13 at all of the distant stations are operated by the application of the ringing modulating frequency to the carrier when the calling operator presses the ringing key'27. Therefore, two of the three normal breaks in the energizing circuit of relay 10, namely, those associated with the rela s l1 and 13 respectively, are closed at this moment at all of the distant stations. But the third break in this energizing circuit, namely, that at a front contact and armature of relay 65, is closed only at the station whose relay 65 is adapted to be selected by the application of the 800 cycle station selecting frequency determined by the pressing of the operators station selecting key 53 that was assumed to have been chosen. Consequently the operating combination necessary for the energization of the relay 10 is effected only at the station determined by the pressing of the particular selecting key 53.

When the rela 10 operates, it closes a locking circuit or itself by way of its lower armature and front contact and the outer right hand armature and front contact ofrelay 11 to ground. Relay 10 in operating also closes a circuit that extends from battery through the calling lamp 40 (Fig. 3) associated with the answering jack AJ that serves as the answering extension of this channel, conductor 83, inner upper armature and front contact of relay 10, armature and back contact of relay 49, and left hand back contact and armature of relay 12 to ground. This lights the lamp 40, and indicates to the answerin operator of channel C at the selected tol station that she is being called over that channel. At the same time relay 10 has closed the talking conductors of channel C through to conductors 79, 80. Consequently when the operator inserts the plug 25 of one of her connecting cord circuits into the answering jack AJ the cord circuit is connected by way of channel C with the calling operator at the distant station; The insertion of plug 25 by the answering operator at the called station closes a circuit from battery through resistance 98, registering sleeve contacts of plug 25 and answering {ack AJ conductor 81 and winding of reay 12 to ground. Relay 12 in operatin breaks at its left hand armature and bae contact the direct connection o ground over which the calling lamp 40 was energized,

leaving the lamp connected to ground by way of the winding of relay 49, which re-- lay Winding is of high enough resistance to reduce the current flow through the lamp sufiiciently to extinguish it. The current flow through the lamp and the relay 49, in series, however, is sufficient to energize relay 49 and hold open the energizing path for lamp 40 by way of the back contact of relay 49, as long as the-controlling contact by way of the inner upper armature of relay 10 remains closed. Consequently thereafter the lighting of the lamp 40 is removed from the control of relay 12.

Relay 12 in operating also closes a circuit that extends from battery through the winding of relay 24 (Fig. 5), conductor 89 and outer right hand front contact and armature of relay 12 to ground. The resultant'energization of relay 24 energizes the transmitting branch of channel C at the called station. When this occurs, relay 19 in the receiving branch of channel C at the calling station is energized as a result of the energization of the transmitting branch at the called station, and in operating closes the energizing circuit of the associated relay 11 at the calling station. When relay 11 at the calling station operates, the movement of the outer right hand armature of relay ll-breaks the locking ath for re lay 10 that was initially close to ground by way of the left hand front contact and armature of relay 12 and closes a locking path for relay 10 directly to ground at the front contact of the outer right hand armature of relay 11. The contacts of the outer right hand armatureof relay 11 are so arranged that the front contact closes before the backcontact opens, so that ground is maintained continuously upon the locking contact of relay 10, and also upon conductor 88 through which relays 26, 27, 36 and 37 are locked up, as will subsequently be explained.

In the condition which now exists, the relays 10 associated with the selected channel at the calling and called stations are energized to extend the channel at these two stations to the associated spring jack terminals, and consequently to the connecting cord circuits at the calling and called stations, plugs of which are inserted in multiple and answering spring jacks respectively, of the channel. The relays 10 at these two stations are maintained energized to keep the re ective channel extensions closed sub ect to t e'control of the associated relays 11 and 12, relay 11 being held 0 rated as long as a plug 1s inserted in a c annel extension spring jack at the remote station and relay 12- being held energized as long as a plug remains in an associated channel extension spring jack at the local station.

At all the other stations on the toll line the relays 11 of the channel that is in use are held energized to keep the associated guard or busy lam s of the channel illuminated to indicate its unavailable condition and also positively to prevent operators at other stations from gaining access to the busy channel by depriving relay 12 of control over the channel controlling relay 10. At the calling and called stations each of the relays 11 and 12 keeps closed a branch of the energizing circuit of the busy lamps 41 of the associated channel, so that these lamps are not extinguished to indicate the available condition of the channel until the plugs are withdrawn from the channel extension spring acks at both stations.

The calling and called operators who are now connected over the chosen channel may communicate with each other by connecting their telephone sets 39 through the medium of the respective listening keys 28 with the cord circuits used for extending the connection. The path taken by the outgoing speech is from the operators set, contacts of key 28, through the repeating coil 37 and plug and jack connection 25, MJ conductors 79, 80, upper front contacts of relay 10, upper back contacts of relay 13, through the inductive coupling of repeating coil 20, back contacts of .inner upper and lower armatures of relay 21, conductors 84, 85, potens tiometer 113 and transformer 112 to the input side of modulator MC. -The modulated waves pass through the output transformer 114, adjustable attenuation network controlled by relays 26, 27, filter MFG, branch T, and into the line L at the repeating coil 100.

The path of the incoming speech is from coil 100 into receiving branch R, filter DFC, attenuating network controlled by relays 36, 37, input coils 116, to the detector input DC. The detected speech waves are amplified at AC; and are passed through filter' LPG, and conductors 91, 92 to the bridge terminals of repeating coil 20. From then on to the operators set the path is the same .as that traced for outgoing speech. At the distant selected station the paths for out- 1 going and incoming speech correspond to those above traced except that the plug 25 of the connecting cord, as'stated, is at that station inserted in jack A 1. When the operator at the called station learns from the 1 operator. at the calling station the particular local connection that is desired, she may complete this connection by inserting the lug 26 of the cord circuit into a multiple pack of the called line and apply ringing 1 current by means of the ringing key 29. The local subscribers at each end of the connection have control over their su ervisory lamps 31 through the medium 0 theassociated supervisory relays 32.

When either operator desires to signal the other for the purpose of disconnecting, or in the supervision of the connection, this may be done by pressing the ringing key 27 of the cord circuit that is being used. As has previously been described in detail, the pressing of ringing key 27 applies ringing current to channel talking conductors 79, 80, thus energizing relay 15, which deenergizes slow-release relay 17, which operates relay 21 to energize the ringing frequency oscillator circuit and apply the 133 cycle current from this source to the outgoing conductors of-the transmitting branch of the channel. At each of the distant toll stations, including the station with which connection has been established over this channel, the 133 cycle modulation of the channel carrier frequency is detected and amplified, and causes the operation of the relay 18 bridged across the receiving branch of the channel As previously indicated, the electrical characteristics of the relay 18 and the branch circuit in which it is included are such as to tune the relay and its circuit to be selective of the 133 cycle ringing frequency. The relay 18 at each of the distant toll stations in responding opens the normally closed energizing circuit of and releases relay 16; and the release of relay 16 effects the energization of the associated relay 13. The response of the relay 13 at each one of the distant toll stations applies ringing current from the ringing generator 119 to the channel extension conductors 79, 80. This ringing current is without effect at each distant station except the one that has been selectively called, and since at that station a cord circuit plug 25 is inserted I in one of the spring jacks to which conduc- 36 and extinguishes lamp 30. Relay tors 79. 80 extend, the ringing current from generator 119 energizes relav 36 of the cord circuit. This relay upon being energized closes a circuit that extends from battery through supervisory lamp 30, relay 36, and normally closed local contacts of the cord circuit listening key 28 to ground. Consequently the relay 36 is locked up and maintains the lamp 30 illuminated. When the operator responds to the lighting of lamp 30. the breaking of the local contacts in the operation of listening key 28 releases relay;

13 o the distant selected station in operating also acts through the medium of relay 14 to disconnect the branch of the talking conductors including relay 15 to prevent any accidental interference of this relay with the continuity of the circuit controlled by the associated relay 17 that would prevent the proper closure of the previously traced energizing circuit of relay 13. Relay 13 also acts through relay 14 to close a short circuit of the serles winding of the two-way repeater 20 so as to revent the condition of unbalance created y the operation of relay 13 from causing the 133 cycle ringing frequency incoming by way of the channel receiving branch to be diverted back to the line by way of the transmitting branch of the channel.

As has been stated, the application of the 133 cycle ringing frequency to the channel causes the operation of the relay 18, the release of relay 16 and the o eration of relay 13 at each of the other toll stations, on account of the normally receptive condition of the channel at each station, whether in use or not. At each of these other stations, the operation of the associated relay 13 connects ringing generator 119 with the conductors that are connected with the two upper armatures of the relay: but as the relays 'at the non-selected stations are unenergized, the ringing generator is not applied to the channel extension conductors 79, 80 of these stations. However. at the non-selected stations,

the operation of relay 14, in response to the operation of the associated relay 13, disconnects relay to prevent any accidental re- 1 ase of relay 17, and consequent operation of relay 21: and also closes the short circuit of the series winding of the associated twoway repeater to prevent the unbalanced condition from communicating the incoming ringing frequency to the transmitting or outgoing branch of the channel and setting up sustained oscillations.

'hen the operators take down the con nection in response to the display of one or another or both of the supervisory signals and 31 associated with their connecting cord circuits, the removal of the plug 25 from the channel extension spring jack in which it has been inserted breaks the energizing circuit of the associated relay 12 and releases that relay. The release of relay 12 breaks the energizing circuit of relay 21- (Fig. 5), and the consequent release of relay 24 disconnects the plate and filament batteries from the transmitting branch of the channel and effects the withdrawal of the carrier frequency. Upon the withdrawal of the carrier wave, the relay 19 at the other one of the connected toll stations is deenergized and releases associated relay 11. If the plugs at the two connected toll stations are both withdrawn, therefore the relay 12 and the relay 11 are both released. When the relays 11 and 12 at each of the connected stations are released, both branches to ground from conductor 82 are opened, and consequently the busy or guard lamps 41 associated with the multiple jacks MJ of the corresponding channel are extinguished, thus indicating that the channel is again available for use. The relays 11 and 12 also jointly control the maintained energization of relay 10, the relay 10 being held operated as long as either of the relays 11 and 12 is energized. To-prevent the redisplay of the calling lamp in the interlift)

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