US2064938A - Common medium multichannel exchange system - Google Patents

Description

il -r 22, 1936. N. MONK COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26, 1934 2 Sheets-Sheet l J'zascrzber Receiving Eeguerz cz'es -Jm lt r. we w JTF WW Ill- Central /722 Jubscrz'ber Z5 From Selecn'rgy Circuit ATTORNEY 5 ZbEea INVENTOR BY f? FV d J dA mv w Mr W m IF m M m f 2 m? F r T l aab-L 3 N. MONK 2,064,938 COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26: 1934 2 Sheets-Sheet 2 0.1.7,

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1720116 Circuit's 1 06c: fieguency, Radio, Wire Carmel" or Other- 671621272668) 0 Central Office o K W o INVENTOR .71 Jlfiw/c BY ATTORNEY Patented Dec. 22, 1936 V UNITED STATES PATET OFFICE COMlVION MEDIUM MULTICHANNEL EXCHANGE SYSTEM Application May 26, 1934, Serial No. 727,799

Claims.

This invention has for its object the provision of an exchange system in which communication between subscribers is carried out over channels of diiferent frequency derived from a common transmission medium. More particularly, the invention is designed to utilize as a substitute for the many subscriber pairs which are employed in the ordinary telephone exchange system the many channels which are obtainable in the high frequency art as it is now unfolding and to provide means whereby these channels may be employed by subscribers for interconnection.

In the telephone exchange systems which have heretofore been provided it has been the practice to group subscribers in central ofiice areas. Each subscriber is connected to his own central office by means of an individual wire circuit or by a party line arranged for non-simultaneous use by a few subscribers. When a subscriber desires to call another subscriber he utilizes his own line to the central ofice, and upon passing the desired number either to an operator or to an automatic switching mechanism is connected to another line which extends either directly or via another office or offices to the called party. For a system of this kind many thousands of subscriber circuits are necessary in each central oifice area. These are commonly provided in the form of a network of multiple pair cables extending over the area.

In accordance with the present invention, it is proposed to substitute for such individual wire circuits extending to each subscriber a transmission medium which is common to all subscribers and which will accommodate a range of frequencies sufficiently wide for a large number of telephone channels. Interconnection between subscribers is then carried out over these high frequency channels.

The essence of the invention resides in making available to a large number of subscribers a transmission medium capable of accommodating a wide range of frequencies so that the subscribers may utilize, for the purposes of intercommunication, a plurality of signaling channels derived from the wide band medium. A transmission medium capable of handling the large number of channels required for the exchange system of the invention may be obtained by utilizing either radio transmission in space or high frequency transmission over a conducting or guiding medium.

Considering, first, the use of radio as the medium for transmitting the wide band of frequencies contemplated by the invention, the invention is designed to employ whatever part of the frequency range may be available. Heretofore, however, the radio frequency spectrum has scarcely afforded suflicient frequency space to yield the channel capacity required for a local telephone exchange system. With the opening up of the field of ultra-short waves this limitation as to channel capacity is removed. Thus, for example, between one and two meters there is a total frequency range of 150 megacycles while the range between 10 and centimeters is 1500 megacycles. The frequency ranges obtainable through the use of still shorter waves are much greater.

With these ultra-high frequencies, however, nature imposed another limitation, i. e., the tendency of the waves to take on quasi-optical characteristics, so that they project only in straight lines and do not bend around corners to any considerable extent. Thus, it becomes necessary that subscribers utilizing such waves have a fairly clear line of sight between their antennas. Some diffraction around obstacles may, of course, be obtained, but, in general, reasonably clear line-of-sight projection without intervening obstacles appears to be called for. This, of course, has the advantage of making it possible to utilize the same ultra-high frequencies in diiferent areas.

The line-of-sight characteristics of the ultrashort waves must be recognized in the design of a telephone exchange system utilizing this technique. In one form of the invention it is proposed to provide in the center of a community a central station with antennas mounted sufficiently high above the surrounding buildings to make possible the necessary directness of transmission between these antennas and practically any point in the territory to be served. One subscribers station is then placed in connection with another by transmitting first to the central station and thence through central station to the desired other subscriber located in another part of the community. The central station acts as a means for catching the waves from one subscriber at a relatively high point and transmitting them down again to a second subscriber, thus lifting the transmission path above any obstacles lying between the subscribers antennas.

In order that such a central station may be economical, it is proposed to make it capable of handling simultaneously a wide band of frequencies such as would accommodate hundreds or perhaps thousands of channels. The particular arrangement of the central station will be hereinafter described.

As an alternative to the use of a radio transmission medium, the invention contemplates the employment of a guiding or conducting transmission path suitable for the required band of frequencies. Such a transmission path would be provided in the form of a network, extending to all the subscribers within a given area. Conceivably, such a network might be composed of ordinary wire circuits branched and interconnected so as to make the common circuit available to each subscriber. Thus, the wire network might be somewhat similar to the network employed for distributing light and power currents, except that a simple two-wire network without multi-phase connections, voltage transformation arrangements, etc., would suflice. It would, however, be possible to carry out the invention by employing the light and power network itself and this is contemplated within the scope of the invention.

The preferred form of guiding transmission medium, however, is one which is designed for the transmission of a wide band of frequencies with comparatively low attenuation and preferably also with shielding to minimize the effect of external disturbances. Such a transmission medium may be found in a circuit consisting of two conductors disposed coaxially with respect to one another. In a properly designed coaxial circuit, low attenuation at high frequencies may be attained by the employment of conductors of suitably low high frequency resistance and the use of a substantially gaseous dielectric between the conductors. With this type of circuit the outer conductor serves also as a shield whose protective effect becomes more nearly perfect as the frequency is increased, so that at high frequencies practically complete immunity from external disturbances may be obtained.

The invention contemplates also as an alternative to the coaxial circuit a high frequency transmission medium comprising a circuit consisting of two conductors surrounded by a shield. Here, again, the high frequency attenuation may be minimized by proper design with substantially gaseous insulation and the thickness of shield may be determined so as to minimize interference from external disturbances.

Yet another form of transmission medium suitable for carrying out the invention is a dielectric wave guide comprising a cylindriform dielectric material which may or may not be surrounded by a conductor. An advantageous form of such a transmission path is a hollow cylindrical conductor containing air or some other gas as the dielectric. This type of circuit is particularly adapted to transmit waves of Very high frequencies, for example, of the order of a few centimeters or less in wave length.

Using any of the above types of transmission media, the interconnection of subscribers at will from the available channels involves a number of new and difficult problems which are solved by the various features of the invention as hereinafter disclosed.

In general, the invention contemplates the assignment to each subscriber of two particular frequencies which might preferably be related to his telephone number. The assignment of frequencies should be such as to facilitate the interconnection of subscribers and one scheme of frequency allocation is suggested in the invention for this purpose.

The foregoing outline having indicated some of the principles of the invention as well as a reases few of its major objects, the complete invention with its various details, features and purposes will now be understood from the following description when read in connection with the accompanying drawings, in which Fig. 1 shows a schematic representation of the principal idea of the invention wherein a plurality of stations are connected to a common transmission medium, Fig. 2 shows in schematic form a system for connecting subscribers through a central oifice using space radio transmission; Fig. 3 shows one possible allocation of frequencies for a multi-channel exchange system; Fig. 4 shows a subscriber station arrangement which may be used in the system of Fig. 2; Fig. 5 shows a modulating device which may be used in various applications of the invention; Fig. 6 shows a central ofiice arrangement which may be used in association with the subscriber station arrangement of Fig. 4; Fig. '7 shows an antenna arrangement for use in a system of the type described; Fig. 8 is a schematic diagram indicating how connections between stations in different areas may be completed through central stations over trunk circuits; and Fig. 9 is a schematic diagram of a guiding or conducting network which is available in common to a central station and a number of individual stations.

Referring to Fig. 1, there is illustrated one of the principal aspects of the invention. In this figure is shown a common medium CM, to which are connected a plurality of stations S1, S2, S3, etc., each of which includes transmitting, receiving and associated apparatus. This apparatus, which will be hereinafter described, is arranged to permit intercommunication between different pairs of stations over the common medium CM employing different frequency bands to permit simultaneous communication between different pairs of stations. Connection between any pair of stations, for example, S1 and S3, takes place through the common central station CS.

Fig. 2 shows in schematic form a telephone exchange system in which a number of subscribers may be interconnected through a central oflice, the connection between each subscriber and the central office being accomplished by radio transmission through space. Each subscriber is provided with transmitting apparatus including an oscillator-modulator and receiving apparatus including a demodulator, together with the necessary selecting circuits. At the central office a plurality of circuits such as the one shown are provided for interconnecting between subscribers. Each connecting circuit includes two separate sets of apparatus, each of which comprises a radio transmitter and receiver, and a connection, to which the operator has access, between the two sets of apparatus.

Transmission from subscriber A to subscriber B is effected by means of the transmitter STl, the oscillator-modulator SM1, the tuned circuit or filter SP1, the antenna SAI, the antenna 0A1, the tuned circuit or filter OF1, the demodulator 0131, the hybrid coil 0H1, the connecting circuit CC (across which is bridged the operators set OS), the hybrid coil 0H2, the oscillator-modulator 0M2, the tuned circuit or filter OF4, the antenna 0A2, the antenna SA2, the tuned circuit or filter SF4, the demodulator SDz and the receiver SR2. Transmission in the opposite direction is accomplished in an analogous manner, employing the transmitter STz, the oscillatormodulator SM2, the selecting circuit SFa, the antenna SAz, the antenna 0A2, the selecting circuit OF's, thedemodulator ODa'the hybrid coil 0H2, the connecting circuit CC, the hybrid coil 0H1, the oscillator-modulator 0M1, the selecting circuit OFz, the antenna 0A1, the antenna SAi, the selecting circuit SFz, the demodulator SDI and the receiver SR1.

It is proposed that different frequencies be employed for the oppositely directed transmissions in Fig. 2 and that the frequencies employed between the calling subscriber and the central office be different from those used between the central oflice and the called subscriber. Transmission might be on either a double or a single sideband basis with the carrier frequency transmitted.

In order that simultaneous conversations may take place between a number of pairs of subscribers, it is necessary that the frequencies employed in the system of Fig. 2 be properly selected. One way of arranging the frequencies would be to assign to each subscriber a separate pair of frequencies, one for transmitting and one for receiving, and to provide apparatus at the central office for receiving and transmitting the various required frequencies. Thus, subscriber A might be assigned a transmitting frequency f1 and a receiving frequency f2, and subscriber B transmitting and receiving frequencies is and f4, respectively. This would mean that in Fig. 2 the modulators SM1 and SMz would operate with carrier frequencies f1 and f3, respectively, and the modulators 0M1 and 0M2 with carrier frequencies f2 and f4, respectively.

' With such a scheme it might prove desirable in order to facilitate the allocation of frequencies and the interconnecting of subscribers, to have a constant difference between the transmitting and receiving frequencies of each subscriber or, in other words, to make The frequencies f1, f3, etc., might then be grouped in one part of the frequency spectrum and the frequencies f2, f4, etc., in another part. A diagram of a frequency allocation of this sort, assuming a total of 99 subscribers, is given in Fig. 3. It will be noted that the order of the frequency assignments corresponds to that of the subscriber numbers. This scheme, while not essential to all embodiments of the invention, is a very desirable feature.

Detailed arrangements of apparatus which might be used at the'subscribers station and at the central oflice in a scheme of this sort are illustrated in Figs. 4 and 6. While some of the apparatus shown in these figures is more particularly adapted to radio frequencies lying below the ultra-short wave range, it will be understood that such apparatus is shown merely to illustrate the principles of the invention, and is not intended in any way to limit its scope.

The operation of the subscriber's apparatus illustrated in Fig. 4 is as follows: When the subscriber desires to make a call, the receiver SR1 is removed from the switchhook SH. Through the contacts of the switchhook this closes a circuit which furnishes power supply to operate the oscillator and modulator 0C and MO. The operation of the switchhook also provides D. C. supply for the transmitter ST1, this supply being derived through a rectifier RF which is connected to the A. C. supply. The speech currents produced in the transmitter, after passing through the transformer TRl, are 7 modulated upon the assigned frequency, here assumed to be ii. The oscillator and modulator may be separate units of any desired types, or a self-oscillating modulator might be used. The carrier and sidebands in the modulator output, after passing thru the selecting circuit SF1, are radiated by the antenna SA1 to the central oifice. When communication with the central oflice is established in a manner hereinafter to be described, the number of the called subscriber is passed orally to the operator and after the proper arrangements are set up at the central office, communication is established with the called subscriber who is provided with apparatus identical'with that of Fig. 4 except for the frequency assignments used.

In the case of an incoming call, the carrier frequency 1: and its associated sidebands are received by the antenna SA1, selected bythe selecting circuit SF2 and demodulated by the demodulator SD1. This demodulator may be of the vacuum tube type, in which case it should be kept always in operating condition so that a call may be received. Preferably, however, the demodulator might comprise some passive element or elements, as, for example, copper-oxide units as shown in Fig. 5; Associated with the demodulator is a relay RL1 which is operated by the rectified current resulting from the incoming carrier frequency and which in turn operates the 'call bell SB. When the subscriber answers the call, the switchhook SH is operated, which, in addition to completing the contacts previously mentioned, operates the slow-release relay RLz, which breaks the circuit of the bell SB. The received voice frequency currents, after passing through the transformer TRz, are applied to the receiver SR1 and two-way communication is thus established. If desired, a low pass filter LP (shown dotted in Fig. 4) may be included in the receiver circuit to suppress unwanted high frequency components in the demodulator output. It will be understood that a similar filter may be provided in the arrangements hereinafter described. When the subscriber hangs up, the slow-release feature of RLz prevents his bell from ringing before the other party also hangs up.

Fig. 6 illustrates central office apparatus which may be used in conjunction with the subscriber's apparatus shown in Fig. 4. The apparatus is so arranged that a call coming in at any frequency is automatically picked up and brought to the operator, who completes the connection to the called subscriber. Referring to Fig. 6, the antenna 0A0 is designed to receive incoming carrier frequencies within a range corresponding to all of the subscribers assigned to the central oflice or to a group of subscribers. The received frequencies are delivered to the tuned circuit OFo whose frequency of response is being continuously varied by the driving motor DMo. The same motor also rotates the switch SW0 which is provided with a. number of contacts, one for each subscriber whose incoming frequency may be selected by the selecting circuit CE). The switch is arranged so that the contact corresponding to any given subscriber is made at the same moment that the frequency of that subscriber is picked out by the selecting circuit.

The carrier frequency of an incoming call is selected momentarily by OFo and is rectified in the rectifier ODo. The resultant current operates the relay 0R0, which closes a circuit through the winding of the slow release relay 0R1, the back contact of the relay CR2 and the switch SW0.

The operation of the relay 0R1 lights the sub scrlbers line lamp 0L1 thereby calling the attention ofthe operator to the incoming call. Similarly, as the selection and switching are being varied, the line lamps corresponding to other incoming calls will be lighted.

When the operator observes thelamp (0L1 she inserts a plug, for example, 0P1, which is associated with one end of a cord circuit, in the subscribers jack 0J1. This action completes a circuit through the relay 0R2, whose operation deenergizes the relay CR1 and thus extinguishes the lamp 0L1. .After insertion of the plug in the jack 0.11, the operator adjusts manually by means of the control C'Ii, the response frequency of the variable selecting circuit OFl to the transmitting frequency of the calling subscriber, which at the same time brings the oscillator 001 and the selecting circuit OFz to the receiving frequency assigned to the calling subscriber. The incoming carrier. frequency of the calling subscriber is rectified in the demodulator OD1 and operates the relay 0R3, thereby lighting the supervisory lamp OL-zduring the time whenjthe calling subscribers receiver is off the hook.

Having ascertained from the calling subscriber the number which is desired, the operator completes the connection by inserting the other cord circuit plug 0P1 in the jack 0J2 which corresponds to the number of the called party and at the same time adjusts by means of the manual control CTz the frequencies of the oscillator 002 and the selecting circuits OF: and OF4 to the frequency assignments of the called subscriber, The connection established by the jack 0J2 energizes the relay 0R6, thereby preventing the line lamp of the called subscriber from lighting when he answers the call. The supervisory lamp 0L3 is lighted when the called subscriber answers, and remains lighted until he hangs up. The conversation then proceeds with the apparatus functioning in a manner similar to that described in connection with Fig. 2. When the conversation is completed the lamps 0L2 and 0L3 are extinguished as the subscribers hang up and the operator takes down the connection. By means of keys such as 0K2 the operator may connect her set to different cord circuits and using the key 0K1 she may split the connection and talk to either subscriber without the other. I

As noted, the selecting circuit OFo and the switch SW0 might serve for all the subscribers assigned to the omce, or these subscribers might be divided into groups, with one selecting circuit and switch taking care of each group, and all such circuits and switches driven by a common motor.

It is apparent that the arrangement of Fig. 6 permits the employment of a relatively simple arrangement at the subscribers station, inasmuch as the frequencies used by the subscriber are al- Ways the same. The cord circuit arrangement at the central office is somewhat complicated, but the number of cord circuits required will be only a small fraction of the total number of subscribers. I

In the various arrangements shown in Fig. 2 to Fig. 6, radio transmission between the subscribers and the central oflice may be accom plished directly using any suitable type of antenna. However, in order to permit the use of ultra short waves for which substantially lineof-sight projection may be desired, it may be necessary to carry the transmission path above any obstacles lying between the antennas of individual subscribers. For this purpose an arrangement-of-the type shown in Fig. '7 might-be employed.

. Referring to Fig. '7, it will be seen that in this case a single central ofiice antenna OA serves for.

receiving all frequencies from subscribers and transmitting all frequencies to subscribers. This antennamay be mounted on a tall building or towerso that substantially a line-of-sight transmission path is obtained between the central office and each subscribers antenna within the central ofiice area. The subscribers antenna should preferably be designed for radiating a beam toward the central oflice antenna and for receiving radiation from that antenna. Thus the subscribers antenna might be a balanced doublet SA as shown in Fig. 7, placed near the focus of a parabolic reflecting surface, PR, directed toward the central oflice antenna 0A. The central ofiice antenna, however, is designed for radiating to and receiving from all subscribers. For this purpose there might be employed an antenna capable of radiating in all directions and receiving radiations from all directions throughout the range of frequencies utilized. Such an antenna, for example, might be a balanced doubtlet CA as shown in Fig. 7. Another possibility would be to employ a central office antenna so designed that it is capable of radiating into and receiving from a conical space which comprehends all subscribers locations. This might be accomplished by adding the reflector RE above the doublet antenna 0A.

It is also contemplated that instead of employing a single central ofiice antenna a multiple antenna arrangement might be used atthe cen tral oflice building,- each antenna being designed for handling a part of the total range of frequencies. It would also be possible to employ two antennas for each subscriber instead of one.

Although the arrangements described in connection with Figs. 2, 4 and 6 are designed to provide a radio connection between the subscribers within a given area, it is proposed to interconnect such stations with subscriber stations in other areas by providing in each area a central ofii ce, so that a radio connection may be established between a subscriber and hiscentral ofiice, using a frequency or frequencies assigned for that purpose, and the connection completed over suitable interoflice trunks. The method is illustrated schematically in Fig. 8. The central office apparatus to be used in carrying out this method might be patterned after that shown in Fig. 6. The types of trunks which may be used to interconnect the central ofiices are well known in the art.

In the arrangements thus far described, with the exception of Fig. 1, it has been assumed that free space is employed as the common transmitting medium. It is contemplated, however, that transmission over a high frequency conducting or guiding path may be employed instead of radio transmission. Such a path would take the form of a network extending to all subscribers within a given area and to the central ofiice for that area. A network of this kind is shown schematically in Fig. 9, where each line represents a complete circuit, C designates the central ofiice and S a subscriber's station.

Probably the simplest form ofsuch a network would be one comprisedofordinarytwo-conductor circuits suitably interconnected and branched so that the common circuit will be available at all desired points. The circuits comprising the network might, for example, consist of pairs of open-wire or cable conductors.

It is contemplated also in accordance with the invention that the wire network employed for distributing light and power currents might be used as the common medium from which to derive high frequency channels for the telephone exchange system. The high frequency channels would be superposed upon the power network by carrier methods which are well known in the art.

Another form of transmission medium, and one which is peculiarly advantageous in that it is capable of transmitting a wide band of frequencies with comparatively low attenuation and which may be so shielded as to be practically immune to external disturbances, may be found in a network of circuits consisting of two conductors disposed coaxially with respect to one another. A circuit of this type has been disclosed in the patents to L. Espenschied and H. A. Affel, No. 1,835,031, December 8, 1931, H. A. Affel and E. I. Green, No. 1,781,092, November 11, 1930, etc.

As an alternative to the coaxial circuit, the high frequency transmission medium might comprise a network of circuits, each consisting of two parallel conductors surrounded by a shield. Circuits of this type are disclosed in the patents to Green, Curtis and Mead, No. 2,034,032, Green and Curtis, No. 2,034,033 and Green and Liebe,

No. 2,034,034, all granted March 1'7, 1936.

The invention contemplates also utilizing as the high frequency transmission medium a dielectric wave guide comprising a cylindriform dielectric material which may or may not be surrounded by a conductor. Such dielectric guides are disclosed in the patent applications of G. C. Southworth, Serial No. 661,154, filed March 16, 1933 and Serial No. 101,711, filed December 9, 1933.

It is contemplated in accordance with the invention that the subscriber set and central office arrangements which have been described may be employed with any of the above types of transmission paths. For this purpose, it is necessary merely to substitute the transmission network for the radio path, replacing the connection of the apparatus to the radio antenna by -a connection suitable to the type of medium employed.

It will be obvious from the preceding discussion that the general principles herein disclosed may be embodied in many other organizations widely diiferent from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. A system for the exchange of intelligence over a common medium, said system comprising a plurality of more than two individual subscribers stations and a common central station through which any two individual subscribers stations may be connected, said individual subscribers stations employing different fixed frequencies for transmission and other and different fixed frequencies for reception, and means at the central station for identifying any of a plurality of said individual subscribers stations whenever said stations are calling.

2. A system for the exchange of intelligence over a common medium, said system comprising a plurality of more than two individual subscribers stations and a common central station through which any two individual subscribers stations may be connected, said individual stations employing different fixed frequencies for transmission and other and diiferent fixed frequencies for reception, and means at the central station to indicate and identify each of a plurality of calling subscribers stations upon receipt of unmodulated carrier frequencies from said calling stations.

3. A system for the exchange of intelligence over a common medium, said system comprising a plurality of individual subscribers stations and a common central station through which any individual subscribers stations may be connected, said individual subscribers stations employing different fixed frequencies for transmission and other and different fixed frequencies for reception, and means at the central station for identifying any of a plurality of calling subscribers stations upon receipt of unmodulated carrier frequencies from said calling stations, said means including an adjustable selective receiving circuit, and means for varying the frequency of response of said circuit continuously and systematically.

4. A system for the exchange of intelligence over a common medium, said system comprising a plurality of individual subscribers stations and a common central station, said individual subscribers stations employing different fixed frequencies for transmission and other and different fixed frequencies for reception, means at the central station including a receiving circuit, means to vary the frequency of response of said circuit continuously and systematically, and means controlled by said receiving circuit to indicate and identify a calling subscribers station upon receipt of unmodulated carrier frequencies from said calling station, said last mentioned means being unresponsive to said unmodulated carrier frequencies after said indication has been acted upon by establishing a carrier current connection to said subscribers station, said last mentioned means being also arranged to remain unresponsive so long as said calling station may be interconnected with any other station.

5. A system for the exchange of intelligence over a common medium, said system comprising a plurality of more than two individual subscribers stations and a common central station through which any two individual subscribers stations may be connected, said individual subscribers stations employing different fixed frequencies for transmission and other and different fixed frequencies for reception, and means at the central station for interconnecting in pairs said individual subscribers stations including a plurality of sets of transmitting and receiving equipment for two-way carrier transmission to and from subscribers stations, each set being adjustable to the frequencies of any subscribers station which is to be connected to any other subscribers station, each set being connected together at voice frequencies, and means associated with each receiving equipment for supervising the connections.

6. A system for the exchange of intelligence over a common medium, said system comprising a plurality of individual subscribers stations and a common central station, said individual subscribers stations employing difierent fixed frequencies for transmission and other and different fixed frequencies for reception, means at the central station whereby a plurality of calling subscribers stations may signal an operator, said means including a continuously varying frequ y se ecting and receiving circuit, said circuit controlling means arranged to operate a plurality of relays, at least one corresponding to each subscribers station, upon receipt of unmodulated carrier frequency from said calling stations, said relays being arranged to close circuits individual to the subscribers stations to indicate and identify the calling subscribers stations.

'7. A system for the exchange of intelligence over a common medium, said system comprising a plurality of individual stations and a central station, said individual stations employing different fixed frequencies for transmission and other and different fixed frequencies for reception, and means at the central station for indieating a calling station including a continuously and systematically varying frequency selecting circuit for selecting in rotation the transmitting frequencies of said individual stations, a rectifier, a plurality of relays including at least one for each of said individual stations, indicating circuits and apparatus associated therewith, and a switch comprising a plurality of fixed contacts one for each individual station; said contacts being continuously swept over by a moving contact in such a manner that a connection is established between the moving contact and each fixed contact when the variable frequency selecting circuit is tuned to the transmitting frequency of the station associated with that contact.

8. A system for the exchange of intelligence over a common medium, said system comprising a plurality of individual stations and a central station, said individual stations employing different fixed frequencies for transmission and other and diiferent fixed frequencies for reception, and means at the central station for indicating to the operator a calling station including a selecting circuit, the frequency of response of which is being continuously and systematically varied by a motor, a switch having fixed contacts one for each station and a rotating contact also connected to said motor and so arranged that a circuit will be established between the moving contact and the fixed contact associated with any one station at the moment when the variable selecting circuit is tuned to the transmitting carrier frequency of said station, a rectifier and relay connected to said variable frequency circuit, a set of relays one for each individual station, a second set of relays also one for each individual station, the windings of each of said second set of relays being separately connected to one of a plurality of jacks, one for each station, and so arranged that when a carrier frequency is received from a calling station a signal identifying the calling station is given and means whereby when a plug is inserted in the jack corresponding to said calling station a circuit will be established through said switch to remove said signal and hold it inoperative until said plug is removed from said jack.

9.'A system for the exchange of intelligence comprising a plurality of individual stations, a central station, and a common transmission medium, each of said individual stations employing different fixed carrier frequencies for transmitting and' other and different fixed carrier frequencies for receiving, means at the central station for determining automatically the number of a calling station, means for adjusting manually variable frequency transmitting and receiving apparatus associated with the common medium to the frequencies of the calling station, means for ascertaining from the calling subscriber the number of the called station, means for adjusting manually a second set of variable frequency transmitting and receiving apparatus,v

mon transmission medium, each of said stations employing different fixed carrier frequencies for transmitting and other and difierent fixed carrier frequencies for receiving, means at the central oifice for determining automatically the number of a calling station, means for adjusting manually variable frequency transmitting and receiving apparatus associated with the common medium to the frequencies of the calling station, means for ascertaining from the calling subscriber the number of the calledstations, means for adjusting manually a second set of variable frequency transmitting and receiving apparatus, also associated with the common medium, to the frequencies of the called station and means for interconnecting the two sets of transmitting and receiving apparatus to establish a connection between the calling and called stations.

-11. A telephone system comprising a plurality of individual stations, a central ofiice, and a common transmission medium, each of said stations employing different fixed carrier frequencies for transmitting and other and different fixed carrier frequencies for receiving, means at the central ofiice for determing automatically the number of a calling station, means for adjusting manually variable frequency transmitting and receiving apparatus associated with the common medium to the frequencies of the calling station, means for ascertaining from the calling subscriber the number of the called station, means for adjusting manually a second set of variable frequency transmitting and receiving apparatus, also associated with the common medium, to the frequencies of the called station and means for interconnecting the two sets of transmitting and receiving apparatus to establish a connection between the calling and called stations and means for monitoring said connection.

12. In a telephone system comprising a plurality of individual stations greater than two, a central ofiice having a tunable element for identification, a common transmission medium, each of said individual stations employing different fixed carrier frequencies for transmitting and other and different fixed frequencies for receiving, and translating apparatus at said central station adjustable to the transmitting and receiving frequencies of any calling station and capable of translating such frequencies to the corresponding frequencies of any called station for two-way communication, the method for indicating and identifying at the central ofiice any of a plurality of calling stations which consists in continuously cyclically adjusting said tunable element to periodically and successively tune it to the transmitting carrier frequency of each of said stations, indicating the receipt of any of said carrier frequencies, identifying the stations transmitting said received frequencies, and ad justing said translating apparatus to the transmitting and receiving frequencies of the calling station identified, without interrupting the continuity of the adjustment of said tunable element.

13. In a telephone system comprising a plurality of at least three individual subscribers stations, a common central oflice and a common transmission medium, each of said subscribers stations employing diiierent fixed carrier frequencies for transmitting and other and different fixed carrier frequencies for receiving, means at the central ofiice for determining automatically at the central office the frequencies of any one of a number of subscribers stations, means at the central ofiice for establishing communication with the calling subscribers station, means at the central oiiice for establishing carrier frequency communication with the called subscribers station, means for interconnecting the calling and called subscribers stations at the central office at a common frequency, and means for effecting simultaneously a plurality of such interconnections.

14. Means for indicating transmission from and identifying any of a plurality of carrier frequency transmitting stations, said means including a variable frequency selecting and receiving circuit, means for continuously varying said circuit, a plurality of relays controlled by said circuit, at least one corresponding to each transmitting station, and means responsive to the receipt of unmodulated carrier frequency from said transmitting stations for operating the corresponding relays, said relays being arranged to close circuits individual to the transmitting stations to indicate transmission from and identify the transmitting stations.

15. Means for the reception of signals on each of a plurality of frequencies during successive intervals, a plurality of relay devices operated by said means, one relay corresponding to each of said frequencies and operable during the receipt of a respective frequency.

NEWTON MONK.

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