US2202474A

US2202474A - Telephone system

Info

Publication number: US2202474A
Application number: US249230A
Authority: US
Inventors: Vroom Edward
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1939-01-04
Filing date: 1939-01-04
Publication date: 1940-05-28
Anticipated expiration: 1957-05-28

Description

May 2s, 1940. E ROOM TELEPHONE SYSTEM 2 Sheets-Sheet l Filed Jan. 4, 1939 @QN @NJ EN R y -fri 2 2 W @QNS ud .wir R w Y @my mm m .51.50 ...d .55 NV f .l wl- /5 f mow NQS wow EN Qu Sw m, ..0\||\ v. @OUD HHN KQQ wm. mm. QN Ov bm.) wm. s @l S8? a @l S May 28, 1940.

E. VROOM TELEPHONE SYSTEM Filed Jan. 4, 19:59

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Patented May 28, 1940 UNITED s'rrss Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application January 4,

14 Claims.

This invention relates to telephone systems and particularly to telephone systems wherein a plurality of telephone messages may be simultaneously transmitted over a single pair of wires. In

a yet more limited sense it is concerned with the connections between individual telephone subscribers and their central oice, especially where these are party-line connections.

Telephone service to subscribers may be provided on an individual line basis or on a partyline basis. In the former a separate pair of wires is ordinarily provided from the central ofce to the subscriber and he has thereby continuous service and complete privacy. A party .line may comprise a single line from a central oice on which, for example, four separate subscriber stations may be bridged. Selective ringing is ordinarily provided so that a desired one of the four subscriber stations may be rung without signaling the other stations. However, telephone service is available for a given one of four subscribers on a party line only at such times as the' line is not in use by one of the other three, and, furthermore, there is no provision to insure that a given conversation by one party-line subscriber shall not be overheard by any or all other` subscribers on the same line.

It is an object of this invention to provide partyline subscribers with individual line service.

Another object is to provide additional facilities over a single existing line.

Another object is to provide an improved methodof ringing over carrier frequency channels.

Another object is to provide means at a subscribers station whereby simple and automatic transformation is provided between voice frequency signals and carrier frequency current modulated by those signals.

A further object is to provide means readily adaptableto standard central oilice equipment whereby voice frequency signals may be readily transformed to and from carrier modulated signals for transmission over a parity line.

An additional object is to provide simple means for the automatic derivation of carrierfrequences for use on a telephone line from a commercial low frequency source'continuously applied to the said 1939, Serial N0. 249,230

whereas their distance from a central telephone station may be so great as to make the cost of such service very high. There arises therefore the problem of providing such service in such a way as to avoid the necessity of Stringing a separate pair of wires for each subscriber.

An effective method for the provision of multiple transmission channels over a single pair of wires consists in the employment of carrier current. 'Ihis method is widely employed on long distance lines where the large unit cost of the carrier current equipment may be justied in view of the fact that the great expense involved in the construction of additional long distance line facilities is thereby avoided.` Thus whereas the basic cost of the carrier current equipment is large, the cost per circuit mile compares favorably with the cost of additional Wires to provide the same number of additional circuits. On the other hand, if this conventional carrier equipment were to be employed in providing additional circuits over the comparatively short line involved in the suburban subscriber loops referred to above, it would be found that the cost per circuit mile of the carrier equipment was so great as to provide little or no advantage.

Not only is the basic cost of the usual carrier equipment too high to justify its use in party line service, but it is also impractical for such service because of the considerable maintenanceordinarily required. Thus maintenance which may be readily provided in a telephone office under expert supervision would present a major, if not economically insuperable, problem if it must be provided on the subscribers premises. A further consideration is to be found in the fact that the maintenance of carrier equipment is ordinarily provided under the special facilities and personnel associated with the long distance telephone test board. The usual local telephone central oflice is not and ordinarily could not readily be so v stantially no maintenance is required, which does not involve a continuous absorption of expensively controlled power in its standby or operating condition, which comes automatically into operation for either speech or signal transmission, and which may be simultaneously employed for both voice frequency and carrier frequency transmission. Thus in a specific embodiment 60-cycle alternating current potential from a commercial power source is bridged across a line supplying telephone service to several subscriber stations. When a subscriber takes his receiver off the hook 60-cycle current enters his telephone set and through the medium of a harmonic generator of the type disclosed in Wrathall Patent 2,117,752, May 17, 1938, the high frequency assigned to that particular subscribers station is generated and selected and goes over the line to the central oice where means is provided whereby the line relay associated with the particular subscriber is thereby operated. Thus the operator is signaled and the connection may then be established in the ordinary way.

To call a subscriber the ordinary 20cycle central office ringing current is translated to 1000- cycle current and the carrier assigned to the particular subscriber station being called is modulated by this 1000-cycle current. 'Ihe modulated current goes over the line, is accepted by the subscribers station tuned to receive it, is demodulated in a combined modulator and demodulator of the type disclosed in H. S. Black Patent 1,983,528, December 11, 1934, and conventionally called a Modem, and the 1000-cycle current thus recovered at the subscribers station operates means which closes the 60cycle line current through the station bell.

Likewise, the Modem is employed to modulate carrier waves with speech or to demodulate modulated carrier waves into speech. The same carrier wave is used for speech and for ringing or signaling and the frequency thereof is characteristic of the particular subscribers station.

For a more complete understanding of the nature, objects and operation of the invention as illustrated by particular embodiments, reference is made to the drawings in which:

Figs l and 2 when placed side by side with Fig. 1 at the left show in schematic diagram a party line according to the invention in which are shown a voice frequency subscribers station and a carrier frequency subscribers station together with the central oliice termination thereof; and

Fig. 3 is a diagram of a harmonic generator which may be employed to produce the frequencies employed in the specific illustration of Figs 1 and 2.

Referring to Fig. l, carrier frequency subscribers station No. l is bridged by conductors II, I2, across party line lf3 which is continuously energized with 60-cycle alternating current applied thereto at the central office. Conductors II, I2 are connected to low-pass lter i3, bandpass filter i4 and transformer I5. A low frequency circuit from the line is established through the low-pass filter I3 into harmonic generator 24 when Vthe receiver 20 is lifted from its hook 2) thus closing

contacts

33, 34. The composite output of harmonic generator 24 is reduced to a single carrier frequency by the band-pass filter 23 and supplied through transformer 22 to conductors 35, 35 from which it passes to Modem 2| and also through band-pass lter I4 and conductors II, I2 to line I0. Bandpass lter i4 is designed to pass carrier plus side-band Whereas band-pass filter 23 passes only current of the carrier frequency itself. Lifting receiver 25 from hook 29 also completes the circuit of transmitter 27 through the primary winding of induction coil 25, switchhook contacts 3i, 32 and local battery 28 while the secondary circuit of induction coil 25 is closed through receiver 2G and

switchhook contacts

29, 3| to speech output leads 37, .3Q- to low-pass filter 2C and then to Modem ZI. Condenser i9 is adapted to tune transformer I8 to a frequency of 100G cycles per second and thus output at that frequency from Modern 2| may pass through low-pass filter 20,

conductors

40, 39 and condenser IS to one primary terminal of transformer I8 and through conductor 31, contacts 2S, 30 if the receiver is on its hook and thence through conductor 33 to the other primary terminal of transformer i3. The secondary of transformer I8 is connected across the control gap of the double gap cold cathode ionic tube Il. Such tubes are well known in the art, are ordinarily gas-filled, the particular gas composition determining the breakdown characteristics. For a description of the operating characteristics of tube of this type reference is made to Holden Patent 2,004,244 June 1l, 1935. The main gap circuit of tube I'. is through the bell E6 and the secondary of (iD-cycle transformer I5.

A voice frequency subscribers station is bridged cross party line I0 by the conductors 4G, 41 to which are connected GO-cycle transformer I5', low-pass filters 4I. 45 and band-pass filter 44. With the receiver 25 on its hook 29', as shown, an alternating current of frequency below its cut-off frequency of 3000 cycles may be fed through low-pass filter and then by conductor 48 and condenser I9 and by conductor 49, closed contacts 2t', 32 and conductor 5I! to the primary of transformer I8. However, since condenser I2' is adapted to tune transformer I8 to a frequency of 1000 cycles per second, only currents of that frequency will be transformed to appear in substantial amount across the control gap of the cold cathode ionic tube Il'. The main gap circuit of tube I8' is through bell I6' and the secondary of Gil-cycle transformer I5.

When receiver 26 is lifted from its hook Z9 contacts 29', 30 are opened. Connection is made between contacts 29', 3I, 32 and between contacts 33', 34. Under this condition current at a frequency below the filter cut-off point of 80 cycles per second may flow through conductors 46, 4'! from line I0, through low-pass filter III to harmonic generator 42. Output current from harmonic generator 42 flows through bandpass filter 44, conductors 46, 41 and over line Hl to the central station. Likewise, sound waves picked up by transmitter 21 may be with the aid of battery 28 translated into voice frequency currents. These currents may then flow through the induction coil 25 and then through receiver 2G' and conductors 49, 5I-48 to low-pass filter 45 and thence through conductors 46, 4'! to line I Similarly voice frequency current traversing line I 0 will be conducted through low-pass filter 45 and thence through the conductors 4S, iS-5I. and the secondary of inductance coil 25 to receiver 26.

Referring to Fig. 2, the central station termination equipment for a party line giving individual line service is shown for four channels comprising a voice frequency channel and three carrier frequency channels. 60-cycle alternating current power is supplied over

conductors

52, 53 to party line I0, to frequency doubler 10, and through the contacts of relay |34 when operated, to harmonic generator 1|. y

Voice frequency channel equipment is bridged on the party line l0 through

conductors

54, 55 and 91, 98. Conductors 54, 55 lead to band-pass lter 56 which passes the carrier frequency associated with the voice channel for signaling purposes. The output of band-pass filter 56 is connected through condenser 51 and transformer 58 to the control gap of double gap cold cathode ionic tube 59. The circuit to the main gap of tube 59 includes battery |41, relay 6|, limiting resistance 60 and secondaryY of transformer 69. The primary of transformer 60 is supplied from the 60-cycle power main through the full-wave rectifier 10 and thusA the output from the secondary of transformer 69 has a strong 1Z0-cycle component combined with other even harmonics f of 60-cyc1es. The

contacts

62, 63 of the relay 6| when closed form a bridge across conductor ||6 through the choke coils 64, 65.

contacts

The central office termination of carrier chanf nel No. 1 is bridged across party line i0 to conductors |01, |08 and then through band-pass filter |56 to

hybrid coil combination

95, 96 and balancing network |00. Thus waves incoming from line I0 through lter |56 will appear in the secondary |0| and in the secondary |06. Resistance |01 terminates the secondary |06 and allows transformer 94 to be tuned by condenser 93 to the frequency of the carrier assigned to carrier channel No. 1, whereupon the potential of this frequency if present upon the line will appear across the control gap of double gap cold cathode ionic tube 81. 'The main gap circuit of tube 81 comprises a source of 1Z0-cycle current from transformer 69 and a relay 88 having

contacts

89, 90 which, when closed, bridge talking circuit ||1 through

chokes

80, 8| to allow operation of a line relay |39 as in the case of the voice frequency channel already described.

The secondary |0| is bridged by a carrier coil 86 supplied through band-pass filter 12 from harmonic generator 1| and by a Modem 85 connected through low-pass filter 84 to talking circuit ||1 which terminates in jack |35'. The

, cycles.

ringing and supervisory equipment for carrier channel No. 1 and also for carrier channels Nos. 2 and 3 which are not shown in detail, is identical with that with which the voice frequency channel referred to above is equipped with the single exception that the cut-off relay |5| has an extra pair of make contacts |49, |50 which close the circuit of relay |34 thus supplying 60-cyc1e power to harmonic generator 1| whenever the central station operator plugs into jack |35 associated with a carrier frequency channel.

In Fig. 3 are two harmonic generator units which may be of the type disclosed in Wrathall, supra. These are HGI comprising magnetic cored induction coil 204, condensers 200, 20| and HG2 comprising magnetic cored induction coil 206 and

condensers

202, 203. Power at 60 cycles per second is supplied to HGI. From its composite output band-pass lter 205 selects 1020 cycles per second which is the seventeenth harmonic of 60 cycles per, second. The output of lter 205 at 1020 cycles per second is applied to HG2 in combination with copper-oxide bridge rectifier 201 from which an output of the even harmonics of 1020 cycles per second is obtained according to Wrathall, supra.

A party-line system such as that shown in schematic diagram in Figs. 1 and 2 might employ as carrier frequencies even harmonics of 4080 cycles per second which is the sixty-eighth harmonic of the 60cycle current continuously supplied to the line. Thus channel No. 1 might operate on a carrier of 8160 cycles per second, carrier No. 2 on 16,320 cycles per second and channel No. 3 on 24,480 cycles per second. Current at a frequencyof 4080 cycles per second may be'employed for signaling from the voice frequency subscribersstation.

In such a party system it will ordinarily be advantageous vto reverse the order indicated in the diagram, of Figs. 1y and 2. Thus, the No. 3 carrier substation would beA that electrically nearest. the central oice, followedin order by carrier substation No. 2, carrier substation No. 1,

and last the voice frequency station. Line attenuation may be expected to increase with frequency and this may become important in the application of carrier to circuits ordinarily employed for subscriber loops. Thus, if a carrier of comparatively high frequency is employed it may be necessary to take precautions to hold line attenuation at a minimum value between the central office and a substation employing such carrier whereas these precautions are less important as the frequency of the carrier decreases and are least important at voice frequency. Size and spacing of conductors are factors affecting line attenuation.

In order to make a call from the voice frequency subscribers station, the receiverl 26 is lifted from` hook 29 whereupon contacts 29', 30' are opened, connection is made between contacts 29', 3|', 32' and contacts 33', 34'. Closing contacts 33', 34 completes connection between line I0 and harmonic generator. 42 through lowpass filter 4| which, having a cut-off frequency of 80 cycles per second, passes 60-cycle current from the line I0 to harmonic generator 42. Harmonic generator 42 may be of the type shown in Fig. 3 thus producing even harmonics of 1020 Band-pass lters 44 then select from the output of generator 42 current of the frequency of the fourth harmonic or 4080 cycles per second and pass it through conductors 46, 41 to line I0.

The line l is thus energized with voltage at a frequency of 4080 cycles per second which finds outlet by central ofce conductors 54, 55 through band-pass filter 5G and tuned transformer 53 to the control gap of cold cathode ionic tube 59. The potential delivered by harmonic generator 42 is so proportioned to the attenuation of line I0 and of band-pass filter 55 and to the transformation ratio of transformer 58 that the potential appearing across the control gap of tube 59 is sufficient to ionze this gap. Under this condition the main gap of tube 59 breaks down under the influence of battery |41 and the 120- cycle output of transformer G9 thus operating relay 6|. Operation of relay 3| closing

contacts

52, 63 permits the operation from battery |53 of line relay |39 whereupon signal lamp |135 lights from battery |44 thus signaling thc operator.

The voice frequency subscriber is called by operating cord circuit key after making connection between plug |55 and jack |35, thus applying -cycle ringing power between conductor |36 and ground. The ZO-cycie current is rectified by tube |21 and in conjunction with battery |26 operates relay |26 which connects the 1000-cycle source |26 across conductors H5. Condenser |25 smooths out the rectified current applied to relay |24. Current at 1000 `cycles per second ows through low-pass filter 38 which is designed to pass all frequencies below 300C cycles per second thence by conductors 97, 98 to line |0. At the voice frequency subscriber-s station the 1000-cycle current passes through lowpass lter and then, since the receiver 2G is assumed to be on its hook 29 thus holding connection between contacts 29', 39', through tuned transformer i8' to the control gap of ionic tube Il'. The control gap is ther-eby ionized thus permitting (iO-cycle current from line I0 to pass through transformer I5 and operate the bell |6 through the main `gap of tube Vf'.

In answering, receiver 26 is removed from hook 29' thus breaking the connection between contacts 29', 30 and establishing connections 29', 3|', 32 and 33', 34 whereupon a voice frequency talking circuit is established from transmitter 2 and receiver 26' through

conductors

49, 5|-48, low-pass lter 45, conductors 46, 4l, line i0, to conductors 91, 98, low-pass filter 68, condensers 65, 6l, circuit H6, and jack |35 and thence by plug and operators cord circuit to the calling station.

A call from carrier frequency subscribers station No. 1 is made by lifting receiver 26 from its hook 20 thus breaking

connections

29, 30 and making connections 2S, 3|, 32 and 33, 34' Completion of

connection

33, 34 admits Sil-cycle current from line |0 through conductors I2 and low-pass lter |3 to harmonic generator 24. From the output of generator 24 band-pass lter 23 passes current at a frequency of 8160 cycles per second through transformer 22, band-pass filter I4 and conductors I2 to line Hl. This 8160- cycle current is accepted by band-pass filter |56 at the central station and passed into the

hybrid coil combination

95, 96 emerging from Winding |09 to enter transformer 94 which is tluied to this frequency, whence it appears across the control gap of cold cathode ionic tube 81. From this point the sequence of operation resulting in the lighting of signal lamp |40 is identical with that already described for the Voice frequency channel.

The subscribers station on carrier channel No.

1 Ais rung by rst translating 20-cycle ringing current into 1000-cycle current as in the case of the voice frequency channel. Current at 1000 cycles per second appears on conductor ill, is passed through

condensers

82, 83 and low-pass filter 84 which has a cut-ol frequency of 3000 cycles per second to Modem 85. Operation of cut-off relay |5| which occurs when connection is made between plug |55 and jack |35' closes contacts |49, |50 thus causing the operation of relay |34 and closing the -cycle supply circuit of harmonic generator 1|. From the output of generator 1| into high frequency bus ||2 the band-pass lter 72 accepts through conductor ||3 current of the carrier frequency 8160 cycles per second and this also is applied to Modem through transformer 86. Thus the 8160 cycles per second carrier is modulated with 1000 cycles per second current and this modulated current flows

hybrid coil

95, 96, band-pass lter |56 and over line I0 to input leads |2 of the No. 1 carrier frequency subscribers station whence it is passed through band-pass filter i4 and transformer 22 to Modem 2| where it is demodulated into 1000cycles current which, flowing through low-pass lter 20 is applied to the control gap of the cold cathode ionic tube Il', the connection to transformer I8 being complete by reason of the fact that receiver 2G is assumed to be on hook 29 thus making

contacts

29, 30. The 1000-cycle potential ionizes the control gap of tube and bell I6 is rung through the main gap of tube by60-cycle curient entering from line I0 to transformer l5.

When receiver 26 is removed from hook 29 voice frequency current from transmitter 21 is applied to Modem 2| together with 8160-cycle current from harmonic generator 24, and the modulated product ows through band-pass filter |4 to line |D and thence to band-pass lter il at the central station whence it flows

throughhybrid coil

95, 96 to Modem 85, is there demodulated and the voice frequency component flows through low-pass filter 84 to jack |35 and then through the cord circuit to the operator or connected subscriber. At the same time, of course, a portion of the carrier frequency component of a transmission from the subscribers station of carrier channel No. l ows through to the transformer 94, maintaining the control gap of ionic tube 81 in ionized condition. Line relay |39 will not operate, however, as long as cut-off relay |5| is operated.

Voice frequency current coming in from the cord circuit through jack |35' passes through low-pass filter 84 to Modem B5 where it modulates the 8160-cycle carrier supplied from harmonic generator 7| through band-pass filter I2 and the modulated wave passes through hybrid 95, and band-pass lter 1| over line Hl to the No. 1 carrier channel subscribers station. Entering through band-pass filter i4 it is demodulated by Modem 2| and the low frequency demodulation product passes through low-pass filter 20 to receiver 2B if the receiver is off its hook.

In the same way talking and signaling circuits may be established on carrier frequency channel No. 2 employing a carrier frequency of 16,320 cycles per second and over carrier frequency channel No. 3 employing a carrier frequency of 24,480 cycles per second and likewise additional carrier channels might be established at other carrier frequencies.

I have thus shown in detail and by specific illustration an embodiment of my invention whereby the well-known principles of carrier current all) transmissionmay be economically and effectively applied to local subscriber loops enabling the establishment over a single pair of Wires of the practical equivalent of individual line service to a plurality of subscribers.

It is to be understood that although I have disclosed a preferred embodiment of my invention I am not limited thereby since it is obvious that I may use many modifications and many other arrangements Without departing from the spirit of my invention which I have dened in the appended claims. I

What is claimed is:

1. A single circuit telephone system comprisingv a plurality of subscribers stations connected respectively to a central'station through a plurality of channels superposed on said vcircuit at least one of which is a voice frequency channel and the remainder carrier frequency channels, means at each of said subscribers stations for producing and transmitting to said circuit a modulated carrier Wave for signaling said central station, the carrier channel subscribers stations including means for modulation and demodulation of voice frequency waves from individual carrier frequencies, a source of 60-cycle waves floating across said single circuit and means for converting said 60-cycle waves into the desired individual carrier frequency.

2. A single circuit telephone system providing private wire service between a telephone central office and aplurality of subscribers comprising at least one voice frequency channel and a plurality of carrier channels superposed on said circuit, said channels being equipped with subscriber sets, means controlled by each subscribers set for generating and transmitting to said circuit a distinctive modulated carrier wave for signaling said central office, the carrier channel subscriber sets including means for modulation and demodulation of voice frequency waves on individual carrier frequencies and means for automatically producing said individual carrier frequencies in said carrier channel subscriber sets from a 60- cycle supply activating said single circuit.

3. A multiparty. telephone system comprising a single pair of Wires connecting a central office and a plurality of subscribers stations, a plurality of channels at least one of which is a voice frequency channel and the remainder carrier frequency channels superposed on said pair of wires thus providing private wire communication to each subscribers station over a different one of said channels, and means at each of said subscribers stations including those associated with a voice frequency channel, to generate a distinctive modulated carrier wave for signaling the central office.

4. A multiparty telephone system comprising la plurality of channels one of which is a voice frequency channel and the remainder carrier frequency channels providing individual line communication between a central oflice and a plurality of subscribers stations over a single pair of wires, means at each of said subscribers stations for calling the central oiiice through the medium of locally generated carrier waves of distinctive frequency modulated with a low frequency wave and responsive means at said subscribers stations whereby said stations may be called through the medium of a low frequency wave transmitted from said central oice as a l ing a plurality of subscribers stations toa central oice over a single pair of wires and means to provide selective ringing for said subscribers stations comprising at said central office a source of 20cycle current, means including a cold cathode tube for translating said. 20cycle current to a 100G-cycle wave and means for modulating a carrier frequency distinctive of the station to be called with said 100G-cycle wave and for transmitting the modulated wave to said pair of wires for transmission thereover, and at said station to be called means for selecting and demodulating the received modulated wave to reproduce the lOOO-cycle wave, ringing apparatus and means for utilizing the reproduced 100G-cycle wave to control the operation of said ringing apparatus.

6. A multiparty` telephone system comprising a .plurality of channels atleast one of which is a single line, a 'continuous low frequency supply to said line and means at each of said subscribers stations to receive said low frequency and to transmit to said line a carrier current derived from said low frequency to call said central office.

'7. A multiparty telephone system according to f claim 6 in which the last-mentioned means is controlled by switchhook operation.

8. A multiparty telephone system according to claim 6 in which the transmitted carrier current has a frequency which is different for each subscriber station. i

9. A multiparty telephone system comprising a plurality of carrier frequency and low frequency channels providing over a single line "individual line communication between a plurality'of subscribers stations and a central station, a continuous low frequency supply on said line and means including impulse generators operating on magnetic saturation for generating carrier waves of different frequencies for transmission over said channels, from said low frequency supply, said carrier Waves being utilized for transmitting call signals.

10. A multiparty-/telephone system according to claim 9 in which said impulse generators are located at said central station and at said subscribers stations.

ll. A multiparty telephone system according to claim l0, in which said means include modulatordemodulators operating in commotion with said impulse generators to provide reciprocal transformation between carrier frequencies and voice frequencies. l

12. A multiparty telephone system comprising a plurality of communication channels including thereamong a voice frequency channel providing over a single line private communication between a central station and a plurality of subscribers stations respectively, means at the said central station for calling a carrier subscriber station comprising means for transmitting a carrier wave of the frequency assigned tov that subscribers station modulated with a 100G-cycle wave, means for obtaining said 100G-cycle Wave by translation from low frequency ringing current, and means at said subscribers station for demodulating the said modulated lOOO-cycle Wave and for translating it to a low frequency cui-rent for bell operation, said. frequency translating means comprising cold cathode ionic tubes.

13. A multiparty telephone system comprising a plurality of carrier communication channels providing over a single line private communication between the central station and a plurality of subscribers stations respectively, a continuous low frequency supply on said line, impulse generators at said stations for generating from the low frequency line supply carrier current of a plurality of frequencies, selective circuits for selecting desired carrier frequencies from the outputs of said impulse generators, modulator-demodulators at said stations for effecting transformation between modulated carrier currents and voice current reciprocally, and a voice frequency channel superposed on said line utilized for carrier frequency supervisory and signaling operations.

14. A carrier frequency signaling system comprising a plurality of stations connected by a carrier line, one of said stations including means for generating carrier Waves of selected frequency, means for generating low frequency ringing current, means to translate said ringing current to a higher 10W frequency current, means ,f-

to modulate said carrier Waves with said higher low frequency current and means to transmit the modulated carrier waves to said line, and another station comprising means to select said modulated carrier Waves from said line, means to demodulate the selected Waves to reproduce said higher low frequency current, means to translate the reproduced current into low frequency ringing current and ringing apparatus responsive to said low frequency ringing current, the frequency translating means at said one and said other station including a cold cathode ionic tube.

EDWARD VROOM.