US2376352A - Telephone or like system - Google Patents

Telephone or like system Download PDF

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Publication number
US2376352A
US2376352A US489136A US48913643A US2376352A US 2376352 A US2376352 A US 2376352A US 489136 A US489136 A US 489136A US 48913643 A US48913643 A US 48913643A US 2376352 A US2376352 A US 2376352A
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Prior art keywords
relay
contacts
switch
circuit
frequency
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US489136A
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English (en)
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Gillings Charles
Beale Charles Edmund
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Automatic Electric Laboratories Inc
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Automatic Electric Laboratories Inc
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q1/00—Details of selecting apparatus or arrangements
    • H04Q1/18—Electrical details
    • H04Q1/30—Signalling arrangements; Manipulation of signalling currents
    • H04Q1/44—Signalling arrangements; Manipulation of signalling currents using alternate current
    • H04Q1/444—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies
    • H04Q1/45—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies using multi-frequency signalling
    • H04Q1/453—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies using multi-frequency signalling in which m-out-of-n signalling frequencies are transmitted
    • H04Q1/4535—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies using multi-frequency signalling in which m-out-of-n signalling frequencies are transmitted with an additional signal transmitted for voice protection

Definitions

  • the present invention relates to telephone or like signalling systems and is more particularly applicable to the setting and control of automatic telephone switching apparatus over long lines, which may include speech transmission ther mionic repeaters, by means of voice frequency the chief object of the invention to provide a.
  • a telephone system suitable for long distance i automatic working and employing signals of a plurality of voice frequencies for effecting the selective setting of automatic switches over a trunk line, a variable length train of impulses received from a calling party by equipment at the outgoing end of the trunk line istransmitted over the trunk line in the formof a single pulse of substantially constant length comprising one or more voice frequencies the nature of which varies in accordance with the number of impulses in the train.
  • a pulse comprising a plurality of frequencies is transmitted prior to the selective pulses to prepare the equipment for response thereto.
  • a further feature of the invention is that in a telephone system suitable for long distance automatic working and employing signals of a plurality of voice frequencies for effecting the selective setting of automatic switches over a trunk line, each train of impulses received from a calling party by equipment at the outgoing end of the trunk line is transmitted over the trunk.
  • Figs. 1-4 when arranged in the manner indicated in Fig. 3a. show the circuits of an outgoingmanual-to-automatic relay set which connects over a V. F. signalling line with a distant incoming relay set, the circuits of which are shown in Figs. 5-8 which should be arranged in the manner shown in Fig. 7a.
  • the local outgoing anddistantincoming relay sets together provide for V. F.”signalling and coded impulsing from an outgoing trunk line operators position of the sleeve controlled type as already employed for on demand trunk service.
  • each coded digittransmitted is preceded by a preparatory signal comprising all four frequencies W-Z, which is effective at the distant exchange to prepare the voice frequency receiver thereat to receive the coded digit which follows.
  • each received coded digit 3 has to be converted back to a train of direct ourrent loop impulses to actuate the switching apparatus thereat, and the receiving equipment is so arranged that after a preparatory signal comprising all fourfrequencies together has been received, the digit code following and consisting of not more than three frequencies W-Z will operate a W, X, Y, Z relay contact pyramid by means of which the code information can be decoded and converted into a marking on a uniselector for controlling the sending out of a corresponding train of loop impulses.
  • the second coded digit is transmitted after an interval sufficient to meet the numerical setting and hunting requirements of the selector and at the terminating end of the line it is decoded as before. Similar remarks apply to the succeeding digits, and when all have been received at the distant end, and when the switching equipment thereat has been set to connect with the required subscriber, the calling operator will'receive ringing tone, busy tone, or number unassigned tone to denote the state of the called line. In the mean time, on completing the dialling of the required digits, the operator will have restored the dialling key, and will thus have caused her supervisory lamp to glow. When the called subscriber answers, a Y tone of 200 In. s. length will be sent back to the originating end to extinguish the supervisory lamp and to establish conversational conditions thereat. i
  • Relay MM in operating, at its contacts mmZ, connects a flicker earth source via lead ill to the left-hand low resistance winding of relay 2 lar function at this time but relay FXY at contacts fryl, Fig. 1, connects X frequency to one winding of transformer TI, the return path being completed over lead RTN.
  • the Xfrequency is thereupon applied to the outgoing trunk line conductors l4 and i5, whereupon at the voice frequency receiver VFR2, Fig. 5, associated with the distant incoming relay set, an X frequency responding relay or its equivalent is operated and at contacts 83:! brings up its relief relay XR.
  • Rela XR in operating, at contacts m3, Fig.
  • relay FXY At the outgoing end relay FXY at contacts fry 3, Fig. 4, completes a self-driving circuit for switch TS to advance its wipers to position 3, whereupon the circuit of relay FXY is opened at wiper and bank TSi, Fig. 2, an d this relaycommences to release slo-wly.
  • Relay FXY also at contacts fan/l reconnects a suitable V.
  • relay XR At the distant incoming end where relay XR has been held operated in response to the X fre-. quency and where relays K'and PY areal-so op erated, the cessation of the X frequency results in the release of relay XR, whereupon at contacts M3, Fig. '7, earth is extended forward over operated contacts k2 to bring up relay AR.
  • Relay AR in operating, at contacts arl, Fig. 6. extends a seizing earth via the winding of relay IR and one winding of the repeating coil RPCZ over'.
  • relay KR is introduced into the sleeve cir-
  • the operator now dials the first digitof the required number; and when the dial is moved off-normal, the battery connection to the tip and ring conductors at the operators position is removedand these conductors are now; looped together. over the dial impulsing springs so that 3 relay AM is nowheld energised over both its windings in series.
  • relay AM in releasing-at contacts aml, Fig.4, extends earth over contacts .bl and low resistance winding of relay C, wiper and bank DDZ of the digit distributor switch DD to the magnet DSAM of digit switch DSA, while on the subsequent re-operationof relay A the magnet is de-energised to advance its wipers to position 1. If the digit 5 is dialled, the DSA switch wipers will be advanced to position.5. RelaysB andC hold operated during the train and contactsicz en'ergise the magnet DDM out, the leads representing these frequencieswill of.1th.e .digitdistributor switch DD during this time.
  • magnet DDM On the release of relay C at the end of the train, magnet DDM is dc-energised and advances the DD switch wipers to position 1, while at contacts cl Fig. 3, earth iseXtended via'corltacts anal and wiper DSA2 in position 5 to operate relays .CNA and CNR in series.
  • Relay CNA in operating,-at .lts contacts cna l locks itself over wiper and bankDSAZ independently of the contacts cl, at contacts cnaZ prepares a"fur DSB, DSC (not shown) and to digit switch s1),
  • relay ST upon it operates relay ST by way of both its windings in series with magnet SCM which cannot. operate in this circuit.
  • Relay ST thereupon at contacts stl short-circuits its left-hand high resistance winding, whereupon magnet SC-M is operated and switch SC self-drives to position 4 where the holding: circuit for relay ST is opened and this relay commences to release.
  • contacts st2-st5
  • SWitCh SC will commence'to self-drive to position 9', Relay ST in releasing disconnects the four-frequency signalling circuit and relay S in operating, at contactss4, Fig. 1, connects up to line a frequency combination in accordance with the digit dialled as controlled by the DSA switch banks DSA3-DSA6 which connect up with contacts 34 over operated contacts cna4 and resting contacts 273.
  • relay ZR operates over its lower high resistance wind ing in series with magnet SCM whichis inopera tive owing to the high resistance of relay ZR.
  • Relay ZR thereupon at contacts zrl completes a homing circuit for the digit switch DSA y ia its upper low resistance winding, contacts cnal5 and wiper and bank DSAl, at contacts 272 completes a locking circuit for relays CNA and CNR in series and relay CNR will also. release if no further digits have yet been dialled.
  • relay CNB will have been operated inseries with relay CNR and consequently with relay CNR still held, switch SC on reaching its home position will commence another cycle of opera tion.
  • Relay ST is again operated and functions in the same manner as before to send the prepare pulse and is followed by relay S to sendout the code corresponding to digit 8 which is
  • the SC switch on advancingto contact 9 ter- "minates the sending out of the W, X and Y frequencies corresponding to digit 8 by.
  • the interdigital pause is "partly determinedby the bathing time of the digit switch controlling the coded digit which has just been sent out, the time concerned being the homing time from the position to which this switch has been set by diallingplus the homing time of switch S from
  • This arrangement provides a suitable interdigital pause for-digits up to 5. In the case of digits 6-0 this time would be slightly less and in fact a longer time is required to cover the correct functioning of the switches at the distant end. This is taken care of by arranging to operate a relay EP, Fig. 3, when relay S operates to initiate the sending out of a coded digit of a value from 6-0.
  • relay S operates to initiate the sending out of the frequencies W, X and Y corresponding to the second dialled digit 8, on the operation of relay S, (3011-. tacts s3 complete a circuit to operate relay EP from earthed operated contacts cnb'l, wiper D832 of the first digit 5, or the reduction 'tO-f'PEQIZEHe cies W, X and Y in the case of the second digit 8, the short-circuit will be removedfifrom relay CB which now operates.
  • Relay CE in operating, at
  • relay CNB will have'been operated byway of its right handwinding, the .left hand winding only being used for digits up to 5.
  • Relay EP in operating, at its contacts epl locks itself independently of relay S and at its contacts e123 ,(not shown, but assumed to be located in Fig.
  • switch DSB restores to its home position after the sending out of the coded digit 8
  • the switch Wipers are again advanced to position 1 from which the switch continues to rotate through another full cycle.
  • 'relay ZR will be maintained operated and will maintain relays CNB and CNR from earth over contacts 2T2, cnaZ, cn-b-Zleft-hand winding of relay CNB to battery via relay'CNR.
  • switch D83 is stepping to position 6, the holding circuit for relay EP will be opened.
  • the WXYZ .prepare signal which precedes each digit operates all four relays Wit-ZR, Fig. 5, contacts of which bring up relay CA, Fig. 6. It. will be remembered that at this incoming end relays K, PY, AR, IR andNN are operated as a result of the seizure of the trunk line. On the operation of relay XR, relay AR, Fig. V7, is released but without eflect, while on the operation of relay CA, contacts cal,
  • Relay CB also at contacts 0225, Fig. 7, brings up relays Pand C.
  • Relay P in operating looks over its contacts pl and at .its contacts p5, Fig. 8, operates relay CK, contacts old and 0703, Fig. '5,
  • Relay GY has no direct function at this stage but relay CO in operating; 'at'its conta'cts'cc'l, Fig. 8, connects earth to the motor driven mag net impulse springs IM'PZ.
  • the impulse springs IMPI and IMP2a're' operated ten times per second,"the former being closedfor 33 of. the period of each impulse and the latter being closed for the remaining 66% of the period. If on the operation of relay CO the impulse springs IMPZ are open, or if not, when they next open',,relay IG operates and at contacts the next closure of the springs IIMPZ, magnet SSM is energised and on the next opening, magnet SSM is de-energised and advances the SS wipers to position 1. At this point the loop impulse springs IMPI are closed, so that when springs IMPZ again close to re-energise 'magnet SSM, springs IMPI in'opening produce tion of the first output impulse.
  • the SS switch wipers are advanced to position 2 while springs-IMPI nowclose to terminate the break portion of the first output impulse.
  • the second impulse is sent out and so on until five impulses have been sent out corresponding to, the received V. F.'coded digit.
  • Wiper SS2 on reaching position 6 at this time will encounter an earth marking which will be con-- nected thereto over ⁇ contacts'of relays WR and ZR. within the known pyramid arrangement.
  • lay SZ thereupon operates and looks over its contacts sel, while at contacts s22 it short-circuits the loop impulse springs IMPI to prevent furvther impulses being sent out'to the first selector, and at contacts s23 it releases relays CB and CA.
  • the first selector is stepped up to level fi and then-proceeds to perform an automatic rotary the break porhuntingioperation to find a succeeding selector infltheselected group.
  • relay SZ releases and re-preparestacircuit for relay CB. .011 the re.
  • Relay CK Fig. 8
  • Relay CK is connected upfoverlead 12 to an interrupted earthsourcewhich gives one second pulses at four second intervals and therefore slowly, pulses but without e'fiect" at this stage except under specialtonditions when busy tone is returned from "the selector as described later.
  • FC, AM and Bar held operated the operator after, dialling the required digits will restore her diallingkey, whereupon a comparatively low resistance battery potential is momentarily applied whereitis further slowly interruptedby contacts cm and 0763 though withoutany particular purpose in thisinstance.
  • the tone then extends back tothe outgoing relay set where it finds a circuit to the outgoing operator'via transformer Tl, the singleway orso-called stopper valve SV and repeating coil RPCL
  • Theprimary function of valve. SV is l to prevent transmission from a the cord circuit to the trunkfl line so as to prevent noise or'the like at the outgoing end switching the linefecho. suppressors against the passage of tonesand signals from the distant incoming end.
  • Y frequency subscriber answerf signal is sentfback to theoutgoing end to dim the op'erators calling supervisory lamp and tocomplete the conversa at the operator's position circuit to the sleeve of the calling plug and this battery extendsover jackOGJ to bring up relay KR.
  • Relay KR thereupon at contacts l crz prepares for called party supervision, at contacts krl removes the battery feed for relay AM which remains held on its upper winding to battery applied to the tip and ring conductors in the position circuit and at contacts Fort provides a local locking circuit for relay KR After the momentary application ofresistance battery to the sleeve circuit, the high resistance originally introduced into this circuit on operation of the dialling key is short-circuited, while ashortinterval later the battery applied to the tip and ring'condu'ctors in the position circuit is removed and, relay AM releases. On the reticnalconditions thereat.
  • contacts I connect earth over contacts ctZi, Icri, b5 and mmZ, and the left-hand low resistan ce winding of relay M to the sleeve circuit so asfag'ainto bring about the lighting r the calling supervisory lamp. This condition persists until the called; subscriber answers.
  • the 'selector train at the distant exchange will have connected with the required subscriber and if he is free, the final selector will return the interrupted ringing tone back over the speaking conductors to the incoming relay 'set,
  • relay RS commences to release slowly and during this time relay RD operates and maintains relay CIHover contacts 1111' and ctl.
  • relay CT now remaining held to earth via contactssal and bank andWipe'rTSLQ f H Conversation nowproceeds between the calling and' called parties, relays AR, NN, I, P, and GK being held operated-at the incoming end.
  • relays AR, NN, I, P, and GK being held operated-at the incoming end.
  • relay IR again operates and relayI releases, results in the transmission of a clearing signal comprising llO m. s. pulses of Y frequency at spacings of 360 in. s. back to the outgoing endto eifect the lighting of the outgoing operatorslsu pervisory 8 ml Relay I in releasing, atlcontacts iZ liig; 8, brings-up relay OK and this relay at contacts clcl connects up a 14.0 m. s. on, 360' m. s. ofl' earth source over lead B to relay FY, whereupon at contacts i1 2, Fig. 5, 140 m. s.
  • Relay RS operates on this and" subsequent Y frequency pulses and relay RD remain steadily operated since it is energisedduring the release time of relay Rs duringeach interval and remains hel'd due to itsslug during each impulse.
  • the conversational condition atthe outgoingend is broken down at contacts @154 and ct5 and at contacts ct3 earth is extended forward over the sleeve circuit to give the clearing'signal to the operator by lighting the-calling supervisory lamp.
  • relay MM On receipt of the clearing signal, the operator will withdraw her calling plug from jack OGJ' and relay M thereupon releases, whereupon relay MM commences to release slowly; The otherrelays SA, CO, F'C' remain held to bank and wiper TSZ, relay CO, on the release of relay MM; main taining an engaged test battery via resistance YB on the sleeve ofj'ack OGJ. Relay MM also releases relay KR. Relay M at contacts mZFi'g. 4; causes the TS switch to advance'to position 7 from which earth over contacts sa4 "causes'it to self-drive to position where its'driving circuit is opened at bank TS4. During thistime relay B, Fig; 2, is operated over bank.
  • switch TS when switch TS reaches contact I5, it proceeds to advance onward" to position 19 under control of a 140' ms. on, 360' m; s, olfinterrupted' earth supply connected to lead I'l whichextends to magnet TSM viabank and' wiper TS3.
  • relay B When position I9 is reached; relay B has its circuit opened and commences to release slowly, while relay FXY is now operatedlj' This operation of relays B- and in turn serves to send forward a clear down? signal comprising, a 1% second pulse of X frequencyfoll'owed bya 300 m. 's.”pul'se of Y frequency, 'It will be seen from Fig.
  • relay B in operating, at contacts b6 extends X frequency over contacts mm l; st2', fxyl and resistance YD to the winding] of transformer Tl from whence-it extends to line. This, circuit is maintained until relay FXY operates and opens the)! frequency circuit at contacts ,frtyl, the time being determined by the stepping of switch TS from position I to position 19. On' the operation of relay FXY, contacts ,fwy'l connect Y- frequency toline.
  • theposition is thatrelays AR, NN', P; IR, or: and (so; are held operated while relay FY is pulsing; .
  • relay XR- On receipt of the X frequency which will occur during the silent period of the Y' clearing signal being sent fromthe' incoming end, relay XR- operates and at its contacts m3, Fig. '7, operates-relay GY which disconnects the pulsing circuit of relay FY and so terminates the sendingoftheY pulses, while at the same contacts the circuit of relay GX is opened so that it commences to release slowly.
  • relayGXrel'ay CC On the release of relayGXrel'ay CC is operated and the equipment now awaits the Y frequency from the outgoing end; On'receipt of this frequency, relay YR in operating, at contacts 3 1'3 brings uprelay CR.
  • This relay atcontacts er operates relay 0 and at contacts crl locks itself and disconnects relay NN which commences to release slowly.
  • Relay C -in operating releases relays IR and AR; so that when relay NN releases, the forward holding loop to the selector train is opened and this train. now restores to normal.
  • Relay NN- inrel'easing alsoreleases relays P, CC, GY and C and on therelease of relays P and CC, relay CR releases. frequency,v relay YR releases and the incoming equipment is ready-for further use.
  • the primary function of'rela-ys GK and" GY at the incoming end is to prevent false operation due to components of speech simulating the X and Y signalling frequencies and the arrangements are such that afalse X frequency followed by a false Y frequency will be ineffective unless the false X frequency is of at least 300* milli' seconds duration. It will be appreciated that the possibility of a false X frequency of at least 300 milli seconds immediatelyfollowed by a false Y frequency is extremely remote.
  • the operation of the supervisory relay I' will have restored relay K tonormal so that if a component of speech during this time simulates the X frequency, relay GX will start to release slowly owing to the operation of relayXR).
  • relay CC On the withdrawal of the plug, relays On the terminationof the relay GX releases, relay CC operates.
  • Relay M in releasing causes the TS switch to advance to position 7, but in this instancesince relay SA has not been operated owing to non-receipt of a subscriber answer signal, the switch TS will not new selfdrive to position .15 as before, but insteadwill advance step by step from the. interrupted earth supply on lead H to this position from which it will again advance under control of the same earth supply to position 19 as before.
  • theswitch moveson .to its home position as already described.
  • the increased stepping time of switch TS gives approximately a fi-second period of transmission of X frequency.
  • the 3007111. s. Y frequency then follows in themanner already described.
  • relay CK will be in operated condition and relay CK will be being slowly pulsed.
  • the reception of the X frequency results in the operation of relays XR, C, CC and CY in turn.
  • Relay C in operating causes the steady operation of relay CK, releases relay; IR and transfers the selector holding loop to wiper and bank SSI.
  • the receipt of the Y frequency brings up relays YR and CR, whereupon relays K, NN, CK, C,,CC, PY, P and CR are released and the loop to theselector train is broken.
  • relay YR releases and the incoming equipment isrestored to normal.
  • relay I In case busy tone is returned from the final selector, or a preceding selector, the circuit operations at the outgoing end are the same as described for the number unassigned or ringing tone, but at the incoming end beforereception of the clearing signal, relay I will beimpulsed from the busy flash battery applied atthe selector in addition to the relays held as already mentioned.
  • the first full operation of relay I re leases relay K which cannot re-operate and relay K in releasing operates relay GK and opens the slow pulsing circuit for relay CK so that this relay now remains normal.
  • the busy tone now extends through without further interruption from the final selector to the operator, the interruption at the incoming relay set being unnecessary with busy tone as the echo suppressors can readily restore during the silent periods.
  • relay XR responds to the X frequency and brings in relay GY, while when In response to the Y frequency relay YR brings up relayCR which operates relay C and also releases relay NN which disconnects the circuits of relays C, CC, CR, P and GY.
  • relays K, PY, AR, NN, IR, and P will be operatedand relay CK will be pulsing, whilerelays CA, 2GB and C will be normal. ates, at contactsdcoa it opens the circuitof :relay K, and on release v ofthis relay contacts k3 connectup relay GK and ,contactslc5 release relay CK. Thereupon atcontactsckZ and'ck3 the incoming trunk conductors. and 15 are extended" through via repeating coil RPCZ and the tandem selector train to the tandem outgoing relay set taken in use. Hence received codedV. F.
  • DCQ oper 9 "bed it has been ar incoming equipment; transients of the order of the dialing frequencies. are encountered, these might. be stored by the incoming voice frequency receiver and sent outin corresponding impulse forms With the illustrated arrangements however, this trouble is avoided since the incoming equipment is not properly responsive unttlsafter a prepare signal has been received. Though it ispossible to avoid trouble due .to signal clipping by the echo suppressor by making each coded d'igit or su-fficien't duration as to be substantially.
  • a trunk line Ina telephone system, a trunk line, aresponsive device, means in the trunk line responsive to different length series of impulses received from-a calling party for transmitting over the frequencies in said single impulse varying in accordance with the number of impulses in said re? ceived series.
  • a trunk line In a telephone system, a trunk line, auto matic switches, means for simultaneously transmitting a plurality of voice frequencies to said automatic switches to. extend a connection over said trunk line and means for transmitting a supervisory signal back over thev trunk line after the connection is extended comprising a single one of the voice frequencies used in the opera- 8'.
  • a trunk line, an automatic switch, a plurality of sources of-"alter nating current of difierent frequencies means of connecting one of said sources to said trunk line to seize said switch, means for simultaneously connecting all of said sources to the trunk line to prepare the switch for operation, and means for simultaneously connecting particular combinations of said sources to the trunk line" to selectively operate the switch to extend a connection.
  • two control offices having an interconnecting line, a plurality of sources of signalling current each source having different electrical characteristics than the other sources, devices in one of saidoflicacid devices respon sive to a plurality of received signals for transmi tting said signals over the line by meansof' a signal of substantially constant length'including a'code impulse of current from all ofsaid sources, a receiving device at the other office responsive to said code, and means controlled by said receiving device to translate the code to signals corresponding tosaid receivedsignals;
  • a trunk line connecting two exchanges, a plurality of frequencies of alternating current, aplurality of automatic switches successively controlled by received'impulses to select a plurality of particular combinations of said frequencies, means for successively controllingeach of the switches connected tosaid particular frequencies to individually apply to said line a signal including a code pulse of the selected frequencies, the interval between the same, means in the register, for simultaneously transmitting a, plurality of voice frequencies to operatethe switches, each plurality of'voice frequencies simultaneously transmitted varying in composition in accordance with the number of impulses in the series registered.
  • a register means in the register responsive to a plurality of received series of impulses for transmitting an impulse of substantially constant length foreach series received', regardless of the number of impulses in the series, and means in. the register for varying connection of each of the successive signals con trolled by the switch that applied the'precedi-ng code, a device responsive to said code signals; other automatic switches, and means controlled by said-responsive device to operate said other switches by pulses corresponding to the received impulses used to operate each of said first automatic switches to select the connected frequen cies.
  • a toll line terminating on a device responsive to particular voice frequencies for controlling switches from received digital information, means for transmitting over said line particular digital information as a voice frequency signal of constant length comprising a preliminary portion of said voice frequencies to prepare the device on said: line for the. digital portion comprising a lesser number of said frequencies for operating the switches.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
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US489136A 1942-07-28 1943-05-31 Telephone or like system Expired - Lifetime US2376352A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2566009A (en) * 1947-12-12 1951-08-28 Automatic Elect Lab Interexchange switching telephone system
US2607854A (en) * 1947-07-22 1952-08-19 Automatic Elect Lab Automatic toll telephone system
US2626996A (en) * 1947-02-07 1953-01-27 Int Standard Electric Corp Conversion from direct current to double voice frequency impulsing

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2626996A (en) * 1947-02-07 1953-01-27 Int Standard Electric Corp Conversion from direct current to double voice frequency impulsing
US2607854A (en) * 1947-07-22 1952-08-19 Automatic Elect Lab Automatic toll telephone system
US2566009A (en) * 1947-12-12 1951-08-28 Automatic Elect Lab Interexchange switching telephone system

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