US2492344A - Line finder control circuit for communication systems - Google Patents

Description

Dec. 27, 1949 P. R. ADAMS T AL 2,492,344

. LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS Filed Nov. 14, 1945 '7 Sheets$heet l I FIG-1 COMMON EQUIPMENT TO OTHER LINKS SECOND LINK CIRCUIT LIN REG SELEC RCUIT CIRCUIT FIRST LINK CIRCUIT FIG. 2

FROM POWER SUPPLY 7 Sheets-Sheet 2 P. R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS mmnzIm INVENTORS PAUL R. 40,4445 DA V/D H. FHA/50M ATTORNEY Dec. 27, 1949 Filed Nov. 14, 1945 Dec. 27, 1949 P, R, ADAMS ET AL 2,492,344

LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS Dec. 27, 1949 Filed NOV. 14, 1945 P R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT '7 Sheets-Sheet 4 :5 1 J IO KC PEAKED CLIPPER LOCK-IN 0s AMPLIFIER AMPLIFIER l0 KC +o-o21. x PHASE CORRECTOR A TTORNE) Dec. 27, 1949 Filed Nov. 14, 1945 P. R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS '7 Sheets-Sheet 5 vvvv CIRCUIT SECOND REGISTER CIRCUIT REGISTER CIRCUIT FOURTH REGISTER CIRCUIT i FlF-TH I28 REGISTER INVENTOR5 PAUL R AO/I/ IS fl/lV/D H. FAA 50M A TTOPNEV Dec. 27, 1949 Filed Nov. 14, 1945 P. R. ADAMS ET AL LINE FINDER CONTROL CIRCUIT FOR COMMUNICATION SYSTEMS I '7 Sheets-Sheet 6 :42 44 --T L I Q! I r----"-'-- [so I I I I I I I 5 MS I DELAY LINEI I I48 l I I l FIRST z I DELAY I IRsT ZERO GATE J I GATE I I C I 5+ l i I I l I 1 l l I54 I v 8+ I- THIRD I I44 ZERO GATE IDELAY GATE I I 50 MS LA I34 E Y 155 FOURTH FOU RTH |s| I35 I45 ZERO GATE IDELAY GATEI 40 MS I36 DELAY J FIFTH v FIFTH I53 I 6 ZERO GATE DELAY GATE :1 so MS DELAY INVENTORS ATTORNEY 1949 P. R. ADAMS ET AL 2,492,344

LINE FINDER CONTROL CIRCUIT I 7 FOR COMMUNICATION SYSTEMS Filed Nov. 14, 1945 7 Sheets-Sheet 7 IOA E89 T98 H-69 Patented Dec. 2 7,v 1949 lllTED STATES TEN OFFICE LINE FENDER CONTROL CIRCUIT FOB COMMUNICATION SYSTEMS Application November 14, 1945, Serial No. 628,612

This invention relates to separation circuits 1 and more particularly to separation or dividing pulses according to their signal carrying and other functions for use in telephone exchange systems.

In telephone systems generally in use, interconnection between subscribers lines through the various trunking lines in telephone exchange requires considerable mechanical switching and a large plant set up. Furthermore, large numbers of interconnecting lines are generally required in the exchange so that connection may be made between any two lines incoming into the system. Likewise, considerable complication is presented in the signalling system for line selections and ringing.

Some replacement of mechanical switching systems by electronic switches have been proposed but, in general, all of these systems require still the mechanical selection of lines for interconnection. The electronic switches as proposed generally are used simply to replace some of the mechanical switches in the system. Furthermore, ringing and other signalling is carried through conventional switching circuits in the same manner as in the telephone systems generally in use.

It is an object of our invention to provide a switching circuit for interconnection of channels wherein signal pulses may be used to establish and maintain connections and to carry the signals through the system.

It is a further object of our invention to provide a system for distributing an incoming signal in the form of pulses, and to use these pulses for establishing and maintaining the complete communication connections.

It is a still further object of our invention to provide means for producing pulses, modulated less that 70% in amplitude for conveying information, and using the lower unmodulated portions of these pulses for establishing a circuit connection, while using the modulated portions of these pulses for conveying the signals through such connections.

It is a still further object of our invention to provide a pulse separating or dividing circuit wherein the lower unmodulated portion of a series of pulses forming a pulse train is obtained in one output circuit, and another series of pulses varying in amplitude in accordance with signals isobtained in a second output circuit.

According to a feature of our invention, a train of pulses for each channel of communication is established. These pulse trains are differently 8 Claims. (Cl. 179-18) timed so as to be successively effective. The pulses are preferably given such a bias that they contain, a substantial amplitude component even when fully amplitude modulated. The percentage modulation is maintained under 70%. and preferably under The pulses of each train 'serve to operate selector circuits, such as telephone line finders and simultaneously to carry the signals and communication energy. To perform these two separate functions, the modulated pulses are clipped at a level below and above the modulation envelope. The constant amplitude pulses below the modulation level are usedto establish and maintain the line finder connections to complete a circuit for transfer of the signals and communication.

T The divisionfor clipping of the pulses is best accomplished by asingle tube circuit provided with a cathode resistor output and a normal anode output. The pulses are applied to the grid of this tube. Control potentials are provided such that the cathode resistor output will pass only those portions of the applied pulses below a given level, preferably such that none of the modulation amplitude variation is passed. The control potentials to the tube also assure that only the upper laortionjs of the pulses will appear in an anode output. limitedso that the effective percentage modulation is high, up to 100% at the highest signal levels. The constant amplitude pulse portions serve continuously to maintain connections for the variable amplitude portions to pass. Because of the systemused in selection and maintaining of the connections, it is desirable to have these pulses of equal amplitude at all times.

In a telephone system incorporating our invention, signal or speech currents in the various lines or other channels may be replaced at the exchange by a series of narrow pulses of amplitude corresponding to the amplitude of the original current at the corresponding time. The pulses are producedat suiiicient rapidity so that they define substantially the signal envelope. In this manner by allotting different time positions to each line, the signal or voice currents within the exchange may be distributed over a common channel. each signal being repeated by a series of pulses displaced in time in accordance with theldistributor time position. This distribution may be readily accomplished by means of a cathode ray tube serving as a distributor which will sequentially scan the lines connected to predetermined terminalsand respond if there is a signalling voltage on the line. The channels Preferably, the anode output is may be separated by time selection and may be applied through time displacement means and a low-pass filter which serves to reproduce the audio envelope to the same or another distributor also coupled to the lines. The incoming signals may serve to adjust the time displacement means so that they will represent the time difference between the time position of the calling line and the selected called line. The time displacement means may be an actual delay line of some form or an equivalent circuit which, while not pro ducing an actual delay of the signals, will 'efiectively serve to store the energy and release it after a predetermined interval equal to the desired delay. In this manner, the interconnection of any one line with any other line of the system may be accomplished. Upon making this interconnection, the communication signals may pass through the same delay means between the interconnected lines. Furthermore, since the scanning cycle covers each of the lines connected to the distributor, as many simultaneous connections may be made as there are time displacement trunking channels within the exchange.

Preferably, means are provided responsive to the interconnection of the lines to tie up these lines so that they cannot be selected by another subscriber attempting to get the connection. If desired, any conventional type of busy signal may be applied to the subscrib'ers line when this 'condition exists so that he will know that he must wait an interval for the line to become free so that he can make the desired connection.

While we have broadly outlined certain objects and features of our invention, a better under standing of our invention and the objects and features thereof may be had from the particular description of an embodiment of a telephone exchange incorporating our invention made with reference to the accompanying drawings, in which:

Fig. l is a block'diagram illustrating the gen eral circuit set up; D

Figs. 2 and 3 are sectional circuit diagrams and views respectively, of a'distributor tube used in our system;

Figs. 4 to 8 inclusive, constitute a circuit diagram of a link exchange in accordance with our invention;

Fig. 4 illustrating the common equipment,

Fig. 5 showing the pulse forming-equipment,

Fig. 6 the line finder equipment, 7

Fig. 7 the dial register equipment, and

Fig. 8 the line selecting e'quip'mentt Fig. 9 is a diagram illustrating how Figs. 4 "to 8 inclusive, should be arranged to illustrate the complete circuit; V

Fig. 10 is a set of curves used explaining the operation of certain parts or the system; and

Fig 11 is a diagram in section of a delay line suitable for use in the equipment shown in Fig. 8.

In the example outlined above, the system may be divided into three parts "as shown in Fig. l: first, all the subscribers lines, twenty for ex ample, assigned numerals -1 to-20, each-of these lines having a subscriber sub-set equipment such as 21; second, the equipment common to all line circuits, hereafter referred to asfconir'non e'quipment 22; and third, a group of link circuits one 'of which is needed for each simultaneous call. Each of the link circuits may be'further subdivided into line finder circuit 23, dial pulse forming circuit 24, dial register circuit 25 and line selecting circuit 26. These several major components are interconnected by wires 21-38 inclusive, as shown in Fig. 1. For the sake of simpilicity in the description only one-way conversation is provided for.

As shown, all lines I to 28 terminate in common equipment 22. This equipment 22 performs a scanning function, preferably by means of a suitable tube having an electronic beam which sweeps each of the lines in turn.

7 When one of these lines has a potential indicative of a calling condition, the common equipment 22 applies signals over wires Ti and 28 to all the link circuits in parallel and specifically to the line finder circuit 23 of the first link (chosen for discussion) This line finder 23 operates to find the calling line and transfer the signals over wire 33 to the dial pulse forming circuit 24.

When dialing ensues, this circuit 24 produces dial pulses which are counted and stored in dial register circuit 25. The dial pulse register 25 then serves to control the line selector circuit 26 which may comprise a delay line or other time displacement apparatus.

The incoming speech signals are then transferred from common equipment 22 over 'wire 28, line finder circuit 23, wire 33, line selector -circi'iit '26 and thence over wire 36 back to the common equipment 22, from whence they are applied to the selected outgoing line. The part of Fig. 1 comprising line finder '23, dial pulse forming circuit 24, dial register 25 and line selector circuit '26 may be considered together as a link circuit. For certain embodiments of the system, a synchronizing frequency may be fed from common equip ment 22 over lead 29 to line selector circuit 26 and line finder circuit 23 respectively. The five leads 2?, 2-8, 29, 3'1 and 38 from common equipment '22 may also be multipled to other link circuits of the system as shown.

The distributor function of common equipment 22 may be performed by a. rotating distributor in the form of a cathode ray tube as illustrated in detail in Figs. 2 and 3. The distributor tube is indicated generally at 39 and may comprise a cathode it, the usual grid 4|, focus and anode electrode :32, horizontal deflector plates 43 and vertical deflector plates 44. Two-phase distributor currents from a suitable sweep control may beapplied over leads 45, 45, 47 and '58 to the horizontal and vertical deflector plates respectively, so as to produce a cyclic rotation of the electron beam. At the target end of tube 39 are provided twenty coupling targets 49 to 68, respectively, which are coupled with the individual lines I to 20 inclusive. These targets may comprise secondary electron emissive elements associated with a common anode 69 to provide dynodes all having "a common output. A mask or screen is may be provided, if desired, having apertures therein so that the electron beam will impinge on each dynode only when the beam is aligned therewith thus preventing possible secondary emission from others. The output of the distributor tube 39 is connected from anode 69 over lead ll, then signal isolating circuits hereafter described to leads 21 and 28 which go to the line finder circuit as shown in Fig. 1. The output from the line selecting circuit 26 may be applied as indicated over line 36 to the grid 41 serving to modulate the beam in accordance with the selected signal energy. Thus, referring to Fig. 1, the output from lead H may be applied after suitable delay (produced in line selecting equipment 26 as hereafter described) over lead 36 to gridII I to provide the desired communication channel between the chosen pair of lines.

The common equipment 22 is illustrated in Fig. 4. For illustrative purposes a base frequency of 10,000 cycles per second has been selected as the scanning rate of the rotating distributor. This frequency is sufficiently high to reproduce voice frequencies with adequate fidelity for transmission of speech. For the twenty-line system the base frequency is derived from a 200 kilocycle stable oscillator I2 preferably crystal controlled. This higher frequency is preferably utilized since it is generally easier to build a more stable oscillator at the higher frequencies than at the lower 10,000 cycle frequency which is to be used. Furthermore, in certain of the modifications illustrated, the 200 kilocycle wave may be utilized for other control purposes. The sinusoidal frequency generated in master oscillator 12 is reduced to the base frequency of ten kilocycles in frequency divider I3.

The output of frequency divider 13 is applied over 90 phase shifter I4 to the vertical and horizontal sets of deflecting plates 43 and 44 of distributor tube 39 herein diagrammatically illustrated. This will serve to rotate the beam at a frequency of 10,000 revolutions per second so that each of the dynodes 49 to 68, illustrated in Figs. 2 and 3 and in this figure, will be scanned once every 10,000ths of a second. Incoming lines I, 5 and are shown connected to the respective dynodes 49, 53 and 68.

At 2| is illustrated a typical subscriber sub-set (shown connected to line 5) for use in the system according to our invention. Such a sub-set will be connected to each of the incoming lines I to 20 inclusive. The voice transmitter I5 is connected in series with dial I6 and the normally open switch hook TI. The receiver I8 is bridged permanently across the line, since, for simplicity of illustration, no separate ringing equipment has been illustrated. Accordingly, the signal for summ'oning a called subscriber may be applied as a special tone which will be reproduced in receiver I8 to call the listener to the phone.

As in the usual equipment, switch hook I1 is normally open. However, upon initiating a call, the switch becomes closed, completing a circuit in the calling line loop over low-pass filter I9 and the associated lines at the sub-set, applying a negative potential from battery 80 to the associated dynode 49. Normally the dynode electrodes "49 to 68 are at the same potential as anode 68 so no current flows. This negative potential will produce a difference in potential and cause secondary emission current to flow from the dynodes upon impingement of the beam of tube 39 thereon, producing a negative output pulse in output line H. The pulses are preferably signal modulated to a depth of only to 50 per cent so that there will always be sumcient amplitude to furnish energy to establish and maintain connections regardless of modulating signals. The negative pulses resulting from operation of the selected dynode 49 are fed to the grid of inverter tube 8I. The anode circuit of tube 8| is coupled to the grid of clipper tube 82 which serves to clip these pulses at a predetermined level to pass only the modulated portions of the incoming pulses. Thus, the output of this tube, representing the speech signals, may be substantially 100 per cent modulated. Theseclipped pulses are then applied to a cathode follower tube 83 and from there to all of the link circuits over the cathode follower output lead 28. A second output is taken across the cathode resistance of inverter tube 8|, these pulses being applied to a clipper tube 84 which serves to clip the pulses to a constant level eliminating modulation effects therefrom. The anode circuit of tube 84 is coupled to the grid of a cathode follower tube 85 which serves to apply pulses 86 through common feed resistor 81 over wire 21 to the grid of line finder gate tube 88 (shown in Fig. 6) of line finder 23 (shown inFigs. 6 and 1) in the first link circuit (now underconsideration) and in parallel to the grids of the corresponding line finder gate tubes in all other link-s. The pulse 86 after passing through resistor 81 may be called 89, so that the pulse actually arriving at the grid of tube 88 and of the other similar tubes is pulse 89. Under the conditions now assumed, when none of the grids of the line finder gate tubes is drawing grid current, pulse 89 is nearly as strong as pulse 86; but under other conditions it may be much weaker than 86 as hereafter explained. In the absence of any signals on the cathode of this line finder gate tube 88, the above traced pulse 89 on its grid is insufficient to cause the flow of plate current, because of the bias applied to the grid is sufficiently far below cutoff.

In the line finder 23 (Figs. 1 and 6) is provided an oscillator 90 normally operating at a fre-'- quency slightly lower than the output frequencyfrom frequency divider I3 in Fig. 4. This oscillator may, for example, operate at one-fiftieth of one per cent below the frequency of the frequency divider. The output energy from oscillator 90 is applied to a clipper amplifier 9| which serves to produce rectangular selecting pulses. These pulses are differentiated in a differentiating network consisting of condenser 92 and re-,

sister 93, to produce the pulse formation 94 which is applied to the control grid of clipper tube 95. The output pulses 96 from tube (corresponding to the leading edge of pulse 90 and the posi-.

tion part of formation 94) are applied to cathode follower tube 91. The resulting pulses 98 are applied to the cathode of tube 88 normally tending to make the cathode of this tube more negative so that the tube will be more nearly conductive. However, except when the pulses 98 applied to the cathode of tube 88 coincide with the previously traced incoming pulses 88, applied via wire 21 to the grid thereof, tube 88 is ineffective. Sufiicient bias is applied to the grid of tube 88 from battery 99 so that it requires the combined amplitudes of the two pulses 89 and 98 to operate this tube. As oscillator 90 continues to drift relative to the output of frequency divider 13, the pulses 98 will commence to coincide with the pulses 89 incoming from the calling line, overcoming the bias in tube 88 and producing output pulses I00 in line 32. These output pulses I00 then are applied over condenser IN to a peaked amplifier and phase corrector circuit I02 which serves to lock oscillator 90 into step with the incoming pulses 89 so that its output is in synchronism with the frequency from divider I3, and pulses 98 will then continue to coincide regularly with the incoming pulses 89 from the predetermined calling line. As soon as the oscillator is locked into step, the pulses from line 32 also are applied over rectifier I03 and an integrating network I04 to a control grid of delayed gain control tube I05. Operation of tube I05 increases the positive voltage on the screen of clipper tube 95 increasing the amplitude of the output pulses 96 and hence 98. The value of resistor 81 and the a grid current characteristics of tube 88 are such that the total positive swing of its grid with respect to its cathode cannot exceed a predetermined small amplitude regardless of :the magnitndes of pulses '98 and 86 which are applied respectively to the cathode and via resistor 87 to the grid of tube '88. However, the square pulses 38 from tube 91 will increase in amplitude with the change in bias of tube 95. Thus, since the sum of pulses 89 and .93 roughly constant, while the value of the component 98 is rising, it is clear, that the magnitude of pulses 89 must-be correspondingly decreasing. This decrease in amplitude of pulse 89 is effective to prevent other line finder gate tubes (similar to 88 but in other links) from responding .as more fully explained hereafterinconj unction with Fig. 10.

This decrease in pulse 89 does not, however, reduce the response of tube 88 in the first link (now :under consideration) since the "total :input between grid and cathode is not decreased. Thus, pulses :IOII .are roughly constant in amplitude. These pulses I01! from the line finder gate tube 88 are applied also over line '32 and coupling circuit I 06 to gate control tube I! which serves to control the suppressor bias on the input gate tube I08. Tube I08 is normally conditioned by suppressor grid bias so that the pulses applied thereto from the output of cathode follower 83 over line v28 will not be passed by the tube. However, upon operation of tube [81, by selection of a predetermined incoming line as described above, the suppressor grid'of tube I 88 has applied to it such a potential that the tube becomes con-- ductive during the instants corresponding to the time-channel of such predetermined line. Accordingly then, combined dial-and-speech pulses I09 will be applied from the output of tube IIlBcver line 33 to the pulse forming equipment 24 of Figs. 1 and '5 and to the line selecting equipment 26 of Figs. 1 and 8. However, theenergy applied to the line selecting equipment of Fig. 8 willnot be passed until such time as line selection has beenv effected which will be described later.

Line finder 23 having now operated, pulses I89 from fine 33 corresponding to the time channel-- individual to the predetermined line assumed to. be calling are applied to an integrating network H0 which may or may not be preceded by a pulse stretching circuit similar to a peak voltmeter. These pulses are then amplified in tube III and and are applied over transformer H2 to the control grid of the clipper tube I I3 and to the control grid of a second tube H4. The integrating network H0 in the input circuit of tube HI functions as a low-pass filter which will pass the dial pulses but will not pass the higher frequency communication signals. The clipper H3 serves to shape and clip the incoming dial pulses. to form square wave pulses H5 which in turn are difierentiated in network H6 and applied to the control grid of dial gate tube H1. Tube H1 is biased so as to suppress the negative part of the difierentiated pulse (corresponding to the leading edge of the square dial pulse I I5) and to pass only the positive part of the dilferentiated pulse, corresponding to the trailing edge of such square wave pulse I I5. Normally tube II! is nearly cut-off by the voltage drop in its screen grid resistor H8 which is common with the plate of a normally conducting tube H9 of a flip-flop circuit which operates in conjunction with tube If}. This cut-off bias serves to prevent low amplitude signals which may precede the dialing pulses from passing and falsely operating the pulse registcr. Time constants of this filter are so adjusted that the leading edge of the first'dial pulse serves to cause tube H4 to operate, cutting on tube H9. Low-pass filter and time constant circuit I20 "in the grid circuit of tube H'9 causes this condition to be maintained for the interval of the pulse signal series until shortl after "the last pulse has passed, when the flip-flop circuit will return to normal, again rendering the dial gate' tube HI insensitive. By provision of this special blocking circuit, transient effects before and after dialing will not afiect the register. The output pulses from dial gate tube H! are applied over line 35 to the dial pulse register circuits 25 of Fig 1, this pulse passing through resistors I2I and I22 to grids of the first register stage.

The dial pulse register circuits consist of a series of tubes of which I23, I24, I25 and I26 are shown in detail connected as conventional trigger circuits for operation as a binary counter. Blocks I21, I28 and I29 constitute further register trigger circuits not shown in detail, there being a sufficient number of these register circuits to count any dialing number in the exchange. With the system shown for twenty lines the five shown are sufficient. Initially, the tubes on the right hand side such-as I24 and I26 are conducting serving to bias tubes I23 and I25 to cut-0th Furthermore, voltages developed in the register' circuits are applied as will be described later in: more detail over lines I30-I39 to bias the various delay gate tubes to cut-off and the'zero .gate tubes to conduction in the line selecting circuit of Fig. 8.

The negative pulses incoming over line 35 are applied to the first register circuit including tubes I23 and I24. When the register circuit is in its normal condition, that is with tube I24 conducting and tube I23 biased to cut-off, voltage is applied to line I38 maintaining the associated zero device of Fig. 8 in operation and over line I31 blocking a delay gate to be described in more de- The first incoming pulse on line 35 tail later. passes through resistance I 2| to the grid of tube I24 thus causing this tube to cut-off rendering, however, tube I23 operative and applying control voltages to lines I36 and I3I which serve to block the first zero gate and open the first delay gate. 1

The output from tube I24 is applied over a line- I40 to the second register circuit comprisingtubes I25 and I26 serving to transfer conduction from tube I26 to I25 and from I25 to I 26' alternately each time the trigger circuit I23, I24 re stores to normal condition (i. e. each time tube It will thus be clear" I24 becomes conductive). that the second register shifts its condition for every second pulse applied to the first register while the first register changes its condition for every incoming pulse.

the second register circuit restores to normal (i. e. eachtimetube I23 becomes conductive) making register I21 shift its condition once for every two The operations of the second register circuit. 7 fourth register I28 is similarly caused to shift its condition each time the third register I21 restores to normal and the fifth register I29 is similarly controlled from the output of the fourth" register I28.

The third register IZ-I-is similarly controlled over line I 4| so that the reg ister circuit I 2-! changes its condition each time as I6, Fig. 4, for each line are numbered with digits from 1 to 20 representing the twenty lines. Each dial for any particular line is set so that when a called line is dialed, a number of pulses corresponding to the difference between the calling line and the called line will be transmitted to the exchange. It thus becomes necessary to produce time displacements in the communication energy corresponding to the difference in timing between the scanning of the two lines in the cathode ray scanning circuit 39. The difierent signalling pulses operate through the pulse register circuit of Fig. 7 as described above, to select the desired time displacement in accordance with the line which is being called. To this end, each of the register circuits is provided with a zero gate I42, I43, I44, I45 and I46 associated with the first, second, third, fourth and fifth register circuits respectively. Likewise, associated with each of these respective registers are different delay gates I48 microseconds for the twenty line system), I49 (10 microseconds), I50 (20 microseconds), I5I (40 microseconds) and I52 (80 microseconds). Each of these delay gates includes a delay line. In the output of each of these delay lines are delay gate tubes I53 and I54 being ilustrated in the case of gates I48 and I49. It is understood that similar delay lines and gate tubes are provided for the other delay gate circuits. In the normal condition, before any pulse arrives, the system is biased so that the zero gates I42 to I46 are all operative so that no delay will be provided in any of the pulses I09 incoming over line 33 from the line finder circuit of Fig. 6. These pulses I09 therefore will be applied directly from line 33 through the zero gate circuits I42 to I46 inclusive, and from there over line I55 to the output gate tube I56. Assuming for the moment that tube I56 is not disabled, its plate delivers corresponding pulses I51 over line 36 to the control electrode of tube 39, Fig. 4, and thence back onto the calling line. The first time the first register operates, the control potential is transferred from line I30 to line I3I rendering tube I53 conductive and biasing tube I42 to cutoff. Thus, if one pulse only is dialed, a delay of five microseconds is produced so that the energy incoming over line 33 will pass through the first delay gate I48 and the remaining zero gates I43 to I46 inclusive. The second pulse transfers the control potential from line I3I back to I30 causing zero gate I42 again to become operative and blocking tube I53 in delay gate I48. At the same time, the second register operates transferring the potential from line I32 to line I33 blocking the second zero gate I43 and opening gate tube I54 in the second delay gate I49 introducing a ten microsecond delay between line 33 and line I55. Thus, the second pulse will produce zero delay in I42, ten microsecond delay in I49 and zero delays in I 44 to I46. The third incoming pulse will not affect the second register circuit but will again operate the first register circuit introducing the five microsecond delay gate I48 as well as the ten microsecond delay gate I49 producing a fifteen microsecond delay in the incoming energy. The fourth pulse then will return both the first and second register to normal but will operate the third register I2'I producing a twenty microsecond delay at delay gate I50. The fifth pulse will again insert the five microsecond delay gate I 48 so that there will be five and twenty microsecond delays producing a total of twenty-five microseconds. The next pulse will switch out the five microsecond delay line and switch in the ten microsecond delay line producing a total delay of thirty microseconds. The next pulse will switch in the five microsecond delay line while leaving the ten and twenty microsecond delay ineffective thus producing thirty-five microsecond delay. The next successive pulse will then render delay lines I48, I49 and I50 ineffective but will bring into circuit the fourth delay gate I 5| with its forty micro-,

second delay. The other pulses will then bring in, in similar sequence, the five, ten and twenty microsecond delay gates I48, I 49 and I50 introducing in sequence five microsecond delays until delay gate I52 is operated whereupon theprocess will again be repeated in five microsecond steps. Thus, with the five delay gates it is possible to produce any desired delay condition in the twenty iines. It will be clear that if a different number of lines are provided, additional stages for the binary counting system and additional zero gates and delay gates similar to those outlined herein may be provided to secure the proper delay in interconnection for any number of lines.

After the desired number has been dialed, the signalling energy from the calling subscriber Will be transmitted as described over the common equipment circuit and line 33 in the link circuit to the grid of tube I 56. The output pulse I5'I from tube I56 is then transferred over line 36 to the control electrode of tube 39 as illustrated. The voice modulations of pulses I51 incoming over line 36 will then produce variations in the electron stream of tube 39 each time the beam is aligned with the called line electrode and this variation in energy will be passed over the line to the corresponding low-pass filter I9 of the called subscriber to the receiver circuit I8. For the purpose of calling, a tone frequency may be transmitted to operate any suitable tone control apparatus at the called subscribers line or the output of receiver 18 may be such that attention is directed to the phone directly by whistle or other call transmitted by the calling subscriber.

In the foregoing it has been assumed that tube I56 was conducting, for the purpose of simplicity of explanation. Actually this tube is normally biased to cut-off in order that the dialing pulses incoming over link circuit 23 do not affect other lines during the dialing. This cut-off bias of output gate tube I56 is controlled by the gate control circuit comprising tubes I58 and I59. Tube I58 is normally conducting maintaining the grid of tube I56 biased to cut-01f. These tubes I58, I59 in turn are controlled by tube II9 as follows: As explained above tube II9 of Fig. 5 becomes cut-off at the beginning of a series of dial pulses. At such time it sends out an ineffective positive pulse through condenser I69 to the grid of tube I58. As soon as the dialing operation is complete, however, tube II9 returns to conducting condition sending out a negative pulse. This negative pulse cuts off tube I58, which in turn renders tube I59, and also gate tube I56, conductive. This permits the message energy to be transferred over line 36 to the called subscribers line.

In order to protect the called line from being seized by the line finders of other links when the called subscribers receiver is removed from the hook, a portion of the delayed pulse I5! is tapped from line 36 over line 31 through isolating resistors I6I in Fig. 4 to a busy pulse shaper I62 from whence it is .conducted to the grid of busy gate tube I63. This limits the maximum possible value of the positive line finder pulse 89 from tube which will be applied, after the called sub- 13 tube I58 conducting. Thus, the whole link circuit is restored to normal.

In order that the pulses from any one incoming linemay be effectively reduced in amplitude so as to prevent other line finders from thereafter seizing the same calling line I, the delayed gain tube I and associated circuit are provided. It will be clear from the above description that when two or more subscribers are using the exchange at the same time there will be a plurality of differently timed pulses in the line circuits of the common equipment of Fig. 4. These pulses from the output of cathode follower 83 are applied to all of the link circuits in parallel. When one link circuit, however, has taken hold it is necessary that the pulses of this selected circuit be made inefiective to seize other links. A better understanding of the operation of the system to prevent this operation may be had by reference to Figs. 4 and 5 and the curves illustrated in Fig. 10. a

The pulses from the anode 69 of tube 39 are applied to the grid of tube 8! which has separate late and cathode outputs. The pulses from the plate output of tube 8I varying in amplitude in accordance with an incoming signal are shown are passed out through the plate circuit of this tube to cathode follower 83. Preferably, the

energy is only about 25% modulated so that the modulation variations will constitute the minor portion of the pulsing energy. These pulses are used for transmitting speech and are not of interest in connection with the feature now being considered.

The pulses from the cathode output of tube 8| are the ones of primary interest. These pulses are clipped in tube 84 and passed through cathode follower 85 so as to produce a series of equal amplitude pulses 86 as shown in curve IIlB. These pulses 86 are applied through resistors 81 as pulses 89 to the grids of all line finder gate tubes 88 in Fig. 6. Lock-in oscillator 90 produces an output wave I12, curve IIlC, whose period is slightly longer than the time interval between two pulses 89. Wave I12 is clipped :at clipping levels I13 and I14 then differentiated and again clipped to produce pulses whose leading edges substantially coincide with the instant of rise of wave I12 between the clipping levels. These pulses which are preferably substantially wider than the incoming pulses 89, pass through cathode follower 91 and the resulting pulses 98 are applied to the gate tube 88. Since the frequencies are slightly different, the phase or time position of pulses 89 will continually shift with respect to pulses 98 until pulse 89 coincides with pulse 98 as shown in curve IUD. When this occurs, the line finder gate 88, Fig. 6, is operated so that the pulses may pass through peaked amplifier I02 to the oscillator 98 locking it into step with the pulses. The phase correction of peaked amplifier I92 is so adjusted that sine wave I12 will rise through zero slightly before the time of arrival of pulse 89. The pulses 98 will then be produced in fixed time relationship with pulses 89 as shown in first waveform of curve IOE. Once these pulses are synchronized, the delay gain tube I05 cuts off increasing the screen bias of tube 95 so that the selecting pulses 98 increase from their normal search" amplitude to a much higher holding amplitude as shown in the second wave form in curve HIE, thus reducing the effective height of 4.. pulses 89. Thus, pulses 89 applied to the grids of the line finder gate tubes (corresponding to tube 88) in all other line finders will be very small as shown in-the third waveform of curve 18E. Then even if coincidence between these pulses 89 and the normal or search selecting pulse 98 of such other line findersdoes occur, no signal will be passed through the gate tubes of such other line finders as shown in the fourth waveform of curve IllE.

When the called party answers, the closure of I his line loop 5 places on the dynode 53 a potential similar to that of a calling line. If no special precautions were taken this would cause another line finder to seize the called partys line thus tying up an additional link. To avoid this, the busy shaper I62, and busy gate I63 are provided which function as follows:

After the completion of dialing the outputgate,

ergy of these pulses I51 is branched from line 38 in Fig. 8 and passes over line 31 and isolating resistor I 9| to the busy gate shaper I62, which amplifies, clips and reshapes these pulses into strong, sharp constant amplitude pulses. (For this purpose the clipping level of speech clipper tube 82 should be set so that the speech modulation never reduces pulses I1I below a small fixed minimum value.) The reshaped pulses from I82 are applied to the grid of busy gate tube I63 to make this momentarily highly conductive. This gate tube I63 then imposes a fixed upper limit upon the amplitude of the positive pulses 89, so

" that these cannot attain an amplitude sufficient to cause seizure of the called line by another line finder. Preferably, however, this upper limit is high enough to hold a line finder which has already locked itself to the called line (in order that the act of selecting a line already engaged as calling line in a previous connection shall not break down such previous connection).

Turning to Fig. 11, there is illustrated a delay line in the system where the longer delays are required. For the shorter intervals shown in delay gates I48, I49 and I58 of five, ten and twenty microseconds, artificial delay lines of known form may readily be used. However, for the longer delays, acoustic delay means may be preferable. The line may, for example, comprise a container I15 filled with mercury I16, having a length where V is the velocity of sound in the liquid and D is the desired delay time. At the input end is provided a crystal, for example a quartz I11, in a suitable mounting ring I18, with an electrode I19 coupled with line I89 for the input signal.

While we have disclosed a particular embodiment of our invention and a particular telephone exchange equipment in which this embodiment is used, it should be distinctly understood that many modifications and applications of our invention other than those shown in the specific examples may occur to those skilled in the art. The particular embodiment described herein is given merely by way of example and is not to be considered as any limitation on the scope of our invention as set forth in the objects thereof and in the appended claims.

is we claim:

1-. 'Ina communication exchange system, a p1u-- means for producing discrete pulses having per-- tl'ens "varying in amplitude inaccordance with signals; means including a normally ineffective line finder circuit for" connecting a line to an outgoing circuit, means for using a portion of said pulses below said varying portion to operate and maintain efiective said line finder cir--- cult; and a circuit to-carr said signals to'said outgoing circuit by" said varying portions.

The communication exchange system according to claim 1", and in which the unm'odulated portions of said pulses areof' constant 'amplitude and operate and maintain eifective the line finder circuit, and the modulated portions carry the signals to said outgoing circuit.-

3. In lei-communication exchange system, a" pluralit'y of channels; means for applying signals to the channels, means for successively scanning said channels, means for placing in signalling condition one of said channels, means operative thereupon for applying a voltage of a predetermined level to said one channel whereby pulses are produced when said channel is scanned, the signals applied to said channel modulate said pulses and are' below said voltage level, a multielectrode di'schargedevice having two output circuits, the first output cireuit-conditionedto pass portions-of saidpulses at a substantially constant amplitude level below said signals; and the second output circuit conditioned to pass portions of said: pulses above-said constant amplitude level, anetwork terminating said channels for applying the produced pulses to-said discharge device, means including a linefinder responsive to said pulses the'first output circuit" for establishing 5'; The system according to claim 3; and in" Wlilchthe signals applied to said channel are'not greater than fifty percent of said voitagelevel;

6; The system according to claim 3, and in which'the discharge device-has an anodein one and a" cathode in the other output circuit, and

a gi i'cl for said discharge device connected with said network.

"7i The system according to claim 6; and in" whichthe cathode isconnected withthe-fi'ist and the'j anode with the" second output circuit.

8. The system accordin to claim 6, and in" which the line finder responds to the output cif= cuit connected with the cathode.-

EATJ'LL. R. ADAMS; DAVID HZ RANSOM;

REFERENCES CITED The following references are of record-inthe file of this patent:

UNITED STATES PATENTS

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