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Publication numberUSRE23313 E
Publication typeGrant
Publication dateDec 26, 1950
Filing dateMay 13, 1941
Publication numberUS RE23313 E, US RE23313E, US-E-RE23313, USRE23313 E, USRE23313E
InventorsD. Mitchell
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Mitchell
US RE23313 E
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Description  (OCR text may contain errors)

Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM 3 Sheets-Sheet 1 original Filed May 13, 1941 Q ./NVENTOR o M/ TCHEL l.

ATTORNEY Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM Original Filed May 15, 1941 3 Sheets-Sheet 2 Il Ill NN .S lll I im w ,Nut a :m

Dec. 26, 1950 D. MITCHELL TRANSMISSION SYSTEM Original Filed May l5, 1941 5 Sheets-Sheet 5 /NVE/VTOR D. M/ TCHE L L NN QN bm ...53am ..8 kauwgum.

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AT TORNEV Ressued Dec. 26, 1950 UNITED STATES PATENT OFFICE Bell Telephone Laborat cries, Incorporated, New

York, N. Y., a corporation of New York Original No. 2.301,223, dated November 10, 1942,

Serial No. 393,196, May

13. 1941. Application for reissue February 9, 1950, Serial No. 143,311

9 Claims.

Matter enclosed in heavy brackets appear reissue specification; matter printed in italics indie This invention relates to a transmission system and more particularly to a speech transmission system of the type utilizing a plurality of oneway speech transmission channels.

An object of the invention is the achievement of increased efliciency in the operation of a speech transmission system by reduction of the idle time of the various one-way speech transmission channels.

Another'object of the invention is the transmission of messages in such a manner that intelligible reception of the messages by unauthorized persons will be rendered diflicult.

A more specific object of the invention is reduction of the frequency band required for transmission of the signals necessary for control of switching operations incidental to the speech transmission.

In accordance with a specific embodiment of the invention, a plurality of one-way speech channels are associated with a larger number of subscribers telephone circuits; for example, six speech channels (three west-east channels and three east-west channels) may be associated with ten different subscribers telephone circuits (five at the west terminal and five at the east terminal). Switching means are provided whereby an idle speech channel is selected for transmission of each different speech portion of vocal interval duration occurring in the various subscribers telephone circuits. The switching means provided is effective, when a speech fragment of vocal interval duration is applied to a speech channel, to preselect the next idle speech channel, that is, the channel to which the speech fragment of vocal interval duration occurring next will be applied.

It will be obvious that the arrangement is effective as a speech channel saver from the statement above which points out that the number of subscribers telephone circuits served by the speech channels is greater than the number of said channels. This makes it possible to take advantage of the now well established fact that during a so-called conversation period on any four-wire circuit, either one-way channel is actually in use only about one-third of the time. Privacy of transmission is attained by the arrangement contemplated in View of the fact that snatches of several different conversations appear in a random manner on any one of the various speech channels.

A feature of the invention is switching means effective, when a speech fragment of vocal interval duration isv applied to a speech channel,

s in the original patent but forms no part of this ates the additions made by reissue 2 to preselect both the near, or transmitting, end and the distant, or receiving, end of the next idle speech channel, that is, the channel to which the speech fragment of vocal interval duration occurring next will be applied. l

Preselection of the distant end of the next channel at the same time the near end of the channel is preselected is a particularly valuable feature of the invention as it makes possible a substantial reduction in the frequency band required for transmission of the signals necessary for control of the switching apparatus. Obviously, if a channel saving arrangement is to function with full eiiiciency the frequency band required for the control signals must be kept within relatively narrow limits. v v Y In general, the term vocal interval as used herein means the duration of one or a succession of syllables no one of which is separated from the preceding syllable by an interval as great as the hang-over time of the system, weak beginnings and endings, i. e., portions not capable of causing relay operation, being considered parts of 'the silent interval. Hang-over time is a term commonly used in connection with systems including voice operated devices and it means the period that a voice-operated device remains, due to its own characteristics, in operated position after the operating impulse has been removed. Hangover time insures that weak endings of syllables, not capable by themselves of holding the relays in operated position, will not be lost by clipping and may be increased or decreased in duration as the type of system demands.

Full understanding of the operation of the arrangement contemplated by the invention as well as appreciation of the various advantageous features thereof may be gained from consideration of the following detailed description in connection with the accompanying drawings in which:

Fig. 1 shows diagrammatically a two-way telephone system embodying features of the present invention;

Fig. 2 shows in ing arrangement associated with detail the transmitting switch'- one of the subl scribers telephone circuits of Fig. l; and

Fig. 3 shows in detail the receiving switching arrangement associated with one of the subscribers telephone circuits of Fig. 1.

Referring now to the drawings there are represented in Fig. 1 two terminals, west and east, of a two-way telephone system which may be, for example, a two-Way radio or line wire telephone system. Five subscribers sets Il, l2, I3, I4 and I5 are shown at the west terminal, each and 25, one West-east control signal channel 2l, three eastwvest speech transmission channels 4|, 42 and 43 and one east-west control signal channel 44.

The West terminal switching apparatus is represented by box 45 and Ythe `east `terminal switching apparatus by box 46; details of these switching arrangements will be brought out by subsequent description of Figs. y2 and 3.

The telephone circuit-of -each of the west'subscribers sets has associated therewith an oscillator; each of these oscillators produces a dis- .tinctive tone frequency. For example, "the tel'e'- phone circuit of subscribers set II hasassociate'd Ytherewith yoscillator 5I which `produces a tone of `frequency' f1,the telephone circuit of subscribers set I2 has associatedztherewith oscillator 52 'which 'produces atene of frequency f2 and soon. ASimilarly, the telephone circuit of each of the feast :subscribers sets has associated therewith .an os- 'cillator which .produces a 'tone of the saine ffnequency .as vthat "produced by Vthe Yoscillator asso'ciated'with the corresponding west subscribers set. For example, the telephone circuit of subsoribers .set I6 has associated therewith 'oscillator :53 which produces vatone fof frequency tf1, the telephone circuit of subscrib'ers set Il has associated therewithl oscillator '54 "which produces .a ltone of frequency f2 and so on. Oscillator 55 ywhich produces a vtone of frequency "fs is Aprovided `at the westzterminal and oscillator "Slfwhich :likewise produces a 'tone of Vfrequency fa 4is prol-videdat the east terminal; the :purpose :of these tones will `be described subsequently.

It Iwill be observed Ythat'in accordance with the arrangement contemplated by the present inven- "tion yeach yof 'the ycontrol signal channels .or wpaths 27 and 44 need transmit a frequency iband only of suicient breadth `to include six different distinctive tones whereas in prior vsystems it `was necessary to `provide for transmission of a -substantially greater frequency band. This reducltion in 'the control frequency 'band results from Vthe :nevel varrahgen'ient whereby not only tis the `sendir-.g end of the next speech transmission channel preselected but vthat the receiving rend -of the narse speech ychannel is also preselected.

The operation of the terminal switching 'ar ranmments 45 ard 4S will `be described in detail i=1 connection with FigsZ and 3*; their operation will be described only briefly at this point. Let us assume that a conversation is taking place -b^tween subscriber A (set it) and subscriberAl (set iii) and that a speech fragment of nvocal interval duration originating at set H has `just been anplied'to speech-channel 24. Switching arrangement 55 operates at this time Vto do several things` among them being the application'to control `signal path 2'! of the tenes produced by oscillators 5I and 55. Tone produced by oscillator 5i actuates proper control apparatus of east terminal arrangement in order to cause thespeech fragment Yof vocal interval which it accompanied to be :applied -to the .receiving Aside `.of the telephone circuit of subscribers set i5; the .tone produced by oscillator v55 performs certain .regulatory functions k.which will .be described .subsequently in reference to Figs. 2 and 3. Switching arrangement 45 also acts at this time to preselect the transmitting .end of the idle West-east speech channel (let us say channel 25) to which the next speech fragment of vocal interval duration originated by subscribers set Il or any of the other westsets will be applied. At the same time certain apparatus of switching arrangement 45 is .conditioned to preselect the receiving end of that same-channel whose sending end was preselected by 'apparatus of the west switching arrangement.

The next speech fragment of vocal interval -duration originated by any of the West [subscribers] subscribers sets will be applied to speech channel 25, this channel having been preselected as described above. We will assume that this speech fragmentcomes from subscribers set l2, subscriber B .and subscriber B1 being conversing. At the time the speech fragment is applied to speech channel 25 tone f2 produced by 'oscillator'52 -is applied to lcontrol Vsignal .path y2'l. This tone causes operation V*of certain apparatus .of :switching `arrangement -46 effective to Vapply the speech fragment-ofvocal interval duration Icoming 1in over channel 25 to be applied to Vthe receiving side of the telephone vcircuit of subscribersfset il. Itshould be observed here that in view `of the fact that both Yends of Aspeech channel 25 were preselected it is necessary to transmitonlyone-control signal tone, i. e., a tone to identify the west subscriber, and that nochan- `nel identifying tone need be transmitted.

Speech fragments of vocal interval duration as they -origi-nate inany ofthe telephone circuits of the west terminal are, therefore, placed on .the .particular west-. eastchannel whichhappened to .be last preselected.; .no .two consecutive l.speech fragments of vocal interval duration can be placed on the same .speech lchannel .in View of the preselection feature.

The .arrangement functions in a similar manner for .east-west transmission, thespeech .fragments of vocal interval duration .together with .the respective control tones being applied to speech-channels 4|, 42 and 43 in accordance with presLlection of .the sending .and receiving ends `thereofby switching arrangements 45 Aand 45, respectively.

t will be clear from the above that it will `be very diiicult for unauthorized persons who may listen in on the speech channels -to interpret the messages being transmitted thereover as there will occur on .any one of lthe channels snatches of several different conversations tbes; conversae tion-parts appearing in a purely random manner.

It will also be apparent that the arrangement is effective as a speech channel saver. Thus in the specific syst'm disclosed, only six one-Way speech transmission channels and two control signal channels are utilized for interconnection .of ten subscribers. With a larger number of subscribers it is possible to closely app-roach the ideal condition of having only one-third as many speech channlls in one direction as there are subscribers and yet provide adequate facilities for Knninterinpted transmission; fthe feasibility of such an arrangement is indicated by the previously Arecognized fact that during the time tivo .subscribers are connected over a four-wire circuit .either side .of the circuit is actually in use .only about one-third of the time. The substantial reduction in the frequency band necessary lfor the control signals which results from the novel .arrangement of the present invention .75 vwhereby both .the sending andthe .receiving end lcepis of the speech channels are preselected makes adoption of the system to use by a relatively large number of subscribers commercially feasible.

Referring now to Figs. 2 and 3, which should be joined left to right, respectively, a detailed showing is set forth of a portion of the switching apparatus of the west terminal (Fig. 2) and a portion of the switching apparatus of the east terminal (Fig. 3). In order to avoid confusing duplication of similar apparatus, the detailed disclosure has been confined, so far as clarity and completeness of description will permit, to a showing of the transmitting switching apparatus of west subscriber A (Fig. 2) and the receiving switching apparatus of east subscriber A1 (Fig. 3).

Subscribers set is `coupled to transmitting line 1| and receiving line 12 by transformer 13, network 14 serving to provide a proper balance in the well-known manner. Transmitting line 1| may be connected through operation of certain relays, which will-be described in detail subsequently, to any one of the three west-east speech channels 24, 25 or 26; receiving line 12 may be connected through operation of other relays to any one of the three east-west channels 4|, 42 or 43.

It will be observed that the relays of the transmitting switching apparatus illustrated in Fig. 2 may, in general, be separated into three groups, one group being associated with each west-east speech channel. For example, it will be apparent from the subsequent description that relays 15, [16,] 11, 8|, 82, 83, 84, 85 and 86 are associated with speech channel 24; that relays ||l|, |62, |53, |04, |65, |06, |01 and are associated with speech channel 25; and that relays ||2, ||3, ||4, ||5, |I6, ||1, |2| and |22 are associated with speech channel 26.

Speech energy emanating from subscribers set Il follows two parallel paths, one path through delay network |24 and out over channel 24, 25 or 26 depending upon which group of relays is operated, and the other path through amplifier-detector |25 to the various relay circuits.

In general, selection of the west-east speech channel is dependent upon the position of brush |26 of rotary selector switch |21; operation of this selector switch when a speech fragment of vocal interval duration is applied to a channel is effective to preselect the next speech channel in a manner that will be clear from subsequent description. The switch may be of the general type disclosed in O. F. Forsberg et al. Patent 1,472,465 and comprises two banks |3| and |32, two brushes |26 and |33 cooperating therewith and stepping magnet |34. It will be understood that the two brushes are coupled mechanically so that energization of stepping magnet |34 causes rotation of both brushes. Selector switch |21 is common to all the [subscribers] subscribers telephone circuits of the west terminal.

For purposes of further illustration of the invention the operation of the circuits of Figs. 2 and 3 will now be described in detail. In the condition illustrated, relays 11, 8| and |31 (Fig. 2) are operated over energizing circuits traced from battery |5|, brush |25 and first contact of selector bank |3I, line |52 and thence over two paths, one comprising an armature and break contact of relay 82, the armatures and break contacts of similar relays in other subscribers circuits and thence through the winding of relay |31 to ground |53 and the other through break contact of relay ||1, break contact of relay |65,

6 operating winding ofy relay 8| to ground |54 and operating winding of relay 11 to ground |55.

' `Relay |31 being operated, control tone of fre- ;quency fe produced by source 55 is applied through band-pass iilter |56 to west-east control signal path 21 for a purpose which will be described in detail subsequently.

Assuming now that subscriber A speaks, speech energy produced by subscribers set is transmitted through transformer 13 to transmitting line 1|. As the energy is delayed somewhat in its passage to the west-east speech channels by `delay network |24, let us see first what actions result from passage of a portion of the energy through amplifier-detector |25.

In view of the fact that relay 8| is in operated position, we have obvious completed paths through the operating windings of relays 82, 83, 84, 85, 86, [15 and 16] and 75 all of which relays are therefore now operated by the energy from amplifier-detector |25. An obvious path is also completed from the battery 164 to the operating winding of relay 76 by the operation lof relay 75 so that relay 76 is also in operated position.

Operation of relay 16 applies control tone of frequency f1 produced by source 5| to west-east control signal path 21 through band-pass filter |61; the purpose of this control tone will be described subsequently. Operation of relay 66 connects transmitting line 1| to west-east speech channel 24 so that the speech energy after traversing 'delay network |24 is sent out over speech channel 24.

Operation of relay 82 releases relay 137 and establishes a locking path'over which relays 11 and 8| are held operated, this path being traced from battery |62, make contact of relay 11, make contact of relay 82, line |63 and 'through the operating windings of relays 8| and 11 to respective grounds |54and |55.

Operation of relay 15 is4 effective to remove battery |64 from the make contact of relay |65. Relays |55, |66 and |61 are associated with the switching circuits of a second subscriber and correspond respectively to relays 11, |52 and ||3 of subscriber A. Relays |8|,v |82 and |83 are also associated with the circuits of a second subscriber and correspond respectively to relays 15, I3! and |I2. In the instance of relays |65, |66 and |51 howevena connection to battery is not obtained directly through a contact of the respective relay but only from the break contact or a relay in the circuits of subscribers set and, then, only if said relay be in unoperated position. This arrangement is provided so that evenv if two subscribers were to start talking at the saine instant there would Abe no chance of the speech of both being placed on the same channel. For example with the circuits in the condition just described there is no chance of a second subscriber breaking in on channel 24 with subscriber A as operation of relay 15 has removed battery from the make contact of relay |55` whereupon the holding circuit for relays in the other subscribers circuit corresponding to relays 11 and 8| cannot be established. In the instance of the switching circuits of the third subscriber, the battery supply path also includes the break contacts of relays ISI, |82 and |83 of the second subscribers circuit; in the instance of subsequent subscribers circuits the path includes the break contact of a corresponding relay of the preceding circuit. Thus, for example, if thesecond subscriber (B)v andthe fourth subscriber (D) were `to start talking simultaneously. subscriber -A being idle, subscriber B would 'be .pu't on the next idle channel and the connection of subscriber D 'thereto at the `same time would 'be prevented due to the break in'the battery .supply path resulting from operation .of relay '|81 Operation of /relay 83 causes ra break in the path .connecting certain of the relays ofthe group associated with channel 25 tothe second contact of selec-tor'bank I3 I; operation `oi" relay 84 causes 'a similar break in thegroup of relays associated with channel l26.

Operation of relay v85 vcompletes an Venergizing path for selector stepping magnet |34 which may be traced from ground |84, make contact v.of relay 85, 'line |85, first Ycontact of selector bank |32, brush |33, operating =winding of stepping magnet |34 .toibattery |86. Brush '|33'is therefore stepped around to the second contact of bank 32 where it comes to reset. 'Had this contact been lmarked busy by the connection of rground thereto due to the .operation of relay |101 (ora corresponding -relay of a switching circuit of another subscriber) brush .|33 would, of course, continue its rotation .until it reached an idle contact of bank |32.

As 'brushes |26 and |33 are coupled mechanically, rotation of ybrush |33 to the second contact of lbank |32 will cause a corresponding rotation of brush |26 to Ithe second Contact of bank I3|. This rotation of brush r|26 to the second contact 'of ibank |3I is effective to preselect the transmit- `ting vend of west-east channel 25 so that the next speech fragment of vocal interval duration pro- .duced by `any of the [subscribersfl subscribers sets vof the West terminal will be applied thereto. This preselection of the transmitting end of Achannel 25 is accomplished by setting Vup an en.- .ergizing path for those relays associated with other subscribers" vsets which correspond to relays I02 and |03 of subscribersset II. Inv-the :instance .of Yrela-ys |02 yand |03 this energizing path is :prepared but not completed as relay 83 is Inovv 1in .operated position; the path when completed, i. e., when relay 33 -is deenergized, `may f be traced from battery I--.I, Vbrush |26, second Acontact of selector bank I 3I, line I9|, break contact .of lrelay -I I6, break -eo-ntact of relay l83, operating winding of :relay |03 to ground |92 ,and operating winding of relay |02 yto ground .1.03. With this condition, therefore, the next Yspeech fragment `Vof vocal linterval duration produced by vany of the other vwest subscribers sets is placed on .channel 2-5; .should lsubscriber A Epause long enough ,for the relays associated with .channel .Z4 to .restore .to .normal position before .any of the .other West Subscribersrstart talk.- ging, relays .|02 and |03 will operate `over the ,path described above and should he then resume speaking the resulting speech fragment will b e placed on channel .25.

Although battery .I5-I .Was removed from line '|52 as soon as brush I 2,6 moved off .the irst contact of bank I 3|, relays .11 and 8| remain .operated due to .the locking vp ath established .by operation `of relay 82.

We ,'have now seen how control .tones .of `fre- ;quency fa and .f1 have been Yapplied to west-east control signal path 21; how the speech energy produced by subscribers set I I has been applied -to the transmitting end of West-east speech chan- *nel 24;k and how the transmitting end of west- 'east speech channel l2,5 has been preselected for application ofthe next speech `fragment lof -vocal linterval duration. lLet us nowsee what has'been happening at the recei'vingendFig 3i) andparlticularly how subscribers set I6 has been connected to vthe receiving end of 4channel 24 and how the receiving `end of .channel 25 has been preselected.

There is included in the switching apparatus illustrated in Fig. 3, channel selector 2II of subscriber A1, stepping selectol1 2|2 of subscriber A1 4and collecting selector 2 I 3 which is common to all ofthe east subscribers sets. Brushes 2 I4 and 2 I5 of selector 2| I (which are coupled mechanically) `and brush 2|'6 of selector 2| 2 are coupled mechanically and are rotated in step by selector stepping magnet 2I1; brush v22| of selector 2I3 is rotated by selector stepping magnet 222. The Acontacts of the bank of selector 21| 2 are connected in multiple to contacts of the banks of the stepping selectors of the other east subscribers.

It -will be recalled `from previous description of Fig. 2 that control tone of frequency f1 is being transmitted over control signal path 21 at this time (when subscriber A is speaking). This control tone passes through band-pass ilter 24|, which is designed to pass only a narrow band of frequencies including f1, to amplifier-,detector 242. 4The Venergy emanating from amplier-detector 242 causes the operation, over obvious paths, of relays 243, 244 and 245. Operation of relay 243 com pletes at its two make contacts la path' over which the speech currents being received over speech channel 24 are applied to the receiving side of the telephone circuit `of subscribers set I6. A portion of this energy passes through amplifier-detector 245 and causes operation of relay `241-which places a shunt on the transmitting side of the telephone circuit of set vlli during the time vspeech ,is being received over the receiving side of -the circuit. The speech currents produced by subscribers set |^I of the West lterminal are noW received by subscribers set I6 of `the east terminal therefore.

Operation of relay 244 removes battery 25.I from o ne side of the operating winding of stepping magnet 2I1 While operation of relay 2,45 places ground 252 on the other side of .the winding of magnet 2 I1 and on the rst contact of the -bank of stepping selector 2I2 and on the corresponding contacts of 'the banks of the Vstepping selectors of the other east subscribers.

Removal of battery Y25| from the operating path of stepping magnet 2I1 prevents rotation .of selectors 2| 2 and 2II during the time speech and the control tone are Abeing received .over channel 2,4. With respect to the stepping selectors of the other east subscribers, however, (assuming that VWest subscriber A is the only one talking) pres.- ence of ground 252 0n the first .contacts of the stepping selector .banks completes energizing paths through the respective stepping magnets (corresponding tomagnet 2 I1) to the respective batteries (corresponding to battery 25|) and causes the stepping Ibrushes (corresponding to brush 2| 6) and the two Ycoupled brushes of the channel selectors (corresponding to brushes 2| 4 and 2I5-) to move to Vthe second position of the respective banks. Ground 252 is also connected -to 4the rst contact of the bank of collecting seflector 2I3 which causes the collecting selector also 'to move to the second contact of its bank as battery 253 is connected to one side of the operating winding of stepping magnet 222. By the above action, it will be obvious that the receiving end `of vspeech channel 25 has'now been preselected.

l"1l-he above described condition prevails so long .as subscriber .A `alone is talking; let us assume g. novi? that subscriber A ceases talking. The removal of control tone f1 from path 21 results in relays 243, 244 and 245 dropping back to unoperated position.

Restoration of relay 245 to normal position removes ground 252 from the contacts of the stepping selector banks and from one vside of the winding of stepping magnet 2|1; restoration of relay 244 reconnects battery 25| to the other side of the winding of stepping magnet 2|1. The brushes of selectors 2|2 and 2 are now moved to the second position of the respective banks as an operating path for magnet 2|1 is completed traced from battery 25|, break contact or relay '244, winding of magnet 2|1, brush 2|6 and rst contact oi' selector 2|2, resistance 254 to ground 255. It should be kept in mind that at this time relays 21| and 212 are in unoperated position as the control tone fa, which is applied to control path 2l whenever brush |26 (Fig. 2) is in its iirst position, is not now being transmitted. The brushes of selectors 2 l2 and 2li will stop in the second position therelore due to the fact that a high potential is placed at the junction of resistance 213 and the second contacts of all the stepping selector banks by the connection thereto of battery 253 through resistance 214 and the winding of magnet 222 in parallel. The constants of the system are so arranged that the diierence between the battery voltage on stepping magnet 2 'l and the voltage at the junction of resistance 213 and the second contact of the banks will be less than the value required for stepping action. The above is true also of collecting selector 2 I3 so that brush 22| remains in the second position; unless a dead ground (corresponding to ground 252) is placed on a contact of one of the stepping selectors, magnet 222 is in eect shunted by resistance 214 and the current through resistance 213 is not sufficient to cause the collecting selector to operate.

Recapitulating the detailed description of Figs. l and Z: the transmitting switching apparatus of the west terminal operated to place the speech energy produced by subscriber set on the transmitting end of channel 24 while the receiving switching apparatus oi the west terminal operated to connect the receiving end of channel 24 through to subscriber set l5 which is paired with subscriber set the transmitting switching apparatus of the West terminal operated to preselect the transmitting end of channel 25 for all west subscribers except the busy subscriber who was left on channel 24 while the receiving switching apparatus of the east terminal operated to preselect the receiving end of channel 25 for all east subscribers except the busy subscriber who was left on channel 24; and certain switching equipment of the busy west subscriber was conditioned so that as soon as he stopped talking preselection of the transmitting end of channel 25 would be effective for his station also while, in view of the position assumed by the collecting selector at the east terminal, the circuits of the busy east subscriber are so conditioned that, as soon as the speech energy being received from the west terminal ceases, preselection of the receiving end of channel 25 becomes effective for this subscriber also. The above is on the assumption that only sets and I6 are busy.

Let us consider now the purpose of control tone fa which, it will be recalled, is applied to control signal path 21 whenever selector brush |26 is in engagement with the rst contact of selector bank |3l. The purpose of this control tone is, briefly, to insure that the brush oi' collecting selector 2|3 is in step with the operation of the system and, if not, to restore synchromsm at least once in each revolution. Let us assume that the circuitv is in the condition illustrated ln Figs. 2 and 3 except for the fact that brush 22| of the collecting selector is on the second contact of the bank instead oi' on the nrst contact as illustrated, i. e., in the assumed condition the collecting selector is out of step by one position. The control tone fa received over control signal path 2| is passed by band-pass filter 28|, which is designed to pass a narrow band o1' frequencies including ia, to ampliner-detector 282. The energy emanating from amplifier-,detector 282 causes the operation, over obvious paths, of relays 21| and 212. Operation of relay 21| connects battery 215 through resistance 215, which is of relatively small resistance, to the junction of resistance 254 and the first contact of collecting selector 2|3 thereby, in effect, designating this contact as the idle position. Operation oi relay 212 removes the shunt path through resistance 2'l'4 so that the full potential o battery 253 is now applied to magnet 222. If brush 22| is on the second contact therefore, as we have assumed, it will be stepped around until it reaches the first contact thereby restoring synchronism. Upon reaching the first contact it will stop due to the balancing potential applied thereto through resistance 21 E. This same action will take place, of course, with respect to the stepping selectors so that if any of them are idle and out of step they will also be collected and brought to the i-lrst position. Busy stepping selectors will not be aii'ected as operation of the relay corresponding to relay 244 will in each case have removed the stepping battery.

This synchronizing action takes place once during each revolution of selector |21, that is, whenever brush |25 is in engagement with the nrst contact of bank I3|.

It will be understood, of course, and is so indicated schematically, in Figs. 2 and 3 that subscriber A is provided, in addition to the transmitting switching apparatus which is illustrated, with receiving switching apparatus (indicated by boxill) which is generally similar to that illustrated in detail in Fig. 3 `and that subscriber Al is provided, in addition to the receiving apparatus which is illustrated, with transmitting switching `apparatus (indicated by box Which is generallyA similar to that illustrated in detail in Fig. 2. It follows from this that transmission irom subscriber A1 to A (east to West) is accomplished in a manner generally similar to west-east transmission which has been described in detail above. During reception by subscriber A, a portion of the speech energy being received over line 12 is passed into amplierdetectorl and the energy emanating therefrom causes operation of relay Sir; operation of relay SI2 places a shunt on transmitting line 'il during. reception over line 12.

While a specific embodiment of the invention has been selected for detailed description it will be understood that the invention is not limited in its application to the embodiment described. The embodiment described should be taken as illustrative of the invention and not as restrictive thereof.

What is claimed is:

1. In a speech transmission system, a plurality of transmitters, an equal number of receivers,

11. aV plurality of speech transmission channels, means for applying a speech fragment of a` dura-1 tion equal to a vocal interval produced by one of said'transmitters to a. selected one of said speechv transmission channels, and means oper-v ated by energy resulting from production of a speech fragment of vocal. interval duration by one of said transmitters to preselect both the sending end and the receiving end of an idle speech channel for transmission of the next succeeding speech fragment of such duration.

2. In a speech transmission system including a plurality of transmitters, an equal number of receivers and a plurality of speech transmission channels between said transmitters andI said receivers, the method of maintaining secrecy during the transmission of speech from said transmitters to said receivers which comprises impressing each successive speech fragment of a duration equal to a vocal interval produced by said transmitters on a dilerent one of saidj channels and preselecting both the transmitting' end and the receiving end of an idle channel for transmission of the next succeeding speech fragment of vocal interval duration each time a speech fragment of such duration-1S applied t0 a channel.

3. In a speech transmission system, a plurality of transmitters, a plurality of receivers, a plurality of speech transmission channels between said transmitters and said receivers, the number of said channels being less than that of said transmitters or said receivers, means operated by energy resulting from production of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to a selected one of said channels, means for maintaining said connection during the duration of said speech fragment and means also operated by said energy for preselecting both the transmitting end and the receiving end of an idle speech transmission channel for transmission of the next succeeding speech fragment of such duration.

4. In a speech transmission system including a west terminal and an east terminal, a plurality of transmitters at the West terminal, a corresponding number of receivers at the east terminal, a plurality of speech transmission channels between said terminals, means at the West terminal operated by energy resulting from production of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to the trans'- mitting end of a predetermined oney of said speech transmission channels, means at said east terminal for simultaneously connecting the` input.

of a selected one of said receivers to the receiving end of the same channel, means for maintaining said connections during the duration of said speech fragment, means at said west' terminal also operated by said energy for preselecting the transmitting end of an' idle speech transmission channel for application of the next succeeding speech fragment of such duration and' means at the east terminalA for simultaneously preselectng the receiving end of that same channel the transmitting end of which is preselectedY at the west terminal.

5. In a speech transmission system including a west terminal and an east terminal, a plurality of transmitters at the west terminal, a corre-Y sponding number of receiversat the east lterminal, a plurality of speech transmission channels between said terminals, means at said West terminal forl applying. a speech fragment` ofvocal interval duration produced by onev of. said., transmitters to a-selected one of said speechtransmission channels, means including a rotatable ele-f ment at said westterminal operated by energy resulting from. production ofv a speech fragmenty ofY vocal.k interval duration by one of said transmitters to p reselect the transmitting end of an idleY speech transmission channel for applicationY of thenextsucceeding speechnfragmentlof such duration, means at said eastterminalincluding a rotatable element for simultaneously preselecting the receiving end vof, the same idle speeehVV transmission channel, and means operating fonce during each rotationof saidrotatable element of saidwest terminal preselectingl means for bring,- in-g said tworotatable elements into positional synchronism. l l v 6. In` a speech transmission systemv including awest terminal and an east terminal, a plurality of transmitters at thewest terminal, a, correspending number of receivers at the eastterminal, a plurality of speech transmission channels between. saidterminals, a control signal. path between said terminals, means operated by the energy resulting fromproduction of a speech fragment of vocal interval rduration by one of said transmittersI for connecting the output of said transmitter tothe west terminal. end of a selectedA one of said speech transmission channelsmeans at said, west terminal for applyinga tone Signal of distinctive frequency path, means at. the Veast terminal operated by said tone signal to connect the input of a selected one of said receivers to the east terminal end of production of said speech fragment to preselect` the west terminal end of another of saidspeech transmission channels for subsequent transmission of speech energy produced by oneV ofA said transmitters and` means at said east terminal for simultaneously preselecting the east terminal end of4 that speech transmission channel the west terminal. end of which was preselected by said previously mentioned means.

7.k In. a speech transmission system including apair of separated terminals, a plurality of subscribers. telephone sets at each of said terminals, a plurality of speech transmission channels be. tween. said terminals, the number of said channels being less. t-han the number of said sets, means for establishing varying interconnections between. said subscribers: sets and said.V channels whereby each successive speech fragment of a` duration equal to a-` vocal interval produced by any of said setsYat-one of. said. terminals is placedl on adifferentspeech channel than thaton whichl the preceding fragment of such duration. wasA placed, said means operating eachrtime aspeech fragment. of vocal interval duration is applied to oner of" said channels to preselect both the sending endl and theA receiving end channel for transmission of the next succeeding speech fragment of. such duration produced by any of'said sets at saidv terminal.

8; In a speech transmission system including a west terminal and an east terminal, a plurality to` said control signal of an idle speech mais '1S duction of a speech fragment of vocal interval duration by a busy one ot said transmitters for connecting the output thereof to the west terminal end of a predetermined one o! said speech transmission channels, means at said east terminal for simultaneously connecting the input of a selected one of said receivers to the east terminal vend oi' the same speech transmission channel,

means 'for maintaining said connections to both ends o! said channel during the duration oi said speech fragment, means at said west terminal also operated by said energy for preselecting the west terminal end of an idle one of said speech channels for `transmission of the next succeeding speech fragmento! vocal interval duration pro- "duced by any 'of the `other of said speech transmitters, and means'at said east terminal for pre- -selecting the east terminal end -speech `channel for reception 'ny-any of the 'of said receivers,

of the same idle other said last-mentioned west terf nal means `being eifectivealso to `condition circuits of said busy transmitter whereby upon ter- I mination of the speech fragment produced thereby the west terminal end of said preselected -channel is made available thereto. said last-mentioned east terminal means being effective also to condition circuits oisaid selected one oi said receivers whereby upon termination of the speech fragment produced by said busy transmitter the east terminal end oi said preselected channel is lmade available to said selected receiver.

'9. 'In a speech transmission system including if...

'a west terminal andan east terminal, a plurality ofspeech transmitters at the west terminal, a y I corresponding number of speech receivers at the east terminal, a plurality of speech transmission channels between said terminals, means operated by energy caused by production of a speech fragment of vocal interval duration by a busy one oi. said transmitters for connecting the output thereof to the west terminal end of a predetermined one of said speech transmission channels and for simultaneously connecting the inputfof a selected -;one of said receivers to the east terminal end of the same speech transmission channel, means also operated by said energy for placing the outputs of all idle transmitters in connectible relationship to the west terminal end lof an idle one Tof said speech transmission channels and for :placing the inputs of all idle receivers in connectible relationship to the east terminal end of Vfthe same last-mentioned channel'afnd means operated by .energy caused by production of a speech ragment of vocal interval duration A.by one of said ast-mentioned transmitters for completing the onnection of the output thereof tothe west ter-l` minal end of said last-mentioned Achannel and or simultaneously completing the connection or 'the input of a selected one of;said last-men- "25f'j5tioned receivers to the east terminal end of the same last-mentioned channel, fsaisl last-menioned means also placing the outputs of all transmitters still idle in connectible relationship to he west terminal end of anotheridleone of said channels and the inputs of all receivers still idle vr in connectible relationship to the east terminal jend of the same last-mentionedchannel.

I DOREN MITCHELL.

No references cited.