US2129860A

US2129860A - Privacy system

Info

Publication number: US2129860A
Application number: US142816A
Authority: US
Inventors: Mitchell Doren
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1937-05-15
Filing date: 1937-05-15
Publication date: 1938-09-13
Anticipated expiration: 1955-09-13

Description

Sept. 13, 1938.

D. MITCHELL PRIVACY SYSTEM Filed May 15, 1957 4 Sheets-Sheet 1 //v VENTOR 0. MI TCHE L L A TTTJRIVE) Sept. 13, 1938. D. MITCHELL 2,129,850

PRIVACY SYSTEM Filed May 15, 1937 4 Sheets-Sheet 3 ATTORNEY Sept. 13, 1938.

D. MITCHELL PRIVACY SYSTEM Filed May 15, 1937 4 sheets-Sheet 4 H mmimuwm fi atM mvm EVM L mvm rt .36 n3 n Am mkm U mmNAl mow AH BM av 6 m2" haw a H. NQN m MON E R7 J J New kmm Num \mN WW N ac k 2% wmwNn wmmv VQNY N Nix By D.M/7'CHEL L Patented Sept. 13, 1938 UNITED STATES PATENT OFFlCE PRIVACY SYSTEM Application May 15, 1931, Serial No. 142,816 23 Claims. (c1.1'z9 1.si

This invention relates to a transmission system and particularly to a method of and means for assuring secrecy in the transmission of messages.

An object of the invention is the assurance of secrecy in the transmission of messages and the prevention of intelligible reception of the messages by unauthorized listeners.

In accordance with a feature of the invention noise or spurious speech is applied to the transio mission channel during the silent intervals, 1. e., when the regular talkers are not speakin whereby there is produced on the channel a practically continuous train of sound composed of the useful speech and the noise or spurious speech.

This train of sound will be inthe nature of a meaningless jargon, the useful speech beingisoblended with the noise or spurious speech that it will be practically impossible for unauthorized listeners to interpret the message. In accordance with a. further feature of the V invention means are provided for preventing the noise or spurious speech from reaching the authorized listener at the distant end of the channel thereby avoiding subjecting him to the annoyance and inconvenience of listening to the unwanted speech or noise.

In accordance with a further feature of the invention the frequency of a current used for certain control purposes is periodically changed whereby selection of the proper control current by unauthorized persons is rendered particularly difficult.

In accordance with a particular embodiment of the invention the output of a phonograph pick-up is connected to the lineduring the silent periods, 1. e., when the regular talker is not speaking. Noise or spurious 'sp'eech has previously been recorded on the phonograph record. At this .same time a control current is applied to the line, being effective to operate switching devices at the distant terminal in order to prevent the noise or spurious speech from reaching the regular talker at the distant end of the channel.

Upon the regular talker speaking, voice operated devices function to remove the spurious speech from the line, replacing it with the useful speech, and also to remove the control current from the line thereby permitting completion of a trans? mission path to the distant talker.

features thereofmay be gained from consideration of the following detailed description in con- A full understanding of the operation of the" nection with the accompanying drawings in which;

Fig. 1 shows diagrammatically a two-way, four-wire radio telephone system embodying features of the invention;

Fig. 2 shows in greater detail one terminal of the system diagrammatically illustrated in Fig. 1;

Fig. 3 shows diagrammatically a modification of the system shown in Fig; 1 in accordance with which a periodically changing control current is utilized; and

Fig. 4 shows one terminal of a system embodying features of a modification of the invention in accordance with which pre-recorded speech is transmitted when the regular talker is not 16 speaking.

The diagrams of Fig. 1 and Fig. 3 are not actual circuit diagrams but rather single line layouts each line indicating a transmission path. In these layouts a normal break in a path is indi- 20 cated by separated arrowheads and a normal make by connecting arrowheads. To show that a transmission element 'is to be controlled by a device, an arrow points from the device towards the element. An arrow directed to a make 25 point indicates that the path will be disabled at that point by operation of the control device and an arrow directed at a break point indicates that the break in the path will be eliminated by operation of the control device. Two arrowheads 30 shown actually separated or wedged apart by the third arrowhead indicates that a normally make point is in operated or open condition.

In Fig. 1 the arrangement contemplated by the invention is shown incorporated in a two-way. 35 four-wire circuit forming apart of a radio telephone system. The two terminals "west and east of the system are represented, the circuit at the west terminal comprising a one-way transmission circuit EA for connecting the output of 40 subscriber's station W to the west-east radio transmitter II and a one-way transmission circuit WB 'for connecting the east-west radio receiver l2 to the input of subscriber's station W. Similarly, at the east terminal. a one-way transi mission circuit EB connects the west-east radio receiver l3 to the input of subscriber's station E while the output of the subscribers station is connected to the east-west radio transmitter l6 by one-way transmission circuit WA.

The west terminal-transmitting circuit EA is normally disabled, (1. e. when the regular talkers are not speaking) so far as the output of subscribers station W is concerned, at control point H by suitable means, for example a short circuit circuit EB, 25 of circuit WA, and 30' of circuit 15 W13. Among the privacy devices commonly used at present are the so-called band-splitting type and the so-called speech inverter type. Full disclosures of two Well-known types of privacy devices are furnished respectively by R. W. Chesnut et al., Patent 1,829,783 and D. Mitchell et al., Patent 1,981,114.

An unauthorizedperson listening-in at this time on the transmission from the west terminal, therefore, would hear only the noise or spurious speech emanating from source 2| and, further, if the system included other types of privacy arrangements in addition to that invented by applicant, it would of course be necessary to overcome the masking efiects thereof before the bare spurious speech in the form transmitted would be heard. In order that the authorized talker at-subscribersstation E will not be subjected to this noise or spurious speech, circuit EB is held open at control point 26 by suitable means, for example by a short circuit applied through operation of a switching relay. The switching relay, in turn, may be controlled through amplifier-detector 21 which is operated by the control current f1 emanating from source 22 and passed 44 to the amplifier-detector through filter 28.

Let it be assumed now that the authorized talker at subscribers station W speaks. A part of the resulting voice currents passes over line 30 through amplifier-detector I8 and causes the opthe short circuit at control point H and enable" circuit EA so far as the output from subscribers station W is concerned, (b) open line 3| at control point til thereby disconnecting source 2| from the circuit, open line 50 at control point 42 thereby removing the control current f1 from the circuit and ((1) open circuit WB at control point 43.

Removal from the circuit of control current f1 results in enabling circuit EB at control point 25, the circuit previously being held open at this I point, as pointed out above, by certain opera- :tions resulting from the passage of the control gi'irrent ,fi through filter 28 to amplifier-detector 6|) The useful speech from subscribers station W passes, therefore, over circuit EA to radio transmitter H, is radiated therefrom, picked up by radio receiver I3 and passed over circuit EB to subscribers station E. A portion of. the speech energy is by-passed through amplifier-detector 46 and 'causes, through operation of a switching relay or other suitable means, disabling of line it at control point 87. This prevents undesirable operation of the transmitting controls of the east terminal by echoes.

When the regular talker at station W pauses in his speech, the circuits immediately return to the condition illustrated and spurious speech or noise is again applied to the line. An unauthorized eration of switching relays which (a) remove aiaaeoo listener therefore upon gaining access to the transmission channel would hear a practically continuous jargon" comprised partly of useful speech and partly of noise or spurious speech.

Interpretation of the message by recognization of the lwtimate portions and mental masking of the spurious portions would be very difiluclt there-- by rendering the message highly secret so far as unauthorized listeners are concerned. Interpretation will, of course, be even more diificult if the arrangement be used in conjunction with other types of privacy arrangements as the masking effects thereof must first be overcome. As pointed out above, however, the noise or spurious speech is not allowed to reach the distant authorized talker since control point 26 is held open" by the tone f1; only the actual message reaches his station. Reception of the message by the authorized person is not interfered with, therefore.

The circuits for east-west transmission function in amanne'r similar to that described above. For example, when the regular talker at station E is not speaking, noise or spurious speech from source is applied to circuit WA and transmitted thereover to the east-west radio transmitter l6. Control current f2 produced by oscillator 52 is also applied to circuit WA after being passed by filter 53.

An unauthorized person listening-in at this time on the transmission from the east terminal to the west terminal would hear only the noise or spurious speech emanating from source 5|. In order that the regular talker at station W will not be subjected to this noise, circuit W3 is held open at control point 56 by suitable means, for example, by a short circuit applied through operation of a switching relay. The switching relay in turn may be controlled through amplifier-detector 57 which is operated by the control current is emanating from source 52 and passed to the amplifier-detector through filter 58.

.Upon the regular talker at station E speaking, the circuits operate in a manner similar to that described above, circuit WA being enabled" due to the action of amplifier-detector 59 for speech transmission at control point 6|, the spurious speech or noise from source 5| and the control current f2 being removed, and circuit WB being enabled at control point 56 (through removal of control current f2) for transmission of the speech to the regular talker at station W. A portion of the speech energy is shunted through amplifier-detector 62 and causes line 30 to be disabled at control point 63. Circuit EE is opened or disabled at control point 56 at this time also.

Upon the regular talker at station E ceasing to speak the circuits immediately return to the condition illustrated and the spurious speech or noise is again applied to the line. An unauthorized listener gaining access to the east-west channel therefore hears only the same meaningless jargon as is heard by unauthorized listeners on the westeast channel. By the action of the means controlled through amplifier-detector 51! however, the regular talker at station W receives only the useful speech and is not subjected to the noise or spurious speech.

Referring now to Fig. 2 a more detailed showing is furnished of the apparatus at one terminal,

forexample the west terminal of the system illustrated in Fig. 1. Subscribers station 8| is coupled to the transmitting line 82 and the receiving line 83 by transformer 86, network till serving to provide a proper balance in the wellknown manner. Speech energy emanating from which may then be transmitted over line 82.

A suitable source of constant frequency I88,

' which may be, for examples. vacuum tube oscillater, is provided the output of which is connected to line 82 through wave filter I2I. This filter may be of the type well known in the art and described, for example, by Patent 1,227,113, issued May 22, 1917, to G. A. Campbell and is so designed that it will pass only a small band of frequencies including the desired control current of a frequency f1.

Radio receiver I22 is associated with receiving line 83 which line, in turn, is inductively coupled .to subscribers set 8| through transformer 88,

amplifiers I24 and I25 being included in theline. A path in shunt to line 83 is provided through filter I23 which is designed to pass only a small frequency band including the control frequency f2 transmittedfrom the distant terminal. The current passed by filter I23 is rectified by amplifier-detector I28 and controls operation of relay I21. A second path in shunt to line 83 pggses through amplifier-detector I3! and relay Privacy devices I34 (transmitting) and 135 (receiving) may be of any of the well-known types, for example, the type disclosed in Patent 1,981,114 issued to applicant and another.

For purposes of further description the actual operation of the circuits illustrated in Fig. 2 will now be described in detail. The condition illustrated is that existing when the regular talker is not speaking. Line 82 is disabled, so far as the output from subscribers station 8I is concerned,- by the short circuit path completed through the break contact of relay 96. Spurious speech or noise from record I88 is picked up by unit I81 and applied to line 82 over a circuit completed through the break contact of relay 91 While the control current f1 produced by source I88 is applied to line 82 over a circuit completed through the two break contacts of relay 98, filter I2I being included in this circuit. The spurious speech or noise and the control current ii are therefore supplied over line 82 to radio transmitter 88 and radiated thereby to the distant radio receiver.

The receiving circuits of the distant terminal, which are not illustrated, are the same as the receiving circuits of the terminal illustrated in Fig. 2 and it is believed that the effect of the currents transmitted by radio transmitter 88 upon the distant terminal circuits may be better understood if we transfer our attention for the present to the receiving channel of Fig. 2 and assume that spurious speech or noise is being received thereby from the radio transmitter of the distant terminal (corresponding to radio transmitter 88) as well as a control current i; (corresponding to control current f1). Control current f2 received by radio receiver I22 from the distant terminal is passed by filter I23, which is designed to pass only this particular frequency, and after rectification by amplifier-detector I28, is effective to cause operation of relay I21. At this time, therefore, receiving circuit 83 is disabled by a short-circuiting path completed through the make contact of relay I21 and the spurious speech or noise is not transmitted to the subscriber's station 8|.

Returning now to the transmitting channel of the terminal it will be understood from the above that the spurious speech or noise radiated from transmitter 88 is picked up by the radio receiver of the distant terminal and that the control current I1 is also picked up by the receiver and is effective to disable the receiving channel so that the distant talker is not subjected to the spurious speech or noise.

Assuming now that the regular talker at station 8| starts to speak. A portion of the speech energy passes over line 9| and after rectification by amplifier-detector 92, is effective to cause operation of

relays

98, 91, 98 and I8I.

Operation of relay 98 interrupts the shortcircuiting path previously established through its break contact, line 82 now being enabled" therefore for transmission of the speech currents from station M to radio transmitter 88.

Operation of relay 91 interrupts at its break contact the path which previously connected the 3 output of phonograph pick-up I81 to line 82 and the spurious speech or noise produced by record I88 is now removed from the transmitting channel.

Operation of relay 98 interrupts at its break contacts the path previously connecting the output of oscillator I88 to line 82, control current fr also being now removed from the line.

Operation of relay I8I completes a short-circuiting path through its make contact which disables line 83 while the regular talker at station BI is speaking thereby assuring against interruption and confusion should the distant talker also start speaking as well as preventing echoes and singing.

During the actual time the regular talker is speaking, therefore, useful speech currents alone are radiated by radio transmitter 88 and received by the distant terminal radio receiver (corresponding to radio receiver I22). In view of. the fact that the control current i1 is not transmitted at this time the receiving line (corresponding to line 83) at the distant terminal is enabled" permitting reception of the useful speech by the distant talker. A portion of the received speech energy is passed through a shunt path and causes operation of a relay efiective to disable a portion of the transmitting channel of the distant terminal thereby preventing undesirable operation of the transmitting controls by echoes. believed these operations of the receiving circuits at the distant terminal may best be visualized if we again turn forthe moment to consideration of the receiving channel of the terminal illustrated in Fig. 2 and assume that useful speech currents are being received from the distant radio transmitter, the spurious speech or noise and the control current 7: not being transmitted.

The absence of the control current I: results of course in relay I21 falling back to unoperated position (this relay was previously held operated by control current I: after rectification by amplitier-detector I215) thereby removing the shortcircuiting patli previously completed through its make contact and enabling line 83 for transmission of the received speech to subscriber's sta- It is tlon M. A portion of the speech energy is shunted through amplifier-detector I3I and, after rectification thereby, is effective to cause operation of relay. I32. Operation of relay I32 com- 4 pletes through its make contact a short-circuiting .IIID

path which disables line 6i in order to prevent undesirable operation of the transmitting controls associated with station 6| by echoes.

Immediately upon the regular talker at station 6| ceasing to speak, relays 96, 91, 98 andIfiI drop back to the unoperated positions illustrated and the spurious speech or noise and the control current 11 are again applied to line 62 and radiated by radio transmitter 68. Therefore in accordance with the features of the system, operating as described above, the regular authorized talkers receive only the useful speech and,,are subjected to.

no noise or spurious speech. Unauthorized persons listening-in on the channel, however, hear a meaningless, substantially continuous jargon comprising the useful speech interspaced, during what would otherwise be silent periods, with the spurious speech or noise. As a result intelligible reception of the message by unauthorized persons is .very difiicult and unpleasant.

Referring now to Fig. 3 a modification of a part of the system is schematically illustrated. In ac? cordance with this modification the outputs of a plurality of oscillators are combined to produce a complex frequency wave whichisused as the control current. The frequency of this complex 'wave, further, is periodically changed in order possible only that part of the apparatus used in transmission from one station and reception at the other station is shown and, further, only parts of the system directly associated with the control current production and utilization have been shown. The system'in so far as the general arrangement is concerned is the same as that described above and it is not considered necessary to again describe all the features" of the present modification which are common to both systems.

In the condition illustrated in Fig. 3, that is when the regular talker at substation I62 is not speaking, line I QI which connects the substation to radio transmitter I63 is disabled at control point I66. Noise or spurious speech from source I61, a phonograp'li for example, is applied over line I416 to line MI and conveyed thereover to radio transmitter I43. It will be noticed that .in accordance with the present modification, the single control current source of the previous arqzm rangement has been replaced by a plurality of oscillators I6I, I62, I63, I66 and I61. Associated with each oscillator is one of a series of relays I1 I I12, I13, I16 and I11. Certain of the make contacts of the relays are connected to line MI by line I16 while the break contacts are connected to line MI by line IBI.

The operating circuits of relays I1I, I12, I13, I16 and I11 are controlled through contacts of a telegraph tape transmitter I65 which4nay be of the type well known in the telegraph art, for example, the general arrangement may be as described in E. F. Watson Patent 2,055,567. The tape I 86, which is punched to control the position of the contact fingers, is periodically stepped ahead by drive wheel I81 which is operated in araaeeo turn by stepping magnet I66. Motor I69 drives shaft I96, discs I9I, I32 and I93 being carried by the shaft.

In the condition illustrated in the drawings, it will be noticed that relays I12 and I11 are in operated position while relays I1I, I13 and I16 are in non-operated position. Relays I12 and I11 are held in the operated position illustrated over locking paths traced from battery I36, the respective right-hand windings and right make contacts of the two relays, line I95, disc I93 to ground I96. The outputs of oscillators I62 (f2) and I61 (is) are, therefore, applied over line I18 to line Il at this time and conducted thereover to radio transmitter I63. I

At the time the regular talker at station I62 is not speaking; therefore, and when tape transmitter I85 is in the position illustrated, radio.

frequency f3, filter 2I8 passes frequencyfr and filter 22I passes frequency is. The output of each amplifier-detector is connected to an armatu:e of a respective one of five

relays 23L

232, 233, 236 and 231.

A tape transmitter 24 is provided at the east station which is similar in arrangement and 'operation to tape transmitter I65 at the west station. Tape 262 is driven by drive wheel 2% which is driven, in turn, by stepping magnet 265. 6 Motor 266 drives shaft 261,

discs

268, 269 and 250 being carried on the shaft.

The two motors I89 and 266, are provided with suitable control means whereby they are operated synchronously and the arrangement of the two tape transmitters is such that when relays I12 and I11 at the west station are operated, relays 232 and 231 at the east station will be operated and when relays I1I, I13 and I16 at the west station are operated, relays 23I, 233 and 236 at the east station will be operated.

As previously stated, noise or spurious speech is being sent out from radio transmitter I43 as well as a control current comprising frequencies in and is. This control current is picked up by radio. receiver 26I, the spurious speech also being received of course. time at control point 263 thereby preventing transmission of the spurious speech to substation The control frequency fz is passed by filter 2I6 to amplifier-detector 223 but as relay 232 is in operated condition a path is not provided over which the output of the amplifier-detector may be transmitted to cause operation of the control device at point 263 to closed or enabled posi-- tion. The control frequency is is passed by filter 22I to amplifier-detector 226 but as relay 231 is operated, here again no path is provided for operation of the control device at point 263.

Assuming now that the regular talker at station I 52 starts speaking, a part of the speech energy passes through amplifier-detector 261 and operates control devices effective to (a) close or enable line What control point I66 thereby closing a path for conveyance of the useful speech from station I62 to radio transmitter I63, (b)

f1, f2 and f4. Frequency I1 is passed by filter 2I3 to amplifier-detector 222, frequency 1; is passed by filter 2I1 to amplifier-detector 226 and freguency I4 is passed by filter 2I8 to amplifier-detector 221 and as

relays

23I, 233 and 236 are in non-operated position, paths are provided through the respective break contacts to line 216 over which the outputs from the respective ampli tier-detectors are applied to the control device at point 263. These output currents are effective to cause operation of the control device to close or enable line 262 at control point 263. The line is enabled, therefore, for transmission of the useful speech to substation 266.

Immediately upon the talker at station I42 ceasing to speak the various elements of the circuit revert to the condition illustrated, noise or spurious speech from source I41 is reapplied to the line and the control current comprising frequencies f1, f3 and I4 is replaced by that comprising'frequencies f2 and fswhereby line 262 is again opened or disabled at control point 263.

Assuming now that shaft I90 has been rotated sufficiently to bring the conductive portion of disc *I9I into contact with spring 211, an operating circuit for stepping magnet I88 is momen tarily established traced from battery 218, winding of stepping magnet I88, spring 211, conductive segment of disc 19!, shaft I90 to ground I96, tape I86 beingstepped ahead one space by operation of stepping magnet I88. The tape is now in position to allow the first, third and fourth (counting from the left) contact fingers of tape transmitter I85 to make contact with conductive bar 25I, the other two contact fingers being held out of contact with the bar by imperforate portions of tape. A moment after this presetting of the tape transmitter, the insulated segment of disc I93 is brought into contact with spring 252 whereupon the holding circuits of relays I12 and I11 (to ground I96) referred to above are inter-- rupted and the relays drop back to normal,

'unoperated position. Simultaneously, the conductive segment of disc I92 is brought into contactwithspring 253, whereupon an operating path is. completed for relay I16, traced from battery I94, left winding of relay I 16, fourth contact finger from left of tape transmitter I 85, contact bar 25I, spring 253, conductive segment of disc I92, shaft I90 to ground I96. Similar operating paths for relays I13 and HI are completed through the third contact finger from the left and the first contact finger from the left respectively of the tape transmitter, relays I16, I13 and HI operating at this time therefore. Before the conductive segment of disc I92 breaks contact with spring 253, the conductive portion of disc I93 has again been brought into contact with spring 252 thereby establishing a holding circuit for relays III, I13 and I16 (over conductor I95 to ground I96). At this same time, relays 23I, 233 and 236 at the east station are operated due to the operation of tape transmitter 24I in synchronism with tape transmitter I85.

. The operation of the arrangement is similar whichever group of relays is operated; the object of periodically changing the frequency combinations is, as pointed out above, to defeat the possible attempts of unscrupulous persons to remove the undesired spurious speech or noise by utilizing for this purpose an amplifier-detector operating on the control current. Obviously this would be diificult if a control current be utilized which is periodically changed with respect to its frequency characteristics. Thus it 'would often occur that the eavesdropper would make a-mistake and use a wrong control current to operate his relay corresponding to 263. In this case he would hear spurious speech.

It will be appreciated that the showing of Fig. 3 is schematic and that only sufllcient parts of the system necessary to illustrate the operation of the control current arrangement have been shown. The system, so far as the general features are concerned should be, preferably, as shown in Figs. 1 and 2 and as described in detail above.

In the instance of the arrangements described above, the material prerecorded on the phonograph record and impressed on the channel during the silent or idle intervals has been either noise or spurious speech, the transmission of which 'has served no useful purpose except in the attainment of secrecy. However, in accordance with the modification of the invention illustrated in Fig. 4, useful speech is prerecordedon the record and is impressed onand transmitted over the channel during the silent intervals, means being provided at the distant terminal for recording the speech as received. This modification has one particularly advantageous feature, namely, that the material transmitted during the idle intervals is not wasted after accomplishing the, purpose of attaining secrecy but is recorded for future reference. The prerecorded matter may be, for example, a transcription of an address by some notable which it may be desired to transmit to some distant point 1 during the next few hours after delivery, there to be recorded for various purposes such as rebroadcasting.

The arrangement disclosed in Fig. 4, is, in general, similar to that disclosed in Fig. 2 except for the changes occasioned by the fact that pre recorded useful speech is to be transmitted instead of spurious speech or noise and the fact that means'for recording the transmitted prerecorded speech have been provided. Only one terminal of the system, for example, the west terminal, is disclosed; it will be understood that the distant terminal, that is, the east terminal, is of similar arrangement.

Subscriber's station 28I is coupled to the transmitting line 282 and the receiving line 283 by transformer 286, network 281 serving to provide a proper balance in the well-known manner. Speech energy;emanating from subscribers station 28I follows two parallel paths, one path 282 leading through amplifiers 284 and 285 to radio transmitter 288 and the other path 29I leading through amplifier-detector 292 to

relays

293, 296, 291 and 298. Privacy device 299 may be of any of the well-known types, for example, the type disclosed in Patent 1,981,114 issued to applicant and another. a

A phonograph turntable 32I is adapted to support a record 322 of prerecorded useful shaft 331.

'gagement with the periphery of cam 333 by spring Upon operation of relay 291, however, arm 331 is rotated about pivot 338 in a counter-clockwise direction, bringing the end of the arm into contact with the shoulder on cam 336 and preventing further rotation of the cam, thereby.

stopping turntable 321. This resilient driving arrangement is similar, in general, to that disclosed in Patent 2,055,567, issued September 29, 1936 to E. F. Watson.

A suitable source of constant frequency 362, a vacuum tube oscillator for example, is provided, the output of which is connected to transmitting line 282 through wave filter 3413. Filter 363 is so designed that it will pass only a narrow band of frequencies including the desired control current of a frequency f1.

Radio receiver 366 is associated with receiving line 233, which line in turn is inductively coupled to subscribers station 281 by transformer 238, amplifiers 364i and 3 35 and receiving privacy device 343 being included in the line. A path 361 in shunt to line 233 leads to filter 311 and amplifier-detector 3116. Path 350 leads to filter 338 and is also in shunt to line 283. Current passed by filter 3G3 traverses recording unit 312 which is operatively associated with recording blank 313. Current passed by filter 311, which is designed to pass only anarrow frequency band including control current of frequency f2 transmitted from the distant terminal, is rectified by amplifier-detector 316 and operates relays 311 and 313. A third path 331, in shunt to receiving line 233 passes through amplifier-detector 382 and relay 333.

Motor 333 is provided, driving shaft 381, which is resiliently coupled to shaft 383 by clutch 39L Shaft 333 is geared to drive shaft 392 of the recorder turntable 333. Stop cam 395 which is carried by shaft 383, is provided with a peripheral shoulder. Stop arm 336, which is rotatably' mounted on pinion 391, is normally urged into engagement with the periphery of stop cam 395 by spring 398. Relay 311 when operated, however, rotates stop arm 396 about pinion 391 in a clockwise direction out of engagement with cam 395. This resilient driving arrangement of the recorder is similar to that of the reproducer except for the fact that operation of the control relay 311 permits operation of the recorder while operation of the reproducer drive control relay 231 stops the reproducer. V

For purposes of further description the actual operation of the circuits illustrated in Fig. 1 will now be described in detail. The condition illustrated is that existing when the regular talker at subscribers stationfitl is'not speaking. Transmitting line 282 is disabled, so far as the output from the subscribers station ,is concerned,

7 by the short-circuiting path completed through the break contact of relay 293. The control current f1 produced by source 342 is applied to line 231 being in normal unoperated condition and stop arm 331 being heldout of the path of stop cam 336 by spring 34, allowing rotation of shafts 33! and 333 and, therefore, rotation of reproducer turntable 321. The control current f1 and the prerecorded useful speech from record 322 are thereforc applied to line 282, conducted thereover to radio transmitter 288 and radiated thereby to the distant terminal.

The receiving circuits of the distant terminal (not illustrated) are the same as the receiving circuits of the terminal illustrated in Fig. 4 and it is believed that the effect of the currents transmitted by radio transmitter 283 on the receiving circuits of the distant terminal may be better visualized if we turn our attention for the moment to the receiving channel of Fig. t and assume that prerecorded useful speech is being received by radio receiver 365 from the radio transmitter of the distant terminal (corresponding to radio transmitter 288) as well as the control current f2 (corresponding to control current ii).

The control current I: received by radio receiver 336 passes over line 361 and through filter 31E, which is designed to pass only a narrow frequency band including frequency is, and, after rectification by amplifier-detector 315, causes the operation of relays 313 and 311. Operation of relay 318 completes through its make contact, a short-circuiting path for receiving line 233 which is therefore disabled at this point, thereby preventing transmission of the received current to subscribers station 28!. Operation of relay 311 rotates stop arm 393 out of the path of stop cam 395, thereby allowing rotation of recorder turntable 393. The received prerecorded useful speech passes over line 330 through filter 3G8 and actuates recorder unit 312. The received prerecorded useful speech is therefore recorded on record 313.

If desired the reproducer may be operated at a low speed so as to cause the output therefrom to occupy a smaller frequency range. In this case the recorder will also be operated at a low speed and filter 333 may be designed to pass only this small frequency range excluding noise in'the unused portion of the frequency range. This will give a. favorable signal-to-noise ratio and will tend to improve the quality of the recorded speech when selective fadlng is present. When the recorded speech is played back the record will, of course, be rotated at normal speed.

Returning now to consideration of the transmitting channel of the terminal illustrated in Fig. 4, it will be understood from the above that the prerecorded useful speech transmitted from record 322 is received and recorded at the distant terminal and that the control current i is also received at the distant terminal, disabling a part of the receiving channel so that the regular talker at the distant terminal is not subjected to the prerecorded speech.

Let us assume-now that the regular talker at subscribers station 231 starts to speak. A portion of the resulting speech energy passes over line 291 and. after rectification by amplifierdetector 292, causes operation of

relays

233, 296, 291 and 298.

Operation of relay 293 interrupts at its break contact the short-circuiting path previously applied to line 28", thereby enabling the line for transmission of speech energy from subscribers station 28l to radio transmitter 288.

Operation of relay 296 interrupts, at its break co tact, the path over which control current 11 was previously applied to line 232, the control current therefore beingremoved from the transmitting channel.

Operation of relay 291 causes rotation of stop arm 33'! about its pivot 338 in a counter-clockwise direction, bringing the end thereof into engagement with the peripheral shoulder on stop cam 338, whereby further rotation of the cam as well as shaft 33l is prevented. The reprocam and of shafts 388 and 392.

ducer turntable 32! is stopped, therefore, and the prerecorded useful speech of record 322 is removed from the transmitting channel.

Operation of relay 298 completes through its make contact a short-circuiting path which "disables" receiving line 283 at this point, thereby assuring against interruption and confusion should the distant talker also start speaking as well as preventing echoes and singing.

During the time the regular talker at subscribers station 28l is actually speaking, therefore, only currents originated by his speech are radiated by radio transmitter 288 and received by the radio receiver of the distant terminal (corresponding to radio receiver 348). In view of the removal from the channel of the control current f1, the receiving line at the distant terminal (corresponding to line 283) is enabled for transmission of the received useful speech to the distant talker and the recorder turntable (corresponding to turntable 393) is stopped. A portion of the received speech energyis passed through a shunt path and causes operation of a relay eilective to disable a portionof the transmitting channel of the distant terminal, thereby preventing undesirable operation of the transmitting controls by echoes.

It is believed that .these operations of the receiving circuits may best be visualized if we again turn -for the moment to consideration oi the receiving channel of the terminal illustrated in Fig. 4 and assume that currents originated by speech of the regular talker at the distant terminal are being received by radio receiver 346 from the radio transmitter of the distant termine], the control current fa and the prerecorded useful speech not being transmitted.

The absence of the control current I: results, of course, in deenergization of relays 311 and 318, these relays previously having been held operated by the control current I; after rectification by amplifier-detector 316.

Relay 318 by dropping back to unoperated position removes the short-circuit path previously applied through its make ,contact to line 283, thereby enabling the line for transmission of the received useful speech to subscriber's station 28L A portion of the speech energy is shunted through path 38| and, after rectification by amplifier-detector 382, causes ope tion of relay 383. Relay 383 upon operation com letes through its make contact a short-circuiting path which disables" transmitting line 282 in orderto prevent undesirable, operation of the transmitting controls by echoes.

Relay 3'" upon deenergization allows stop arm 398 to be rotated in a counter-clockwise direction by spring 398, bringing the end of the arm into engagement with the peripheral shoulder on stop cam 395 and preventing further rotation of the The recorder turntable 393 is therefore stopped, thereby preventing the recording on record 313 of any portion of the incoming speech energy which may chance to be passed by filter 348.

Immediately upon the regular talker at station 28l' ceasing to speak, relays 293, 288, 281 and i in close succession.

and 298 drop back to the unoperated position illustrated, the reproducer is set into operation to impress upon the transmitting channel prerecorded useful speech from record 322, the control current i1 is also reapplied to the transmitting channel, and the recorder at the distant terminal is set into operation. In accordance with this modification of the invention, therefore, unauthorized persons listening-in on the channel will hear a substantially continuous train of sound comprising the regular talkers speech interspaced, during what would otherwise be the silent" periods with the unrelated prerecorded speech. The result will be, to say the least, confusing to the unauthorized listener and in many cases he will be unable to mentally unscramble the two sections of speech in order to intelligently interpret either. ever, receive only the regular talkers speech. The prerecorded speech, in addition to serving as an aid to attaining privacy, is recorded as received at the distant terminal for future reference. As a matter of fact, the arrangement has commercial merit entirely apart from a privacy standpoint; it is commercially valuable also from the standpoint of attainment of a more continuous revenue paying use of the costly apparatus which is involved in long' telephone circuits such as the transoceanic radio systems.

While certain specific embodiments of the invention have been selected for detailed description, the invention is not, of course, limited in its application to the embodiments described. These embodiments should be taken as illustrative of the invention and not as restrictive thereof.

What is claimed is:

1. In a speech transmission system including a transmitter, a receiver, and a channel between said transmitter and said receiver, the method of maintaining secrecy during the transmission of speech from said transmitter to said receiver which comprises impressing noise on the transmission channel during each pause in the speech whereby a practically continuous train of sounds is produced on the channel comprising periods of speech and-periods of noise following one another 2. In a speech transmission-system including a transmitter, a receiver, and a channel between said transmitter and said receiver, the method of maintaining secrecy during the transmission of speech from. said transmitter to said receiver which comprises impressing. spurious speech on the transmission channel during each pause in the useful speech whereby a practically continuous jargon is produced on the channel comprising periods of useful speech and periods of spurious speech following one another in close succession.

3. In a speech transmission system, a source of useful speech waves, a source of spurious speech waves, a transmission channel, means for applying spurious speech waves to said channel, means for applying useful speech waves to said channel, and means controlled by application of said useful speech waves to said channel for removing said spurious speech waves from said channel.

4. In a speech transmission system, a source of speech waves, a source of artificial noise currents, a transmission channel, means for applying noise currents to said channel, means for applying speech waves to said channel, and means controlled by application of said speech waves to said channel for removing said noise currents from said channel.

5. In a signaling system, a transmission channel, a source of spurious signal waves, a source of useful signal waves, means at the sending end of said channel for applying spurious signal waves to said channel, additional means at the sending end of the channel for applying useful signal waves to said channel, said first-mentioned means being renderedineffective to apply said spurious signal waves upon application of said useful signal waves by operation of the second-mentioned means, and means at the receiving end of the channel controlled from the sending end thereof for disabling a portion of said channel at the receiving end during application of the spurious signal Wavesat the sending end. v

6. In a speech transmission system, a transmission channel, a phonograph record having spurious speech recorded thereon, means for driving said record, a pick-up device in operative relationship to said record, means for connecting the output of said pick-up device to said transmission channel whereby spurious speech waves are applied to said channel, means for applying useful speechwaves to said transmission channel, and means controlled by application of said useful speech waves to said transmission channel for disconnecting the output of said pick-up device from said channel.

7. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, means at the west terminal for disabling said line at the west terminal at a point intermediate said radio receiver and said telephone receiver, means at the east terminal for applying spurious speech waves to said line at the east terminal and for causing operation of said disabling means at the west terminal, said means at the east terminal being rendered ineffective to apply spurious speech waves or to cause operation of said disabling means upon application of useful speech waves to said line at the east terminal.

8. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a control device at the west terminal operable by current of a predetermined frequency to disable said line at a point intermediate said radio receiver and said telephone receiver, a source of spurious speech and a source of current of said predetermined frequency at the east terminal, means for applying spurious speech waves and current from said respective sources to said line at the east terminal for conductance to the radio transmitter and transmission thereby to the radio receiver at the west terminal, said means being rendered ineffective for application of the spurious speech waves and said current to said line upon application to said line of useful speech waves.

9. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and araaeco said radio transmitter at the east terminal, a

radio receiver, a telephone receiver and a line.

for connecting said radio receiver and said telephone receiver at the west terminal, a control device normally in a, condition disabling said line at the west terminal at a point intermediate said radio receiver and said telephone receiver, said control/ device being operable by currents of a predetermined combination of frequencies to enable said line at said intermediate point, a source of spurious speech at the east terminal, a source of control current'of said predetermined combination of frequencies at the east terminal, means for applying spurious speech waves to said line at the east terminal for conductance thereover to said radio transmitter and transmission thereby to said radio receiver, additional means" for applying control current of said predetermined combination of frequencies to said line at the east terminal for conductance thereover to the radio transmitter andtransmission thereby to said radio receiver, said first-mentioned means being rendered inefiective and said second-mentioned means being rendered effective upon application of useful speech waves to said line at the east terminal for conductance thereover'to said radio transmitter and transmission thereby to said radio receiver.

10. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a control device normally in a condition disabling said line at the west terminal at a point intermediate said radio receiver and said telephone receiver, said control device being operable by current of certain predetermined combinations of frequencies to enable said line at said intermediate point, a pinrality of groups of input circuits associated with said control device onlyone of which groups is I connected to said device at one time, a source of spurious speech at the east terminal, a source of control current of certain predetermined combinations of frequencies at the east terminal, means for applying spurious speech waves to said line at the east terminal for conductance there- 0 over to said radio transmitter and transmission thereby to said radio receiver, additional means for applying control current of a predetermined combination of frequencies to said line at the east terminal for conductance thereover to the radio transmitter and transmission thereby to said radio receiver, said first-mentioned means being rendered ineffective and said second-mentioned means being rendered effective upon application of useful speech waves to said line .at the east terminal for conductance thereover to said radio transmitter and transmission thereby to said radio receiver, additional means at the east terminal for periodically changing the combination of frequencies applied to the line at the east terminal, and means at the west terminal for periodically changing the group 'of input circuits connected to said control device whereby it will be operable by the particular combination of frequencies applied at any particular instant to the line at the east terminal.

11. In a speech transmission system including a transmitter, a receiver, and a channel between said transmitter and receiver, the method of i secrecy during the transmission of speech as uttered by a talker from said transmitter to said receiver which comprises impressing speech unrelated to said uttered speech on the channel during each pause in the uttered speech whereby a substantially continuous train of sounds is produced on the channel comprising periods of speech as uttered and periods of unrelated speech following one another in close succession.

12. In a speech transmission'system including a transmitter, a receiyer, and a channel between said transmitter and receiver, the method of maintaining secrecy during the transmission of speech as uttered by a talker from said transmitter to said receiver which comprises impressing prerecorded speech unrelated to said uttered speech on the channel during each pause in the uttered speech whereby a substantiallycontinuous train of sounds is produced on the channel comprising periods of speech as uttered and periods of unrelated speech following one another in close succession. l

13. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver, at the west terminal, a source of prerecorded speech at the east terminal, means for reproducing said prerecorded speech and for applying the resulting speech waves'to said line at the east terminal for conductance thereover to the radio transmitter and radiation thereby to the radio receiver at the west terminal, a recorder at the west terminal forrecording said speech waves, means at the. east terminal for applying speech waves originating at said telephone transmitter to said line at the east terminal and control means effective upon application of said last-mentioned speech waves to said last-mentioned line to rendersaid reproducing means and said recorder inoperative.

14. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said 'radio receiver and said telephone receiver at the west terminal, a source of prere corded speech at the east terminal, means for i reproducing said prerecorded speech and for applying the resulting speech waves to said line at the east terminal for conductance thereover to the radio transmitter and radiation thereby to the radio receiver at the west terminal, a recorder at the west terminal for recording said speech waves, means at the east terminal for\applying speech waves originating at said telephone transmitter to said line at the east terminal, and means for controlling the operation of said reproducing means and said recorder, said controlling means allowing operation of said reproducing means and said recorder during periods when said telephone transmitter is not operating to originate speech waves and preventing operation of said reproducing means and said recorder during periods when said telephone transmitter-is operating to originate speech waves.

, .15. In a speech transmission system including a transmitter, a receiver, and-a channel between said transmitter and said receiver, a plurality of privacy devices of diiferent types operating in the channel comprising periods of speech and periodsof noise following one another in close succession.

16. In a speech transmission system including o a transmitter, a receiver, and a channel between said transmitter and said receiver, a plurality of privacy devices of different types operating in conjunction, one of which types is effective to impress spurious speech on the transmission channel during each pause in the speech whereby a practically continuous train of sounds is produced on the channel comprising periods of speech and periods of spurious speech following one another in close succession.

17. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for conmeeting said radio receiver and said telephone receiver at the west terminal, a control device at the west terminal operable by current of a predetermined frequency to enable said line at a point intermediate said radio receiver and said telephone receiver, a source of spurious speech at the east terminal, a plurality of sources of currents of different frequencies at the east terminal, means for applying spurious speech waves from said source of spurious speech and current from one of said plurality of current sources to said line at the east terminal for conductance to the radio transmitter and transmission thereby to the radio receiver at the west terminal, said applied current being of a frequency different from that effective to operate said control device, said means being rendered ineffective for application of the spurious speech waves and said current to said line upon application to said line of useful speech waves.

' 18. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for Y connecting said telephone transmitter and, said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a control device at the west terminal operable by current of a predetermined frequency to enable said line at a point intermediate said radio receiver and said telephone receiver, asource of spurious speech at the east terminal, a plurality of sources of currents of different frequencies at the east terminal, means for applying spurious speech waves from said source of spurious speech and current from one of said plurality of current sources to said line at the east terminal for conductance to the radio transmitter. and transmission thereby to the radio receiver at the west terminal,.said applied current being of afrequency different from that effective to operate said control device, and means effective upon application of useful speech to said line for removing the spurious speech and the applied current from the line and for applying thereto current from another of said plurality of current sources, said last current being of the frequency effective to operate said control device.

19. In a speech transmission system including a transmitter, a receiver, and a channel between said transmitter and said receiver, a plurality of privacy devices of difierent types operating in conjunction, one of which types is effective to connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a source of prerecorded speech at the east terminal, means for reproducing said prerecorded speech, said reproducing means being operated at a comparatively low speed causing the output therefrom to occupy a comparatively small frequency range, means for applying the resulting speech waves to said line at the east terminal for conductance there'- over to the radio transmitter and radiation thereby to the radio receiver at the west terminal, a recorder at the west terminal for recording said speech waves, said recorder being operated at a speed comparable to the speed at which the re producer is operated, means at the east terminal for applying speech waves originating at said telephone transmitter to said line at the east terminal and control means effective-upon application oi said last-mentioned speech waves to said lastmentioned line to render said reproducing means and said recorder inoperative.

21. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a control device at the west terminal operable by current of a predetermined frequency to enable said line at a point intermediate said radio receiver and said telephone receiver, a source of spurious speech at the east terminal, a plurality of sources of currents of difierent frequencies at the east terminal, an equal number of filters at the west terminal, a relay associated with each of said sources at the east terminal, a relay associated with each of said filters at the west terminal, means at the araaaec ductance to the radio transmitter and transmission thereby to said radio receiver at the west terminal, said last-mentioned means being rendered ineffective upon application to said line of useful speech waves.

22. In a speech transmission system including an east terminal and a west terminal, a telephone transmitter, a radio transmitter and a line for connecting said telephone transmitter and said radio transmitter at the east terminal, a radio receiver, a telephone receiver and a line for connecting said radio receiver and said telephone receiver at the west terminal, a control device at the west terminal operable by current of apredetermined frequency to enable said line at a point intermediate said radio receiver and said telephone receiver, a source of spurious speech at the east terminal, a plurality of sources of currents of diirerent frequencies at the east terminal, an equal number of filters at the west terminal, a relay associated with each of said sources at the east terminal, a relay associated with each of said filters at the west terminal, means at the east terminal for-causing the periodic operation of said relays in groups, one group including more than one of the relays and the other group including the rest of the relays, said means comprising a tape controlled contact mechanism, means for presetting the tape thereof in contact controlling position, means effective after said presetting operation for completing the operating circuits of certain of saidrelays through predetermined combinations of the contacts of the tape controlled mechanism, and means becoming effective after completion of said operating circuits and before interruption thereof for establishing holding circuits for those relays operated through said previously mentioned means, means at the west terminal for causing operation of said relays at said west terminal in groups corresponding to the group operation of the relays at the 'east terminal, .means for applying spurious speech waves and current from one of said plurality of current sources to said line at the east terminal for conductance to the radio transmitter and transmission thereby to said radio receiver at the west terminal, said last-mentioned means being rendered ineffective upon application to said line of useful speech waves.

23. In a speech transmission system including a transmitter, a receiver, and a channel between said transmitter and said receiver, a plurality of privacy devices of difierent types operating in con- 'junction, one of which types is effective to impress spurious speech on the transmission channel during each pause in the speech whereby a practically continuous train of sounds is produced on the channel comprising periods of useful speech and periods of spurious speech following one another in close succession, another of said privacy devices acting to change the characteristics both of said spurious speech and said useful speech.

DOREN MITCI-ELL.