US2107756A - Privacy system - Google Patents

Description

Feb. 8, 1938. B. w. KENDALI. ET AL PRIVACY SYSTEM Filed May 15, 193'? 5 Sheets-Sheet l z BWKENDALL NVENTORS D. M/TCHELL A TTOPNFV Feb. 8, 1938. B. w. KENDALL ET AL PRIVACYA SYSTEM Filed May 15, 1937 3 Sheets-Sheet 2 B WKENDALL /NVENTORS' aM/TCHELL A TroRA/Ey 5 sheets-sheet 5 B W/(ENDALL 0./14/ TCHELL A TTO/QNEV PRIVACY SYSTEM Filed May l5, 1957 B. W. KENDALL ET AL Feb. 8, 1938.

Patented Fer. s, i938 UNITED STATES PATENT ori-tice PRIVACY SYSTEM Burton W.V

Kendall, East Orange, and Doren Application May 15, 1937, Serial No. 142,815

9 Claims.

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

`An object of the invention is to disguise or 5 mask the transmitted messages so as to render reception of the kmessages by unauthorized listeners very diilicult.`

In accordance with a feature of the invention spuriousspeech is applied to the transmission 10 channel during the silent periods, i. e., when the regular talkers are not speaking, whereby there is present on the channel a practically continuous jargon composed of the useful speech and the spurious speech, the useful speech being so blend- 15 ed with the spurious speech that interpretation of the message by unauthorized listeners will be very difficult.

In accordancev with a further feature of the invention the jargon lapplied to the channel is 0 rendered even more coniusingand diiiicult to interpret by unauthorized listeners as the spurious speech applied to the channel is of a pitch closely simulating the pitch of the useful speech with which it is interspaced. l5

the invention a phonograph record is provided having several individual sound tracks thereon each of whichis a record of spurious speech of a different characteristic pitch. A suitable picko up unit is associated with each sound track. During the silent intervals, i, e., when the regular talker at the substation is not speaking, spurious speech from the phonograph record is applied to the transmission channel, a pitch selective circuit 5 being provided which is efective to select for application to the channelthe output of the particular pick-up unit which is associated with. the soundtrack having recorded thereon spurious speech of a characteristic pitch most similar to J the characteristic pitch of the regular talkers speech. During the time the spurious speech is applied to the channel, a control current is' also applied thereto being effective to operate switching devices at the distant end of the channel. in i order to prevent the spurious speech from reaching the regular talker at the distant end. Upon the regular talker again speaking, voice operated devices function to remove the spurious speech from the channel, replacing it with the useful speech, and also to remove the control current fromthe channel thereby permitting completion of a transmissiony path to the regular talker at the distant end oi the channel.

A full understanding of the operation of the S arrangementy contemplated by the invention In accordance with aspecific embodiment ofwell 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 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; and

Fig. 3 shows in detail the pitch selective circuit Which'is associated with the terminal equipment as indicated schematically in Figs. 1 and 2. f

'I'he diagram of Fig. 1 is not an actual circuit -diagram but rather a-single line layout, each line indicating a transmission path. 1n this layout a normal break in a path is indicated by separated arrowheads and a normal make by arrowheads in contact. To show that a transmission element is to be controlled by a device, an arrowhead points from the device towards the element. An arrow directed at a make 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 or the control device. Two arrowheads shown actually separated or wedged apart by the third arrowhead indicates that a normal make point is in operated or open condition.

In Fig. l the arrangement contemplated hy the invention is shown incorporated in a twom way four-wire circuit forming a part of a radio telephone system. The two terminals, west77 5 1; and east, of the system are represented, the circuit at the west terminal comprising a one" way transmission circuit EA for connecting .Y output of subscribers station W to the if est-ea radio transmitter it and a one-way tra' n circuit WB for connecting the east-west radio receiver l2 to the input of subscribers station W. Similarly at the east terminal, a one-way transmission circuit EB connects the west-east radio receiver i3 to the input of subscribers station E while the output of the subscribers station is con nected to theeast-west radio transmitter i5 by one-way transmission circuit WA.

The west terminal transmitting circuit EA is normally disabled (i. e. when the regular talker is not speaking) so far 'as the output of suhscribers station W is concerned, at control point il by suitable means, for example a short circuit applied by switching relays operating, in turn, under control of amplifier-detecter it. At this .applied to circuit EA.

time, however, spurious speech of appropriate pitch is selected by pitch selecting circuit 2| from a plurality of sources 22, 23 and 26, which may be phonographs for example, and applied to circuit EA and conducted thereover to radio transmitter II. A control current f1 produced by oscillator 2l is also applied to circuit EA at this time after being passed by iilter 28. Pitch selecting circuit 2| acts in a manner to be described in detail subsequently to select from a plurality of sources spurious speech of a characteristic pitch most similar to the pitch of the useful speech which was last An unauthorized person listening-in" at this time on the transmission from the west terminal, therefore, would hear only 'the spurious speech. In order that the authorized talker at subscribers station E (east terminal) will not be subjected to this spurious speech, circuit EB is held open at control point 3| by suitable means, for example by a short circuit applied through operation of a switching relay. The switching relay, irr turn, may be controlled through amplifier-detector 32 which is operated by the control current f1 transmitted from the west terminal and passed to the amplifier-detector through lter 3.3.

The privacy arrangement ofthis invention may be used tov advantage in conjunction with other types of privacy arrangements which are now well known, such an arrangement being indicated schematically at 34 of circuit EA, 35 of circuit WA, 38 of circuit EB and 39 of circuit WB. 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.

Let it be assumed now that the regular talker at subscriber-s station W starts to speak. A part (in a manner which will be described in detail subsequently) to select and associate with line v6I the output ofv that particular one of the plurality of spurious speech sources 22, 23 and 26 which most nearly approximates, with respect to characteristic pitch, the speech of the regular talker at subscribers station W. v

The part of the speech energy passed by line 31 to amplifier-detector |8, after rectification thereby, causes operation of switching relays effective to (a) remove the short circuit at control point I 'l thereby enabling circuit EA for transmission of speech energy fronrsubscribers'station W to radio transmitter II, (b) open or disable line 6| at control point 62 thereby removing the spurious speech from circuit EA, (c) open or disable line 63 at control point 66 thereby removing control current f1 from circuit EA, and (d) open or disable circuit WB at control point 61 thereby preventing echoes and singing.

Removal of control current f1 from the transmission channel results in enabling circuit EB at control point 3|, the circuit previously having been held open or disabled at this point due to the action of control current f1 passed by filter 33 to amplifier-detector 32. A circuit is now established, therefore, from radio receiver I3 to subscribers station E.

The useful' speecl'from subscribers station W passes, therefore,l over circuit to radio transmitter II, is radiated therefrom, picked up by radio receiver I3 and passed over circuit EB to subscribers station IE. A portion of the received speech energy is shunted by line 1I through amtransmitting controls of the east terminal by echoes.

When the regular talker atl subscribers station W ceases to speak, the circuits immediately return to the conditionillustrated and spurious speech is again applied tothe transmission channel. Due to the action of pitch selecting circuit 2| previously referred to, the spurious speech now applied to the line closely simulates with respect to characteristic .pitch the useful speech previously transmitted. An unauthorized listener, therefore, upon gaining-access to thewest-east transmission channel would hear a practically continuous jargon comprised partly of useful speech and partly of spurious speech. Interpretation of the message by recognition of the useful portions and mental masking of the spurious portions is rendered much more difficult by the fact that the spurious speech is so selected that the characteristic pitch thereof closely simulates Vthe characteristic pitch of the useful speech. The message is vtherefore rendered private so far as unauthorized listeners are concerned. As pointed out above, however, the spurious speech is not allowed to reach the distant authorized talker (due to the action of the control current f1), only the useful speech portions reaching his station. Transmission and reception from one authorized person to another is not interfered with in any Way by the privacy feature of the'invention therefore. V

As pointed out above, the privacy arrangement invented by applicants may advantageously be used in conjunction with other well-known types of privacy arrangemeiitsthereby making it even more difficult for unauthorized persons to interpret the message.

The circuits for east-west transmission function in a manner similar to that described above. For example, when thel regular talker at station E is not speaking, spurious speech of appropriate pitch selected by pitch selecting circuit II from sources 18, 8| and 82 is applied vover line 63 to circuit WA and conducted thereby to east-west radio transmitter I6. Control current f2 produced by oscillator v864is also applied to circuit WA after being passed by filter 81.

relay in turn may be controlled through ampli.

fier-detector 9| which is operated by the control current f2 emanating from source -86 and passed to amplifier-detector 9| through filter 92.y

Upon the regular talker at subscribers station E speaking, the circuits operate in a manner similar to that described above in connection with west-east transmission, circuit WA being enabled for useful speech transmission at con' trol point 93 due to the yaction of amplifier- Point |2.

detector 96, the spurious speech and the control current f2 being removed from the line and .cir- ,cuit WB (west terminal) being enabledA at control point 88 (through removal of the control current'fz) for transmission of th useful speech to the regular talker 'at Subscribers station W. A portion .of the speech energy is shunted by line 91 through amplifier-detector 98 and causes line 31 to be opened or disabled at control control point |22 at this time also.

Upon the regular talker at station E ceasing to, speak the circuits immediately return to the condition illustrated and spurious speech of a 15 characteristic pitch closely simulating that of the regular talkers speech 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 thev west-east channel. Due to the action of the means controlled through amplifier-detector 9|, however, the regular talker at station W receives only the useful speech and is not subjected to the spurious speech.

l Referring now to Fig. 2, 'a more detailed showing is set forth of one of the terminals, for example, the west terminal, of. the system illustrated in Fig. 1. Subscribers station |26 is coupled to the transmitting line |21 and the receiving line |28 by transformer |3|, network |32 serving to provide a proper balance in the wellknown manner. Speech energy emanating from Subscribers station paths, one over transmitting line |21 through amplifiers |34 and |35 to radio transmitter |33, a second over line |36 to pitch selecting circuit |31 and a third over line |38 through amplifierdetector |4| to relays |42, |43, |46 and |41. (The pitch selecting circuit |31 is shown in detail in Fig. 3 and will be described subsequently in connection with that ligure.) Piivacyarrangements (transmitting) and |5| (receiving) may be of any Well-known disclosed in D. Mitchell et al. Patent 1,981,114.

A phonograph turntable |1| and'suitable driving means |12 therefor are provided, phonograph record |13 being rotatably supported on turntable |1|. The phonograph record has three individual sound tracks |16, I 11 and |18 recorded thereon each sound track being a record of spurious speech having adifferent characteristic pitch. For example, we shall designatethe pitch of track 16 as P1, the and the pitch of track |18 as Pa; P1 representing low pitch, P2 representing medium pitch and P3 representing high pitch. Associated with each sound track is a phonograph pick-up device, pick-up |8| being associated with sound track |16, pick-up |82 with sound track |11 and pickup |83 with sound track |18.

The respective output paths |86, .|81 and |88 of. the pick-up units, after leading through respective one-way transmission devices |9|, |32 and |93, are connected in parallel to common output line |96 which is connected under certain circuit conditions to transmitting line |21.

The operating windings of relays 22|, 222 and 223 are associated with pitch selecting circuit,

|31; this circuit acting in such a way (manner of operation will be described subsequently with reference to Fig. 3) that one of these three relays will be operated depending upon the characteristic pitch of the regular talkers speech y transmitted over line |36. For example, we may Circuit EB is opened or disabled at |26 follows three parallel,

A path type, for example the type` pitch of track |11 as Pz,

. 3 assume that relay 223 will operate if the pitch of the regular talkers speech be P1,.that relay 222 will operate if the pitch of the regular talkers speech be Pz and that relay 22| will operate if the pitch of the regular talkers speech be Pz; P1, P2 and P3 representing low pitch, mediumpitch land high pitch as before.

Relays 22|, 222 and 223 control respectively by their operation, operation of relays 226, 221 and 228. Each of the three last-mentioned relays controls through the right break contact, a short-circuiting path for thel respective output lines, |88, |81, |86, of pick-up devices |83, |82 and 8|. From the subsequent description it will be apparent that two oi' these output paths are at all times short-circuited due to the action of relays 226, 221 and.228 leaving the output of only one of the pick-up units effective fortransmission to common line |96. Determination of the output path which is effective at anyparticular instant is dependent upon which of the three relays 22|, 222, 223 controlled byipitch selecting circuit |31 has been last operated.

A suitable source of constant frequency current 23| is provided which may be, for example, a vacuum tube oscillator. 'Ihe output of source 23| is connected to line |21 through the break contact of relay |46, filter 232 also being included in the output circuit. This lter 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 designed to pass only a small band of frequencies including the control frequency fr.

Radio receiver 233 is connected to receiving line |28 which line, in turn, is associated with Subscribers station |26 by transformer |3|,` ampliers 234 and 235 being included in the line. in shunt to line |28 is provided through filter 236 which lta' is designed to pass only a small frequency band including frequency f2, the frequency of the control current transmitted from the distant terminal. The current passed by filter 236 is rectified by amplifier-detector 231 and controls operation of relay 238. A second path in shunt to line 28 passes through amplifier-detector 24| and relay 242.

For purposes of further description, the actual operation of the circuits illustrated .in Fig. 2 Willnow be described in detail. The circuit condition illustrated ls that existing when the regular talker is not speaking. Line |21 is disabled, so far as the output from Subscribers station |26 is concerned, by the short circuit applied through the break contact of relay |42.

It will be assumed that relay 22| was operated when the useful speech was last removed from the transmission channel (through action of pitch selecting circuit |31, the operation of which will be described subsequently) it being assumed that the usefulospeech last applied to the transmission channel was of the high pitch which We have designated P3. Operation of relay 22| established an energizing path for relay 226 traced from battery 243, Winding of relay 226, make contactof relay 22| to ground 246, relay 226 operating over this circuit and locking-up over a of pick-up |83 is effectively associated with line |96 and thereover with transmitting line |21. It will be remembered that pick-up |83 is associated with the high pitch (P3) sound track so that spurious speech of pitch P3, the pitch of the useful speech last applied, is now applied to transmitting line |21.

Pick-up |8| (P1) and pick-up |82 (P2) are not effectively associated with line |96 at this time, their output lines |86 and |81 being short-circuited by paths completed respectively through the right break contacts of relays 228 and 221.

Control current fi is applied to line |21 at this time also over a path completed through the break contact of relay |46, lter 232 being included in this path.

At this time therefore spurious speech of pitch Ps and the control current f1 are supplied to radio transmitter |33 over line |21 and radiated thereby to the radio receiver of the distant terminal.

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 eiect of the currents transmitted by radio transmitter |33 upon the receiving circuits of the distant terminal may be better understood if we transfer our attention for the present to the receiving channel of Fig. 2 and assume that spurious speech of pitch P3 is being received thereby from the radio transmitter of the distant terminal (corresponding to radio transmitter |33) as well as a control current f2 (corresponding to control current f1).

Control current f2 received by radio receiver 233 from the distant terminal is passed by filter 236, which is designed to pass a small band including this particular frequency, and after rectification by amplier-detector 231 is effective to cause operation of relay 238. At this time, therefore, receiving line |28 is "disab1ed by a short-circuiting path completed through the make contact of relay 238 and the received spurious speech is not transmitted to the subscribers station |26.

Returning now to the transmitting channel of the terminal it will be understood from the above that the spurious speech radiated from transmitter |33 is picked up by the radio receiver of the distantl terminal and that the control current f1 is also picked up by the distant receiver and is effective to disable the receiving channel so that the distant talker is not subjected to the spurious speech.

An unauthorized person listening-in on the transmission at this time, therefore, hears only the spurious speech of high pitch (P3L.

Let us assume now that the regular talker at subscribers station |26 starts to speak. The speech energy is divided between three shunt paths |21, |38 and |36.

The speech energy present in path |38, after rectification by amplifier-detector |4|, is eiective to cause operation of relays |42, |43, |46 and |41. Operation of relay |42 removes the shortcircuiting path, previously completed through its break contact, thereby enabling line |21 for transmission of the speech energy to radio transmitter |33. Operation of relay |43 interrupts at its break contact the connection of line |96 to transmitting line |21 thereby removing the spurious speech from the transmitting channel. Operation of relay |46 interrupts at its break contact the connection of oscillator 23| to transmitting line |21 thereby removing the control current f1 from the transmitting channel. Operation of relay |41 Acompletes a shortcircuiting path through its make contact which disables receiving line |28 While the regular talker at station |26 is speaking thereby preventing echoes and singing.

The speech energy in shunt path |36 is transmitted to pitch selecting circuit |31 which functions in a manner to be described in detail subsequently to cause operation of either relay 22|, 222 or 223 depending upon the pitch of the talkers speech. It will be recalled that in the instance previously described, relay 22| had operated, as the pitch of the talkers speech was hig (P3), and that as a result of operation of relay 22|, relay 226 had been operated and had locked-up. Let us assume now that the pitch of the speech applied through line |36 is medium (P2), relay 222 therefore being operated. Operation of relay 222 completes an energizing circuit for relay 221 traced from battery 25|, Winding of relay 221, make contact of relay 222 to ground 252. Relay 221 upon operation (1) interrupts at its left center break contact, the locking circuit of relay 226 Which relay thereupon drops back to normal position, (2) locks-up in operated position over a circuit traced from battery winding of relay 221, left inner make contact of relay 221, left middle break contact of relay 228, left outer break contact of relay 226 to ground 253 and (3) interrupts at its right break contact the short-circuiting path previously applied to output line |81 of phonograph pick-up |82.

Relay 226 upon restoring to normal position reestablished through its right break contact the short-circuiting path for line |88 so that We now have only output line |81 effectively connected to line |96. Output line |81 is, as previously stated, associated with the medium pitch (P2) source of spurious speech so that when line |96 is again connected to line |21, spurious speech of medium pitch (P2) will be applied to the transmitting channel. It will be remembered, however, that relay |43 is operated at the present time so that spurious speech is not being applied to the transmitting channel.

During the actual time the regular talker at subscribers station |26 is speaking, therefore, useful speech currents alone are radiated by radio transmitter |33 and received by the distant terminal radio receiver (corresponding to radio receiver 233). In view of the fact that the control current f1 is not transmitted at this time, the receiving line (corresponding to line |28) 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 eiective to disable a portion of 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 at the distant terminal may be better visualized if We again turn for the moment to consideration of the receiving channel ofthe terminal illustrated in Fig. 2 and assume that useful speech currents are being received from the distant radio transmitter, the spurious speech and the control current fz not being transmitted.

The absence of the control current ,f2 results, of course, in relay 238 dropping back to normal, unoperated position (this relay was previously held operated by the control current fz after rectication by amplifier-detector 231) thereby removing the short-circuiting path previously complted through its make contact and "enabling line |28 for transmission of the received useful l speech to .subscribers station |26. A portion of the speech energy is shunted through amplierdetector 24| and after rectification thereby is effective to cause operation of relay 242. Operation of relay 242 completes through its ymake contact a short-circuiting path which disables line |21 in order'to prevent undesirable operation of the transmitter controls associated with station |26 by echoes.

Returning now to the transmitting channel, immediately upon the regular talker at 'subscribers station |26 ceasing to speak, relaysr |42, |43, |46 and |41 drop back to their normal, unoperated positions illustrated and spurious speech and the control current f1 are again applied to line |21 and radiated by radio transmitter |33. As pointed out aboverelay 221is now in operated position (due to previous operation of relay 222) so that the spurious speech applied now to the line is of medium pitch (P2), that is the pitch simulating most nearly the pitch of the useful speech last applied to the transmission channel. Had the pitch of the useful speech last applied been high (Pa) relay I226 would liave been operated thereby removing the short circuit from output line |88 associated with pickup |83 (P3) while if the pitch of the useful speech last applied had been "low (P1), relay 228 would have beenloperated thereby removing the short circuit from output line |86 associated with pick-up |8| \\(P1). It will be apparent from examination oil the operating circuits that only one of the three relays 226, 221 and 228, is operl ated at any one time, as operation of cme-of the relays interrupts the locking circuits of the other two.

In accordance with the features of the system operating as described above, therefore, the regular authorized talkersv receive only the useful speech and are subjected to no spurious speech. Unauthorized persons listening-in on the channel, however, hear a meaningless substantially constant jargon comprising the useful speech interspacedfduring what would otherwise be the silent periods, with the spurious speech. 'I'he spurious speech is, as pointed out above, so selected that it closely simulates with respect to characteristic pitch, the useful speech lastv applied to the line; application of useful speech of a different pitch immediately results in selection of spurious speech of a corresponding pitch which is applied to the channel upon disconnection of the useful speech., As a result of the above actions, intelligible reception of the message by unauthorized persons is very dinicult.-

Referring now to Fig. 3, a detailed showing is set forth of a pitch selecting circuit of the type shown schematically in Fig. 2. Three band pass filters 28|, 282 and 283 are shown connected in parallel across line 286 (corresponding toline |36 of Fig. 2). "These filters serve to divide the frequency range of the useful speech transmitted over line 286 into three smaller bands, it being assumed that filter 28| passes the high pitch (P3) band, filter 282 the medium pitch (P2) band and filter 283 the low pitch (P1) band. A rectifier is associated with each of the three filters (rectifier 29| being associated with lter 28|, rectifier 292 with filter 282 and rectier 283 with filter 283) and serves to change the alterhating current output therefrom into direct current. ,A shunt or hy-pass provided through condensers 296, 291 and 298 for the alternating current components of the rectified current. Differential relays 32|, 322, 323, 326, 321 and 328 are provided; theirfunction and method of operation will be apparent from the subsequent description. The small arrows shown on these relays lindicate by their direction the effect of current passing through the windings; for example, referring to relay 32|, current passing through the left-hand winding tends to operate the relay, that is to move the armature to the left or away from the contact While current in the right-hand winding tends to oppose operation of the relay, i. e., to hold the armature in engagement with the contact as illustrated.

Casual Iinspection will reveal that the relays are so arranged that current from each filter passes through one winding of each of four relays, but that in each instance the current in two relays tends to operate the relays and in the other two relays tends to oppose operationthereof.

To the right of the broken-line enclosure, three relays 33|, 332 and 333 are shown, relay 33| being associated with relays 32| and 322, relay 332 being associated with relays 323 and 326 and relay 333 -being associated with relays 321 and 328. Relays 33|, 332 and 333'correspond respectively to relays 22|, 222 and 223 of Fig. 2.

Resistance 336 and battery 331 are associated with the line leading to relay 33|, resistance 338 and battery 34| with the line leading to relay 332 and resistance 342 and battery 343 with the line leading to relay 333.

Let us assumethat the useful speech transmitted over line 286 is of a predominately high (P3) pitch, that is that maximum power is present in the frequency range passed by filter 28| andlet us see how operation of relay 33| is accomplished (it will be remembered that in the description of Fig. 2, it was pointed out that relay 22|, corresponding to relay 33|, was operated ing) of relay 328 and left winding (aiding) of relay 322 and back to line 286. Now as the current through relays 32| and 322 is in the aiding direction, these two relays will tend to operate while as the current through relays 323 and 328 is inthe opposing direction, these two relays will tend to remain unoperated.

Assuming for the moment that relays 32| and 322 operate, the short circuit previously applied to battery 331, over the respective paths,break contact of relay 322 to ground 346 and break contact of relay 32| to ground 341 is removed and an eifective operating path for relay 33| tracedfrom battery 331, resistance 336, winding of relay 33| to ground 348 is established. Relay 33| (corresponding to relay 22| of Fig. 2) therefore operates and brings about the desired operations in the associated circuits (as previously described, operation of relay 22|, Fig. 2, results in operation of the associated relay 226).

Now let us see the effect upon the circuit of the medium powered current (Pz) and the low powered current (P1) passed respectively by l- :ers 282 and 288. 'I'he medium powered current passed by lter 282, after rectification by rectier 292, passes through the left-hand winding (aiding) of relay 323, right-hand winding (opposing) of relay` 321, right-hand winding (opposing) of relay 322 and left-hand winding (aiding) of relay 326 and back to line 286. Now while this current passes through relays 323 and 326 in the aiding direction, only one of the relays, i. e. relay 326, operates, relay 323 being held from operating by the more powerful high pitch currents passing through the right-hand winding in the opposing direction. Therefore while the shortcircuiting path previously applied vto battery 34| .through the break contact of relay 326 to ground 35 I' is interrupted, a short-circuiting path through the break contact of relay 323 to"ground 352 is still applied so that an effective operating circuit for relay 332 is not established and the relay remains in non-operated position. The opposing effect of the medium powered current passing through the right-hand winding of relay 322 does not, of course, prevent operation of the relay by the more powerful current passing through the left-hand winding of the relay in the aiding direction.

Considering now the effect of the low powered current (P1) passed by lter 283, this current, after rectication by rectier 293, passes through the left-hand winding (aiding) of relay 321, right-hand winding (opposing) of relay 326, right-hand winding (opposing) of relay 32| and left-hand winding (aiding) of relay 328. None of the relays is operated by this current, the aiding effect in relays 321 and 328 being overcome by the opposing effect of the more powerful currents in the right-hand windings. Neither of the short-circuiting paths applied to battery 343 (break contact of relay 328 to ground 353 and break contact of relay 321 to ground 356) is interrupted therefore and an effective operating circuit fo relay 333 is therefore not established. The opposing effect of the low powered current on relays 326 and 32| does not, of course, prevent their operation by the nfore powerful currents passing through the windings in the aiding direction.l

It will be evident from the above that only one of the three relays 33|, 332, 333 will operate, namely, that relay associated with the frequency range P1, P2 or P3 in which the power is maximum. It will be understood. further, that the circuit operates in a similar manner for other distributions of power. For example, should medium pitch (P2) currents predominate, i. e. the frequency range passed by filter 282, relay 332 would be operated and should low pitch (P1) currents predominate, i. e. the frequency range passed by lter 283, relay 333 would be operated.

While certain specic embodiments of the invention have been selected for illustration and detailed. description, the invention is not, of course, limited in its application to these embodiments. aFor example the invention is applicable to systems other than a two-way four-wire radio telephone system, more than three (oi` less than three) sources of spurious speech may be provided, other pitch selective circuits may be utilized and so on. In short, those embodiments which have been selected for detailed description should be looked upon as illustrative of the invention and not as restrictive thereof.

A pitchlselective circuit of the type described above is disclosed and claimed in the copending application of L. Schott and S. B. Wright, Serial No. 142,832, filed May 15, 1937, entitled Transmission circui 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 lsaid receiver which comprises applying useful-speech to said channel, selecting fromr a plurality of sources of spurious speech of different characteristic pitches that source most nearly approximating in characteristic pitch the useful speech and applying spurious speech from said selected source to said channel upon removal therefrom of said useful speech.

2. Ina 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 cf speech from said transmittento said receiver which comprises impressing spurious speech on the 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, each period of spurious speech being .of a pitch closely simulating that of the immediately preceding periodof useful speech. l

3. In a speech transmission system including a transmitter, a receiver and a channel between said transmitter and said receiver, a source of useful speech waves, a plurality of sources of spurious speech each of a different characteristic pitch, means for applying useful speech waves to said channel, means for selecting the source of spurious speech of characteristic pitch most nearly like that of the useful speech waves, and means effective upon disconnection of said useful speech waves from the channel for applying spurious speech from said selected source to said channel.

4. In a speech transmission system including a transmitter, a receiver and a channel between said transmitter and said receiver,"a source f useful speech waves, a pluralityJ of records of spurious speech of different characteristic pitch, a pick-up device associated with each record, an output line for each pick-up device, means for applying useful speech waves to said channel,

means for determining the record of spurious speech of a characteristic pitch most nearly like that of the useful speech waves, and means effective upon disconnection of said useful speech waves from the channel for connecting the output line of the pick-up device associated with the determined record to said channel.

5. In a speech transmission system including a transmitter/a receiver and a channel between said transmitter and said receiver, a source of useful speech, a plurality of records of spurious speech of diiferent characteristic pitch, a translating device associated with each record, an output line for each'translating device, means for connecting each of said output lines to said channel, means for applying useful speech to said channel and means for rendering one of said translating device output lines effective for transmission of spurious speech to said channel, the others of said translating device output lines being renderedineffective for transmission lating device associated with each record. an

output line for each translating device, means for connecting each of said output lines to said channel, means for applying useful speech to said channel and means for rendering one of said translating device output lines effective for transmission of spurious speech to said channel, the others of said translating device output lines being rendered ineffective for transmission to said channel, said last-mentioned means operating in accordance with the pitch of the useful speech applied to said channel.

7.In a speech transmission system including a transmitter, a receiver and a channel between said transmitter and said receiver, a source of useful speech, a plurality of records of spurious speech of different characteristic pitches, a translating device associated with each record, an output line for each translating device, means for connecting each of said output lines to said channel, means for applying useful speech to said channel, means for analyzing the applied useful speech in order to determine the relative characteristic pitch thereof, a short-circuitingr path applied to each of said translating device output linesv for rendering said lines ineffective for transmission of spurious speech to said channel, and means operating under control of said analyzing means for removing the short circuit from one of said translating device output lines whereby said line` is rendered effective for transmission of spurious speech to said channel.

8. In a speech transmission system including a transmitter, a receiver and a channel between 4 said transmitter and said receiver, a source of usefulspeech, a plurality of records of spurious speech of different characteristic pitches, a translating device associated with each record,

an output line foreach translating device, means for connecting each of said output lines to said channel, means for applying useful speech to said channel, means for analyzing the applied useful speech in order to determine the relative characteristic pitch thereof, a relay associated with each of said translating device output lines effective when in normal non-operated position to disable said line for transmission of spurious speech to said channel, and means operating under control of said analyzingmeans for caus- "ing operation of one of said relays whereby the associated translating device output line is enabled for transmission of spurious speech to said channel.

9. In a speech transmission system including a transmitter, a receiver and a channel between said transmitter and said receiver, a source of useful speech, a plurality of records of spurious speech of different characteristic pitches, a translating device associated with each record, an output line for each translating device, means for connecting each of said output lines to said channel, means for applying useful Ispeech to said channel, means for analyzing the applied useful speech in order to determine the relative characteristic pitch thereof, a relay associated with each of said translating device output lines eiective when in normal non-operated position to disable said line for transmission of spurious speech to said channel, and means operated under control of said analyzing means for causing operation of one of said relays whereby the associated translating device output line is enabled for transmission to said channel during idle message `transmission periods, of spurious speech of pitch corresponding to that of the useful speech last'applied to the channel.

BURTON W. DOREN MITCI-IEIL.

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