US2402059A - Secrecy communication system - Google Patents

Description

SECRECY COMMUNICATION SYSTEM Jane n, 19

CRAIB sheets-sheet 2v Filed .April` 29, 1942 IW o.

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m'mooooooommmmm m MOE NOE INVENTOR JAMES F. CRAIB BY QW ATT RNEY N). 4+ 0.0 OO

IMT oO June 11, 1946- J. F. cRAlB 2,402,059

SECRECYQOMMUNICATION SYSTEM Filed April 29, 1942 s sheets-sheet s JAMES F. GRA|B Patented June ll, 1946 carton srs'rniu Application April 29, 1942, Serial No. 441,024

(Ci. Mii- 1.5)

3l Claims. l

The present invention relates to secrecy cornmunication systems and, particularly, to such systems which are especially suitable for the transmission of intelligence in the form of speech signals. In greater particularity, the invention relates to secrecy communication systems of the type in which frequency-component subbands of a speech signal are periodically modified, as by sluiting, inverting and interchanging the subbands in the frequency spectrum, thus tp modify the intelligibility of the signal when reproduced in conventional manner.

Many systems have heretofore been proposed by which to effect, to a greater or lesser degree, secrecy of communication of speech signals, especially for radio communications which are easily intercepted by unauthorized persons. A secrecy communication system is considered satisfactory from a practical standpoint only when intelligible reproduction of communications by an unauthorized person is practically impossible, even though such person has a complete receiving apparatus capable of rendering the received signals intelligible and lacks only information concerning the preestablished procedure by which the transmission is rendered unintelligible.

t is desirable that a secrecy communication system interrupt and otherwise modify the communication to be transmitted at such a rapid rate as to render the communication so unintelligible that an unauthorized person receiving the communication is unable to understand even syllables of the speech'signal .when reproduced in conventional manner, or when reproduced merely with frequency-component subbands of the received signal displaced in the frequency spectrum since, where this is possible, the speech signal is' in general reproducible as intelligible speech portions against a background of severe noise and a skilled person is frequently able to understand the communication Further, it is desirable that the rapidity of interruption and modification of the speech signal be so high that intelligibility of the communication cannot be restored without f great expenditure of time and effort when'accoin plished by the method of recording the communication on both sound and visual records, analyzing the visual record to determine the preestablished plan of procedure by which the transmission is rendered unintelligible, and. using this information, reproducing the'recorded sound record with completely restored intelligibility of communication. Lastly, it is desirable that the apparatus necessary to effect secrecy of 'communication be relatively light in weight, compact.

inexpensive, and adapted satisfactorily to operate Without any adjustments other than the initial adjustments usually made by the manufacturer of the apparatus; that the plan, of procedure of rendering the communication unintelligible and of restoring intelligibiiity be easily, quickly and readily accomplished; and that the band width required to translate the unintelligible signal should not appreciabiy exceed the band width required to translate the speech ,signal before it is rendered unintelligible.

It is an object of the present invention, therefore, to provide a new and improved secrecy communication system of the type described and one characterized by a higher degree of secrecy of communication than has heretofore been at- A tainable by known methods and apparatus.

it is a further object of the invention to provide a secrecy communication system particularly suitable for the transmission of speech signais and one in which one or more groups or subbands of the frequency components .of the speech signal are interchanged relative to each other, shifted or inverted in the frequency specgtrurn, or interchanged, shifted and inverted in a predetermined order at intervals not substantially longer than the longest syllabic interval of the speech signal.

It is an additional object of the invention to provide a new and improved secrecy communication system in which the signal to be communicated is rendered unintelligible by modifying the frequency components thereof in accordance with any selected one of a very large number of preestablished plans `of procedure which are adapted to be readily and quickly changed from time to time, as desired.

It is an additional object of the invention to provide a new and improved secrecy communication system having any or all of the features specified above as desirable in a system of this nature. f

In accordance with one embodiment of theinvention, e. secrecy communication system comprises an input circuit adapted to have applied thereto a speech-signal to be transmitted, the signal comprising frequency components extending over a predetermined frequency band, The system includes meansA for deriving from the signal a second signal comprising a plurality of frequency-component subbands corresponding to individual` portions oi the aforesaid frequency band, the frequency-component subbands of the second signal being successively modified relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of the speech signal. The system also includes means for translating and receiving the second signal and for deriving therefrom the speech signal.

In a particular form of the invention, a secrecy communication system of the type described -includes a plurality of band-pass filters for separating the speech signal into a plurality of subbands corresponding to individual portions ,of the speech-signal frequency band, a pluraltiy of modulators individually coupled to the band- '\pass filters, means for deriving a plurality of displacement carrier waves, means, including electronic switching means, for successively applying the carrier waves individuallyY to themodulators in a predetermined order during intervals not substantially longer than-the longest syllable interval of the speech signal to derive a second signal having modified intelligibility, and an output circuit coupled to the modulators to have applied thereto the second signal, whereby the signal applied to the output circuit is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy of communication.

, For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken inconnection with the accompanying drawings, and its scope will be pointed out in the appended claims. v

Referring now to the drawings, F18. 1 is a cir- 'cuit diagram, partly schematic, of a complete carrier-wave transmitter suitable for use in a secrecy communication system embodying the in-4 vention; Fig. 2 comprises graphs representing the manner in which a speechsignal may be `divided linto frequency-component subbands and the man- -gram of a portion of a secrecy communication 'system and represents a modified form ordis-- placement carrier-wave generator suitable for use in the transmitter and receiver arrangements of Figs. 1 and 4.

Referring now more particularly to Fig. 1, there is represented, schematically, a complete carrierwave transmitter suitable for use in a secrecy communication system embodying the invention in a preferred form. The transmitter includes an input circuit, comprising input-circuit terminals I0, II, ladaptedto have applied thereto a speech signal to be transmitted. This speech signal has somewhat modified intelligibility and is derived from a speech signal of full intelligibility picked up by a microphone I2 and applied through an audio-frequency amplifier I2, of one or more stages, to an input circuit of a modulator I4, the latter unit having another input circuit to which, is coupled a displacement carrierwave generator I5. The speech signal picked up by the microphone I2 has frequency components extending over a predetermined frequency band,

.for example from 300 to 3000 cycles, and the speech signal to 'be transmitted and which is applied to the input-circuit terminals I0.. I I, has

For example, frequency components of 300 and 3000 cycles in the speech signal correspond to i the respective frequency components of 8700 and 6000 cycles in the speech signal to be transmitted when the generator I5 has a frequency of 9000 cycles. The transmitter includes means for separating the speech signal to be transmitted into a plurality of frequency-component subbands corresponding to individual portions of the frequency band of this signal. This means comprises a plurality of band-pass filters I6, Il and I8 having their input circuits connected to the output circuit of the modulator I4. Thus, assuming the speech signal to be transmitted has a frequency band from 6000 to 8700 cycles, the band-pass filter I8 may derive, for example, a frequency-component subband corresponding to the portion extending from 8700 to 7800 cycles of the frequency band, the band-pass filter I1 may derive that portion fro'm 7800 to 6900 cycles, and the band-,pass filter I8 may derive the portion from 6900 to 6000 cycles.

There is also included in thetransmitter a plurality of modulators I8, 20 and 2|, each having a signal input circuit, and these input circuits being individually coupled to the output circuits of the band-pass'filters I6, I1 and I8. The output circuits of the modulators I8, 20 and 2l are coupled to the input circuit of a low-pass filter 22 through respective amplifiers 22, 24 and 2l. The output circuit of the filter 22, in turn,l is coupled to the input circuit of a carrier-wave transmitting apparatus 28 which includes a source of carrier waves, a modulator, and one or more stages of radio-frequency amplification. The output circuit of unit 28 is coupled to a radiating antenna system 21, 28.

There is also provided in the transmitter means for deriving a plurality of displacement carrier waves, this means comprising a plurality of carrier-wave generators 28 to 28, inclusive, having respective individual frequencies f1 to fa, inclusive. The frequencies f1 to fa. inclusive, may havethe respective values. for example, of 10,800 cycles. 9900 cycles, 7500 cycles, 6600 cycles, 8100 cycles, 4800 cycles, 7200 cycles, and 3900 cycles. There is also included in the transmitter a carrier-wave repeater unit 1 comprising a plurality ofyrepeater devices 31 to 48, inclusive, arranged in groups, lfor example 31 to 40, inclusive. 4I to 44, inclusive. and 45 to 48, inclusive. Each such group has a common output circuit which is coupled to a carrierwave input circuit of an individual one of the modulators I9, 20 and 2l. Each o f the repeater devices 31 to 48, inclusive, has its input electrodes coupled to a given one of the generators 28 to 28, inclusive,` and is, in turn, effective to couple the generator. to which it is so coupled, to an associated one f the modulators I9, 20 and 2| when 'the repeater device is suitably energized and has a suitable bias applied between its input electrodes. The repeater devices 31 to 48, inclusive, are energized from sources of space current, indicated as +B,` but the input electrodes of each asoaoeo are normally biased below anode current cutoff from sources of biasing potential indicated as -C. At any given instant during the operation of the transmitter of Fig. l, one repeater device in each of the groups 31 to Q0, inclusive, il to im, inclusive.

" and Q5 to 68, inclusive, is biased above cuto by electronic switching means 39, and the interval during which each tube is so biased is not substantially longer than the longest syllabic interval of the speech signal. The term syllabic interval of speech will hereinafter be discussed in greater detail and defined during the description of the operation of the invention. Unit 69 has a plurality of control circuits successively energized at intervals not substantially longer than the longest syllable interval of the speechsignal. each such circuit being coupled to the one of the aforesaid repeater devices of the before-mentioned groups through manually-adjustable switching means 513. The manually-adjustable switching means 5B is provided so that the order of coupling of each or' the control circuits of unit te to the repeater devices 31 to i, inclusive, can be established and modied, as desired, from time to time.

The electronic switching means i9 comprises a plurality of identical switching units iii to inclusive, only the ilrst of which is shown in detailed schematic form. Each oi the switching units 5i to 553, inclusive, includes an input circuit, comprising input-circuit terminals 59, 59', and an output circuit comprising output-circuit terminals d, Si. Each of these units also includes means responsive to a pulse of negative potential applied to the input circuit thereof for developing in the output circuit thereof a pulse of positive potential. This means comprises a pair of vacuum tubes 62, S3, the input electrodes of tube e2 being coupled to the input-circuit terminals 59, 59' and the output electrodes o f tube 63 being coupled to the output-circuit terminals tu, 6l. The lastnamed means also comprises means intercoupling the input and output electrodes of the tubes 62. 63

to develop the positive-potential pulse in the output circuit of the unit in response to the application of a negative-potential pulse to the input circuit thereof. The intercoupling means of each of units 5l to 59, inclusive, comprises a voltage divider, comprising serially-connected resistors Gil,

65, which couples the output circuit of tube-52 to y the input circuit of tube 53 and a time-constant circuit, comprising a condenser vlit and an adjustable grid resistor 61, which couples the output circuit of tube 53 to the input circuit of tube 62. The control circuit of unit 5l, which comprises control-circuit terminals 6i land SB, is coupled to the output circuit of tube 63. The electronic switch also includes means for coupling the .units 5i to 53. inclusive, in cascade., In order to effect this connection, the output circuit of eachunit is coupled to the input circuit oi' a succeeding unit, the coupling means comprising, in each case, a condenser te.

There is also included in the electronic switch fle, for a purpose presently to be considered, an additional switching unit 1t comprising an input circuit including input-circuit terminals ll, 1i. an output circuit including output- circuit terminals 12, 13, and means responsive to a negative-potential pulse appliedto the input circuit for developing a negative-potential pulse in the output circuit. The last-named means comprises a pair of vacuum tubes 1d, 15, the output circuit of tube l being coupled to the input circuit of tube it by a voltage divider, cmprising series-con- 6 nected resistors 1t, 11, and the output circuit of tube 15 being coupled to the input circuit of tube 14 by a time-constant circuit comprising a condenser 18 and an adjustable grid resistor 19. The

input electrodes of tube 14 are coupled to the input-circuit terminals 1|, 1|', whilethe output electrodes of tube 15 are coupled to the outputcircuit terminals 12, 13 and the latter terminals are coupled, in turn, to the input-circuit terminals 59, 59' of unit 5I by means of a coupling condenser 68. Thus, units 10 and 5| are similar in circuit arrangement, except that the outputcircuit terminals 12, 13 of the former are coupled across the output electrodes of tube 15, whereas the output-circuit terminals eil, 6l of unit 5! are coupled to the cathode circuit impedance of tube 153. The cascade connections of the units of the electronic switch 39 are completed by coupling the output circuit of the last cascaded unit 5t to the input circuit oi the additional unit l@ through a coupling condenser 6e.

The manually-adjustable switch 5h includes a plurality of vertical conductive bars @il to t1, inclusive, which are connected to individual ones o the control circuits of units 5l to t, inclusive, of the electronic switch lli?. The unit t@ also includes a plurality oi. horizontal conductive bars et to et, inclusive, which are connected to the control electrodes of individual ones of the repeater devices 31 to et, inclusive. The latter bars may be divided into groups I, l, and III of four bars each to designate, for convenience, the grouping of the bars ,with relation to the respective groups of repeater devices 3l to l, inclusive, il to titl, inclusive, and 65 to 453,.

inclusive. Manually-adjustable switch elements are included in unit ell so that the operator can electrically connect any one of the vertical bars t@ to u?, inclusive, to any one of the horizontal bars et to @9, inclusive. These switch elements may comprise a spring-biased contact, not shown' for simplicity, which normally would electrically connect each vertical bar to every one of the horizontal bars at their cross-over pointsL However, only selected ones of the switch elements are permitted to complete the electrical connection between the vertical and horizontal bars. This is accomplished by the insertion or a sheet of insulating material between the switch elements and the vertical or horizontal bars which the switch elements would normally engage, appropriate holes vbeing punched in the sheet of mate- I rial to form a code card and to permit engagement of only selected vertical elements with their cooperating horizontal bars. The engaging switch elements for one such code card are illustrated in Fig. i of the drawings by the solid black circles, and it will be noted that each of the bars Bu to 81, inclusive, is electrically connectedto one, and only one, horizontal bar in each of the three groups I, Il, and III.` Thus, the `:ontrol circuit of each of units El to be, inclusive, is electrically connected through the manually-ML justable switch tu to the control electrode ci one repeater device in each of the three groups oi repeater devices 31 to d, inclusive, il to lii, inclusive, and t5 to (it, inclusive. Preferably, the manually-adjustable switch E@ is constructed with the vertical bars secured on one support member and the horizontal bars secured on another supportmember, these members being hinged lin book-like fashion and one of them supporting the engaging switch elements,V a construction which permits -ready lnsertion of the code card between the vertical and horizontal inafter designated as "speech signal Y.

bars and the switch elements cooperating theref distinguish it from the speech signals of modified intelligibility derived therefrom and having frequency components corresponding thereto. The speech signal X is amplified-by the audiofrequency amplifier I3, and applied to one input circuit of the modulator I4. There is applied to another input circuit of the modulator Il a speech-displacement carrier wave generated by the generator I5. There is thus produced in the output circuit of modulator Il a modulated car- 'rier wave having modulation sidebands, each of which contains frequencv components corresponding to the speech signal X. However, these modulationsidebands are each displaced upward in the frequency spectrum from the speech signal X and the frequency components of the lower sideband are inverted with respect to the corresponding frequency components of the speech signal X. Since such frequency--component inversion is useful as a first step Ain the direction of modification of the intelligibility of the speech signal, the lower sideband alone-is selected and utilized in the apparatus which follows the modulator I4 and such sideband thus comprises another speech signal to 'be transmitted and is here- The purpose in using the displacementcarrier generator I5 to derive the speech signal Y is primarily to shift the frequency components of the 4speech signal X to` a higher portion of the frequency spectrum where they may be more readilyhandled for purposes of interchanging. shifting, and inverting them in the frequency spectrum ultimately to derive the unintelligible speech signal to be transmitted.

. Assuming, as before, that the displacement carrier generator I5 has a frequency' of 9 kilocycles and the speech signal a frequency band from 300 to 3,000 cycles, the modulated displacement carrier wave applied to the input-circuit terminais I0, II of the transmitter has 'a lower sideband extending from 8.7 to 6 kilocycles, comprising the speech' signal Y as hereinbefore stated,

` and an upper sideband extending from 9.3 to 1,2

kilocycles which is not translated. As'previousiy stated, the band-pass filters I6, I1 and I8 translate only the respective pass bands extending from 8.7 to 7.8 kilocycles, 7.8 to 6.9 kilocycles, and 6.9

- to 6.0 kilocycles. Consequently, the band-pass filters I8, I'I and I8 separate-the speech signal Y into a -plurality of frequency-component subbands corresponding to `individual portions of the frequency band of this signal. It may be noted that these subbands correspond to similar subbands into which the speech signal X might be divided. For example, the subbands (3.7-7.8 kilocycles, 7.8-6.9 kilocycles, and 6.9-6.0 kilocycles of the speech signal Y correspond in information content of the signal to be transmitted to the respective subbands .3-l.2 kilocycles, 1.2-2.1 kilocycles, and 2.1-3.0 kilocycles of the speech signal X. The subbands derived by the filters ,|6, I1 and I8 are individually applied to the Vsignal input circuits of the respective modulators I8, 20 and 2|.

Referring now to Fig. 2 of the drawings, the frequency-component subbands derived by the band-pass filters I6, I1 and I8 from the speech signal4 Y are designated in Fig. 2 as A, B and C,

8 respectively. It will be noted that each of these subbands is 0.9 kilocycle wide. Further to modify the intelligibility of the speech signal .Y, the frequency-component subbands A, B and C are successively modifiedrelative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of the speech signal X. This is accomplished by apparatus, presently to be considered, which operatesy to interchange the position of the subbands, invert the frequency components of individual ones of the subbands, or both interchange the position of the subbands and also invert the frequency components of one or more thereof. Actually, as

will presently become apparent, the relative shift... .ing of the subbands A to F, inclusive, does not occurin the frequency band 8.7-6.0 kilocycles, but

takes place effectively as though the subbands had first been 'shifted with relation to each other in this frequency band to derive a signal of modified intelligibility, which may be designated as sigvnal Z, and that thereafter the signal Z had been so shifted in the frequency spectrum that its frequency components occupied the frequency band of .3-3.0 kilocycles. -However, for purposes of simplicity in explaining the manner oflforming thequency spectrum of 8.7-6.0 kilocycles, the inverted subbands .corresponding to the subbands A, B and C then would occupy the position indicated in Fig. 2 as D, E and F, respectively, in which the subbands appear in their original order. It will be evident, of course, that the ultimate speech signal to be derived, -that is, the speech signal having the highest degree of unintelligibility and which is to be transmitted, must include frequency components corresponding to the entire number of frequencycomponents of the speech signal X. This may be effected, for example, by adding the subbands A and C to the inverted subband E or by adding the inverted subbands D and F to the subband B. When the subbands A-F, inclusive, are thus added in all possible arrangements containing the complete original speechfrequency components, it can be shown that it i is possible to provide 64 arrangements of the subbandswhile yet transmitting frequency components corresponding to al1 frequency components of the speech signal X.

However, the transmission of subbands A, B and C in their original position in the frequency band of '8.7-6.0 kilocycles would be intelligible when reproduced by an unauthorized listener if he simply inverted the frequency components of these subbands andshifted them to .the corresponding frequency rangel of .3-3.0 kilocycles of the original speechsignal X. This also applied to the inverted subbands D, E and F when used in their origina1 position, except that these subbands need merelyhe shifted to their corresponding position in the original speech signal X, the inversion step having already been performed on them.

Hence. a higher degree of secrecy may be effected by forming the ultimate signal of modified aeoaose 9 intelligibility which is to be transmitted only of frequency-component subbands in which no frequency-component subband, or inverted subband, of the speech signal Y occurs in its normal position with relation to the other two subbands.

This reduces the number of possible arrangements of the frequency-component subbands which may beused to form the ultimate signal of modified intelligibility to sixteen. These are tabulated in Fig. 3. Considering specically one such arrangement of subbands, which is designated in Fig. 3 las code number I, it will be noted that the sub- ,Y band B has been shifted into that portion of the frequency spectrum normally occupied by-subband A, that subband A is shifted to the position normally occupied by subband B, these two subbands thus being merely interchanged, and that subband C has effectively been inverted and shifted in the frequency spectrum to the position which it normally occupies without inversion.

kBefore considering the precise manner in which the frequency-component subbands are shifted. interchanged relative to each other, inverted, and added to derive the ultimate signal of modified intelligibility, the detailedoperation of the electronic switch de will rst be considered.

In this discussion, frequent reference will be made to the longest syllabic interval of the speech signal and it is,I therefore, deemed advisable at this point more fully to deiine the meaning of this term; It -has been observed that ordinary speech is eilected by. Word syllables and that the lowest frequency of occurrence of such syllables averages approximately twenty times per second. On this basis, the longest syllabic interval is 50 milliseconds. This, of course, is an average value and in isolated cases the syllabic interval may increase substantially to that of the x word interval which, in ordinary speech, has been found to have a value of approximately 70D-800 milliseconds as determined byaverage speech of approximately one-hundred words per minute.- Hence, in the present specification and claims, the term longest syllabic interval" of the speech signal may be defined as an interval of from approximately 50 milliseconds to 700 or 800 milliseconds.

Considering now in detail the operation of the electronic switch t9, the iirst tube in each of the units 5l to E8, inclusive, and 79, for example tube 52 of unit 5| 'and tube ri of unit lt, is normally conductive and thus has relatively high conductance, and the second tube of each of these units is normally nonconductive and hence has low conductance.

Now assume that a negative-potential pulse is applied to the input-circuit terminals 'ii and li of unit 7d. This pulse causes tube le to become nonconductive and applies a positive pulse to tube 'i5 to render the iatr conductive. When tube 'i5 becomes conductive, a negative pulse is appliedto the input-circuit terminals 5t, Se' of unit el, thus causing tube 62 of this unitto be nonconductive, thereby to apply a positive pulse to the control electrode of tube 63. There is coupled from the output circuit of unit El to the input circuit of unit 52 a positive pulse at the time that tube ed becomes conductive, but this pulse has no eiect on the operation of unit 52, since the iirst tube of that unit is already in a conductive state.- At the same time, a negative pulse is applied from the output circuit of tube S3 through the condenser Se and the resistor 61 to the control electrode of the lirst tube 62 of unit lfand this pulse has a duration, determined by a negative pulse is applied from the output circuit f 62 to the control electrode of tube 63 to reduce the conductivity of the latter, the action being rapid and cumulative to render tube` 62 fully conductive and tube 63 nonconductive. .At this time,

of unit 5l to the input circuit'of unit E2, thus to render the first tube of the latter unit nonconductive and the second tube of such unit conductive for an interval of approximately 50 milliseconds, as in unit 5 I.

Consequently, at the end of each such interval, a succeeding oneA of the units 53 to 58, inclusive, is caused to produce a. positive potential pulse in its output circuit. The last unit 58 is arranged to apply a negative pulse to the input circuit of I unit 10 at the end of the 50 millisecond interval of operation of the former unit. During the time required for the units El to 58, inclusive, to become successively operative, the first tube It of unit 'l0 has been maintained nonconductive by a negative pulse of longer duration, determined by the time-constant of elements i8 and 79, which is applied from the output circuit of tube 75 to'r the control electrode of tube 1li. Tube la becomes fully conductive, however, at a time just preceding the time at which the negative potential pulse is appliedy from unity 5% through the coupling condenser B9' to the input circuit of unit 1li. .Consequently, unit l@ may be considered a barrier unit in that it is not affected by undesired spurious potential pulses applied to its input circuit during the major portion of the interval between the application thereto of desired negative pulses from the output circuit of unit 58. It will thus be evident that the plurality of control circuits of unit da are successively'energized at vintervals not substantially longer than the long- 32 to the modulator it, from the generator tt to y the modulator 2b, and from the generator 3d to the modulator 2 l. Similarly, when the next vertical bar Sil has a positive potential pulse applied thereto from unit t9, therepeater devices 38, d2 and it are conditioned to apply carrier-wave oscillations from the respective generators 30,' 33 and 3G to the respective modulators I9, 2@ and 2i.

The carrier waves applied to the modulators i9. 2li and 2l are modulated therein by the frequency-component subbands applied to the modulators from the filters iii, il and i8; There is thus developed in the output circuit of each modulator a carrier wave having upper and lower modulation sidebands. As will presently become apparent, only the lower modulation sideband of these modulated carrier waves is' used in forming the speech signal of modied intelliglblllty signal of modified intelligibility may readily be translated through conventional audio-frequency translating amplifiers and circuits, but also that it requires minimum band width for the modulated carrier Wave which is radiated by the antenna system 21, 28. Assuming that the signal of modified intelligibility is to occupy a frequency band ofthe same width, and to have the same mean frequency, asthat of the speech signal derived by the microphone I2, for example 300- 3,000 cycles, the frequency that each of the generators 29 to 36, inclusive, should have is governed by two considerations. First, the lower sideband of the modulated carrier Wave developed in the output circuit of each of the modulators I9, and 2| should always lie within the frequency range from G-3,000 cycles; and, secondly. the frequency components of the lower modulation sideband should correspond tothe frequency components of a corresponding one of the subbands A, B and C, or inverted subbands D, E and F, Fig. 2, when the latter subbands are 4 shifted in the frequency spectrum relative to one or more such subbands. To illustrate this by way of example, consider what occurs when the vertical bar 8D of switch 50 is energized with a `positive-potential pulse fromI the lelectronic switch 49. It has previously been shown that this is effective to couple generators 32, 33 and 36 through the respective repeater devices 40, 42 and 48 to the respective modulators I9, 20 and 2|. The lower sideband of the .modulated carrier wave developed in the output circuit of modulator I9 includes frequency components in the band of 2.1-1.2 kilocycles in-l which the frequencies 2.1 and 1.2 kilocycles correspond, respectively, to the frequencies 8.7 and 7.8 kilocycles of the subband A of speech signal Y. Now if the entire speech signal Y were to be shifted into the frequency bandof .3-3.0 kilocycles, as by the use of a 5.7 kilocycle carrier wave and the selection of the lower modulation sideband thereof, the subband A would then occupy the portion 3.0-2.1 kilocycles of this frequency band, subband B the portion 2.1-1.2 kilocycles thereof, and subband C the portion 1.2-.3 kilocycles thereof. However, this has not been done as only the subband A has been so shifted, but in doing so it will be apparent that the frequency components of the lower sideband of the carrier wave developed in the output circuit of modulator I 9, under the actual conditions assumed, correspond to the subband A shifted in the frequency spectrum to the relative position normally' occupied by the subband B. The term "relative position,'as here used, refers to the related positions of the subbands as they occur in the speech signal Y. v Similarly, the lower sideband of the modulated carrier wave developed in the output circuit of modulator 20 includes frequency components in the band .3-1.2 kilocycles in which the frequencies .3 and 1.2 kilocycles correspond, respectively, to the frequencies 7.8-6.9 kilocycles of the subband B of speech signal Y. The frequency components of this lower sideband thus correspond to thesubband B inverted as subband E and shifted into the relative position normally occupied by subband C. Lastly, the lower sideband of the modulated carrier wave developed in the output circuit of the modulator 2| includes frequency components in the band 3.0-2.1v kilocycles in which the frequencies 3.0 and 2.1 kilocycles correspond, respectively. to the frequencies 6.9 and 6.0 of the subband C of speech signal Y. The

frequency components of the latter sideband thus correspond to the subband C shifted to the relative position normally occupied by the subband A. Thus, it will be evident that when these lower modulation sidebands are subsequently selected and added, in a manner presently to be considered, there is derived the ultimate speech signal of modified intelligibility comprising subbands arranged in the manner designated as code No. 7

in the ltabulation of Fig. y3. It can similarly be shown that when the vertical bar 8| of switch 50 is energized by unit 49, the subbands E and C vmodified intelligibility and occupies the relative position normally occupied by the subband B. This arrangement of subbands is designated in Fig. 3 as code No. 5.

Since the subbands are interchanged at the end of 50millisecond intervals as each of the control circuits of the electronic switch 49 becomes successively energized, and in accordance with the particular code card used in unit 5l, it is possible to combine the subbands in conformance with the tabulations of Fig. 3 in a very large number of permutations and combinations. It may here be stated that from such possible combinations and permutations, it is possible to use approximately 500,000 different code cards, each of which defines a preconceived plan of `operation. Each such code card, of course, will be used for adesired interval of a few minutes, an hour, or a day, but is preferably changed at the end of every few minutes according to a prearranged plan by which a limited number of such code cards, for example 10, could be utilized over a twenty-four hour operating period.

The modulated carrier waves developed in the output circuits of the modulators I9, 20 and 2| are amplified by the respective amplifiers 22, 24l

and 25, added in the common output circuit of the latter units, and translated through the lowpass filter 22 which selects the lower sideband p of each, as previously stated, to derive a second speech signal having frequency components lying only in the frequency band of .33.0 kilocycles as do the frequency components of the original speech signal X, but the second speech signal, unlike the original speech signal, has greatly modified intelligibility.v 'I'his signal is .applied to the modulator of unit 29 to modulate the carrier wavegenerated therein. There are also applied to the modulator of this unit, simultaneously to modulate the carrier wave thereof, synchronizing potential pulses derived from orderat inervals not substantially longer than the longest syllabic interval of the speech signal to derive an ultimate or second signal to be transmitted Vhaving its intelligibility modified to lators i9', 20' and 2i are coupled through india high degree. It will further be evident that this means includes means for deriving a plurality of displacement carrier waves, comprising units 29 tov 36, inclusive. It will alsob be evident thatunits 1, t9 and 50 comprise means including electronic switching means for successively applying the carrier waves of units 29 to 38, inclusive, individually to the' modulators i9, 20 and 2l in a predetermined order during intervals not substantially longer than the longest syllabic interval of the speech signal, thus to .derive the speech signal of modified ntelligibility to be transmitted.

In connection with the electronic switch t9, it will -be seen from the description thereof that this switch comprises a plurality of cascadeconnected vacuum tubes l, 15, 62, 63, etc., the

first and alternate ones of the'tubes, for example` tubes 1B, 62, etc., in the cascade arrangement normally having high conductance and the other of the tubes normally having substantially zero conductance. The switch also includes means for coupling the tubes in pairs to cause the tubes successively in pairs to traverse a complete cycle of change of conductance in response to the application of a negative control potential or control signal to lthe first tube. This meansr com'- prises the circuit elements 6G, 65, 66, 6l in'units 5| to 58, inclusive, elements 1S. l1, 'i8 and 'i9 in unit 10, and the couplingcondensers 69. The electronic switch further includes a plurality of control circuits individually so coupled'to the pairs of tubes that each of the control circuits is energized during thechange of conductance of one of the tubes ofthe pair associated therewith, such control circuits comprising control-circuit terminals 6i, S8 of the units 5l to 58, inclusive, whereby the control circuits are successively energized. There is additionally included in the electronic switch means for coupling the output circuit of the last of the tubes in the cascade arrangement to the input circuit of the first tube thereof to cause the control circuits to be successively and repeatedly energized, this means comprising the coupling condenser S9'.

Fig. 4 is a circuit diagram, partly schematic, of a complete carrier-wave receiver suitable for use .with the transmitter arrangement of Fig. 1 in'a secrecy communication system embodying the invention. Certain of the elements of this receiver correspond to similar elements of the transmitter arrangement of Fig. l andare designated by the same reference numerals primed;

' The receiver includes means for receiving the signal of modified intelligibility comprising a. carrier-wave receiver Il 'having an input circuit connected to an antenna ground system ill, H2. The receiver also includes frequency-responsive means for deriving from the received signal frequency-component subbands thereof corresponding to individual portions of the frequency band of the speech signal toy be reproduced. This means comprises band-pass filters its, il@ and H5 corresponding, respectively, to band-pass filters I 6, i1 and i3 of the transmitter arrangement but having respective pass bands of 2.1-3.0 kilocycles, 1.2-2.1 kilocycles, and .31.2 kilocycles. Each of the band-pass filters has an input circuit which is coupled to the output circuit of the carrier-wave receiver H0 and each includes an output circuit which is coupled to a signal input circuit of an individual one of the modulators i9', 20 and 2l'. The output circuits of moduband of 6.0-8.7 kilocycies. The output circuit of unit I I8 is coupled to an input circuit of a modudit vidual amplifiers 23', 24' and 25' to the input circuit of a band-passfllter H6 having a pass lator i4', the latter having an additional input circuit to which is coupled a carrier-wave generator l5. The output circuit of modulator i4' a pass band of SOO-3,000 cycles, to the inputcir-` cuit of an audio-frequency amplifier H8, and

there is coupled to the output circuit of the-lattery l unit a suitable reproducing device comprising a v loud-speaker HB.

As in the transmitter arrangement of Fig. l, there are coupled to carrier-wave input circuits of the modulators i9', 20' and l2| a carrier-wave repeater unit 'i'. includes means for separating the synchronizingsignal components of the received carrier wave, and these components are applied to the input circuit including input-circuit terminal 'il' of electronic switch t9', there being the difference in the electronic-switch 69 of the receiver that the output of the last unit thereof is not coupled to the input circuit of the additional unit, asv in the electronic switch t9 of the transmitter 'ar' rangement: This arrangement insures operation of unit 59 only in synchronism with unit d@ of the transmitter. The control circuits of the electronic switch i9 are coupled to the vertical Ybars of the manually-operable` switch and the horizontal bars thereof are coupled to the input electrodes of the relay devices of unit l' as at the transmitter. Also coupled to the input electrodes of the repeater devices of unit l are a plurality of displacement carrier generators 29' to '36', inclusive, in the same manner as at the transmitter.

Considering now the operation of the receiver arrangement just described, the carrier wave radiated by the transmitter is received by the carrier-wave receiver H0 and the modulation com- POnents thereof, comprising the signal of modied intelligibility, are derived in unit H0 and applied to the band-pass filters H3, iil and H5. TheA latter units derive the frequency-component subbands of the signal of-modiiled intelligibility and apply these subbands to the respective modulators i9', 2li and 2l'. The synchronizingsignal components of the received carrier wave accordance with the plan of transmission determined by the manually-operable switch 5t', and the relay devices, upon being thus energized, op-

lerate to apply such of the carrier waves of units 29 to 3S as are necessary successively to interchange the frequency-component subbands derived by units H3, llt and H5 relative to each other at intervals corresponding to the intervals at which these subbands were interchanged at the transmitter, but in reverse order to that at the transmitter, thus to derive in the output cirl cuits of modulators I9', 20' and 2l modulated The carrier-Wave receiver H0 bands are amplied by amplifiers 28'. 24' and 25 and applied to a speech-signal input circuit of the band-pass filter IIB which selects the proper sidebands and applies them to an input circuit of modulator lI4. There is also applied to'a carrier-wave input circuit of the latter unit a car- `rier wave generated by the carrier-wave generator |5, thereby to provide a modulated carrier wave, the lower sideband of which lies in the frequency band of .3-3.0 kilocycles and correspends to the speech signal to be reproduced, which, of course, is the speech signal X of full intelligibility derived by the microphone I2 of the transmitter. This sideband is selected by W-pass illter H1 and applied to the audio-frequency amplifier H8 where it is amplified and the amplified signal is then applied to the reproducing device ||9 for reproduction in conventional manner.

From the above description of the receiver arrangement of Fig. 4, it will be evident that units 1', Il', I5', I9', 20', 2|', 29' to 36', inclusive, 49', 50', III and |I1 comprise means for successively modifying the frequency-component subbands derived by the band-pass filters H3, H4 and ||5 relative to each other at intervals corresponding to the intervals at which the subbands were interchanged at the transmitter, but in reverse order to the predetermined order of modiiication at the transmitter. and for adding the modified subbands to derive the speech signal to be reproduced. It will further be evident that this means includes electronic switching means and also includes the means for deriving the plurality of disly-tuned band-pass ill'ters |2| to |21, inclusive7 each of which selects and translates onlythe fundamental frequency or a desired harmonic-frequency component of the carrier wave generated by unit |20. The output circuit of each of the band-pass filters |2| to |20, inclusive, iscoupied to the input electrodes of the repeater devices of unit 'l inthe same manner as the carrier-wave generators 29 to 20, inclusive. of the Fig. 1 arrangement. There is the difference in the present arrangement over the use of the'carrier-wave generators to 20, inclusive, of Fig. 1, that since the carrier waves translated by the band-pass filters |2| to |28, inclusive, are harmonically related, lt can be demonstrated that fewer carrier waves are vrequiredl in the Fig. -5 arrangement than in that of Fig. 1, since a greater number of the repeater devices in unit 1 may properly be coupled to the output circuit of certain ones of the band-pass illters |2| to |20,inclusive, in effecting the derivation of the ultimate speech signal of modified intelligibility. 'I'he output of the band-pass filter |21 comprises the fundamentalfrequencyl component of the carrier wave generated by unit |20 and thus is used as a synchronizing signal which is appliedto the modulator of unit 20 of the transmitter toV be transmitted simultaneously with the required synchronizing signal for the electronic switches 40 and 4l'.

16 When the arrangement of Fig. 5 is used at the receiver, unit |21 is dispensed with and the synchronizing-signal componentl of the received carrier wave is applied to a synchronizing circuit of the carrier-wave generator |20 in conventional manner to synchronize its operation with the corresponding unit of the transmitter apparatus. The operation of the Fig. 5 arrangement is otherwise essentially similar to' that described above in connection with Fig. 1 and will not be repeated.

As illustrative of specific frequency values rsuitable for use with the modified form of displacement carrier-wave generating means of the type shown in Fig. 5, the following values may l be used:

Kilocycles Frequency of carrier-wave generator 2 From the above description o f the invention, it will be evident that asecrecy communication system embodying the invention provides an exceptionally high degree of unintelligibility of the transmitted speech signal. 'I'he unintelligibility of the signal is due largely to two factors. The first of these concerns the particular manner, heretofore described, of interchanging and inverting the frequency-component subbands of the speech signal. The second factor'concerns the rate of interruption of the speechsignal: that is, the rate at which the frequency-component subbands are interchanged and inverted relative to each other. In the arrangement hereinbefore described, such interruption occurs at 50 millisecond intervals, which is equivalent to a rate of interruption of twenty times per second. This rate of interruption is one which has been found by actual operation to be quite satisfactory from the standpoint of rendering the transmitted speech signal highly unintelligible. The lower limit at which the rate of interruption is reasonably eiective is perhaps of the order of 'the average word interval which, as has previously been pointed out. is approximately one hundred times per minute or an ining bands of interruption rate at which the intelligibility of the speech is found to be satisfactory, that an upper limit of interruption rate occurs at approximately 2,000 times per second. and that above this limit the intelligibility of the speech is no longer impaired by interruption.

While the invention has been described as em` bodied in an`arrangement wherein the ultimate signal of modied'intelligibility has frequency` components in the frequency band of .33.0 kiloacoaose cycles, it will be evident that the frequencycomponent subbands A to F, inclusive, of the speech signal Y may, by proper choice of the carrier-wave generators 29-36, inclusive, be so shifted with relation to each other that the ultimate signal of modified intelligibility may have frequency components which occupy any deaired band of frequencies located in any desired.

portion of the frequency spectrum.

While there has been described .what is at present considered to be the preferred'embodiment of this invention, it will be obvious to those skilled in the art that various changes and modiiications may be made therein without departponent subbands corresponding to individuall portions of said frequency band, the frequencycomponent subbands of said second signal being successively modified relative to each other' in a predetermi-ned order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for translating and receiving said second signal and for deriving therefrom said first-named signal.

2. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-component subbands f said second signal being successively shifted relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said vspeech signal, and means for translating and receiving said second signal and for deriving therefrom said first-named signal.

3. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for derivingr from said signal a second signal comprising a plurality of frequency-'component subbands corresponding to individual portions of said frequency band,` the frequency-component subbands of said second signal being successively modified relative to each other and the frequency components of at least one` such subband being inverted in the frequency spectrum in a predeter` mined order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for translating and receiving said second signal and fir deriving therefrom said first-named signal.

4. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components. extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said y frequency band, the frequency-component subbands of said second signal being successively modified relative to each other and the frequency components of at least one of such subbands being shifted in the frequencyspectrum in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for translating and receiving said second signal and for deriving therefrom said first-named signal. v

5. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a. plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-component subbands of said second signal being successively modified relative to each other and the frequency.

components of at least one of such subbands being inverted and of at least another of said subbands being shifted in the .frequency spectrum in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for translating and receiving said second signal and for deriving therefrom said first-named signal.

6. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a. plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-component subbands of said second signal being successively modified relative to each other` and thev frequency components of at least one of Vsuch subbands being inverted and shifted in the frequency spectrum in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for translating and receiving said second signal and for `deriving therefrom said first-named signal.

7. A secrecy communication system comprising, an input circuit adapted to have applied thereto a, speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, electronic switching means for deriving from said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-'component subbands of said second signal being successively modified relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, and means for g translating and receiving said second signal' and for derivingi'rom said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-component subbands of said second 'signal being successively 19 modified relative to each other in a predetermined order at intervals of substantially fifty milliseconds durationland means for translating and receiving said second signal and for deriving therefrom said first-named signal.

9. A secrecy communication system comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said. frequency band, the frequency-component sub-bands'of said second signal being successively modified relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, manually-adjustable switch means forsaid frequency band, the' frequency-component subbands of said second Signal being successively modified relative to eachother in a predeterl mined order at intervals not substantially longer said frequency band, 'the frequency-component subbands of said second signal being successively modified relative to each other and the frequency components of at least one of such subbands being shifted in the frequency spectrum in a predetermined order at intervals not substantially longer than the longest syllable interval of said speech signal, and an output circuit .coupled to said means to have applied thereto said second signal, whereby the signal applied to said output circuit has modified intelligibiiity and is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy loi! communication.

13. A secrecy communication transmitter comprising. aninput circuit adapted to have applied thereto a speech signal to betransmittedsaid signal comprisingfrequency components extending over a predetermined frequency band, means for deriving from said signal a second signal comprising a plurality' of vfrequency-component subbands corresponding to individual portions of said frequency band. the frequency-component subbands of said second signal being successively modified relative to each other and the frequency components of at least one of such subiands being inverted and of at least another of such subbands being shifted in the frequency spectrum in a predeterminedvorder at intervals not substantially longer than the longest syllable interval ofv said speech signal, and an output circuit coupled to said means' to have applied thereto said second Isignal, whereby the signal applied to said outthan the longest syllable interval of said speech i signal, and anA output circuit coupled to said means to have applied thereto said second signal, whereby the signal applied to said output circuit has modified intelligibility and is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy ofcommunication.

11. A secrecy communication transmitter comprising. an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency. components extemling over a predetermined frequency band, means for deriving from said signal a second signal comprising a plurality of frequency-component sub- Y bands corresponding to individual portions otiaid frequency band, the frequency-component subbands o f .said second signal being successively modified relative to each other and the frequency components of atleast one of such subbands be- .inginvertedinthefrequencyspectnnnina'predetermined order at intervals not substantially longer than the longest syllable interval of said lspeech signal, and anoutput circuit coupledto said means to have `applied thereto said second signal, whereby the signal applied to said output circuit has modified intelligibility and is suitable for on therefrom to a remote pointwhilemaintainingahighdegreeofsecrecy of communication.

put circuit has modified intelligibility and is suitable for transmission therefrom to a remote point while vmaintaining a high degree of secrecy of communication. Y

i4. A secrecy .communication transmittercomprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, electronic switching means for deriving from said signal a second signal comprising a plurality of frequency-component subbands corresponding to individual portions of said frequency band, the frequency-component subbands of said second signal being successively modiiied relativeto each other in a predetermined order at intervals not substantially longer than the longest syllable inferval of said speech signal. and an output circuit coupled to said means to have applied thereto said second signal, whereby the signal applied to said output circuit has modified intelligibility and is suitable for tron therefrom to a remote point while maintaining a high degree of secrecy of communication.

15. VA secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a speech sign'al to be transmitted, said signal comprising frequency components extending vover a predetermined frequency band. means for deriving from said signal a second signal comprising a plurality of frequency-component subv bands corresponding to individual portions of said 12. A secrecy communication transmitter comprising, an input circuit adapted to have applied theretoaspeechsignaliobetransmitted.said signal comprising frequency component extending over a predetermined frequency band, means for deriving from said signal a second signal comprislng a plurality of frequency-component subbands corresponding to individual portions of.

frequency band, the frequency-component subbands of said second signal being successively modined relative to eachother in a predetermined order at intervals of substantially ility milliseconds duration, and an output circuit coupled to said means to have applied thereto said second signal, whereby the signal applied to said output circuit has modiiied intelligibility and is suitable for transmission therefrom to a remote `.2l point whilemaintaining a high degree of secrecy of communication.

16. A secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a, speech signal to be transmitted, said signal comprising frequency components extend- \\ing over a, predetermined frequency band, means,

ing and modifying as desired from time to time y the said predetermined order of interchangingof said frequency-component subbands, and an output circuit coupled to said means to have applied thereto said second signal, whereby the signal applied to said output circuit has modified intelligibility and is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy of communication.

17. A secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over'a predetermined frequency band, means for separating said speech signal into a plurality of frequency-component subbands corresponding to individual portions of said frequency band,

means for successively modifying said subbands 35 tive to render said one repeater devices operarelative to Veach other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal to derive a second signal having modified intelligibility, an` output circuit coupled to said last-named means to have applied thereto said second signal, whereby the signal applied to said output circuit is suitable for transmission therefrom to a re-' mote point while maintaining a highdegree of secrecy of communication. Y

18. A secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, means for separating said speech signal into a pluralityof frequency-component subbands corresponding to individual portions of said frequency band, means for deriving a'plurality of displacement carrier waves, means including said last-named means for successively modifying said subbands relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal to derive a second signal having modified intelligibility, and an output circuit coupled to said last-named means to .have applied thereto saidr second signal, whereby the signal applied to said `output circuit is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy of communication. l

19. A secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a speech signal to be transmitted, said signal comprisingffrequency components extend*- ing over a predetermined frequency band, a p lurality of band-pass filters for separating said speech signal into a plurality of subbands corresponding to individual portions of said frequency band, a plurality of modulators individually coupled to said band-pass filters, means for deriving leficaces;

22 a plurality of displacementfcarrier waves, means including electronic switching means for successively applying said carrier waves individually to saidmodulators in a predetermined order during intervals not substantially longer than the longest syllabic interval of said speech signal to derive a second signal having modified intelligibility, and

- an output circuit coupled to said modulators to have applied thereto said'second signal, whereby l0 the signal applied to said output circuit is suitable for transmission therefrom to a remote point while maintaining a high degree of secrecy of' communication. Y

20. A secrecy communication transmitter com- -prising, an input circuit adapted to have applied thereto a speech signal to be transmrtted, said signal comprising frequency components extending over a predetermined frequency band, a plurality 'of band-pass filtersv for separating 'said speech signal into a plurality of frequency-component subbands corresponding to individual portions of said frequency band, a plurality of modulators individually coupled to said filters, means for deriving a plurality of displacement carrier 5-waves, a plurality Iof repeater devices arranged in groups each adapted to apply a predetermined group of said carrier waves to individual ones of said modulators, electronic switching means having a, plurality of control circuits successively o energized at intervals not substantially longer than the longest syllabic interval of said speech signal, each of said control circuits being coupled to one repeater device in each of said groups of repeater devices and when energized being effeccessively to modify said frequency-componentV subbands relative to each other at said intervals,

40 and an output circuit coupled to all of said modulators for combining the interchanged frequencycomponent subbands to/ derive a second signal having modified intelligibility, whereby the signal derived in said output circuit is suitable for transmission therefrom to a remote point while maintaining a high degreeof secrecy of communication.

21. A secrecy communication transmitter comprising, an input circuit adapted to have applied thereto a speech -signal to be transmitted, said signal comprising frequency components extending over a predetermined frequency band, a plurality of band-pass filters for separating said speech signal into a plurality offrequency-component subbands corresponding to individual portions of said frequency band, a plurality of modulators individually coupled to said filters, means for deriving a plurality of displacement carrier Waves, a. plurality of repeater devices arranged in groups each adapted to apply a predetermined group of said carrier waves to individual ones of said modulators, electronic switching means having 'a plurality of control circuits successively energized at intervals not -substantially longer than the longest syllabic interval of said speech signal, each o1 said control circuits being coupled to one repeater device in each of said groups of repeater devices and when energized being effective to render said one repeater devices operative to'apply a group of said carrier waves to individual ones of said modulators, thereby successively to modify said frequency-component subbands relative to each other at said intervals, manually-adjustable switching means for establishingand modifying as desired from time to amaca 'component subbands corresponding to individual portions of the frequency band of said speech 24 l lied relative to eachother in a predetermined orderV at intervals not substantially longer than the longest syllabic interval of said speech signal,

a plurality of band-pass filters for deriving said frequency-component subbands of said received Signaka plurality of modulators individually coupled to said'iilters, means for deriving a plurality of displacement carrier waves, and means including electronic switching means for successively signal but4 having said subbands successively modiiled relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval of said speech signal, frequency-responsive means for deriving said frequency-component subbands of said received signal, and means for successively modifying said derived frequency-component subbands Arelative to eachother at intervals corresponding to said nrst-nam'ed intervals but in reverse order to said predetermined order and for adding said last-named modied subbands to derive said Silmal to be reproduced. l

23. A secrecy communication receiver comprising, means for receiving a signal of modified intelligibility which is` derived from a speech signal to be reproduced and which includes frequencycomponent subbands corresponding to individual portions of the frequency band of said speech signal but having said subbands successively modiiled relative to each other in a predetermined order at intervals not substantially longer than the longest syllabic interval oi said speech signal, frequency-responsive means forderiving said frequency-component subbands of said received signal, and electronic switching means for successively modifying said derived frequency-component subbands relative to each other at intervals corresponding to said rst-named intervals but in reverse order to said predetermined order and for adding said last-named modiiied subbands to derive said signal to-.be reproduced.

24. A secrecy communication receiver comprising, means for receiving a signal of modified intelligibility which is derived from a speech signal to be reproduced and which includes frequencycomponent subbands corresponding to individual portions of the frequency band of said speech signal but having said subbands successively modled relative to each other in a predetermined orderat intervals not substantially longer than the longest syllabic interval vof said speech signal, frequency-responsive means for deriving said frequency-component subbands ot said received signal, means for deriving a plurality of displacement carrier waves, and means including said last-named means for successively modifying said 'derived frequency-component subbands `relative applying said carrier waves individually to said modulators in a predetermined order during intervals corresponding to said Brat-named intervals successively to modify said derived frequency-component subbands in reverse` order to said predetermined order and for adding the outputs of said modulators. to derive said signal to be reproduced.

28. An electronic switch comprising, a plurality of cascade-connected vacuum tubes, means inter.. coupling said tubesin pairs to cause saidtubes successively in pairs to traverse a complete cycle of change of conductance in response to the application of a control signal to the ilrst of said cascaded tubes, and a plurality of control circuits individually so coupled to said pairs of tubes thateach of said control circuits is energized during the change of conductance of one of the tubes of the pair associated therewith, whereby said control circuits are successively energized.

27. An electronic switch comprising, a plurality of cascade-connected -vacuum tubes, alternate ones of said tubes in the cascade arrangement normally having high conductance and the other of said tubes normally having low conductance, means intercoupling said tubes in pairs to cause said tubes successively in pairs to traverse a com.. plete cycle of change of conductance in response to the application of a control signal to the first oi' said cascaded tubes, and a plurality of control circuits individually so coupled to said pairs oi' tubes that each of said control circuits is energized during the change of conductance of one of the tubes of the pair associated therewith, whereby said control circuits are successively energized.

28. An electronic switch comprising, a plurality of cascade-connected vacuum tubes, a ilrst and alternate ones of saidv tubes in the cascade arrangement normally having high conductance and the other of said tubes normally having low conductance, means intercoupling said tubes in pairs with a normally high conductance tube as the iirst tube of each pair to cause said tubes successively in pairs to traverse a complete cycle of change of conductance in response to the application of a control signal to the ilrst oi' said cascade tubes, and a plurality of control circuits individually so coupled to said pairs oi tubes that each of said control circuits is energized during the change of conductance of one of the tubes of the pair associated therewith, whereby said control circuits are successively energized.

29. An electronic switch comprising, a plurality Aof cascade-connected vacuum tubes, means 5 dntercoupling said'tubes in pairs to cause said componentsubbands corresponding to individual portions of the frequency band of said speech sig- .1 hm. having said subbands successively moditubes'successively in pairs and at the ends of predetermined equaltimeintervalstotraverseacomplete cycle of change of conductance in response tothe application of acontrol signal to the iirst i of said cascade tubes, and a plurality of cmtrol circuits individually soV coupled to said pairs of 30. An electronic switch comprising, a plurality of cascade-connected vacuum tubes, means intercoupling said tubes in pairs to cause said tubes successively in pairs to traverse a, complete cycle of change of conductance in response to the application of a control signal to the first of said cascaded tubes, a plurality of control circuits ineach of said control circuits ls energized during the change of conductance of one of the tubes of the pair associated therewith, and means cou pling the output circuit of the last of said/cascade tubes to the input circuit of the first tube thereof, whereby said` control circuits are successively and repeatedly energized.

31. An electronic switch comprising, a plurality of cascade-connected vacuum tubes, the first and y dividually so coupled to said pairs of tubes that accanto 26 alternate ones of said tubes in the cascade arrangement normally having high conductance and the other of said tubes normally having low conductance, means intercoupling said tubes in pairs with a normally high conductance tube as the first tube of each pair to cause said tubes successively in pairs to traverse a complete cycle of change in conductance in response to the application of a negative potential pulse to the first of said cascade tubes, anda plurality of control circuits individually so coupled to said pairs of K sized during the change of 'conductance of one X tubes that each of said control circuits is enerof the tubes of the .pair associated therewith; whereby said control circuits are successively s energized.

JAMES F. CRAAIB.

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