US2411206A

US2411206A - Electrical signaling method

Info

Publication number: US2411206A
Application number: US536233A
Authority: US
Inventors: Guanella Gustav
Original assignee: Radio Patents Corp
Current assignee: Radio Patents Corp
Priority date: 1943-04-05
Filing date: 1944-05-19
Publication date: 1946-11-19
Anticipated expiration: 1963-11-19
Also published as: FR902917A; CH238926A; BE455172A; GB582737A; NL63459C

Abstract

582,737. Secret transmission. PATELHOLD PATENTVERWERTUNGS- & ELEKTRO-HOLDING AKT.-GES. April 5, 1944, No. 6377. Convention date, April 5, 1943. [Class 40 (iv)] In a system of secret telephony wherein the speech band is converted into a number of adjacent but transposed sub-bands of which the relative widths are constantly varying, the arrangement is such that at least one subband is inverted and at least one is not inverted. As shown in Fig. 4, the speech band e0 modulates successively a fixed frequency g1 and a varying frequency g2, the lower side-band being selected in each case to give the band e2. This then modulates a pair of frequencies g3, g4 spaced by the width of the band to give an inverted double-band e3 and also another pair of frequencies g5, g6 to give an erect doubleband e5. Complementary portions e4, e6 are selected from these double-bands and combined to form a double-band e7, from which the central portion is taken and shifted to produce a final band e10 comprising four subbands of varying width, two being erect and two inverted. In another arrangement of a similar character, different transpositions of the sub-bands are obtainable by a choice of carrier frequencies at the intermediate modulation stages. One or more of the sub-bands may be retarded ih relation to the remainder of the band ; for example, the selected portion e4 may be retarded by means of a telegraphone before combination with the portion e6. The speech band may be subjected to variable attenuation before scrambling. When the arrangement is applied to a multi-channel system, several speech channels may be grouped together for the purpose of scrambling.

Description

N017. 19, 1946. Q GUNELLA 2,411,206

ELECTRICAL SIGNALING METHOD b3 TCI- 2- d 1 zwlal/ lganella BY J /ldf` Y HTTORN E Y NOV. 19, 1946. Q GUANELLAy ELECTRICAL SIGNALING METHOD Filed May 19, 1944 4 Sheets-Sheet 2 e] i g VE'Z I 3 f g 4 "//f/l/M "f///m Iremn INVENTOR. nd/ella vf /zaf HTTORNE Y NV- 19 1945- G. GUANELLA ELECTRICAL SIGNALING METHOD Filed .May 19, 1'944 4 Sheets-Shee. 4

M nK. m m6. Ww w TTORN EY Y two sub-bands of the scrambled signal.

Patented Nov. 19, 1946 il LZ FFICE ELECTRICAL SIGNALING METHOD Gustav Guanella, Zurich, Switzerland, assignor to Radio Patents Corporation, New York, N. Y., a corporation of New York Application May 19, 1944, Serial No. 536,233 In Switzerland April 5, 1943 11 Claims.

or the like, is involved, and the main object of the invention is the provision of improved and simplified arrangements for systems of this character.

According to known methods of scrambling speech signals, the speech frequency band is divided into several sub-bands of constant and equal width, these bands being interchanged according to a prescribed key. In order to increase the degree of secrecy, individual sub-bands may be inverted. A disadvantage of this method is the fact that, on account of the fixed frequency limits of the individual sub-bands, deciphering by repeatedly reshifting the bands is greatly facilitated.

According to another known method, the speech frequency band is divided into unequal sub-bands of varying width which are transposed by modulating them with auxiliary frequencies. The sub-band with the original high speech frequencies then appears as the lower frequency band, and the sub-band With the `original lower speech frequencies appears as the upper frequency Iband of the scrambled signal, the original lowest and the original highest frequencies adjoining each other at the boundary between the A disadvantage of this method is that decoding is comparatively easy, if this frequency boundary is shifted about until the message is deciphered. Successive and repeated application of this same method does not increase the degree of secrecy.

In order to avoid these disadvantages, the invention proposes a method of scrambling speech signals and whereinv the signal which is to be scrambled is converted before transmission into a signal consisting of several mutually interchanged sub-bands of the original frequency band, whose width is varied and at least one of which is inverted and at least one is not inverted. This new method possesses the great advantage that due to the variable width of the individual bands, reshifting is rendered .diicult As a result of the continuously variable or very finely graduated adjustment of the band width, `a -much greater variety of scrambling possibilities Ais obtained than is possible with bands of constant width.

The objects and novel aspects of the invention will `become more 'apparent from thefollowing detailed description taken in reference to the ,ac-

companying drawings forming part of this specication, and wherein:

Figure 1 is a diagrammatic representation explanatory of a known method of speech scrambling by dividing a signal frequency band into sub-bands of varying relative band widths.

Figures 2-5 are similar diagrams explanatory of the improved method according to the invention.

Figure 6 is a block diagram of a system for carrying out a speech scrambling method of the type illustrated in Figure 5; and

Figure 7 is a further diagram illustrating the reproduction of a message scrambled in accordance With Figure 5.

Referring to Figure l, an loriginal speech frequency band is shown at bo. By modulating band bo with the constant auxiliary frequencies f1, f2 and suppressing the lower side bands or difference Vfrequencies of both modulation products, the frequency bands b1, b2 are obtained. A further modulation of bands b1 and b2 with the variable auxiliary frequency fs results in an intermediate signal b3 after suppression of the sum frequencies of the modulation product. `The frequencies of the signal bs indicated by the broken lines are suppressed by a Vband filter having a fixed band pass characteristic. A further modulation of band b3 with the constant auxiliary frequency f4 then produces as final dilference frequencies the scrambled signal b4. The original signal amplitudes are indicated in all signals by the height of the shaded areas, assuming a linear variation as a function of frequency of the signal bo.

In a system of vthis type, therefore, the signal frequency band is divided into a plurality of subbands mutually interchanged in respect to their initial relative position and having relatively varying frequency such that the total frequency band remains the same and equal to that of the original signal.

The signal b4 may be easily deciphered by reshifting it according to the same method until the original signal bo is obtained. A further application of the same scrambling'method does not increase the secrecy, because no further subdivision of the frequency band is obtained, and the signal ba obtained from b4 by a further modulation process with frequencies ft-fa corresponding to fi-f4 mai7 readily be deciphered by a simple reshifting of the frequency bands.

The secrecy of such a system is greatly improved according'to the present invention by subjecting lat vleast one sub-band .of the intermediate signal `ba Vto .a 'frequency inversion. Thus, as

2,411,2o6 Y f shown in Figure 2, band b3 may for instance be divided into the sub-bands c4 and c5. Inversion of the latter then results in the band cs. By adding the frequency bands c4 and ce, the band c'z is obtained and finally by reshifting the frequency band to the initial position, a nal scrambled signal cev is produced. It is obvious that the scrambled signal cg may be subjected to further scrambling processes, Such as by repeating the above method, whereby contrary to a repeated scrambling process of Figure 1, an increased degree of secrecy is obtained.

Alternatively, it is possible to obtain from signal b3, by inverting one sub-band thereof, a new signal d4 which is shifted to its initial position d5. In this case, too, repeated application of the scrambling process will result in an increase of secrecy.

According to another modification of the invention, a sub-band of the intermediate signal b3, for instance band c5 of Figure 2, may be retarded for a definite interval of time. A signal is then obtained which, in order to increase the secrecy, can be subjected a number of times to the aforementioned scrambling processes.

Another method of carrying out a scrambling process according to the invention is illustrated by Figure 4. The intermediate frequency band e1 is obtained from the original speech frequency band en by modulating it with the constant frequency g1. A further modulation with the variable frequency gz produces the new intermediate frequency band e2 which is shown for an average position of 92. A further modulation of e2 with the auxiliary frequencies g3 and g4 then results,

after suppression of the sum frequencies, in a new intermediate frequency band e3 which is inverted with respect to e0. A limited band e4 is segregated from e3 by means of a filter of corresponding band pass characteristic. By modulating e2 with the further constant auxiliary frequencies gs and ge a further new intermediate frequency band es is obtained which, after the suppression of a sub-frequency range by a suitable filter,

results in the signal es. By adding e4 and es the intermediate frequency band e7 is produced. Renewed modulation with the optional auxiliary frequency g8 results in the difference frequency e8 from which the sub-band e9 is ltered out. Finally, by modulating with the auxiliary frequency gio, the scrambled signal ein in the original frequency band is obtained. It will readily be seen 'that em contains all components of the original signal e as transposed frequencies.

As the frequency of g2 i-s varied, the sub-bands in ein move accordingly in such a manner that the frequencies which pass beyond the upper limit of the frequency band reappear at the lower part of the band and, due to the inversion of part of the sub-bands, it will- :be practically impossible to reproduce the original signal by reshifting of the bands by an unauthorized receiver.l

A further advantage of the scrambling method described hereinabove is due to the fact that by employing the same a number of times in succession, it is possible to increase the degree of secrecy to any desirable degree.

An alternative manner of carrying out a scrambling method according to the invention is illustratedin Figure 5. By modulating the speech .frequency band Zo with a frequency h1, the first intermediate signal Z1 is obtained and as a result of a further modulation with a frequency h2, a second intermediate signal Z2 is produced. Further modulation with a variable auxiliary frequency h3 then results in the intermediate signal ls. The low frequency sub-band of Z3 lying below the xed boundary frequency 1c segregated by means of a similar cut-off filter is moved upwards again by modulating it with h3 to produce the lower sub-band of the new intermediate signal Z4. IThe upper `sub-band of 14 resultsf'rom the inversion of the upper sub-band of la by modulating it with h4.. An intermediate signal l may also be formed by modulating the lower sub-band of la with a frequency h5 and modulating the upper sub-band of Z3 with the frequency h3. The intermediate signals Z4 and l5 respectively thus eachY contain two signal frequency bands of variable width, one of these bands being inverted. If the same process is I"epeated with the auxiliary frequency he and hq or ha, this will result in the intermediate frequency bands le and lv, respectively, and final bands Z8, Z9, Zio, In. When the process is repeated a third time, the intermediate signal ly and the scrambled signal Zz are obtained, which in the case under consideration occurs in eight different combinations.

With this method a deciphering can be rendered more difficult by repeating the procedure,

without the clarity of the transmitted speechv being thereby affected to any noticeable extent.

When modulating with variable frequencies, the lower side bands have always been used as an intermediate signal for scrambling purposes. The upper side band-s may, however, also be used for the same purpose. The shifting of the frequencies by variable amounts may also be accomplished by a modulation frequency whose frequency lies above the range of the signal frequency which is to be shifted.

Figure 6 illustrates in block diagram form an arrangement by means of which the method according to Figure 5 may be realized. The modulators and band filters are indicated by letters M and F, respectively, and the cut-off filter-s by W, while the reference letters l for the frequency bands and h for the modulation frequencies are the same as those in Figure 5. The change-over 7 switch S enables the frequencies h3 and h4 or ha and h5 to be employed for modulation as required,

whereby bands Z4 and l5, respectively, are formed.

Similar change-over switches (not shown) may be provided after the second and third shunting switches by-mean-s of which it is possible to choose between the intermediate signals le, lio and 19,111,

respectively, and the desired constellation of the bands ly.

When applying the method according to the invention, it is desirable that the auxiliary frequencies h3, hs and hg should be finely graduated or made constantly variable so that the band limits in the scrambled signal can be shifted as desired. A scrambled signal is thus obtained, the 50 sub-bands of which have an unequal band width. In order to reconstruct .the original signal, the conversions employed during the scrambling process are eliminated gradually by reversing the operations, Figure 7 illustrates one way of achieving this, the same frequencies (these being indicated again by vertical arrows) and the same y nals is an inverted signal. An advantage of these partial retardationsis due to the fact that the temporal course of the energy in the scrambled signal differs from that of the original signal, so that unauthorized deciphering by studying the energy rhythm is impossible. The energy rhythm can also be made to differ` from its original course if the signal to be scrambled is subjected to a variable damping which may or may not depend upon frequency.

The method according to the invention may also be applied to multi-channel transmission. In this case, it is not necessary for each channel to be scrambled, it being suiiicient if several speech channels are grouped together for the purpose of l scrambling, so that the message frequency band formed from several channels is subjected as a Whole to one of the` scrambling processes described.

YI claim:

g 1. In a secrecy signaling system, the method consisting in separately modulating a pair of fixed carrier frequencies by a signal frequency band, said carrier frequencies being separated by a distance equal to the width `of said signal band, modulating a further periodically variable carrier frequency by the resultant side bands located at the same .side of said fired carrier frequencies to produce a resultant intermediate band comprising two adjacent signal bands movingv back and forth along the frequency scale at the rhythm of said variable carrier frequency, and segregating from said intermediate band a band equal to the original signal band and lyingl within fixed points on the Yfrequency scale located at equal distances from the middle of said resultant band in the position corresponding to the mean value of said variable carrier frequency- 2. In a secrecy signaling system, the method consisting in separately modulating a pairof fixed carrier frequencies by a signal frequency bad,said carrier frequencies being separated by a -distance equal to the width of said signal band, modulating a further periodically variable carrier frequency by the resultant side bands located at the same side of said fixed carrier frequencies to produce a resultantv intermediate band comprising two adjacent signal bands moving backrand forth .along the frequency scale at the rhythm of said .variable carrier frequency, segregating from s aid intermediate band a band equal to the original signal band and lying within fixed points on the frequencyscale located at equal distances from into a plurality of sub -bands rearranged with respect to their originall order to produce a scram- -bled signal of band widthequal to the band width of the original signal band, means for inverting at least one of said sub-bands, and further means for continuously and periodically shifting the individual frequencies of the scrambled signal band between the upper and lower limits thereof and with the frequencies passing beyond the upper end reappearing at the lower end' of the band.

5. In a secrecy signaling system, a source of signals comprising a band of component frequencies, means to provide a pair of Xed carrier frequencies located outside of and separated by a distance equal to said signal band, means for modulating each of said carrier frequencies by said signal band, a source of periodically variable carrier frequency, means for modulating said variable carrier frequency by the resultant side bands 0btained from said rst modulating means and located at the same side of said fixed frequencies to produce a resultant intermediate frequency band comprising two adjacent signal bands moving back and forth along the frequency scale at the rhythm of said variable carrier frequency, and a band pass filter having fixed lower and upper frequency limits comprising a range equal tosaid signal frequency band and spaced by equal distances from the center of said intermediate frequency band in lthe position correspondingto the mean value of said variable carrier frequency.

6. In a secrecyv signaling system; a sourceof signals comprising a bandA of component frequencies, means to provide a pair of fixed carrier frequencieslocatedoutside of yand separated by a distance equal to said `signal, band, means for modulating each of said carrier frequencies by said signal band, a source of periodically variable carrier frequency," meansy for kmodulating said variable carrierv frequency by the resultant side bands obtainedfrom said firstl'modulating means and locatedat the same side of said fixed frequencies to produce a resultant intermediate frequency band comprising two adjacent signal bands moving back and forth along the frequency scale atfthe rhythm of said variable carrier frequency, a band pass filterhaving fixedlower and the middle of said 'resultant bandin the position corresponding, to the mean value of said variable carrier frequency, whereb-y the segregated band is divided inte sub-bands'of constantlyvarying relative band width, and frequency-inverting at least one of said sub-bands.

` 3. In a secrecy signaling system, a source of signals comprising a band of component frequencies, means for subdividing said signal band into a plurality of sub-bands and for rearranging the order of said sub-bands to produce,V a scrambled signal of band width equal to the band width of the original signal, means for inverting at least one of said sub-bands, and further means for continuously shifting the individual frequencies of the scrambled signal band within the upper and lower frequency signal limits thereof k and with the frequencies passing beyond the upper end reappearing at the lower end of the band.

Ll. In a secrecy signaling system, a source of signals comprising a band of component frequencies, means for subdividing said signal band upper frequency limits 'comprising a range equal to said signal frequency band and spacedA byY equal distances from the center of said intermediate frequency bandwin the position oorrespondingto. the mean value lof said variablecarrier frequency, p whereby, the band segregated by said lter is divided into sub-bands of continuously relatively varying band width, and means for inverting at.

least. one of said sub-bands. Y

7L In a secrecy signaling system, a source of signals comprising a band of component. fre,- quencies, means to provide apair of fixed car-y rier frequencies located outside of Vand separated by'a distance equal-tosaid signalA band, means for modulating each of said carrier frequencies by said signal band, a source of periodically variable carrier frequency, means for modulating said variable carrier frequency by the resultant side bands obtained from said first modulating means and located at the same side of said fixed frequencies to produce a resultant intermediate frequency band comprising two adjacent signal bands moving back and forth along the frequency scale at the rhythm of said variable carrier frequency, a band pass filter having xed lower and upper frequency limits comprising a range equal to said signal frequency band and spaced by equal distances from the center of said intermediate frequency band in the position corresponding to the mean value of said variable carrier frequency, further band pass filter means having a band width less than said first band pass filter for dividing said intermediate frequency band into at least two partial bands, and means for inverting at least one of said partial bands and recombining it with the remaining non-inverted partial band. l

8. In a secrecy signaling system, a source of signals comprising a band of component frequencies, means to provide a pair of fixed carrier frequencies located outside of and separated by a distance equal to said signal band, means Vfor modulating each of said carrier frequencies by said signal band, a source of periodically variable carrier frequency, means for modulating said variable carrier frequency by the resultant side bands obtained from said first modulating means and located at the same side of vsaid fixed frequencies to produce a resultant intermediate frequency band comprising two adjacent signal bands moving back and forth along the frequency scale at the rhythm of said variable carrier frequency, a band pass filter having fixed lower and upper frequency limits comprising a range equal to said signal frequency band and spaced by equal distances from the center of said intermediate frequency band in the position corresponding to the mean value of said variable carrier frequency, a further band pass filter having a -band width being a fraction of the band width of and symmetrically located with respect to said iirst band pass filter to produce-a pair of sub-bands from said intermediate frequency band, and means for frequency-inverting at least one of said last-mentioned sub-bands and recombining it with the other sub-band.

' 9. In a secrecy signal system, a source of sigdistance equal to said signal band and v'located symmetrically to said first pair of fixed carrier frequencies with respect to said side bandin the position corresponding to the mean value of said variable carrier frequency, means to modulate each of said pairs of fixed carrier frequencies by l said'side band and for segregating a pair of second modulation side bands from the modulation product, each of said second side bands comprising two adjacent signal side bands with the signal bands of one side band being inverted relative to the signal bands of the other side band, and further means for segregating from each of said pairs of second side bands a predetermined subband and for combining resultant sub-bands to produce a final scrambled signal of band width equal to the original signal band and having subbands varying continuously between the upper and lower frequency limits thereof.

10. In a secrecy signaling system, a source of signals comprising a band of component frequencies, a first source of periodically variable carrier frequency, means for modulating said carrier frequency by said signal band, means to divide a resultant modulation side band at a fixed fre-- quency to produce a pair of sub-bands of continuously varying relative band width, means for inverting at least one of said sub-bands and combining it with the other sub-band to produce an intermediate scrambled signal, a second source of periodically variable carrier frequency differing from said iirst source by a predetermined frequency difference, means for modulating said second carrier frequency by said intermediate signal, and furthermeans for dividing a resultant modulation side band at said fixed frequency to produce a second intermediate signal of width equal to the original signal frequency band and comprising three sub-bands of continuously varying relative band width.

11. In a secrecy signaling system, a source of signals comprising a band of component. frequencies, a first source of vperiodically Variable carrier frequency, means for modulating said carrier frequency by said signal band, means to divide a resultant modulation side band at a fixed frequency to produce a pair of sub-bands of continuously varying relative band width, means for inverting at least one of said sub-bands and combining it with the other sub-band to produce an intermediate scrambled signal, a second source of periodically variable carrier frequency differing from said first source by a predetermined frequencyV difference, means for modulating said second carrier frequency by said intermediate signal, further means for dividing a resultant modulation side band at said fixed frequency to produce a second intermediate signal of width equal to the original signal frequency band and, comprising threesub-bands of continuously varying relative band width, and means for further successively modulating the resultant kintermediate signals against variable carrier frequencies differing from theV preceding carrier frequencies and for'deriving resultant side bands to produce a final scrambled signal of band width equal Ato theforiginal signal band and comprising Ya ldesired number of sub-bands some of which are inverted and all of which vary continuously in relative width within the upper and lower frequency limits of the scrambled signal.

` GUSTAV GUANELLA.