US1542567A - Secret signaling - Google Patents

Description

June 16, 1925.

R (3.. MATE-1E5 SECRET S IGNALING Filed June 30, 1923 a m H ,0? W Ev 5 w Patented June 1%, l gw like a ROBERT C. MATHES, OF NEVJ YORK, N. Y., ASSIGNOR- T0 WESTERN ELECTRIC 601M- ?AN'Y, INCORPORATED, OF NEW YORK, N. '35., A GORPQRATIQN 0F NEW YQRK.

SECREC-i. SIGNALING.

Application filed June so, 1923. Serial No. 648,649,

To all whom it may concern:

Be it known that I, Bonner O. Marissa, a citizen of the .United States, residing at New York city, in the county of Bronx and State of New York, have invented certain new and useful Improvements in Secret Signaling, of which the following is a full, clear, concise, and exact description.

The present invention relates to the secret transmission of waves for any desired purpose, such as signaling;

The invention is particularly applhiahle to the transmissionand reception, with secrecy, of waves such as signals and the like comprised of a considerable band of f equencies, as in the case of speech; An object of the invention is to obtain a high degree of secrecy with comparatively simple terminal apparatus and by such transformations of the signal or other waves as will preserve a high degree of quality in thereproduced signals or waves at the receiver.

In the specific form of embodiment to be described herein, the speech waves toJoe secretly transmitted are subdivided inti) "relatively narrow sub-bands of frequency components. A plurality of these sub-bands non-adjacent to one another in frequency are transmitted overyone transmission path or channel, and other sub-bands are sent over a different channel. I For example, the sub-bands transmitted over the first channel. may comprise every alternate sub-band taken. in the order of ascending frequencies, and the remaining sub-bands may be sent over a different channel. The frequency inter vals between the successive subbands trans mitted over the same channel may be occupied by noise components.

At the receiver arranged in accordance with the invention, the sub-bands received over each path are separated from the noise currents by suitable selective means and the sets of sub-bands transmitted over the difierent channels are combined in the receiver. I

To an outsider who does not have access to all of the channels over which component pasts of the same message are transmitted, or who does not know the. number, width, and arrangement of the various sub-bands, cr has not the necessary filters to separate the speech components from the noise components, it is impossible to receive any inseptation of a transmitting station, and Fig.

2 is a similar representation of a receiving station adapted to cooperate with the station of Fig. 1 in carrying on communica tions with secrecy.

he speech currents developed in circuit 2 as a result of speaking against transmitter l are applied to the series of subdividing o'r analyzing filters F F F Each of these filters is a band transmission filter of theqtype disclosed in the patent to Campbell, No. 1,227,113 of May 525), 191? These filters are for the purpose of subdividing the normal speech frequency band into sub-bands of relatively narrow frequency ran e, each of which subhands by itsell is incapableol reproducing understandable speech,

,Tl'ie two sub-bands selectively transmitted by the respective filters F and ll, nonadjacent in frequency, and are preferal'i y spaced apart in frequency by an interval equal to the width or the band trans sass by the filter F Similarly, the iiiter F, transmits the next higher adjacent band to that transmitted by the filter F it being assumed. that the filter F frequencies than the filter F as shown in this figure, therefore, it will be seen that the lowermost subband of the speech impulse is transmitted by the filter transmits higher With the arrangement of the "four filters F The next higher sub-band. beginning substantially at the upper limit of the first I band is tra'nsmitted hy' the filter F Sinai-J larly, the next higher hand beginning sub stantially at the upper limit of the frequency range of the filter F is transmitted. by thefil ter F and the highest of the four sch-bands is transmitted by the filter E if more than four filters are used, the sub-bands may be of correspondingly narrower frequency range, and, as in the case shown in the drawing', the analyzing filters will be grouped into a plurality of sets, the filters of each set transmitting sub-bands which are non-ad jacent in frequency.

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The sub-bands transmitted by the filters F and F are applied to the modulator M through the input transformer 3. This modulator is supplied also by a carrier wave from the source 4- so that the applied subbands modulate the carrier Wave, and produce for transmission a modulated Wave having two side bands, as is Well known in the art. ."Sources f and f of continuous Waves are also arranged to be applied to the modulator M simultaneously With the sub-bands from the filters F and F These wave sources are preferably vacuum tube oscillators of the type shown in the patent to.

l lartiey, No. 1,356,763, October 26, 1920.,the tuning condensers 5 and 6 of which are arranged to have their movable plates continuously driven by the motor 7 so that the frequency of each generatorvaries cyclically between two fixed limits. The limits between which the frequency of the oscillator f varies are of thesame order of frequency as the frequency limits of the band transmitted by the filter-F Similarly, the frequency of the oscillator f varies over a range equivalent to the transmission range of the filter F The rate of variation of the frequency of these wave sourcesmay be sufiiciently high so that ifthey Were applied to a telephone receiver they would appear as noise currents havinga band of frequencies of the widthequivalent to the frequency separation of the limiting frequencies bet-ween which they are varied.

As a result of applying to the modulator M both the sub-bands from the filters F F and the noise currents from the oscillators f and f the carrier wave 4 is modulated by a band of frequencies of normal speech frequency width. It is preferred to use a balanced modulator of the type indicated so that the unmodulat'ed carrier component is suppressed at the transmitting station. Also by sharply tuning the trans mitting antenna TA, to one of the two side bands resulting from the modulation, the components that are transmitted can be re stricted to those comprising a single one of the two side bands. Instead. of using the type of modulator shown, a modulator circuit of the type disclosed in the patent to Osborne, No. 1,861,488, December 7', 1920, may be employed for the puypose of restricting the transmitted components to those comprised in the single side band. By so restricting the transmitted components, the actual amount of energy transmitted is only about one-sixth great would be the case if bothside bands and the unmodulated carrier omponcnt were transmitted. l v

The sub-bands transmitt d by the filters F, and F, are applied t the modulator M through the transforme- 8. This mod ulator is also supplied wlth carrier waves; from the source 9 and with noise currents trom the oscillators f and 7". These oscillators have their tuning condensers 10 and 11, respectively,continuously varied by the motor 7 so that they furnish noise currents composed of bands of frequency components substantially equal in width to the transmis sion ranges of the filters F and F respec having a side band width equal to the width of normal speech and the unmodulated carrier component and one-of the two side bands are suppressed as described in corinection with the modulator M The remaining single side band is transmitted from the transmitting antenha TA Carrier wave sources 4 and 9 may each be of the type disclosed in the Hartley patent; supra.

To an unauthorized person who intercepts either or both of the modulated waves from transmitting antenna TA, and transmitting antenna TA the received currents will appear only as noise currents incapable of reproducing speech; This is true since the components of any single sub-band represent only unintelligible components and it is impossible to superpose the sub-bands transmitted. over both channels in such a way as to build up the speech range, since the noise components transmitted from the sources f to f cover practically the entire speech range and may readily be made of suclramplitude as to completely drown out the speech components.

The speech transmitted by the system of Fig. l is received in accordance witn the in vention by means of-an arrangement such as is shown in Fig. 2. In this figure the receiving antenna RA, is tuned to receive the mooulated wave transmitted. from the antenna TA, and the receiving antenna HA is tuned to receive the other of the two transmitted waves.

The frequency interval between the two transmitted waves should be large enough to permit of good selectivity between the Waves at the receiving station. relying on receiving circuits tuned to the transmitted wave frequency, a beat oscillator may be used to step down the received frequencies and the selection may be carried out at a frequency lower than the transmitted Wave frequency. II1'Bli1l1Q1 Ci1S8 a practically complete separation will be made between the waves transmitted over one of the high frequency paths or channels and those transmitted over the other channel, so that the waves from one of the channels are applied exclusively through the tuned cir Instead of l if) aws? eating sir 1 Uni Llu 0 D1. m 1 9 .h a a. J p 1 u .1 u m 0 W q, H T O4 C v 1 Wm A .i T Q tv hkmw w mwm m L rnmmmfiuwmuw Hum ww .3 run P ne L t T C a? T T. v .l .L Y a 0.... n 0 I 8.1.1CW? i. 2C 0 sbfific a O in 3 no 0 5 Au 5 A .1 5 5 the sub-bands from the several channels insecretly transmitted by the method defined in claim 6, comprlsing separately detecting said carrier Waves to yield sub-bands of the frequencies used at the transmitter to modulate the respective carrier Waves, selecting the sub-bands and applying them in com- 10 mon to a receiving circuit.

11. In a telephone system, a source of speech currents, a plurality of transmission channels, a plurality of frequency selec tive elements, each-selective of a different I frequencyconnected between each of the transmission channels and "said source of speech currents.

In witness whereof, I hereunto subscribe my name this 26th'day of June A. D., 1923.

ROBERT C. MATHES.

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