Patented Oct. 29, 1946 - UNITED 2,410,27 TES ATENT OFFIC 2,410,276 SECRET COIVIIWUNICATION SYSTEM Stanley D. Eilenberger, Kenosha, Wis., assignor, by mesne assignments, of sixty-?ve percent to S. E. Steen, Kenosha, Wis., seventeen and one half percent to L. G. Voorhees, Macedonia, Ohio, and seventeen and one-half percent to Howard W. Taft, Balboa Heights, 0. Z. Application December 17, 1941, Serial No. 423,384 16 Claims. (01. 250-6) This invention relates to a method of secret communication, principally for military use, where a false or masking signal is provided where in the false signal in itself is a complete, intelligi ble signal which may be designed to mislead unauthorized persons, which method has consid - erable value over so-called masking signals con sisting of noise modulation or other spurious fre quencies, which in themselves provide evidence 2 otherwise shown in this speci?cation, and ref erence should be made to this co-pending appli cation for this additional detail. Referring more particularly to Fig. 1, A—l rep resents the source of the true signal which may well be a microphone, or tone modulated tele graph signals, and B—l an ampli?er for this true signal input. A--2 represents a source of intel ligible false signal and B—-2 an ampli?er for this that communication of a secret nature is being 10. false signal. The output of ampli?er 13-! is carried out. connected to the input of reactance modulator The purposes of this invention are: unit C, which is so designed as to frequency mod To, provide a method of secret communication ulate controlled oscillator D throughout a range in which the communication shall be disguised in such a manner that reception shall be di?icult 15 of 20 C. P. S. above and 20 C. P. S. below a mean frequency of 100 kc. The output of controlled for any except the authorized parties. oscillator D beats with the output of ?xed oscil To provide a method of secret communication lator E, which is adjusted to a ?xed frequency employing a true signal lying in the ultra-sonic of 125 kc. This provides a beat-note having a frequency range and a false intelligible signal mean frequency of 25 kc, and this beat note is lying in the sonic frequency range. 20 frequency modulated between the limits of To provide a method of secret communication 25 kc¢20 C. P. S., by the true signal originat employing two signals, one of which is the true ing in microphone A-l. If desired, this 25 kc. signal, the other being designated as a false sig subcarrier might be obtained directly, without nal, the true signal being so constituted as to be receivable only with special receiving equipment 25 the use of beat frequency oscillators, by adjust ing oscillator D to oscillate at 25 kc. In either case, as shown in detail in my above quoted co tional types of equipment. pending application, the resultant carrier will be To provide a means of secret communication modulated at an index less than unity, whereby in which two diiferent signals are transmitted’ simultaneously without any increase in carrier 30 the modulation index is de?ned as the ratio of frequency deviation from the mean carrier fre or modulation power beyond that which would and the false signal being receivable on conven be required to radiate only one signal. This invention will be best understood from a quency over the signal frequency. Unit BP is a band-pass ?lter designed to pass this frequency modulated carrier, together with consideration of the following detailed descrip its associated sidebands. The output of ?lter tion, in view of the accompanying drawing form ing a part of the speci?cation; nevertheless, it 35 BP is connected to input terminal I of switching unit X, while the output of ampli?er unit B—2 is to be understood that the invention is not con is connected to input terminal 3 of switching ?ned to the disclosure, being susceptible of such uni-t X. ~ changes and modi?cations as de?ne no material Switching unit X is a two pole switch, with departure from the salient features of the inven 40 two input terminals I and 3 and two output ter tion as expressed in the appended claims. minals 2 and 4; this switching unit is arranged In the drawing: that contact must always be between terminals Figure 1 represents one form of a transmitter I and 2, or between terminals 3 and 4. Figure 1 for use with this system. Figure 2_ represents a receiver for use with the 45 shows this switch with contacts closed between input terminal I and output terminal 2, the dotted transmitter of Fig. 1. line from input terminal 3 to output terminal ll Figure 3 shows detail of unit X in the trans mitter of Figure 1. For additional detail regarding this system, ref erence should also be made to my co-pending 50 application, Serial No. 422,915, ?led December 13, 1941, entitled “Signalling by sub-modulation.” All letter designations in the drawing represent units identical with like lettered units in the co representing the alternate position of the switch. With switch X in position l—2 the output of band-pass ?lter BP is connected-to the input of modulation ampli?er H. This switch is designed to change positions at a very rapid rate, such as 50,000 times per second. This could be done mechanically by mounting pending application referred to above, unless 55 the switching blade on a high speed motor shaft at the center terminal 5 and while such switches 2,410,276 3 are practical, and have been used, particularly with a mean resonant frequency of 100 cycles and in commutator form, a mechanical switch moving at such a high speed is not completely satisfac tory. For this reason, an electronic switch is recommended-for this switching unit which accomplishes the same results without moving parts. One simple form of such an electronic switch is illustrated by Fig. 3, and will be de scribed in detail later in this speci?cation. The balance of the system is arranged for con 4 Unit R is a. frequency discriminator designed further designed to provide e?icient recovery of the modulating signal on the 40 C. P. S. frequency swing. Unit S2 represents a high pass ?lter with a low frequency cut-01f at 120 cycles, designed to remove residual subcarrier frequencies. Unit T represents the audio frequency ampli?er, and U the reproducer. The reception of such a modu 10 lated carrier wave, which has been amplitude ventional amplitude modulation at any desired carrier frequency, where unit Mist-he modulator,’ L the power supply, I the carrier-frequency am modulated by a frequency modulated subcarrier, is fullyv describedin my co-pending application referred to above. . A full study of, this co-pending application Will pli?er, E2 the carrier-frequency, oscillator and K‘ - reveal'that numerous additional methods of dis the antenna. All of these units are conventional guising the true signal are possible and it should except that modulation ampli?er H and modu be further. understood that any existing type of lator unit M should be arrangedto carry out. am disguising a signal, such as frequency inversion, plitude modulation at an ultra-sonic frequency, might be used in lieu of the system shown here. since switching unit X is being moved from the If such alternate method» were employed the ap contact position l-—2 to the contact position 3-4 20 paratus used'to disguise the signal would bacon atthe rate, of approximately 50,090 timesper sec nected between the output terminal of ampli?er ond; With this switch in the position 3——fi the B1.- and' the input» to switch terminal I. A re output of false signal ampli?er B—-2 is connected quirement of such alternate method is that the directlyv to the inputv of modulation ampli?er H, switching operation and the converted true sig andis consequently radiated as a conventional 25 nal, as supplied to the» input of modulation ampli amplitude-modulated signal, which may be re ?er H, be at on ultra-sonic frequency. ceived on any conventional type of receiver. Figure 3 shows adetailed arrangement of» a Summarizing the operation of the transmitter simpli?ed electronic switch-for use as switching of vFig. l, the true signal is radiated as amplitude unitzX'inthe transmitter of Figure 1. Termi modulation of the carrier by a frequency-modu 30 nals No, l and 3 represent the input-terminals lated subcarrier having- a mean frequency of 25 and terminals No. 2‘ and'ritrepresent theoutput kc; and-a-total frequency swing of 40 C. P. S.; the terminals, these terminalsbeing numbered iden false signal is radiated-as conventional amplitude tically' with the arrangement shown in Fig. 1. modulation. Only one of these signals is present inthe modulator and carrier units at one time, and therefore the modulator and carrier units neednot operate at increased power, even though two/separate signals are being radiated. Thevacuum tubes-9| and- Hli might be commer cial type 6L7 or any other triple grid type ar ranged as an ampli?er; 9 isthea-mpli?er for in put‘ terminal! and if}: is the ampli?er for input terminal 3. rf‘liese tubes-are arranged to alter Referring to Fig-.- 2, there is- shown one form. nately amplify andblock- atthe frequency of the of 'a receiver for use in recovering the true signal 40 pulses supplied from‘ square-wave generators radiated by the transmitter of Fig. 1. In Fig. 2, K represents the’ receiving antenna, N the radio fre quency ampli?er, O the convertor, P the inter mediate frequency. ampli?er, and .Q the amplitude demodulator‘; all of these units are conventional, except, in the example given, the acceptance band of units N, O, P, Q must be at. least wide enough topassv the received‘ carrier and its associated sidebands, including the switching frequency, SW-—i and SW--2, the frequency and synchro nization of which are- controlled by unit PG, which is a pulse generatoror oscillator, operat ~ ingat-the desired switchingifrequency, which'in this. case is- 50,000 times per- second. Blocking pulses are supplied to auxiliary control-grids l3‘ and l'itso that the signal inputs received at pri rn-a'ry'1Contr01- grids H and» i2‘ are alternately which, in the example given,is 50110.; the carrier 50 ampli?ed, and the signal voltages developed across resistors‘ l'l- applied through coupling con and its associated sidebands alsooccupy 50 kc, if densers E8 to ioutputiterminalsez and ll. Conden both sidebandsare transmitted. If desired, either ser - i9 ‘is arrangedto by-pass any inter-coupling or lower sideband may be removed by between stages. Terminal 2?} represents the com commonly used methods. The original modulated subcarrier, having a mean 55 mon ground connection-and terminal 21 a source ofhigh voltage of approximately 250 volts. Cath of 25- kc., is present at the output of ode, primary control grid and screen grid cir demodulator Q, also the original signal frequen cuits are» conventionally- arranged. Electronic cies impressed as the false signal at microphone switching- devices similar toy that illustrated by A-—2 at the transmitter of Fig. 1; this combined output of demodulator Q is passed through high 60 Figure 3 are in common-‘use and commercial units the upper any of the frequency frequency pass ?lter S-l , which has, for example, a low fre quency cut-off at 5000 cycles, which eliminates the so-called false signal, and passes the 25 kc. subcarrier on to ampli?er-mixer unit F. Here it beats with a constant frequency from ?xed oscil lator E, which has a frequency of 24.9 kc., so that the resultant beat note at the output of, mixer unit F is a frequency modulated audio frequency subcarrier having a mean frequency of 100 cycles. may be obtainedoperating onthis principle. While the foregoing description servesto il lustratethe manner in which the objects'of my invention may be carried-out, nevertheless, it is understood that I do not desirev myinvention to be limited thereby, but only bythe-scopeofthe following claims. I claim: 1.- The method of secret communication, which comprises the steps of p-roducinggan ultra-sonic 70 This frequency modulated’ 100 cycle subcarrier carrier wave, frequency modulating said ultra is impressed on ?lter unit G, which is- a parallel sonic'carrier'by‘a true signal containing the in resonant ?lter with a resonant frequency’ of 108 telligence to be transmitted'toan authorized-re cycles, identical in function and purpose to ‘filter ceiver at a modulation'index smaller than unity, G described in detail in my above quoted co-pend producing a signal containing any arbitrary in_ ing application. 76 telligénce, and alternately amplitude modulating modulated high frequency carrier wav‘e,.l,y?lteringv a'high frequency carrier wave by‘the frequency the demodulated wave through a high pass ?lter modulated ultra-sonic carrier wave and by the to attenuate all frequency components contained in. the false :signal and to. obtain the‘ frequency false signal containing any arbitrary intelligence at an'ultra-sonic rate of alternation; '2. The method of secret communication, com prising the steps of producing a ?rst high fre quency carrier Wave, frequency modulating. said ?rst'high frequency carrier wave by a true signal containing the intelligence to be transmitted to 10 an" authorized receiver at a modulation index modulated ultra-sonic carrier, beating theirecov-' ered ultra-sonic carrier with a constant frequen-. cy Wave‘ to produce an "audio frequency frequencyv modulated subcarrier, and demodulating theatre-' quency modulated subcarrier to recover the true signal; ' .1 ' I _- . ' > 9. In, a transmitting circuit" a source of-true signal; .a source of false signal, a high frequency smaller than unity, beating said frequency mod ulated high frequency carrier with a constant fre oscillator frequency modulated at an index. less quency wave to produce a frequency modulated than unity i" by the true signal, a high‘ speed ultra-sonic carrier, producing a false signal con 15 change~over switching circuit, the input sides of taining any arbitrary intelligence, alternately which being connected to receive both true and amplitude modulating a second high frequency false signals, and a high frequency oscillator am carrier by both the frequency modulated ultra plitude modulated by the output of the high speed sonic carrier and the false signal at an ultra switchingcircuit. sonic rate of alternation. 20 10. In a transmitting circuit a source of true 3. The method of receiving a true signal con signal, a source of false signal, a source of ultra taining the intelligence transmitted to an author sonic oscillations and means for frequency mod ized receiver by the method described in claim 1, ulating said ultra-sonic oscillation at a modula comprising the steps of demodulating the ampli tion index less than unity by the true signal, a tude modulated high-frequency carrier wave, ?l source of high frequency oscillations and means tering the demodulated wave to attenuate all for alternately causing the false signal and the the frequency components contained in the false ‘ frequency modulated ultra-sonic oscillations to signal and to obtain the modulated ultra-sonic modulate the high frequency oscillations at an wave, demodulating the ultra-sonic Wave to re cover the true signal containing the intelligence 30 ultra-sonic rate of alternation, 11. The method of receiving a signal contain intended for reception. ing the intelligence transmitted to an authorized 4. Themethod of secret communication, Which receiver by the method described in claim 2, com comprises producing a true signal and an intelli gible false signal of sonic frequency, converting the true signal to an ultra-sonic signal frequency modulated at an index less than unity, combin ing the sonic signal and the ultra-sonic signal, transmitting thecombined signals to a receiving point, eliminating the sonic signal, and recon prising the steps of demodulating the amplitude modulated high frequency carrier, ?ltering the demodulated Wave to attenuate all the frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic car rier, and demodulating said frequency modulated carrier to recover the true signal. verting the ultra-sonic signal into the true signal. 40 ultra-sonic 12. The method of receiving a signal contain 5. The method of secret communication, which comprises producing an intelligible true signal and an intelligible false signal, converting the true signal into an ultra-sonic signal frequency modulated at an index less than unity, alternate ly transmitting the ultra-sonic signal and the false signal at an ultra-sonic rate of alternation to a receiving point, eliminating the false signal and reconverting the ultra-sonic signal to the true signal. - 6. A system of secret communication, compris ing a source of intelligible true signals, a source of intelligible false signals, means for converting the true signals into ultra-sonic unintelligible ing the intelligence transmitted to an authorized receiver by the method described in claim 2, com prising the steps of demodulating the amplitude modulated high frequency carrier Wave, ?ltering the demodulated Wave through a high pass ?lter to attenuate all frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic carrier, beating the ‘re covered ultra-sonic carrier with a constant fre quency Wave to produce an audio frequency fre quency modulated subcarrier, and demodulating the frequency modulated subcarrier to recover the true signal. _ . 13. The method of secret communication which signals frequency-modulated , at an index less 55 comprises producing a true signal and an intelli than unity, means for alternately transmitting the intelligible false signals and the ultra-sonic unintelligible signals to a receiving point, means for eliminating the false signals, and means for reconverting the ultra-sonic unintelligible signals to true signals. 7. The method of receiving a, signal containing gible false signal of sonic frequency, converting the true signal toan ultra-sonic signal frequency modulated at an index less than unity, combining the sonic signal and the ultra-sonic signal, modu 60 lating a carrier Wave by the combined signals, . the intelligence transmitted to an authorized re transmitting the carrier Wave to a receiving point, then demodulating the carrier Wave to recover ceiver by the method described in claim 1, com prising the steps of demodulating the amplitude the combined signals, eliminating the sonic sig nal, and reconverting the ultra-sonic signal into modulated high frequency carrier, ?ltering the the true signal. demodulated wave to attenuate all the frequency components contained in the false signal and to obtain the frequency modulated ultra-sonic car _ 14. The method of secret communication which comprises producing an intelligible true signal and an intelligible false signal, converting the rier, and demodulating said frequency modulated 70 true signal into an ultra-sonic signal frequency ultra-sonic carrier to recover the true signal. modula'ted at an index less than unity, alter 8. The method of receiving a signal containing nately modulating a carrier wave by the ultra-. the intelligence transmitted to an authorized re sonic signal and the false signal at an ultra-sonic ceiver by the method described in claim 1, com rate, transmitting the carrier Wave to recover the prising the steps of demodulating the amplitude ultra-sonic signal and the false signal, eliminat 2,410,276 7 ing the false signal and reconverting the ultra sonic signal to the true signal. 15. ,A secret communication system comprising a source of true signals, a source of intelligible false signals, means for converting the true sig 8 a ‘source- of true signals, a source of ‘intelligible false signals, ‘means for converting the true sig nal to an ultra-sonic ‘signal frequency-modulated at an index less than unity, means for producing a carrier wave, switching means ‘for alternately nal 'tora'n ultra-sonic signal frequency-modulated causing the false signal and the ultra-sonic signal signal. signal. to modulate the carrier wave for transmission to atan index less than unity, switching means for a receiving point, means at the receiving point alternately ‘introducing the false signal and the for demodulating the carrier Wave to recover ‘the ultra-sonic ‘signal ‘into a channel for transmis sion to a receiving point, means at the receiving 10 false signal and the ultra-sonic signal, means for eliminating the false signal, and means for re— point for eliminating the false signal, and means converting the ultra-sonic signal to the true for ‘reconverting the ultra-sonic signal to the true 16. .A secret communication system comprising STANLEY D. EILENBERGER.