US2400950A - Privacy signaling system - Google Patents

Description

May 28, 1946. v E. s. PURINGTON 2,400,950

PRIVACY S IGNALING SYSTEM Filed Jan. 24, 1942 3 Sheets-Sheet 5 ATTORNEY Patented May 28, 1946 2,400,950 PRIVACY SIGNALING SYSTEM Ellison S. Purington, Gloucester, Mass, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application January 24, 1942, Serial No. 428,056

I 21 Claims. (Cl. 179-15) My present invention 1 t t rivacy iggreat difliculty of analysis of the nature of the naling systems, and more especially to improved trensmlsslon, nd makes it necessary to use a privacy systems of radio telephony of the type h degree of technical slilll eqnstlllet using masking signals, suitable receiver for intelligible reception.

An object of this invention is to provide an lf, the P t illventlen p ov es t at imprgved type of Privacy radio telephony, 1n the carrier need not be artificiallyirequency modwhich t will be m fgr an unwanted ulated for the purpose of preventing reception of server to note thatprivacy telephone is bein the message by continuous wave receivers; or if practical s I it is frequency modulatedfor such purposes,

Another bje t is t m 1 the observer if he 10 that the modulator shall not be of the artificial believes privacy telephone is being practiced nature prevalent in the art, but in accordance and cause him to draw false conclusions as to with the Presence and nature of the amplitude th broad principles of operation or "quasi-phase modulations of the carrier due A further bject is to make it m for an to the signal or mask thereby preventing identiobserver who fail t be misled, to-anslyzs the fication t;of the radiations as being related to a details of the principle of operation and to con- Prlvacy v struct the proper type of receiver for intelligible e hovel features WhlCh I believe to be rrecept10n acteristic of my invention are set forth with In U. s. Patent 1,642,663 to E. L. Chafiee, a was particularity in the appended s; the inventaught that privacy radio telephony can be non ltself however, s to both niza ions and achieved by causing a; carrier to be modulated at methods Of Operation Wlll best be uhderstood y frequencies differing from the speech frequencies, reference to f descrlptmh taken l and by frequency modulating the carrier in an W the drawlflg in which I v artificial manner. .The former process prevented mdlcated dlagmh'lmatlcally Y u reception bylusual'broadcast type receivers, and gamzallohs whereby my lhvehtloh m y be carthe 'latter .by'; continuou siwave type. jreceivers. fled efiecli- I It is,"'als0,' known -that-a,,masking signal may hl drawings! be transmitted together with themessa'geiislgnal Flgure schematically Shows a tr-flllsmltter to aid in preventing the reception of signalsindi '-.---Y }l W .the sk and signals are of an ea P ivacy is being practiced, as taught in lntegralftypel Wlthollt interleaving; I U. s. Patent 2,204,050 to Purington. Figure 2 Shows in atic manner a trans- The present invention provides that a masking l modlflcatlon useful when the l and signal shall be sent, with the option of causing slgnals are interleaved; .th masking signal to be present only when the Figure 3 shows in schematic form the general message is being sent This is for the purpose of receiver arrangement useful when the signals better concealing the message, and, also, for T F by quasl'phase" modulation; suggesting that themasking signal is a chopped Figure 4 Shows schematically an d o a part, of Privacy message, f which the other receiver arrangement for use when the signal is portion is sent by some other channel. lntegl'al: as in Flgule 1; D It further provides for the. option of sending 40 Figure 5 shows an addltlohel Schematic the message on a i ph modulation basis, ceiver arrangement useful when the signal is as explained in U. s. Patent 2,020,327 to Puringdlvidedo Figure ton, hil th masking signal is t on an am 1i It will be understood that the circuit arrangetude modulation basis, thereby reducing the ratio mehts'to be descl'lbed are for illustration O of signal energy to masking energy that results Frequency indications are no definite, u can when the entire radiation is received by the be Orgamzed Otherwlse- For p the audio al amplitude modulation receiver, without, sources for the frequency converter circuits can however, impairing the'signal as properly rebe both the same and derived from a. common i d by a timing modulation receives source. Since in my aforesaid U. S. Patent It further provides the option of interleaving 4, 50, there are shown the circuit details of the signal and masking modulations in such a a e l p i acy system of the mask n si nal manner that the radiated side bands due to t p such circuit details wil n t be r p at d the signal modulation are closely adjacent to, Those skilled in the art Will e able fully and carefully guarded by, the side bands due to to understand the schematic disclosure. In Figthe masking modulations. This, also, results in ure 1 there is provided a signal source I, such as a microphone and amplifier, providing audio signals, for example in the audio frequency range of 200 to 3,000 cycles. There is provided a masking source 2, such as a phonograph or microphone with amplifier, providing masking frequencies, for example in the audio frequency range of 200 to 2,000 cycles. The signal frequencies are impressed upon a signal inverter 3 containing a local audio oscillator source operating at say 5,000 cycles. As shown in my aforesaid U. S. Patent 2,204,050 the signal frequencies and local oscillations are applied to a balanced modulator whose output would contain solely the, original input in inverted form. That is, a 200 cycle signal produces a modulating frequency of 4,800 cycles, and a 3,000 cycle signal produces a modulating frequency of 2,000 cycles. The local oscillation is suppressed in the balanced modulator output."

n the other hand, the masking, signals in original form are impressed upon a mask controller 4. The transmission of electrical energy through controller 4 is under the electrical control of other electrical energy corresponding solely to the original input but in inverted form. In the case shown the controller 4 is under the control of audio energy existing in the output of the signal inverter. Such devices are well known in the art and need not be shown in detail. The controller could consist, for example, of an amplifier normally biased to prevent passa e of mask signals. Passage of the masking signals would bepermitted by a rectifier means operating from the control signals'to vary the gain characteristics of the amplifier for the controlled (mask) signal. In the present case the control signal is preferably derived from the output of the signal intion only. There is no overlap or spread indicated for the modulating frequencies through A and through C, and it will be understood that the system will be operative even if there'is an overlap; possibly of very considerable extent if the two modulations are of different types. There is depicted, in a. purely pictorial manner, the various audio frequencies at different points of the system.

The radio transmitter itself. shown above the dotted line, includes the carrier generator 0, the frequency of which is under the control of the frequency controller 5, flanked on both sides by verter,.instead of from the output of the signal source, completely to minimize the possibility of the message signals getting into the masking channel and thereby cause a loss of privacy.

It could be controlled, if desired, by the output of the mask controller, or in any other-manner, provided it results in the carrier frequency being controlled by the frequency and strength characteristics of the signal or signal and mask source to give a random frequency pattern, preferably with the frequency not changing when the signal source is notv actuated. Themechanism for producing the frequency variation of carrier generator 5 may be, for example, a variable condenser in the tuned circuit of the carrier generator, with electro-mechanical means for changing the capacity in accordance with currents developed by rectifying thecontrol energy from the signal inverter 3.

As a result, referring to the dotted line, modulating current in the range of 200-2,000 cycles in accordance with the mask source passes through junction A when the signal source is operated. The frequency of the currents and voltages at junction B is modulated at slow, or syllabic, frequency rates in accordance with the energy and frequency characteristics of the signal source, or of the mask source as controlled by the signal source, or both. And, finally, a modulating current in the range of 4,800 to 2,000 cycles passes through junction Cr As explained previously that range corresponds to a signal source current in the lower and inversely arranged range oi 200-3,000 cycles. It will be understood these figures are employed for illustrapush-pull modulators I and 0 for the masking and signal channels respectively. The carrier generator may produce oscillations at any desirable radio frequency. Energy from the carrier generator passes directly to a combining amplifier 9, and, also, through adjustable phase shifter networks l0 and H to the push-pull modulators I and 8 respectively. The outputs of the modulators also pass to the combining amplifier. The phase shifter III for the mask channel may be so adjusted that the result of combining the side bands, due to the masking push-pull modulator, with the carrier, in the combining ampliner represents amplitude modulation. In other words, in the output of the modulator I there will exist solely mask side bands M and M+ corresponding to the 200 to 2,000 cycle audio range. The carrier (shown as a dotted vertical line) is suppressed in the modulator output circuit. The phase shifter II for the signal channel may be so adjusted that the result of combining the side bands, due to the signal push-pull modulator, with the carrier in the combining amplifier represents "quasi-phase modulation.

Here, also the S- and 8+ side bands exist in the output circuit of the push-pull modulator 8 with carrier suppressed. In this case, unlike modulator I, the carrier is phased to be in quadrature with the vector sum of modulation side bands S- and 8+. This is quasi-phase modulation. This process, described in U. 8. Patent 2,020,327 to Purington, referring to U. S. Patents 1,935,776 and 1,976,393 to John Hays Hammond, Jr., is now well known in the art, and is believed to require no detailed description. As shown, there is fed to the combining amplifier 9, which may be of any well known construction, the carrier energy from generator 8, the mask side bands from I and the signal side bands from 0. The phase relations between these components are depicted under amplifier 9. 'As a result there is developed in the antenna, as indicated graphically to the left of amplifier 9, a carrier C which is amplitude modulated by the masking signals to produce the masking side bands M and M+ and which is also quasi-phase modulated at higher frequencies derived from the signal frequencies to produce the signal side bands S-' and 8+. By virtue of the action of frequency controller 5, the radiation as a whole is shifted in frequency whenever the carrier conveys 9. signal.

the range up to 5,000 cycles.

to 1,000 cycles.

It is to be clearly understood that no frequency changes occur in the combining amplifier 9. The modulations from the output of modulators I and are phased as shown in the drawing. The essential function ofthe combining amplifier is to put the components C, 8+ S-, M+, M- on the same antenna or line.

In the modification of Figure 2, the radio circuit arrangements are substantially the same as for Figure 1, and need not be repeated. The masking source 2 may provide audio energy in Successive band elimination filters l2 and I3 are employed to extract, or remove two bands of frequencies, say

600-2,600 cycles and 3,500-4,500 cycles so that the carrier will not be amplitude modulated by frequencies in these ranges, but can be quasi-phase modulated in these ranges due to signals from the signal source. The mask controller 4 isinserted in the line to the modulator 1.

The output of signal source l'is diverted to two channels by isolating amplifiers and filters, creating one band of 0-l,000 cycles and another band of 1,000-3,000 for example. This may be done by passing the signals through an amplifier followed by a low pass filter M which passes energy of 0 Another portion of the signal energy is applied to a high pass filter l5 which passes a band of 1,000 to 3,000 cycles. The 0-l,000 cycle band is introduced into a frequency adding converter IS with oscillations from a local oscillator source ll operating at 3,500 cycles. As a result of the conversion at IS, the zero frequency is elevated to 3,500 cycles, while a signal current of 1,000 cycles is converted'over into 4,500 cycles nodulating current. It is, of course, possible to make this frequency adder circuit a frequency subtractor, using a local source of 4,500 cycles and yielding the same output band of 4,500 to 3,500 cycles. The other signal band of 1,000-3,000 cycles is similarly subjected .to conversion into a range of 2,600 to 600 cycles. For this conversion there is used an oscillator l8 operating at 3,600 cycles. The oscillations are fed to the frequency subtractor converter IS. The output of the latter has a range of 2,600 to 600 cycles. The two bands S2 and S1 are applied to a conventional mixer 20. The output of the mixer comprises signal currents in two bands of modulating currents, these bands being in the same ranges as the bands eliminated from the masking source. As shown, the masking bands M1, M2 and M3 are for 0 to 600 cycles, 2,600 to 3,500 cycles and 4,500 to 5,000 cycles.

After the modulation and combining as shown above the dotted line in Figure l, the radiations for Figure 2 are for a carrier modulated by a masking source as to amplitude, creating masking sidebands M1, M2, M3; and modulated by message conveying signals as to phase, creating signal side bands S1 and S2, with the masking and signal side bands interleaved. At the top of Figure 2 there is shown pictorially the nature of the radiation from the antenna of the system of Figure 2. The same convention is here employed as in Figure 1. It will be clear from this graph that the mask and signals are interleaved, and that the S side bands are quasi-phase modulated. For this arrangement of Figure 2, the use of the frequency controller 5 for the carrier is not as necessary as for Figure 1, but can be used by closure of the switch 2| by which it can be energized from the signal source, before or after conversion.

It is noted that all the radiations of Figure 1 or Figure 2 exist simultaneously, and that the masking bands will produce a considerable degree of intelligibility of a chopped nature, but that there will neither be intelligibility nor appreciable noise from the signal bands in the usual amplitude modulation receivers. The natural inference will be that the masking bands are part of a privacy message, chopped up into syllabic lumps of modulations, and the remainder transmitted by some other radio or other channel. As a result, the actual message conveyed by the signal bands may be entirely overlooked. The proper'type of receiver, when the mask and signal are sent one by amplitude and the other by quasi-phase modulations, which is the preferred arrangement, is shown schematically in Figure 3. Thelatter is to be understood as supplemented by-Figure 4 for transmissions by the system of Figure 1. Figure 3 is supplemented by the system of Figure 5 for transmissions .by the system of Figure 2.

r In Figure 3 is provided the usual tuner 22 and heterodyne local oscillator 23 for creating intermediate frequency (I. F.) signals. A split circuit is employed, with the upper sideband filter Fu energized in accordance with the upper sideband energy of the transmitted energy, and the .lower sideband filter Fr. energized in accordance with the lower sideband energy. Both of these channels are provided with individual detectors. The detectors are provided with combining means 24 so that additive effects are produced'in the signal circuit 25 due to the quasi-phase modulation, and subtractive effects are produced therein due to the amplitude modulation. This is the well known frequency discriminator circuit described in U. S. Patent 1,776,065 to E. L. Chafiee, and U. S. Patents 1,935,776 and 1,976,393 to John Hays Hammond, Jr., and need not be described further in detail.

For the radiation of Figure l, the energy of the signal circuit, 25, as shown in Figure 4, may be passed through a high pass, or band pass, filter 26 to remove any audio currents due to the amplitude modulation signals. The remaining audio currents, representing the S components, are then passed through an inverter circuit 217, and, if desired, through a following low pass filter 28 to actuate the indicating headphones 29 in accordance with the message. The hi'gh pass filter 26 would be designed to pass all the audio frequencies above 2,000 cycles. The inverter 21 could be of the same type as inverter 3 of Figure l, but would act to invert the 2,000 to 4,800 cycle range to the original 200 to 3,000 cycle range. The low pass filter 28 would be designed to cut out all audio frequencies above 3,000 cycles.

To receive the radiation of. the system of Figure 2, the energy of the signal circuit, as shown in Figure 5, may be split into two channels by band pass filters 30 and 3| which respectively transmit 600 to 2,600 cycles and 3,500

to 4,500 cycles for example. The separate frequency converters 40 and 4| respectively change the signals from 30 and 3| back into the original signal ranges of 1,000 to 3,000 cycles and 0 to 1,000 cycles, which may then be amplified and combined toactuate the headphones 50. It will be noted that the process is the reverse of the process in Figure 2 between source and mixer 20. The extent of frequency modulation of the carrier to conceal reception by a continuous wave receiver is not sufllciently great to disturb the proper reception of the signals.

The arrangements here shown may, of course,

be extended to yield other arrangements, as for example by providing for the changing of transmitter and'receiver channels; filters, etc. synchronously, or from time to time tocyield even a greater degree of privacy.

' While I have indicated and described several systems for carrying my invention into .effect, it will be apparent to one skilled in the art that my invention is by no means limited to the par- 7 ticular organizations shown and described, but

that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of imparting a high degree of secrecy tosignals desired tobe transmitted between two points, which method consists in producing a carrier wave, amplitude modulating the carrier wave in accordance with a false or masking set of signals and phase modulating the carrier wave in accordance with the true set of current representative of the carrier and upper set of .side bands and a current representative of the carrier and lower set of side bands, de-

tecting both of the produced currents and subtractively combining the resultant detected currents to produce therefrom a current representative of the inverted band of audio frequencies.

-reinvertingv the resulting currents and deriving from the resultant currents the desired set of signals.

4. A method of signaling which consists in generating a carrier wave, producing a band of audio frequencies representative of a first set of signals, transmitting the resultantcarrier and upper and lower sets of side bands, intercepting the transmitted energy at a receiving point, producing from the interceptedenergy, a current representative of the carrier and upper set of side bands and a current representative of the carrier and lower set of side bands, detecting the two produced currents and subtracting one from the other to derive therefrom the desired or true a set of signals. g

2. A method of imparting a high degree of secrecy to signals desired to be transmitted from one point to another, which method con sists in producing a carrier wave, producing a V band of audio frequencies representative of a false or masking set of signals, producing an other band of audio frequencies representative 7 of the true set of signals desired to be transmitted, converting the ,last named band of audio frequencies, to another band ;of related audio signals, producing another band of audio frequencies representative of a second set of signals, simultaneously frequency inverting and converting said last named band of audio frequencies to another band. of frequencies, amplitude modulating the carrier wave in accordance with the first named band of audio frequencies, phase modulating the carrier wave in accordance with the converted and invertedband of frequencies, transmittin the resultant'vcarrier and upper and :lower sets of side bands, intercepting the trans- 5. In a privacy telephonic communicationsystem, the method of imparting a high degree of secrecy to signals desired to be transmitted frequencies, amplitude..modulating the carrier wave'in accordance with-the .flrstnamed 'band of audio frequencies, phase modulating the-ca rier wave in accordance with the converted band of audio frequencies, transmitting theresultant carrier and upper and lower sets of side bands, intercepting thetransmitted energy at a receiving point, producing from the intercepted energy a current representative of the carrier and upper set of side bands and a current representa tive of the carrier and lower set of side bands, detecting the two produced currents, subtracting one of the resulting detected. currents from the other thereof to produce a current representative of the converted band of audio frequencies and reconverting the resulting energy to derive therefrom the true set of signals.

3. A method of imparting a high degree of secrecy to signals transmitted between two points, which method consists in generating a carrier wave, producing a band of audio frefrom one point to another, which consists in producing a carrier wave and two sets of signals.

one of said, sets ofsignals comprising 'the true signals, the other comprising' ma'skihgj signals,

- amplitude modulating the carrier wave by the quencies representative of a false or masking set V r of signals, producing another band of audio frequencies representative of the true set of signals desired to. be transmitted, frequency inpoint, producing from the intercepted energy a verting the last named band of audio frequenmasking signals and simultaneously fquasiphase? modulating the carrier wave by the true signals, transmitting the modulated carrier wa've,

intercepting the. transmitted ene y at a re ceiving point, detecting the intercepted .energy to derive therefrom the two modulation components; separating the upper and lower side' bands of each of the two signal'components,

producing direct current components from each of the side bands and combining the produced two direct current components in additive phase to produce the desired signals therefrom.

6. A receiving system for multiplex telephonic systems of the type wherein a carrier wave is quasi-phase" modulated by one set of signals and amplitude modulated by another set of 'signals, comprising-in combination means for intercepting the modulated carrier wave, a source of local oscillations, means for changing the incoming oscillations to an intermediate frequency modulated in accordance with both sets of signals, a pair of filter circuits, means for impressing said intermediate frequency upon both said circuits, one of said circuits being adapted to pass the intermediate frequencies and frequencies above the intermediate frequency and the other thereof the intermediate frequencies or frequencies below said intermediate frequency whereby one of said circuits passes substantially only the intermediate frequencyand the upper side bands of said signal modulations and the other only the intermediate frequency and the i lower side bands thereof, a pair of detector tubes having separate input circuits, one of said input circuits being coupled to the output of said one of said filter circuits, the other of said input circuits being coupled to the output of the other filter circuit, output circuits for each of said detector tubes including a common portion, a signal utilizing circuit coupled to said common portion for deriving onef of said signals, and a utilizing circuit coupledto both said output circuits for deriving the other signals.

7. A method of multiplex telephony which consists in producing a carrier wave, amplitude modulating the carrier wave in accordance with signals desired to be transmitted, phase modulating the same carrier wave in accordance with other signals desired to be transmitted, receiving the modulated carrier, producing an intermediate frequency carrier havingupper and lower side hands by combining the'jreceived carrier with cally produced oscillations, diverting a portion of the intermediate frequency carrier and the upper side bands to one-channel and another portion of the intermediate frequency carrier and lower side bands to another channel, separately detecting the diverted energies and deriving one set of signals by addition of the detected energies and the other set of signals by subtraction of the detected energies.

8. In a method of Privacy communication wherein a masking, or false, signal is transmitted by amplitude modulation of a carrier wave and a message, or true, signal is transmitted, by phase modulation of the same carrier; the improvement whichincludes the step of normally preventing the said amplitude modulation in the absence of said message signal, and automatical- 1y providing said amplitude modulation in response to said message signal.

9. A method of multiplex telephony which consists in producing a carrier wave, amplitude received energy a current'representative of the carrier and upper set of side bands and a current representative of the carrier and lower set of side bands, separately detecting the two produced currents, additively'combining the resulting detected currents to derive one set of signals therefrom; and subtractively combining the resulting detected currents to derive the other set of signals therefrom.

10. In a privacy transmission system, modulating a common carrier with masking and message signals, preventing modulation by masking signals in the absence of message signals and automatically permitting said masking modulation in response to said message signals.

11. In a secrecy communication system, generating carrier energy, providing separate masking and message signals, amplitude modulating the carrier with the mask signals, subjecting the same carrier to timing modulation with said message signals and automatically controlling the effectiveness of said amplitude modulation in response to the message signals.

12. In a method of secrecy communication, providing a privacy message and a separate masking signal, generating acarrier, dividing said message and masking signal into a pluralitypf spaced modulation bands such that the masking bands are of frequencies above and below the privacy message bands, and modulating said carrier with said spaced modulation bands to provide a resultant modulated carrier wave whose masking side band component frequencies appear in the form of amplitude modulation and whose message side band component fre-- quencies appear in the form of timing modulation.

13. A secrecy method which comprises generatin carrier energy, phase modulating the carrier with a privacy message, amplitude modulating the same carrier with false signals located at frequencies different from message frequencies, and frequency modulating said carrier in accordance with energy derived from said privacy message.

14. A method of imparting a high degree of secrecy to true signals desired to be transmitted between two points, which method consists in producing a carrier wave, amplitude modulating the carrier wave in accordance with a masking set of modulationsignals, translating the true signals into at least two spaced bands of modulation frequencies whose intensities are inverted relative to the original signals, phase modulating the carrier wave in accordance with the two bands and transmitting the resultant carrier and upper and lower sets of side bands.

15. A method of imparting a high degree of privacy to true signals desired to be transmitted from'one point to another, which method consists in producing a high frequency carrier wave,

producing a band of audio frequencies representative of a masking set of signals, converting said band into a plurality of spaced groups of masking frequencies, producing another band of audio frequencies representative of the true set of signals desired to be transmitted, converting the last named band of audio frequencies to spaced groups of frequencies, the masking and true signal frequency groups being interleaved, amplitude modulating the carrier wave in accordance with the groups of masking audio frequencies, phase modulating the carrier wave in accordance with the true signal groups of audio frequencies, and transmitting the resultant carrier and upper and lower sets of side bands.

16. A method of imparting a high degree of secrecy to signalsv transmitted between two points, which method consists in generating a radio carrier wave, producing a band ofaudio g frequencies representative 'of a masking set of signals, producing another band of audio frequencies representative of the true set of signals desired to be transmitted, frequency inverting the true signal band of audio frequencies,

subjecting separate portions of the carrier to predetermined phase shifts, amplitude modulating one carrier wave portion in accordance with the masking band of audio frequencies, and phase modulating the second carrier wave portion in accordance with the inverted band of true signal audio frequencies. a

17. A method of signaling which consists in generating a carrier wave, producing a band of audio frequencies representative of a first set of signals, producing another band of audio frequencies representative of a second set of signals, simultaneously frequency inverting and converting said last named band of audio frequencies to another band of frequencies, amplitude modulating the carrier wave in accordance with thefirst named'band of audio frequencies, phase moan lating the carrier wave in accordancefwith the converted and inverted band of frequencies, and" I transmitting the resultant carrier and upperand;

by the true Signals, automatically permittin'g the modulation of the-carrier by the masking signals coupled to the output ofone'ofgsaid ,fllter circuits theother of said input circuits being coupied to the output of the other filter; circuit, output circuits for each of said etectors; including a common output circuit, a signal utilizing circuit coupledto's'aid common output circuit for deriving one of said signals, and a utilizing cir- '-'*cuit' coupled'to both said detector output'circuits for deriving the other signals.

-20-;'; Af' method of multiplex telephony which ""consists'in producing a carrier wave, amplitude in response to the presence of said'true signals,"

and transmitting the resultant modulated carrier wave.

I 19.A receiving system for multiplex telephonic "other set of signals, permitting modulation by system -of thetype wherein a carrier wave is r quasi-phase modulated-by one set of signals and amplitude modulated by'another set of signals, comprising-"a"sourceof local oscillations, means for'changing the frequency of incoming modulated ca'rrier to a lower frequency modulated in'ac cordance with both'sets of signals, a-

pair of filter circuits,*means for impressing energy of said *lowe'r frequency upon both said-circuits,

"*one of said ii-lte'r cireuitsbeing adapted topass at least frequencies above said lower frequency modulat'ing the carrier "wave inaccordance with one set of signals desired to be transmitted, phase I modulating the carrier wave in accordance with another-"set of signals desired to be transmitted, "transmitting'the resultant carrier-and upper and lowersets of "side bandsfpreventing modulation by said one'set of signals in the absence of the the o'ne set of signals in response to presence of the other set of signals, and providing a degree of frequency modulation of said carrier wave in and the other'thereof at least frequencies below said'lowerzfrequencywhereby'one of said circuits I passes-substantially'the upper side bands of said r signal modulations and the other the lower side bands thereofla pair of detectors having separate input circuits; one 01' said input circuits being 35 modulation. Y

response' to said other set of signals. I

21'."- In a method of secrecy communication, providing a privacy message and. a separate masking signal, generating a carrier, separately dividing said message and masking signal into a plurality ofspaced modulation frequency bands such that the masking bands are of frequencies above and below the privacy message bands, and modulating said carrier with said modulation bands to-pr'ovide a resultant modulated wave wherein the masking bands appear as amplitude modulation" and the message bands appear as phase ELLISON S. PURINGTON.v

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