US1571005A

US1571005A - Secret signaling

Info

Publication number: US1571005A
Application number: US408890A
Authority: US
Inventors: Ralph V L Hartley
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1920-09-08
Filing date: 1920-09-08
Publication date: 1926-01-26
Anticipated expiration: 1943-01-26

Description

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1,571,005 R. v. 1 HARTLEY SECRET SIGNALING Original Filed Sept- 8, 1920 2 Sheets-Sheet 2 lnvenror:

,Qa/,0k V. L. Hanf/ey Patented Jan. ze, 1926.

UNITED STATES PATENT OFFICE.

RALPH V. L. HARTLEY. OF SOUTH ORANGE, NEW JERSEY. ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK. N. Y.. A CORPORATION OF NEW YORK.

SECRET SIGNALING.

Application led September 8, 1920, Serial No. 408,890. Renewed November 14, 1925.

To al? whom z't may concern.

Be it known that I, RALPH V. L. HAnTmir, a citfen of the United States, residing at South Orange, in the county of Essex, State of New Jersey, 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 secret transmission of electrical currents or waves for signaling or other purposes.

More specifically the invention relates to the type of wave transmission in which a sustained wave is modulated or controlled vin accordance with the wave to be secretly transmitted. A sustained wave used in this manner is sometimes called a carrier wave, and generally has a constant high frequency. In the present invention, however, the carrier wave may be audible and is made to have a cyclically varying frequency, the rate of variation being' very low in comparison with the frequency of the signaling waves to be transmitted.

It is an object of the invention to enable audio frequency currents representing intelligence, but having a form which renders them unintelligible when received by ordinary means, to be` eiiiciently transmitted to the practically complete exclusion of components which are unnecessary and which might4 tend to lessen the difficulty of obtaining` intelligible indications from the transmitted currents.

It is well known that when a carrier wave is modulated by a range of low frequency waves such as speech waves, a number of different frequency components result, among which are an unmodulated component of the original carrier frequency and two sidebands representing the sum and difference, respectively, of the carrier frequency and the speech frequencies. -If all of these components are transmitted over a telephone line. the speech frequency can be received by the use of a simple detector. If the unmodulated carrier frequenc1 component is completely suppressed and only one sideband is transmitted. it is impossible to receive the speech by simple detection, but

if aiwave of the carrier frequency is combined at the receiver with the transmitted sideband, adetector will yield the speech waves. A system in which only one sideband is transmitted, therefore, possesses a considerable degree of secrecy, since an outsider would have some difficulty in discovering the frequency of the wave which it is necessary to use at the detector in order to receive speech. It is conceivable, howe-ver, that one might hit upon the proper frequency by trial, and when once this has been discovered there would be but little difficulty in receiving the speech.

It has been proposed to use a continuously varying carrier frequency in order to make it necessary to know not only the carrier frequency at any instant, but the manner in which it is varied before the transmitted message can be received. By varying, the

carrier frequency over a considerable range, 4

and especially by employing a more or less complex scheme of variation of the carrier frequency, a high degree of secrecy is obtainable.

If a wire transmission system is used, it. is desirable to suppress completely the currents of the original voice frequencies. since ifany of these get to the line it might be possible to filter them out and amplify them suiiciently to render them receivable. It is also necessary to suppress practically completely the unmodulated carrier frequency component and one ofthe sidebands. It is desirable also to suppress double frcquency `components and the other frequency components which are not to serve any useful purpose. If the lower sideband is the one which is to be transmitted. and if the lowest essential frequency represented in it is above the highest voice frequency then a high pass and a low pass filter in series in the output of the modulator can be used. The high `pass filter will suppress all currents of the frequencies from zero up to the upper limit of the voice range. while the low pass filter will suppress all currents of frequencies higher than the lower sideband including components of the carrier frequency andthe harmonics. If the lower sideband, however, `contains frequencies so low as to overlap the voice range, a filter cannot be used to suppress the voice, but a balanced modulator may be used fer this purpose as will be described hereinafter. In either case a low pass filter may be used to suppress the umnodulated carrier component, the upper sideband and the high harmonics. It will be shown hereinafter, however, that if a low pass filter having a lfiXed cutoff frequency is used to suppress the carrier it will permit of only a very limited range of Variation of the carrier frequency.

It is a further object of the present invention to overcome this limitation and to make possible a variation of the carrier frequency over any desired range while still preventing the unmodulated component of carrier frequency and other undesired frequency components from being transmitted. In the present instance` this object is accomplished by varying,vr simultaneously with the carrier frequency the frequency above which all frequencies are suppressed. This may be done either by the use of a plurality of filters of successively higher'cutofi' frequencies -or by a filter having a variable cutoff.

In the drawings to which reference will now be made for a more detailed description of the invention` Figs. 1,

A

2, 3 and 4 are schematic circuit diagrams illustrating forms of embodiments which the invention may take` Fig. lf1 is a graphic representar tion of certain frequency relations made use of in describing the invention, Fig. 2a illustrates a modification of certain details of Fic. 2, and Fig. 5 shows a modified form of filter suitable for use in the system of the invention.

In the system to be described, it is assumed that the carrier frequency` varies cvcliffllv in any desired manner between the limits c and fv-I-d where d varies between r'ero nnd a predetermined limiting value. If the sneech frequencvband as a whole is 'renr^sented bv s and if the carrier wave is l nent and the upper sideband. This means that the cutoff frequency of the filter must always lie between the carrier .frequency fv-l-d and the frequency in the band v-l-d-fs lying nearest to the carrier frequency. The

permissible range ofe variation of the carrier frequency is limited.. therefore, to something less than the lowest essential voice frequency. This is illustrated in Fig. la. The upper portion of the diagram represents the position of the speech modulated wave in terms of frequency relative to the cutoffl frequency F of the filter at the instant when the carrier frequency has been varied to its upper permissible limit (dzmax.) and the lower portion of the figure represents the conditions at the instant when the carrier frequency has its lowest frequency value ((1:0). The carrier frequency is in this figure denoted by the heavy vertical line. The sidebands are denoted by s, the one to the left of the carrier 'frequency' being assumed to be the lower and the one at the right the upper sideband. If the filter is so designed that all frequencies to the left of the cutoff frequency F Vof the filter are transmitted while all frequencies to the right are supthan that shown by max.. it will be seen` bearing in mind that the sidebands would be shifted with the carrier, that some of the lower sidebands would be thrown into the suppression range of the filter and this would result in impairing the transmission of the speech. If thecarrier goes to a lower value than that indicated by (Lto, it will pass into the transmission range of the fil' ter. It is evident therefore that with a filter having a fixed cutoff. the maximum permisf sible range of variation of the carrier frequency issomewhat less than the frequency Iinterval between the carrier and the nearer edge of the sideband.

According to the present invention, the cutoff frequency F is changed with the carrier so that to whatever extent the carrier frequency shifts, the cutoff frequency changes in such a manner as to suppress the carrier, but to permit the transmission of one of the sidebands. for accomplishing this is shown in Fig. 1. In this figure a transmittingr arrangement is shown. IThe modulator M is supplied with carrier oscillations from the generator O and with speech frequencies frein the circuit 2. The speech waves originate in the microphone circuit 3, which is coupled to the circuit 2 through a low-pass filter 4. This filter is so designed as to transmit alll of the essential. voice frequencies, but to suppress the high harmonics of the voice so that essential speech frequencies confined to a limited range are used to modulate the carrier. The modulator M is of the balanced type comprising the two discharge devices 5 and G connected so as to suppress transmission of the speech waves. The speech waves, it will be noted, are impressed One arrangement upon the common connection between the filaments and the grids through the coil 7, while the carrier wave from the oscillator O is impressed in an opposite sense upon the two tubes by means of the coupling 8. Due to the mode Aof connection employed here, the unmodified voice frequency components are suppressed, but the unmodified carrier com onent will be transmitted. The carrier equency may be high in comparison with speech frequencies but it may also be of audible frequency. There will be no confusion of the original speech currents and the side band currents for the reason that the former are not transmitted by modulator M. c

A filter 9 is employed to suppress the carrier frequency, but to permit theI transmission of the lower sideband resulting from modulation of the carrier by the speech frequency currents. The filter 9 is, asis also .the filter 4, of the general type described in the patent to Campbell, No. 1,227,113, dated May 22, 1917. but filter 9 is provided with means for making its upper limiting frequency variable at will. The filter 9 comprises a number of. sections each of which includes inductive elements in series with the line and capacitative elements in shunt of the line. The cutoff frequency of the filter depends upon the product of the` inductance and the capacity values, and hence by varying the magnitude of either the inductance or the capacity, it is possible to vary the cutoff frequency. In, the drawing, the capacity in shunt of the line consists of fixed capacity. elements 10 and variable capacity elements 11 in parallel .with each other. The variable capacity elements each comprise a stationary plate and a rotatable plate which is arranged oo be driven by the motor 12, and their plates or armatures may be so designed as to introduce any desired changes into the capacity of these condensers. In this way the cutoff value of the filter may be varied in any one of a number of ways by changing the shape and size of the condensers even though the speed of rotation of the motor remains' the same. As pointed out above, the carrier, frequency in this system changes cyclically between redetermined limits. For introducing t ese I frequency variations the oscillator O may be supplied with a cyclically varying capacity element 13 mounted on the same shaft as the condensers 11, and the system may be so controlled that as the carrier frequency, supplied from the oscillator O, in-

) creases or decreases, the cutoff value of the filter will follow the chan es in the carrier frequency by virtue of t e. variations in capacity of the condensers 11. With an arrangement of this kind it is possible to 5 vary the carrier frequency in accordance with any desired scheme of variation between relatively wide limits, and still permit the transmission of the lower sideband, but suppress the transmission of the unmodulatcd component of the carrier frequency.

Since in order to receive the Wave. transmitted from the circuit in Fig. 1, it is necessai-y to supply to the detector a wave of the carrier frequency, it is necessary to provide at the receiver an oscillator, the frequency of which varies in the same manner as that of the oscillator O. This may be done conveniently by providing at the receiver a constant speed motor similar to 12 and by synchronizing the'movements of thc two motorsy in any well known manner.

For two-way secret communication. a rcceiver may be provided atthe same station as the transmitter shown in Fig. 1, and this receiver may be supplied with the necessary carrier frequency wave from the oscillator O provided the same carrier frequenc is used for transmission in the two directions. The type of receiver necessary will be clear, it is believed, from the description that has been given of the transmitting circuit, but the circuit arrangement preferably used at the receiver will be described more in detail in connection with Fig. 3.

The system of secret signaling according to the invention may be either simplex or multiplex. Fig. 2 shows a schematic layout of a terminal station on the multiplex line 14, in which provision is made for transmitting simultaneously two messages in each direction. By an extension of the arrangement illustrated in this fi ure 4it is evident that a larger number o channels maybe provided. The multiplex line 14 has a balancing network 15 byvirtue of which in connection with the

hybrid coil

16, 17, currents may be impressed on the line and received from the line without interfering with one another. Bridgedacross the line 14 is a transmitting loop S to which the transmitting circuits of all of the channels are connected, and inductively related to the hybrid coils 16, 17 is a receiving loop R to which the receiving circuits of the channels are connected. In this system instead of showing a transmitter and receiver connected with the modulator and detector, low frequency lines v such as ordinary telephone lines L,l and L2 are connected with the high frequency sets to enable two-way repeating between the multiplex line and the telephone lines. Each channel is supplied with carrier frequency oscillations from the generators such as indicated at O1 and O2. The voice currents received over theline L1, for instance, are impressed upon the modulator M1 in the same manner as indicated in Fig. 1, and the lll) carrier. wave from the, oscillator O14 ismodulated in accordance with speech or other low frequency currents transmitted overline L1. A transmitting filter SFl, which may be ofthe type shown in Fig. 1, suppresses transmission of' the unmodulated carrier frequency component, but permits transmission of the lower sideband from the modulator to the line through sending loop S. At the same time speech currents received over the line L2 modulate a car-- may be 4similar in every respect to that' shown in this figure, the nature of the receiving circuit may be understood by referenceito the receiving apparat-us at the terminal station shown. It will be assumed that waves of the same carrier .frequency are used for transmitting in the two directions and that the lower sideband is in each case transmitted, although if preferred, the upper sideband could-be transmitted( and different carrie-r frequencies for. the two directions might be used. Modulated waves received from the line 14 inthe loop R and intended for the low frequency line L1 are selectively received lby the receiving filter RF from which they pass into the detector D,. As pointed out above, it is necessary to supply to the detector a carrier wave component of the same nature as that which was modulated at the distant-'station by the message wave to be detected, but of which 'the unmodulated component was suppressed at the distant station. This -wave may introduced to the detector D1 from the oscil# lator O1, as is shown. In the same manner modulating waves to be retransmitted over the line L2 are selectively received'by thev receiving filter BF2, and' are detected by the detector D2. The receiving' filters, the transf mittingfilters and the oscillators may be controlled cyclically by the motor 12 in the same manner as that indicated in Fig. 1. ,As stated above, a similar motor and similar high frequency apparatus will be provided`` at the distant'station of the line, andas it 1s necessary to maintain synchronlsm between the shafts which drive the high fre-` quency apparatus at the two stations, any suitable method of synchronizing maybe employed.` For urposes of illustration,- however, an impu se sender 18 is shown combe,v transmitted b v the other channels.'v

`contact successively in its rotation with the commutator .segments 19. These contacts close a circult from battery 20 through the low-pass filter LP, thel sending` loop S and the main line 14. The low-pass filter LP prevents waves-of carrier. frequency from nding access to the commutator circuit, but permits Athe transmission of periodic impulses to the main line 14. The commutator may be arranged to transmit any number of these impulses per revolution of the motor, four such impulses being transmitted in the arrangement shown. At the distant station these impulses are received through a filter similar to LP and may be used 1n, accordance with any one of a number of lwell known synchronizing systems for introducing phase corrections into the motion'of a synchronously, driven shaft which may con' trol the oscillators and. the filters at that station. It should be noted that these impulses bear no Idefinite relation to the carrier frequencies or to the scheme of variationv of the carriers, since any desired number of correcting impulses may be'transmtted per revolution, and-'the condensers which control the oscillators and the filters mayl be given any desired shape and may be changed from time to time. It isl evident, therefore that the correcting impulses will give no clue to the'conditions that it is necessary to know in order to receive the message, and that the correct-ing impulses do'not need to be concealed.

The filter that is preferably used in the circuits thus far-shown, is 'a low-pass filter which'transmits vfreely currents of all fre'- quencies between zeroI anda predetermined upper limit.` If the systemis .used on a multiplex-line, as 'indicated in Fig. 2, the filters of 'the different channels will differ as to their cutoff frequencies and the filter having the highest cutoff frequency of any of the channels will admit the frequencies To prevent thehpossibilityQof lcross-talk, band filters BF may be'inserte'd between the variable filtersv and the line. .These filters; may be fixed filters of suitable constants to permit the passage ofafllfrequenciesthatf -it is desired to. ganSmit-{through the channel jto which they are connected, but toysuppress ltransmission of"frequencies4 used "by .each of the other channels.l l I It has' been assumed that the cutoE value ofthe filtrsthat have been described is varied by 1v1/trying the shunt 'capacity only. For most 1purposes thi-s will probably be sufficient. owever, the impedance of a filter whose cutoff frequency isvaried in this manner, would also vary to a considerable extent. Since the expression for both the cutoff frequency andthe impedance'involve the inductance and the capacity of the filter sections, it may be dsirable in some cases to vary cyclically the inductance as well as the capacity in order to maintain the impedance more nearly constant. A filter in which provision is made for doing this is indicated in Fig. 5, which in view of the other figures will be understood without further explanation.

It is, of course, possible to operate one or two channels of the multiple system of Fig. 2 with an audio frequency carrier wave. The first channel, might, for example, use the lower side band of a modulate-d carrier wave of 2,500 cycles frequency and the second channel, the lower side' band of a modulated carrier wave of 5,000 cycles frequency.

Fig. 2 illustrates a modification of Fig. 2 in which a variable band filter having variable series and shunt capacities may be used to replace a pair of lters such as SF1 and BF or RF1 and BF in each one-way channel. The characteristic of the filter of Fig. 2a is such that the Width of its transmission band may remain approximately constant as its upper and lower cut-0E frequencies simultaneously increase or decrease. This type of filter is accordingly well adapted to transmit a modulation side band of fixed range, but of varying carrier frequency.

Instead of varying the cutoff' frequency of the lters which are used to suppress the carrier frequency, arrangements are shown in Figs. 3 and 4 for using a plurality of fixed lters having successively different cutoff frequencies.

In Fig. 3, the modulator M is supplied by the carrier wave from the oscillator O and by speech waves from the circuit 22, as

vin the 4previous figures. The output of the modulator leads to rotating switch arms 23 arranged to connect the leads 24 of the modulator successively to the input terminals of the respective filters SF, SF and SF. The output sides of these filters are connected in parallel to the outgoing line 25. The switch arms 23 may be driven from the shaft of the motor` 12, which also drives the variable condenser 13 for introducing variations into the oscillator frequency. If the cutoff frequency of the successive filters differs by less than the frequency interval between the carrier and the near edge of the side band to be transmitted, it is evident that as the carrier frequency rises to the point where the lower sideband is about to pass into the suppression range of the filter SF, the carrier frequency has exceeded the cutoff value of the filter SF. The filter SF can, therefore, be switched out of the circuit and the filter SF can be thrown into the circuit and the carrier frequency can then be permitted to increase to such a hi h value that the lower sideband is about to is cut-off by this filter when the third filter SF may be switched in. This process may be continued indefinitely by providing enough filters. The switch arms 23 are provided with sufiiciently broad contacts to enable the next succeeding filter to be included in the circuit before the one previously used is disconnected so that no discontinuity in the transmission occurs. As the carrier frequency decreases in value, the filters will be switched into the circuit in the reverse order and any suitable means of controlling the switch arms 23 by the continuously rotating shaft of the motor 12 may be employed. It is considered unnecessary to show further details of such an arrangement.

Referring now to the receiving circuit of Fig. 3, modulated incoming waves in the circuit 26 are impressedupon the bank of receiving filters RF, RF and RF. Switch arms 27 connected to the shaft of the motor 12 and operating in the same manner as switch arms 23, cut the successive receiving filters into and out of the line leading from circuit 26 to the detector I). In the case of a simplex systemit probably would not be necessary to use a plurality of receiving filters, provided arrangements were used for venting transmission of ythe carrier Wave back to the line. However, the three receiving filters have been indicated for purposes of illustration, since in a multiplex system it might be desired to have the frequencies of the currents of all the channels vary together in the same sense, so that at certain instants the channel would be using currents of certain frequencies which at other instants might be used by a different channel, in which case it would be necessary to provide receiving filters corresponding to the transmitting filters. The detector D may be of the type used in any of the circuits heretofore shown. It comprises two

discharge tubes

28 and 29 connected in balanced relation, the received sideband being impressed upon the tvwo vgrid circuits through the coupling 30. The necessary unmodulated component of carrier Wave frequency might be applied either to the input circuit or to the output circuit. It is preferably applied to the output or plate circuit, as is indicated by theconnection 31, since such connection prevents transmission of the carrier frequency back to the line 26 by virture of the unilateral conductivit of the discharge devices 28- and 29. By ta ing 4carrier frequency and the cut-off frequency of the transmitting filters is indicated in Fig. 4. In this figure the d ottedl line 33 indicates the shaft of the motor such as motor- 12 in the other figures.'y This shaft merely serves to drive the commutator 34 which is shown provided with conducting and insulating segments irregular in character. The

, shape and disposition of these segments de- 4 output of the modulator M through thev filter SF. This circuit may be traced from the lead 38 of the modulator to the contact spring 39, relay armature'40 and lead 41 to one terminal of the filter SF and from the other terminal of the filter through the lead 42 to the opposite lead 43 of the modulator M. Relay armature 40 in its retracted position permits the closure of contacts 44 which close a shunt around a portion of the inductance 45 which inductance, together with the condenser shown, controls the period of the oscillator O furnishing carrier waves to the modulator M. The adjustment of the oscillator and the constants of the filter SF areV arranged so that when the shunting contacts 44 are closed, the filter SF suppresses the unmodulated component of the carrier frequency and components of higher frequency "but transmits -the lower sideband. When the rela 37 is energized by closure of the circuit t rough a conducting segment of the commutator, its armatures are attracted and armature 4() shifts the circuit from the contact spring 39 to the spring 46,'

and immediately thereafter breaks the con- -5 tact between the spring 39 and the armature 40` At the same time it separates the contacts 44, thereby removing the shunt connection from the inductance 45. The out# put of the modulator is now connected through conductor 38, contact springs 39 andV 46, conductor 47 to one terminal of the filter' SF and from 'the other terminal of the filter to the opposite lead 43 of the modulator. The filter SF is so designed that when the shunt through contacts 44 `is broken and the corresponding variation is introduced into the frequency of the oscil` lator, this filter will suppress the unmodu lated carrier 4frequency component and components of higher frequencies but will per' mit the transmission of the lower sideband.

l,By alternately making and breakin the'circuit of relay 37, therefore, a sud en shift 1n the carrler frequency is produced and. the filters are alternately controlled to suppress the unmodulated carrier frequency compo-l nentand, to transmit the lower sideban'd. Relay 37 is also shown as provided with an armature 48, and by a set of contacts and circuits similar to those controlled by armature 40, causesthe receiving filters RF and l RF tobe alternately included between the incoming circuit 49 and the detector D. As noted above, it may not in all cases be necessary or desirable to provide a plurality 'of receiving filters, but 1n some cases a single filter will suffice.

The circuit arrangements that have been illustrated and described are intended merely as representative of preferred embodiments of the invention, and it is obvious. that many other arrangements' are 'possible' and will occur to those skilled' in the art. The invention, therefore, is not to be construed Ias limited to the specific circuit arrangements that have beeny disclosed, but only by the scope of the appended claims.

What is claimed is:

1. In asignaling system, a source ofI car- Y rier waves of continuously varying frequency, means to modulate said waves in accordance with signals to produce upper and lower side bands of frequency components, selective circuits for selectively transmitting a'portion'of said frequency components to the practical exclusion of all of the components in one of said side bands, and means coordinated with the'said source of carrier waves for varying the selective range of transmission of said selective circuits in step with the variations in the carrier frequency to cause said selective circuits to suppress substantially the whole of the same side band at all frequencies of the carrier wave.

2. In a signalingsystem, a source of carrier waves of continuously Vvarying frequency, means to modulate said wave linl accordance with signals to produce upper and lower side bands of frequency components, selective circuits for selectively transmittinga portion. of said frequency components to the practical exclusion of the unmodulated carrier frequency component and all of the components in one of said side bands, and means coordinated with said source of carrier waves for varying the selective range of transmission of said selective circuits in stepwith the variations in the carrier frequency to cause said selective circuits to suppress said carrier frequency and substantially the whole of the same side bandcomponents at all frequencies ,of the carrier wave.

3. In a signaling system a source of carrier waves of cychcally varying frequency, a modulator for modulating sald waves'in accordance with signals to be transmitted, filtering circuits for suppressing the unmodulated carrier frequency component while permitting transmission of one of the sidebands, and means for controlling said circuits to vary the limiting frequency between the transmitted and the suppressed ranges as the carrier frequency is varied.

4. A signaling system comprising transmitting and receiving stations, synchronously driven control members at said stations, means for producing a complex wave at a transmitting station, selective circuits at each station for selectively transmitting a portion of said wave and means under control of said synchronously driven' control members for cyclically varying the frequencies of said wave and for varying the transmission characteristics of said selective circuits to transmit the corresponding portion of said complex wave at its different frequencies.

5. In a modulated wave transmission system including a wave transmitting medium, circuits for producing'for transmission a modulated carrier wave, a plurality of ltering circuits arranged for connection between said wave producing circuits and the medium, having different cut-ofi:l frequencies, means for changing the carrier frequency and for simultaneously changing the connections of said filtering circuits between said wave producing circuits and the transmission medium to transmit substantially 2 the same relative components of said modulated wave and to suppress substantially the same relative components of said modulated wave after the carrier frequency is changed as before.

6. The method of modulated carrier wave signaling in which the carrier wave in cyelically varied in frequency, which method comprises modulating the carrier wave of varying frequency in accordance with signals to produce side bands, producing cychc variations in the carrier frequency of greater extent than one half the frequency interval separating said side bands, filtering the modulated waves to suppress the carrier and one of the side bands while freely transmitting the other, and shifting the suppression range of the filtering circuits simultaneously with the cyclical changes in the carrier frequency to enable separation between the carrier and side band to be suppressed. and the other side band at all the values of the carrier frequenc 7. n modulated carrier wave transmission of signals comprising a band of frequencies lying between two finite frequencies neither of which is zero, the method of selecting one side band to the exclusion of the carrier component and the other side band notwithstanding variations in the carrier frequency of greater extent than that represented between zero and the lower limiting frequency of the signals to be transmitted. which method comprises filtering the modulated wave to separate one side band from the carrier component and the other side band, and varying the cut-oil'l frequency of the filtering circuits in synchronism with the variations in carrier fre uency to cause the cut-off frequency of the ltering circuits to be within the frequency interval separating the carrier frequency and the side band to be transmitted at all values of the carrier frequency.

8. The method of wave transmission, which comprises modulating a varying frequency carrier wave by a modulating wave to produce a modulated wave, suppressing from the resulting modulated wave com- -ponents of a range of frequencies including the original carrler frequency, and causing the absolute value of the limiting frequency of the suppressed range to vary in accordance with the variation in the frequency of the carrier wave, whereby suppression of the said range of frequencies including the original carrier frequency may take place Wit a greaterrange of variation of the carrier frequency than -would be possible with a fixed limiting frequency.

9. A transmission system comprising means for producing a carrier wave of varying audio frequency,- modulating circuits for modulating said wave in accordance with audio frequency currents to produce# resultant modulated waves of frequencies overlapping the range occupied by said audio frequency modulatin waves, and means included in said modulating circuits for suppressing transmission of unmodified audio.

frequency modulating currents, while permitting transmission of said ,modulated waves. y

10. Ina secret signaling system, a line, a source of audio frequency carrier waves, a balanced modulator includin input and output individual branches an a neutral in- `'put branch, a source of speech current wavesa said source of speech current waves being associated with said neutral branch, said source of carrier waves being differentially associated with both input individual branches, said lines being associated differentially with both output individual branches, whereby the speech waves are prevented from being transmitted to line, said carrier waves being of such low frequency that the resulting modulated waves overla in frequency the range occu ied by sai speech waves, and a low pass lter connected between said modulator and said line for selectively7 transmitting the lower side band from said modulating circuit, said filter suppressing transmission of the carrier wave component.

11. In a signaling system, a line, modulating circuits for producing a speech modulated carrier Wave having frequencies overlapping the range occupied by the unmodied speech waves, said modulating circuits including means to suppress transmission to the line ofthe unmodified speech waves, and a low pass filter connected between said modulating circuits and said line for suppressing transmission of currents of the carrier Wave frequency and of hlgher frequen- 10 cies.

In witness whereof, I hereunto subscribe my 'name this 1st day of September A. D.,

RALPH V. L. HARTLEY.