US1612090A - Means and method for signaling - Google Patents

Description

Deco 28 1926.

W. E. BEATTY MEANS AND METHOD FOR SIGNALING Filed May 21, 1921 SUPP/755558 o l'oIOOO .S UPPFTE SSE S o ro mun SUPPRESS v 300 man [500 F0 To To 400 I200 1600 UPPRE $2.

300 I000 I509 1"0 f'o To 400 I200 I600 SUPP/9E 555 I000 To 20060 SUPERESSES 55 5 OTnl SUPPHE SUPPHESSES 1000 to 20000 Patented Dec. 2%, 19260 WILLIAM E. rnnrrr, or" BAYSIIDE, nnw

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YORK, ASSIGNOR T0 WESTERN ELECTRIG COMPANY, INCORPGRATED, 0F YORK, N. Y., A CORPORATION OF NEW YORK. 4

MEANS AND METHOD FOR SIGNALING.

Application filed May 21,

This invention relates to means and methods for signaling and more particularly it relates to secret signaling.

One of the objects of this invention is to provide a signaling method and system by means of which signals, such as telephone signals, may be transmitted to a distant station without any danger of the'signals being intelligently received by persons not equipped with the special signaling apparatus of this invention.

Another object of this invention is to provide a secret signaling system which does not require any means for synchronizing the apparatus at the transmitting and receiving stations, which is a necessary element of secrecy systems heretofore proposed.

This invention broadly contemplates transmitting to an outgoing line, a signal of a single frequency or a plurality of frequencies, impressing simultaneously on the outgoing line disturbing currents of other frequencies and providing a receiving device which is responsive to the signaling frequencies but unresponsive to the isturbing frequencies.

The preferred embodiment of this invention by means of which the above objects are obtained comprises, when employed for the transmission of speech, a wave filter at the transmitting station located between the telephone transmitter and the outgoing line and so designed that the filter suppresses certain of the speech frequencies or a band of frequencies within the speech range while allowing the remaining speech frequencies to be transmitted to the outgoing line. Noise currents of frequencies-corresponding to the suppressed speech frequencies are also impressed on the line so that the signaling line at any particular instant contains both speech frequencies and noise frequencies, the noise frequencies being of such an intensity as to render the signals wholly unintelligible. At the receiving station a wave filter similar to that at the transmitting station is provided for suppressing the noise frequencies while permitting the signaling frequencies to pass on to a suitable receiving device.

This invention will be better understood by reference to the following detailed description taken in connection with the accompanying drawings in which in Fig. 1,

1921. Serial No. 471,331. 7

a wave filter is employed at both the transmitting and receiving stations for suppressing a certain portion of the speech frequenciesfmeans being provided for impressing on the signaling line noise frequencies corresponding to the suppressed frequencies of the speech signal; Fig. 2 represents a modification of the system of Fig. 1, in which a plurality of filters are employed at both the transmitting and receiving stations for suppressing bands of frequencies throughout the. speech range, means being provided for impressing on the signaling line, noise currents of frequencies corresponding to the suppressed bands, and Fig. 3 represents a modification of the arrangement of Fig. 1 arranged for two-way opera tion.

The secret signaling system of Fig. 1 illustrates a form particularly adapted for the secret transmission of telephone signals. The transmitting station of the system comprises a telephone transmitter 10 which is connected to the signal line 11, 12 through a wave filter 13. The impedance elements comprising wave filter 13 should be so ar ranged and should have such values that the filter serves to completely suppress an appreciable portion of the speech frequencies generated by the transmitter 10 whereby the transmission line 11, 12 contains currents of only a ortion of the various frequencies generated by source 10. For example, wave filter '13 may have its values so chosen that it will allow frequencies between 1000 cycles and 3000 cycles to be transmitted freely while suppressing frequencies outside this range. The arrangement and the values the wave filter should have to produce this result are described in the Campbell U. S. Patent No. 1,227,113 of May 22, 1917, and, for the purpose of this invention, it is not believed necessary to embody in this specification, a complete description of the desi formulae: for wave filters as is outlined in the above patent.

Inasmuch as telephone currents are still intelligible in a telephone receiver even after.

they have been distorted by the suppression of an appreciable portion of the frequencies representing speech, it follows that the suppression of a portion of the speech frequencies by wave filter 13, does not render, the

speech from source 10 unintelligible although, of course, the quality of the speech will be considerably modified thereby.

In accordance with this invention, the speech currents in line 11, 12 are rendered wholly unintelligible by impressing on the signal line a disturbing noise current corresponding to a range of frequencies suppressed y wave filter 13. For example, in the case where filter 13 suppresses all frequencies below 1000 cycles, the source of noise currents 14 may be so designed as to impress on the signal line 11, 12 currents of frequencies ranging from 0 to 1000 cycles per second. These noise currents, of course, should have such an intensity as to prevent one, by an ordinary receiving device such as a telephone receiver, from being able to distinguish between the noise currents and the speech currents in the signal line, and therefore it will generally be found necessary to make the intensity of the noise currents at least as great as the intensity of the speech currents. One form the source of noise currents 14 may have comprises a vacuum tube oscillation generator 15 of the type now well known in the art in which the frequency generated depends upon the values of the inductance windings 16 and 17, and capacities 18 and 19. Inasmuch as it is preferable that source 14 generate frequencies continuously varying over a portion of the range of frequencies suppressed by wave filter 13, one of the condensers 18 for example, may be made of such a type as to permit a motor 20 to continuously vary the capacity of condenser 18 in such a manner that source 14 through transformer 21 delivers to the signal line currents varying, for example, between 0 and 1000 cycles in the case where such a range is suppressed by filter 13.

It is to be understood, of course, that this invention is not limited to the particular source of noise currents just described since various means, widely different from that described above, may be employed equally well to render the speech frequencies in line 11, 12 unintelligible.

The receiving station ofthe system of Fig. 1 comprises a wave filter 22 which preferably should be of the same type and design as wave filter 13 so that filter 22 transmits, in the assumed case, frequencies only in the range between 1000 and 3000 cycles per second. Since the noise currents impressed on line 11, 12 are outside of this transmission band, it follows that Wave filter 22 will serve to suppress the noise currents and will allow only the speech currents to be impressed by a transformer 23 upon asuitable receiving device such as a telephone receiver 24.

From the above description, it is evident that any one listening in on the signal line 11, 12 will not be able to decipher the distorted signals unless he is provided with a wave filter of the same design as that employed at the true transmitting and receiving stations.

()ne of the chief advantages of the above system is that no synchronizing apparatus is required to synchronize the various parts of the receiving station with corresponding parts at the transmitting station as is generally required in secret signaling systems heretofore proposed. The system of this invention has therefore been described in certain of the appended claims as a system in which the deciphering means at the receiving station acts independently of time, that is, independently of the time the distorting means begins to operate and independently of the rate of its operation.

As shown in Fig. 1, it is preferable that the source of noise currents be located at the transmitting station so that as the noise cur rents and the telephone currents are transmitted along the line conductor 11, 12 both will be attenuated to the same degree and the predominance of the noise currents over the telephone currents will be maintained over the entire length of the signaling c011- ductor 11, 12. It is obvious, however, that if desired the source of noise currents 14 may be associated with the signaling conductor ata point other than at the transmitting station. 7

Fig. 2 illustrates a secret telephone system 1n which a plurality of filters are employed at boththe transmitting and receiving stations for suppressing bands of frequencies throughout the speech range, means being provided for impressing on the signaling line, noise currents of frequencies corresponding to the suppressed bands. The transmitting station of this system comprises a plurality of wave filters 26, 27 and 28 located between the signaling line 29, 30 and the source of telephone currents 31. For illustrative purposes, it may be assumed that filter 26 is so designed that it will suppress currents of frequencies lying between 300 and 400 cycles per second, filter 27 being designed to suppress currents of frequencies between 1000 and 1200 cycles and filter 28 being designed to suppress currents of frequencies between 1500 and 1800 cycles. In order to provide noise currents for the line 29, 30 having frequencies corresponding to the suppressed bands, a plurality of sources of noise currents 32, 33 and 34 are provided, source 32 being adapted in the case assumed, to supply currents of frequencies continuously varying between 300 and 400 cycles, source 33 supplying currents of frequencies between 1000 and 1200 cycles and source 34 supplying currents of frequencies between 1500 and 1800 cycles. These three sources 32, 33 and 34 may be of the type, for example, shown in Fig. 1. The receiving station for the signals from source 31 comprises eraoeo filters 35, 36 and 37 corresponding to filters 26, 27 and 28 of the transmitting station. The insertion of these three filters between the telephone receiver 38 and the signaling conductor serves to prevent thenoise currents from being impressed on the receiver while allowing telephone currents unsuppressed by filters 26, 27 and 28 to be received thereby. It is to be understood, of course, that any desired number of bands of s )eech frequencies may be suppressed by suitable wave filters and that any desired number of sources of noise currents may be employed for generating currents of the suppressed frequencies.

Fig. 3 illustrates an arrangement whereby the system of Fig. 1 may be employed for two-way transmission between two stations. Wave filter 40 at one of the stations may be designed to suppress frequencies between 0 and 1000 cycles thereby preventing currents in this range from being impressed on the signaling line 41, 42 by the telephone transmitter 43. Noise currents corresponding to the suppressed band are impressed on line conductor 41, 42 by the source of noise currents 39 through a transformer 44. At the other station, a wave filter 45 corresponding to wave filter 40 is inserted between the telephone receiver 46 and the signaling line, to allow only currents from the telephone transmitter 43 to be impressed on the re ceiver 46. In order to provide for transmission in the reverse direction, wave filter 47 which is located between the telephone trans mitter 48 and the signaling conductor should be of such a type as to suppress a band of frequencies which is freely transmitted by the wave filter 45. In the case assumed, wave filter 47 should therefore be designed to suppress currents between 1000 and 20,000 cycles or the upper limit of frequency of importance in speech. The connections to the signaling line 41, 42 for the telephone receiver 49 should be such that the noise currents from source 39 are not impressed on the telephone receiver. This precaution is necessary since the associated wave filter 50 is designed to transmit currents of the frequencies generated by source 39. One way this result may be accomplished is to connect one of these leads 51 from the receiver to conductor 41 and to connect the other lead 52 to the midpoint of the secondary winding of transformer 44. If, now, the impedance relations of the system are made such that points 53 and 54 are at equal potential for the noise currents, it follows that currents from source 43 will notbe impressed on receiver 49. The telephone currents, however, from transmitter 48 will not be sub ject'to this neutralizing action and will be freely transmitted by wave filter 50 to the telephone receiver 49. Receiver 49 will not 'freceive any of the transmitted currents from its associated transmitter 43 for the reason that Wave filter 50, as specified above, is so designed as to suppress all of the frequencies transmitted by wave filter 40. Similarly, receiver 46 at the other station will not receive any of the current; generated by source 48 for the reason that wave filter 45 is designed to suppress all frequencies transmitted by wave filter 47. It therefore follows that for the transmission of signals between transmitter 43 and re- .ceiver 46, the signaling line 41, 42 will contain signaling frequencies from 1000 cycles to the limit of audibility, at the same time containing noise currents from 0 to 1000 cycles. For transmission in the opposite direction between transmitter 48 and receiver 49, the signaling line 41, 42 will contain signaling currents from 0 to 1000 cycles and will also contain noise currents within the same range thereby insuring secrecy in signaling in both directions.

It is to be understood that the forms of this invention described above may be variously modified without in any wise departing from the spirit of this invention as defined in the appended claims.

l/Vhat is claimed is:

1. The method of secret signaling over a path extending between separated stations which comprises generating at one station a band of wave frequencies representing sig nals, transmitting to a second station with uniform small attenuation from the generated band a continuous band of frequencies suficient to enable the reproduction of intelligible signals at said second station, suppressing a different band of the frequenc es of said generated band, impressing on the transmission path noise currents of frequencies corresponding to the suppressed frequencies and embracing a total frequency range comparable with that of the transmitted signal band to render the transmitted continuous bandof frequencies diflicult of reception, and suppressing the noise currents and receiving the signaling frequencies at said second station.

2. A secrecy signaling system comprising a transmitting station, a receiving station, a line extending between said stations, said transmitting station comprisng means for generating a wave band of a wide frequency range representing signals, a wave filter between said means and saidline, and arranged a to suppress from said Wave band a ub-band of frequencies wh le freely transmitting to said line a second sub-band of frequencies wider than the suppressed sub-band, said unsuppressed band being sufficient to enable intelligible reproduction of signals, receiving means at saidireceiving station responsive to sa d unsuppre'ssed band of frequencies, a second wave filter between said receiving means and said line and arranged to suppress the same frequency band as suppressed by said first wave filter, and means for impressing upon said line noise currents having frequencies substantially entirely within the frequency range suppressed by said filter and embracing a total frequency range comparable with that of the transmitted signal sub-band.

3. A secrecy signaling system comprising a transmitting station. a receiving station, a signaling path extending between said stations, means at said transmitting station for generating a continuous wave band of signaling frequencies, a wave filter between said means and said path and arranged to suppress a sub-band of said continuous wave band while freely transmitting from said continuous band, a sub-band of frequencies wider than the suppressed sub-band and sufficient to enable intelligible reproductionof signals, receiving means at said receiving station responsive to the unsuppressed frequencies, means for impressing upon said path unintelligible currents of a band of frequencies corresponding to the band sup-v pressed by said filter, the total frequency range of said impressed unintelligible currents being comparable to that of the transmitted signal band, and means at said receiving station for preventing said unintelligible currents from being received by said receiving means.

4. A secrecy signaling system comprising a transmitting station, a receiving station, a signaling path between said stations, means at said transmitting station for generating a continuous frequency band repre senting signals, means for suppressing a small frequency band from said continuous band while. freely transmitting the major portion of the frequencies in said continuous band with substantially the same relative amplitudes as generated by said means, said .major portion being sufficient to enable intelligible reproduction of signals at said receiving station, means for impressing upon said path unintelligible currents of a band of frequencies corresponding to the band suppressed by said second means, said impressed unintelligible currents having a total frequency range comparable with that of the transmitted signal band, and means at said receiving station for suppressing the same frequency band as suppressed by said second means.

5. A signaling system comprising a plurality of separated stations, a telephone transmitter and a telephone receiver at each station, a plurality of wave filters of a type arranged to suppress a wide frequency range while freely transmitting another range, a plurality of filters of a second type to suppress a frequency band freely passed by said first type, a filter of said first type being located between said line and the transmitter of said first station; a second filter of the first type being located between said receiver of the second station and said line. one filter of said second type being located between said line and the transmitter of said second station, another filter of said second type being located between the receiver of said first station and said line, a source of noise currents of a frequency band corresponding to the frequency band passed by said first type of filter, and coupling means between the receiver and the transmitter of said first station, said source and said line arranged to prevent the receiver of said first station from being energized by currents from said source.

6. A secret signaling system comprising a transmission line extending between separated stations, means for impressing on said line at one of said stations for transmission in one direction a signal comprising a band of a plurality of audible frequencies, means for impressing on said line at another of said stations for transmission in the opposite direction a signal comprising a band of a plurality of other audible frequencies, receiving means at each of said stations, means for impressing on said line at the transmitting station noise currents corresponding to one band of said signaling frequencies, and means for keeping said noise currents out of the local receiving means.

7. In a secret signaling system, a subscribers set comprising a telephone transmitter and a telephone receiver, a transmission line associated with said set, means for impressing on said line currents of certain frequencies of importance in speech, and for preventing currents of other frequencies of importance in speech from being impressed on said line by said transmitter, means for impressing on said line noise currents comprising a continuous band of frequencies corresponding to a band of the suppressed speech frequencies to render the transmitted speech signals unintelligible, and means for rendering said receiver unresponsive to currents of frequencies unsuppressed from said transmitter while permitting said receiver to be responsive to currents of the frequencies suppressed from said transmitter.

8. The method of secret signaling over a path extending between separated stations which comprises generating at one station a band of wave frequencies representing signals, suppressing from said band a sub-band of said frequencies, selecting from said band a second sub-band of frequencies wider than said sub-band suppressed, and transmitting the sub-band s0 selected to a. second station, the transmitted frequencies being sufficient to enable reproduction of intelligible signals at said second station, impressing on the path noise currents of frequencies corresponding to those of the subband suppressed, and embracing a total frequency range comparable to that of the transmitted signaling sub-band to render the unsuppressed part of said generated band unintelligible, and at the distant station suppressing said noise currents while permitting the unsuppressed part of said generated band to be received.

9. In a secret telephone system, a transmitting and a receiving station, means at the transmitting station for setting up electrical variations corresponding to speech, means including a band filter for selecting from the variations for transmission to the receiving station at least one continuous band of frequency components, the total components selected for transmission comprising more than one-half the continuous frequency range from 100 to 2200 cycles per second, means at the transmitting station for supplying together with the selected components noise currents of frequencies lying in the range of 100 to 2200 cycles per second but different from those transmitted by said band filter, said noise currents embracing a total frequency range comparable to that of the transmitted speech band, means including a similar band filter at the receiving station for separating the speech components from the noise components, and a telephone receiver actuated by the selected speech components.

10. In a secrecy signaling system, a local station comprising a telephone transmitter and a telephone receiver, a second station comprising a second telephone transmitter and a second telephone receiver, a transmission line between said stations, a wave filter between said line and said'first transmitter for suppressing a certain portion of the frequencies of importance in speech, a similar filter between said second receiver and said line, a Wave filter between said second transmitter and said line for suppressing another portion of the range of frequencies of importance in speech, a Wave filter similar to said last mentioned filter located between said first receiver and said line, means for impressin on said line noise currents comprising a and of frequencies occupying ap proximately the same position in the fre quency spectrum as the speech frequencies suppressed by the first-mentioned wave filter, and means for preventing said noise currents from being impressed on the firstmentioned telephone receiver.

In witness whereof, I hereunto subscribe my name this 19th day of May A. D, 1921.

LIAM E. BEATTY.

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