US1542565A - Secret signaling - Google Patents

Description

June 16,. 1925. 1,542,565

R. c. MATHEs sEoRET SIGNALING Filed June 30. 1923 "Ioni" acting as so much noise in the receiver.

Patented June 16, 1925.

UNITED STATES PATENT OFFICE.

ROBERT C. MTHES, OF NEW YORK, N. Y., ASSIG-NOR T( WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SECRET SIGNALING.

Application filed .Tune 30, 1923. Serial No. 648,647.

To all whom z'zf may concern:

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

The present invention relates to wave transmission with secrecy for any purpose,j

such as for secret signaling.

The invention has particular application in transmitting and receiving signals, or the like, comprised of a considerable band of frequencies, such as speech.

An object of the invention is to obtain a high degree of secrecy with comparatively simple terminal apparatus and by such transformations of the signal or other waves as will preserve a high degree of quality in the reproduction of the signals at the receiver.

A further object of the invention, when it is applied to radio or carrier transmission, is to economize in the energy and in the breadth ofthe frequency band transmitted.

Briefiy and specifically stated, according to the invention, the speech or other wave band is divided as to frequency into a plurality of sub-bands, some of which may be transmitted without modification and others of which first have their frequency order inverted and are then transmitted along with the unchanged sub-bands, or, if desired, each of the sub-bands may be inverted beforf" transmission. Each of the sub-bands is preferably so chosen as vto be by itself in capable of reproducing understandable speech. 'The possibility of understanding any of the message contained in one of the sub-bands is still further lessened by the presence of a neighboring sub-band of inverted frequencies, these inverted frequencies The message is received and understood at an authorized station by reinverting each in` verted sub-band and properly combining it with the other sub-bands.

The various objects and features of the invention will be more fully apparent from the following detailed description in which .reference will be made to the accompanying drawing, showing inF ig. 1, a form of transmitting' station employing the invention and For example, filter F, may

may transmit frequencies from 1500 to 2200 cycles, it being assumed that the essential speech frequencies from 100 to 2200 cycles are to be transmitted. This result may be accomplished by constructing each of these filters in the form of the wellknown band filter composed of a number of sections as disclosed in the Campbell Patent No. 1,227,113, May 22, 1917. If desired, a different number of filters than the three illustrated may be used to sub-divide the speech frequencies, in which case the band Width of the -respective filters will be suitably chosen to correspond with the number of filters used.

In the form of the embodiment illustratedV in Fig. 1, the sub-band from each of the band filters is applied to an inverting modulator, these being shown at M M2 and M3 respectively. For example, the sub-band from the filter F, is applied through the transformer 12 to the common branch on the input side of the modulator M, comprising :the discharge tubes 13 and 14. A continuous wave of a frequency equal to the sum of the filter band edges, in this case 100 plus 800. cycles, equal to 900 cycles, is supplied from the source 15 to the individual input windings 16 and 17. The output windings 18 of the tubes 13 and 14 are connected in common to the outgoing circuit 19 which includes a filter F, having the same transmission frehquency band as the filter F,. The source 15 may be a vacuum tube oscillator of the type shown in the Hartley Patent No. 1,356,763 October 26, 1920 and the modulator M, is o the general type disclosed in Carsons Patent No. 1,343,306, June 15, 1920.

The purpose of the modulator M,L and its associated circuits is to invert the frequency order of the applied waves from the filter F, so that in the outgoing circuit 19 only Wave components which have. a frequency order the inverse of that of the applied waves are present. The action of this modulator as is well known, is to produce an upper side band extending from 1000 to 1700 cycles, and a lower side band extending from 100 cycles to 800 cycles, in which the 100 cycle component is produced as the difference frequency between the 900 cycle con tinuous wave and the corresponding 800 cycle component. Similarly, each component of the lower side band represents the difference in frequency between the 900A cycle continuous wave and a corresponding component of the applied wave. As has been stated, the filter F1 selectively transmits only the range 100 to 800 and, there* fore, transmits only the inverted frequencies.

In a similar manner,the wave transmitted throughthe filter F2 modulates the continu- 'ous wave from the source 20 of a fre uency of 2300 cycles, andthe filter F2 whic may be a duplicate of filter F2, selectively transmits the lower side band, that is, a wave having a frequency order the inverse of that applied to the modulator. The source 22 has a frequency of 3700 cycles and consequently when the Wave transmitted through lter F5 .is applied to the modulator M3, a lower side band within the same frequency limits as the applied sub-band is produced having a frequency order the inverse of the applied sub- `band, and this is selectively transmitted by the citer F3'.

Due to the fact that the sub-bands from .applied in common through the transformer 23 to the input side of the transmitting modulator TM which has its output side associated with the transmitting antenna TA. The common or neutral branch o f the input circuit is supplied with a radio frequency wave from the source 24 so that this radio frequency wave is modulated by a band of frequencies having a width'equal to the sum of the applied sub-bands. This source may be of the type disclosed in the Hartley Patent No. 1,356,763 referred to above.

Due to the use of the balanced modulator TM, the unmodulated carrier or radiov wave component is balanced out in the output circuit of the modulator and is, therefore, prevented from being transmitted. The transmitting antenna TA. is preferably sharply tuned by means of the condenser 25 so as to discriminate in favor of'one of the two side bands of the modulated radio wave, andto suppress the other side band. Where the carrier or radio frequency is relatively low as, for example, 10,000 to 20,000 cycles, this method of suppressing one side band is effective. If very high carrier frequencies are to be used, it is preferable, in order to secure single side band transmission, to employ a modulating system of the type shown in the Osborne Patent No. 1,361,488, December 7, 1920, since at very high frequencies it is impracticable to provide sufficiently sharp tuning in an antenna to discriminate materially between speech side-bands.

It is afe'ature of the invention that it does not depend forsecrecy upon the transmission of all of the components of the modulated Wave nor components in both side bands, nor transmission of the unmodulated carrier component, but that only one side band need betransmitted, and therefore the energy contained in one of the side bands and in the unmodulated carrier component need not be transmitted. It has been found that the energy of one side band and the' unmodulated carrier component comprises about {ive-sixth of the total energy of the modulated wave and applicant is enabled therefore to effect a considerable economy in the energy transmitted by having to transmit only a single Side 4hand or substantially one-sixth of the energy of the entire modulated wave. This feature makes the invention particularly adapted for long distance communication such as trans-atlantic radio telephony. However, the invention is not limited to such use since the economy in the energy and frequency range employed by the invention makes it advantageous for other types of transmission such as wire transmission, either by the carrier current method or by transmission at the signal frequency level.

It is not necessary to invert -the frequency order of eachA of the sub-bands. For example, the frequency order may be inverted in Ithe case of two of the sub-bands rovid-ed for in Fig. 1, while the other subliand may be applied to the modulator TM without first being inverted. To do this, it is only necessary to omit the modulating circuit M1 or M2 or M3, as the case may be, and to connect the corresponding dividing lter such as F1, F2 or F8 directly across to the circuit 19.

If the Wave transmitted from the station in Fig. 1, is received by a simple detector,

itis obvious that no components capable of reproducing intelligible speech will be obtained, since 'each ofthe sub-bands is, by itself, incapable 'of being understood, and the presence of the-inverted sub-band or sub-bands produces noise components which will interfere with the reception of such lil() fragmentary speech components as may be y present'incase' one or more' of the m1li-bands vthrough the filters RF1 RF2 detector D ivith a continuous Wave of the radio or carrier frequency supplied from the source 26. As a rusult of the Well known action of the detector D, the lou7 frequency output Waves contain components corresponding to those which were impressed on the modulator TM at'the distant station. These components are separated and selectively transmitted through the respective filters RF1, RF2 and RF3 to the inverting or demodulatingcircuits DMl, DM2 and DM3, respectively. These filters may be duplicates of the filters F1, F2 and F3, respectively, and the demodulators DMI, etc., may be duplicates of the modulator M1 of Fig. 1. Also, the three Wave sources 27 28 and 29 may be duplicates of the respective Wave sources 15, 20 and 22 of Fig. 1 and are adjusted to have the same respective fre quencies.

As explained in connection with Fig. 1, therefore, the action of the demodulating circuits is to invert the frequency orders of the impressed sub-bands which, it will be recalled, have frequency orders the inversel of the normal frequency order of the subbands as derived from speech. The reinverted sub-bands are selectively transmitted and RF3 to the receivers R. The sub-bands applied in common to the receiver R, it Will be seen, are substantially the same as the three sub-ban ds divided out of normal speech by the' filters F1, F2 and F3' of Fig. l and these received and reinverted sub-bands produce understandable speech in the receiver R.

No amplifiers have been shown, but, it Will be obvious to supply the necessary amplification at suitable points in the system.

That i's claimed is:

1. The method of transmission of Waves having a band of frequencyv components occurring in a characteristic order, which comprises dividing said band into sub-bands, inverting the frequency order of the components of certain of the sub-bands while keeping the respective sub-bands Within substantially the same frequency limits as they occupy before inversion, and transmitting all of said sub-bands simultaneously.

2. The method of secret transmission of speech comprising sub-dividing .the speech waves into sub-bands of frequency components, the components comprised in any one sub-band being incapable of reproducing understandable speech, inverting the frequency order of certain of the sub-bands While keeping the sub-bands Within substantially the same frequency limits, and transmitting all of the sub-bands simultaneously.

3. The method of receiving the Waves secretly transmitted by the method defined n in claim 1, comprising receiving all of the transmitted sub-bands simultaneously, reinverting the frequency order of each of the sub-bands of inverted frequency to restore the characteristic frequency order of each sub-band, and applying the restored subbands together to a receiver.

4. The method of receiving understandable speech from the Waves secretly transmitted by the method defined in claim 2 which comprises receiving all of the transmitted sub-bands simultaneously, separating the individua-l sub-bands by filtering, rein-V verting the frequency order of each of the inverted frequency sub-bands to restore the normal order of frequencies Within each subband, and recombining the restored subbands and applying them simultaneously to a receiver.

5. The method of secret carrier Wave transmission comprising modulating a carrier Wave by the sub-bands resulting from the method defined in claim 1 and transmitting the modulated Wave to-a dist-ance.

6. The method of secret transmission of speech comprising subdividing the speech waves into sub-bands of frequency components, the components comprised in any one sub-band being. incapable iof reproducing understandable speech, inverting the frequency order of certain of the sub-bands While keeping the sub-bands Within substantially the same frequency limits, modulating a carrier wave by the sub-bands so produced, suppressing transmission of the unmodulated carrier component and one of the two side-bands resulting from the modu lation, and transmit-ting the remaining sideband to a distance.

.7. The method of reproducing understandable speech Waves from the side band transmitted by the method'defined in claim 6 which comprises combining the side band with a Wave of the carricr'frequency in a detecting circuit, separating the individu-al sub-bands Within speech range obtained from the detection, reinverting the frequency order of each inverted frequency subband, and combining the resulting subbands in a receiver.

8. The method' of securing secrecy of transmiion Within the frequency limits of a single side-band of a modulated Wave, comprising modulating a carrier Wave by suppressing the unmodulated carrier component and one side-band, and transmitting the remaining side-band.

9. The method of secret radio telephony comprising transposing essential speech requency components to produce a band of unintelligible components occupying substantially the same frequency limits as the original speech frequency components, modulating a radio wave by the band of frequency components so transposed, suppressing 10 transmission of one of the two resulting side-bands of the modulated Wave and the unmodulated carrier Jfrequency component, and transmitting the remaining side-band to a distance. 15 In Witness whereof, I hereunto subscribe my name this 26th day of June A. D., 1928.

ROBERT C. MATHES.

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