US1743710A - Secrecy system for signaling - Google Patents

Description

Jan. M, i930. c. R. ENGLUND ET AL L743y70 SECRECY SYSTEM `FOR SIGNALING Filed Nov. l, 1927 2 Sheets-Sheet 1 l n A 70m/Ey Jam 14? E930 C, R, ENGLUND Er AL 1,743,7@

SECRECY SYSTEM FOR SIGNALING Filed Nov. 1. 1927 2 Sheets-Sheet 2 ,rra STABS ArsN'r f CABLQB. ENGLUND, 0F FBEEHOIJ, AND FREDERICK B. LLEWELLY'N, 0F MON'TCLAIB, NEW JERSEY, ASSIGNORS TO BELL TELEPHGNE LABORATORIES, INCORPORATED, 0F

NEW YORK, N. Y., A CORPORATION 0F NEW YORK SEGREGY SYSTEM FOR SIGNALNG Continuation of application SerialNm 217,180, :Lied September 2, 1927. This application led November Y l 1, 1927. Serial No. 230,222. Y

This invention relates to Wave transmission, especially signaling by electric waves. vAn object is to prevent unauthorized obtaining of information carried by transmit- 5 ted waves.

A further object is to obtain such secrecy without unduly broadening the requisite width of the frequency band comprising the frequencies which the components of a wave transmitting information have in the transmission medium.

In one specific aspect the invention is a radio `telephone system in which a speech wave separately modulates two carrier waves of such frequencies that the resulting upper side bands for example, are contiguous and when selected and added form a band of fre uencies double the width of the speech ban the carrier waves being suppressed in the modulators.

This band of double the speech band width is caused to modulate a variable frequency carrier wave, which is suppressed in the modulator. The resulting upper side band, for example, is a wobbling band, its width in the frequency scale bein twice the width of the speech frequency and at each instant, and its position in the frequency scale varying in synchronism with the variation of the frequency of the carrier wave, through a frequency "range of variation equal to that of the carrier wave.

This wobbling band is impressed on a filter which passes only a frequency band of the speech band width having lts center somewhere between the frequency limits of. the w-cbbling band, as for example', midway between the frequency limits of the wolfbling band. The passed band is radiated from a transmitting antenna and received by .a distant receivlng antenna. The pass 'band of the filter is`of constant width, and at `each instant the radiated band contains components representing all of the components of the speech waye, with their identity preserved. However, the components representing speech components are permuted ina cyclic order in this radiated band. The band consists of two contiguous sub-bands each of varying width, the upper one con- Aband extending npwar taining components representing the components of a variable portion of the speech from the lower limit of the speech band, and the lower of the two sub-bands containing components represent- -ing the remaining components of the speech band. The information carried by the radiated band cannot be obtained from the band without knowledge of various factors, including the frequency of the variable frequency carrier wave.

The band incident on the receiving antenna is demodulated separately by ytwo waves of variable frequencies, the frequency of one of these waves being the algebraic sum of the variable carrier frequency and the lower of the two fixed carrier frequencies employed at the transmit-ter, and the frequency of the other variable carrier wave employed at the receiver being the algebraic sum of the variable'carrier frequency and the higher of the two fixed carrier frequencies employed at the sending station. This demodulation yields the two above mentioned variable portions of the speech band, which together comprise at each instant all of the components of the speech wave. The demodulations also yield distortion coniponents, but certain of these are filtered out, and the remainder do not render the re ceived speech unintelligible.

Other objects and aspects of the invention will be apparent from the following description and claims.

Fig. l of the drawing is a circuit diagram of the. sending station referred to above; Fig. 2 is a similar diagram of the receiving station referred to above; and Fig. 3 is a set of graphs for facilitating explanation of the invention.

In the following description, frequencies are giv'en in terms of radians per second, except when otherwise indicated, and in symbolical expressions for current or voltage waves, coefficients indicating amplitudes are omitteid, only quantities that indicate frequency being given. Specific values of quantities such as frequencies, speeds, etc., are mentioned merely by way of example, and the invention is not limited thereto.

En Fig. 1 a signal wave, for example a speech wave from telephone transmitter 1, is introduced Iinto the s ch or signal frequency input circuitV o' each of twomodulators lill- 2 and M+S. Each of the modul lators ma`l be, for example, of the usual type of ba anced vacuum tube modulator arranged for carrier wave suppression.

The carrier wave for modulator M-2 may be obtained from an'oscillator 0 5 of a frequency c which may be, fo'r example, 2n 20,000. f e

The carrier wave for modulator M may be obtained from an oscillator 0-'6 of a frequency c-i-q, where lis'the difference between the highest Vand t e lowest frequencies of the speech wave that is to be transmitted. rlhus, if is 21r 3,000, then the frcquency of 'oscillator O-,-6 may be 21|- (20,000+3,000). The lowest frequency of the speech band that is to be transmitted may be designated so.

The upper side bands, for example, from modulatorsM-2 and M3 are selected by .filters BF- and BF-S respectively, and

amplified by amplifiers A`9 and A-lO respectively, and applied as a modulating .wave to. a modulator M-ll, which may be, for example, of the usual type of balanced vacuumtube modulator arranged for sup- ,pression of the carrier wave. AThe filter BF-7 may be, for example, a band ter passing only frequencies between c-i-s., and c+s0-j-g. The filter BF--8 may be, for eX- ample, a band filter passing only frequencies between a+s0|g and c+s+2g. The modulating wave supplied to modulator M-ll from amplifiers A-9 and A-lO is then two identical speech side bands one immediately/above the other, the two occupying the /frequency range from G+S., to c+s+2g.

In modulator M-11 this wave modulates a variable frequency carrier wave supplied from source O-12. 'The frequency of this carrier wave may vary in any suitable manner, as for example sinusoidally, ten times per second, between a lower limiting frequency d and an upper limiting frequency d -I- g Then, designating the value ofthis variable carrier frequency la, the frequency h will vary about an average value d through a range g. The source O-12 may be, for example, a variable frequency oscillatorof any suitable type. A suitable type is disclosed. in Kendall Patent 1,571,010, January v26, 1926, or 1,592,940, July 20, 1926. The frequency d may be, for example 21r 20,000.

t The speech wave vthat is to be transmitted is assumed, as indicated above, to have a frequency spectrum extending from a fre- .quency so up to frequency gli-8. and is assumed to consist of two contiguous subbands or portions, s1 being taken as representative of those frequencies between so and Q-i-a) where :u may be a variable number and is never less than 1, and .92 being taken as representative of the remaining frequencies, which are those between (g+s) and gts.V The speech may then be symbolically Written cos slt-i-cos 8215, where, for

generality, s1 and s, may -exist either simultaneously or one at a time.

wave from aplifier A-9, and cos (p g-i-s?) t andcos ,(P+g+s2)t, both due to the modulating wavefrom amplifier A-lO. As indicated in Fig. 3, this side band represents two Variable frequency or wobbling side bands exactly alike except relatively displaced in the frequency scale so that the lowest frequency of the side band (extending from p-i-g-i-s., up to p7l-2g+s) due to the modulating Wave from amplifier A-lO is equal to the highest frequency of the side band (extending from p-i-so up to p+g+s) due to the modulating wave from amplifier A-9.

The average value of frequency p will be designated pa. The band pass filter BF-20 passes only the frequencies between p., -I- g+so, andpa -I- 2 That is, it passes only the frequencies lying l i 2 center frequency, pa-l-g+s. The frequency p may take on any values between radians per second on either side of its pa-gand pa-ivBF-20 consistsof components which may be symbolically Written cos (p-Il-s2)t and cos (p-t-g-I-s.

This output wave is amplified in any desired number of amplifiers A-21, radiated from the sending antenna 25' and received by the receiving antenna 30 at the distant receiving station, shown in Fig. 2. Although the Wave contains componerfts representing all of the speech components, with the identity of each.

preserved, the information which it carries can not be obtained from it Without knowledge of various factors, including the variable frequency p, which is not transmitted.

From the antenna 30 the received Wave is impressed on the input ci'rcuits'of each of two demodulators DM-32 and DNI-33, which may be for example, like modulators M--2 and M3.

The carrier or demodulating wave for demodulator DM-32 has the variable frequenc p, and the demodulating Wave for demo ulator DM--33 has the variable freyIn ther/system of the drawing, t ese demo ating Waves are obtained from variable frequency oscillators 0 45 and 0 46 like the oscillator O--l2 at the sendin r station. l y

he low fre uency output wave fromdemodulator D --32 may be symbolically Written cos ezt-l-cos (g4-8,)t-i-cos (g-l-sl-sgt,

uency pfland the low frequency output wave from demodulator DM--33 may be symbolically written,

cos (q s2) t cos sit cos (q s1 s2) t.

These waves, after amplication by amplifiers A-55 and A-56 respectively, are 1mpressed on low pass filter LPF-60, which has its output side connected to tele hone receiver 61, and which has a cut-off requency g-i-su. The com onent cos (g4-81W is suppressed by the lter. The components slt and cos szt constitute all of the components of the original speech wave, at their original speech frequencies, and reproduce the signal intelligibly in receiver 61, although the component cos (g-s2) and the two components of frequency g-l-sl--sz are distortion components. The two latter components are customarily present in speech demodulators.

It will be apparent that the invention is not limited to radio transmission but is applicable, for example, to Wire carrier systems and to systems in which the fre uencies employed for transmitting speec or other signals between two stations are within the frequency limits of the original frequencies of the signal components, the radio transmission channel between the stations being representative of any suitable type of transmission channel such as a voice frequency or wire carrier frequency channel.

Although in the system shown in the drawing the waves of the frequencies o and c-l-g are generated by separate oscillators, it will be apparent that, if desired, suitable means, as disclosed in Fig. 3 of H. W. Nichols Patent 1,545,270, July 7, 1925, may be employed for deriving the second wave from the first by modulating the first with a wave of frequency equal to the difference between the desired frequency of the second wave and the frequency of the first wave, and selecting the upper side frequency from It will be apparent that the frequencies of the carrier waves at the transmitting station and the demodulating waves at the receiving station may be kept in proper relation b suitable electrical means, as for examp e, in the man-ner indicated in the above mentioned Kendall patents, Voin thc manner in which television transmitting apparatus and receiving apparatus has been synchronized. Means which have been eniployed for synchronizing such television apparatus are disclosed for example in the copending applicatiton of H. M. Stoller and E. R. Morton, Serial No. 200,799, June 23, 1927.

If desired, oscillator O--45 can be a vacuum tube oscillator carefully constructed to have the same frequency variations as oscillator O-12, although in a different frequency range; or if preferred, the demodu lating wave of frequency -p for demodulator DM-BQ may be obtained by having two oscillators (not shown) at the receiving station, identical with the oscillators O-5 and/O=-12 respectively at the sending station, and a modulator (not shown) for producing the frequency p from the rWaves of such two oscillators. Similarly, the oscillator 0 46 can be constructed to have the same frequency variations as oscillator O-12, in a different range; or the demodulating wave of frequency p-i-g can be obtained as aside band resulting from intermodulation of waves of two oscillators (not shown) identical -withoscillators 0 6 and O-lQ respectively.

VAs indicated above, it isnot necessary that the frequency of oscillators 0 12, O45 and 0 46 vary throughout a frequency range equal in width to that from 8 to g|-s. With any smaller variation, if still of considerable magnitude, the system will still afford a large element of secrecy, because all components of the signal undergo variation in frequency, in the Wave transmitted between the sending station and the receiving station. If the frequencies of oscillators O-12, 0-45 and 0 46 vary less than q radians, then the pass band of filter Blf-20 does not have to be located exactly midway in the extreme side band variation range but may be in any position in that range, such that the speech side bandsdo not materially slide off the characteristic selective range of the filter. The rate of variation of the frequencies of oscillators O`-12, 0 45 and 0 46 preferably should be sufficiently high to prevent syllable reception W-ith use of a constant beating frequency.

' Preferably the frequency of variation is k'between about cycles per second and about 15.

cycles .per second. As indicated above,

cycles alsecond is a very satisfactory frequency of variation. It will be apparent that the variation-need not be'sinusoidal, and that if r,desired it may be intermittent ratherthan continuous, and moreover, that the mode of the variation of frequency may be changed, in any suitable manner agreed upon between the operators at the two fstatioris,l to still further increase the secrecy of transmission.

Y For simplicity the drawing shows the ilaria# tion 'as sinusoidal; but for greater secrecy it is preferable to havethe variation irregular withinY each cycle of variation. Y Y

Although the ,specific value of frequency d mentioned above byway of example is equal tothe value mentioned for frequency c, such 'A art that various modifications may be made in the system shown in the drawing, Without departing from the invention. Thus, for example, the filter Bld-20 may be omitted if the antenna-25 is tuned suficiently sharply to suppress the frequencies outside of the band which it is desired to radiate.

This application is a continuation of our application Serial No. 217 ,130, filed Septemberr2, 1927, entitled l Secrecy systems for signaling.

What is claimed is:

1. The method of operating on a Wave having components of different frequencies,. relatively displacingv which comprises groups of said components in the-frequency scale-and progressively varying the width vof the frequency uband formed by the frequencies of the components that form one -of the relatively displaced groups.

, 2. 'The method which comprises changing the order of sequence of portions of a band of frequencies in the frequency scale and varying the widths of said portions.

3. The method which comprises transposing portions of a band of frequencies and cyclically and continuously varying the width of each of said portions while maintaining their combined width constant.

4. The method which comprises so variably permuting, in the frequency scale, currents vhaving frequencies forming sub-bands that form a frequency band, as to vary the widths of the sub-bands and maintain all of the permuted currents Within a frequencyv .band width equal to thatof the lirst nientioned band at each instant.

5. The method which comprises so varying the frequency of each of the components of a band of signal com onents of dierent frequencies as to produce from the first mentioned band a frequency band of the signal components which has the sainev Width as the first vmentioned band, and which contains the signal components of the first mentioned band variably permuted in the frequency scale but at each instant all present that are present `in the yfirst mentioned band at a corresponding instant, and Wliieh'iscomposed of variable 'widthL sub-bands Vof the signal components throughout each of whicli'the signal components have their original order of sequence in thel frequency scale.

6. Themethod which comprises relatively displacing, in the frequency scale, groups of Wave lcomponents that form portions of a band of frequenciesy and gradually varying thewidth of one of the relatively displaced portions, transmitting the relatively disf placed portions to a distantl point, and at that point varying the frequencies of the components transmitted. and restoring the portions to their initial relation.

y 7. The vmethod of transmitting signal components of different frequencies comprised in a frequency band, which comprises transposing groupsof the components that form portions of the band in the frequency scale sand repeatedly varying the width of certain of the frequency bands formed by the transposed groups, fory transf mission of the signal components, and, for

reception of the signal components, restoring the transposed groups to theirinitial relative positions inthe'frequency scale.

8. The method of operating on a plurality of frequency bands of components, eachA band varying in width but all of the bands forming during said` variation a band of constant width, which comprises varying the frequency of each ofthe components of the first mentioned bands and transposing the irst mentioned bands inthe frequency scale.

` 9. `In combination, means for producing tw contiguous frequency bands of signal f components with each band representing the same signal, and means for varying the frequencies of certain of ,the components of each band.

10. In combinationmeans for producing two contiguous frequency bands of signal components with each band representing the same signal and with the signal components of eachl band varying in frequency, and means for selecting two' such contiguous, variable width, frequency groups o f the components, one group from each baud, as to thereby form a frequency band of the components that is constant in 4width and that' has constant frequency limits.

those at the lower end when the frequency of the carrier waves increases, and vice versa. 12. In combination, means for producing two contiguous frequency bands of signal components witheach band representing the same signal and with the signal components of each band varying in frequency, and means for selecting two contiguous frequency groups of the components, one group from each of the two bands.

13. In combination, means for producing two contiguous frequency bands of signal components with each band representing the same signal and with the signal components of each band varying in frequency, and means for selecting two such contiguous frequency groups of the components, one group from each band, as to thereby form a frequency band of the components that is constant in width.

14:. 4In combination, means for producing two contiguous frequency bands of signal components with each band representing the same signal and with the signal components of each band varying in frequency, and means for selecting two contiguous frequency groups of the components, with the components of each group forming a frequency band of varying width.

15. A secrecy signaling system comprising a transmitting station having means for producing from the signal to be transmitted a wave in which complementary portions of the signal frequency band each of which varies in width are reversed as to their order of sequence in the frequency scale, and a receiving station having means for restoring the portions to their original positions in the fre uency scale.

16. pparatus for receiving signal wave components forming a plurality of frequency bands each varying cyclically in width, said apparatus comprising means for cyclically varying the frequency of each of the components at the frequency of said cyclical variation of band width and transposing the bands of variable width in the frequency scale to form a resulting wave, and means for selecting from the resulting wave a frequency grou of wave com onents and reproducing t e signal there rom in a form intelligible to one of the senses.

17. In a method of receiving a plurality of frequency bands each varying cyclically in width, cyclically varying the frequencies of the components at the frequency of said cyclical variations of band width and transposing the bands of variable width in the frequency scale.

18. A method of operating on mutually exclusiveu portions of a band of signal components of various frequencies, which portions are variable in width in the frequency scale in such a manner that the respective frequency ranges over which said portions extend, at least partly overlap, said method comprising varying the frequencies of the components, synchronously with the first mentioned variations and through a frequency range of variation equal to the range of the first mentioned variations, and transposing the bands of variable width in the frequency scale.

19. In combination, means for producing two contiguous frequency bands of signal components with each band representing the same signal and with the signal components of each band varying in frequency, and means for selecting two contiguous frequency groups of the components, one group from each of the two bands, and each group of variable width in the frequency scale.

20. In combination, means for producing two 'contiguous frequency bands of signal components with each band representing the same signal,` means for selecting two contiguous frequency groups of the components, one group from each of the two bands, and means for varying the frequencies of the components of each group.

In witness whereof, we hereunto subscribe our7names this 31st day of October, A D. 192

CARL R. ENGLUND. FREDERICK B. LLEVELLYN.

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