US2395432A - Secrecy signaling system, method, and apparatus - Google Patents

Description

Feb. 26, 1946. R. M. SPRAGUE SECRECY SIGNALING SYSTEM, METHOD, AND APPARATUS 3 Sheets-Sheet 1' Filed NOV. 19, 1943 ATTORNEY .Feb 26, E946. R. M. SPRAGUE SEQRECY SIGNALING SYSTM, METHOD, AND APPARATUS Filed Nov. 19, 1943 3 Sheets-Sheet 2 INVENTOR BYMJ%AM' ATTORNEY R. M. sPRAGuE SECRECY SIGNALING SYSTEM, METHOD, AND APPARATUS 3 Sheets-Sheet 3 Filed NDV; 19, 1943 M m R @f 0. m H m j www www r v mb R m l b mSwwQ E bx N MUMQNQ Y -2| Sx Q B \\\w wim wel im km www mw R 1 w l u v n u .,KQBQNQ m WQFQQQ +e Y Ev m Qwmmwwl Wsw Eva I Qm :@.WVL Vm@ :ww WT-- m -L ATTO R N EY Patented Feb. 26, 1946 TES UETED SECRECY SIGNALING SYSTEM, METHOD, AND APPARATUS tion of Delaware Application November 19, 1943, Serial No. 510,943

19 Claims.

This invention relates to secrecy signaling systems and more especially to secrecy systems of the frequency scrambling type.

A principal object is to simplify and improve the secrecy system and apparatus disclosed in my prior application Serial No. 410,450, filed Sept. 11, 1941.

Another object is to provide a frequency scrambling arrangement for secrecy systems and the like, wherein the band width required for transmitting the scrambled message is approximately the same as the frequency band of the original signals,

Another object is to provide a simplified and improved secrecy system which is useful over radio links as well as over wire lines.

Another object is to provide a system of secrecy transmission which is well suited for commercial wire line communication systems.

A feature of the invention relates to a, modulating arrangement for frequency scrambling systems whereby the number of possible scrambling combinations is materially increased without increasing the band width required for transmission.

Another feature relates to a secrecy system of the general type disclosed in my prior application Serial No. 410,450, but wherein only a single variable frequency modulator is required at the transmitter and a corresponding single Variable frequency modulator is required at the receiver.

Another feature relates to the method of using haphazard recorded frequencies to control a single variable frequency modulator while achieving the scrambling power heretofore requiring two or more variable oscillators.

A further feature relates to a frequency scrambling system of the type employing haphazard frequency recordings as modulator controls, wherein a substantial reduction is obtained in the band of recorded haphazard frequencies.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to produce a simplified and break-proof secrecy signaling system.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawings which are illustrative of one preferred embodiment,

Fig. 1 is a schematic block diagram of a secrecy signaling system according to the invention.

Fig. 2 is a chart showing the relation between original signal frequency and the output or scrambled frequency for three arbitrary values of the haphazard oscillator frequency,

Fig. 3 is a chart showing the relation between input and output frequencies for the various modulator stages, and also the relation between signal input frequency and output frequency for the over-al1 modulator stages.

Fig. 4 is a schematic diagram of a recording blank arrangement from which are derived the variable oscillator frequencies.

Fig. 5 is a modification of Fig. 4.

Referring to Fig. 1 the numeral l represents schematically any well-known form of signal generator such for example as a microphone or other voice frequency source. The original voice frequencies are passed through a low-pass filter 2 to limit the frequency range to some predetermined band e. g. 0-1800 cycles. This 0-1800 cycle band is then passed through another llter network 3 which is logarithmic between input and output so that the fundamental and harmonics of the voice frequencies are equalized in power and this equalization is spread over the 0-1800 band. The signals from lter 3 are divided into two paths each including a double-balanced modulator 4, 5 of the type disclosed in application Serial No. 501,928, filed Sept. 11, 1943. Modulator 4 is supplied with an 1800 cycle carrier from a suit-able source B, while modulator 5 is supplied with a 3600 cycle carrier from source l. If desired, the 1800 and 3600 cycle carriers can be derived from the Same source.

In the output of modulator l there appear substantially only the sum and difference frequencies between the 1800 cycle carrier and the input frequencies from lter 3. Likewise in the output of modulator -ii'there appear substantially only the sum and difference frequencies between the 3600 cycle carrier and the input frequencies from filter 3. Modulator 4 is followed by an 1800 cycle lowpass lter so that there appear at the output of filter 3 only the difference frequencies from modulator 4. Likewise modulator 5 is followed by a 3600 cycle low-pass'filter whereby there appear at its output only the diiference frequencies from modulator 5. The outputs of lters 8 and Sa-re then recombined thus giving in the common output, Voice frequencies in two bands, the original 0-1800 cycle band from filter 3 being now transposed to 3600-1800 and 1800-0.

The transposed and recombined signal from the first modulator stage #I is then fed into modulator stage #2 including a double balanced mod- 5. Modulator ulator l0 similar to modulators 4,

bear the same designation numeral. -ceiver the filter 3b is the converse of the filter 3 I is supplied with a carrier frequency from source II which frequency is variable over the band 3800-5400 cycles but at a haphazard rate. Consequently the dual voice frequency band 3600-1800 and 1800- 0 from modulator #I is changed to the sum and difference frequencies between the carrier from source II and the frequencies from stage virl. The output of modulator I0 is followed by a 3600 cycle low-pass filter I 2 which eliminates all the sum frequencies from At the receiver the received scrambled fre-V quencies are passed through successive stages which are identical with the stages #L #2 and V#3. In the drawings the parts at the receiver which correspond to those at the transmitter At the reat the transmitter so as to restore the original powerV relations in the fundamental and harmonies of the signal frequencies, and of course the microphone is replaced by a suitable voicefrequency recorder or reproducer Ib. It will be understoodthat the variable frequencies from modulator `IIb at the receiver are synchronized in any suitable manner with the frequencies from modulator I I at the transmitter so that at any given instant each modulator supplies the identical frequency to its corresponding modulator I 0 or 0b.

One preferred manner of deriving the variable 3800-5400 cycle carriers at the transmitter and receiver is schematically shown in Fig. 4. A pre- 'viously made identical phonographic recording of the haphazard frequencies on two phonograph blanks I1 and I'Ib is used respectively at the transmitter and receiver.A These phonograph records are rotated by respective motors I8 and I8b which are maintained in synchronism in any well-known manner. Associated with each record is a pair of phonograph pick-ups I9, 20 whose relative spacing is fixed, but whose joint position with respect to the record I'I can be varied as desired. By suitable arrangements as described in my said prior application Serial No. 410,450, the Y corresponding pick-ups I 9b and 200 at the receiver can be located at the proper orientation with respect to the record at the receiver. The haphazard frequencies from the two pick-ups I9, 20 are added by means of a double balanced modulator 2l similar to modulator 4 and the modulater output is passed through a suitable highpass filter for passing `the sum frequencies from modulator 2|. The same appliesto the pick-ups I9b, 2019.

In my prior application Serial No. 410,450, filed Sept. 11, 1941, two variable frequency modulatorsrare required at the transmitter, and two similar synchronized variable frequency modulators are required at the receiver. By using the method of `deriving the single variable frequency above described, the recording frequency of the serambling tracl in each'phcnegrapir reccrd is reduced by approximately 50% and permits doubling the playing time of the records. The same number of scrambling combinations are afforded as are afforded with the system of my prior application even though the phonograph records rotate at the same speed as the rotation of the records in the prior application. However, by rotating the records above -zdescribed at half the speed required in the prior application, the number of scrambling conditions is squared.

Fig. 2 shows in graph form the relation between the original signal voice frequency and the output frequency from the filter I5 or filter I5b. The full-line graphs A show the relation when the oscillators lII and IIb, are each delivering 3800 cycles. The dotted-line graphs B show the relation at a Icarrier frequency of oscillators I I and IIb of 4600 cycles; while the dot-dash line graph C shows the relation at a frequency of 5400 cycles.

'lrhegraphs of Fig. 3 show the individual input and output frequencies for the .three modulator stages at an instant when the oscillators II and Hb are delivering for example 3800 cycles. In this figure, the full-line graph D shows the relation between the input and output frequencies of modulator stage #L The dashed-line graph E shows the relation between the input and output frequencies of the modulator stage #2. The dotdash line graph F lshows the relation between the input and output frequencies -of ,modulator stage #3. The dotted line graphs G show the overall relation between the input frequency to stage #I and the output frequency at stage #3. It will be noted that the graphs G correspond tothe graphs A vof Fig. 2. A similar series of graphs for different haphazard carrier frequencies of lthe oscillators H and H will show that the output frequency of the third stage is confined to the 0 to `1800 cycle band, notwithstanding the scrambling and successive modulations to which the input signals have been subjected.

If there is any disturbance produced by an irregular motion of the record Il, this difficulty is overcome in the following manner (Fig. 5). Assume that the frequencies recorded on the variable frequency track range between 1,900 and 2,700 cycles at some haphazard rate. Instead of adding these frequencies directly the output from one vof the pick-up arms e. g., pick-up I9, is beatv in the modulator 25 against a fixed frequency of 4,600 cycles from source 2.6, and the differency frequency taken therefrom, When the pick-up arm I9 is scanning 1,900 cycles, the output from the modulator 25 delivers 2,700cyc1es (4600- 1900), and when the pick-up arm I9 is scanning 2700 cycles. the output from the auxiliary modulatoris 1,900 cycles. quencies is the same; The output of this auxiliary rnodulator 25 is then added to the output of the other unaffected pick-up arm 20 and as before, the sum output frequencies `of from 3,800

' to `5,400 cycles result. Now the variable frequency modulator is receivingthe'same range .of frequencies as before .but the following has'been accomplished. When the driunorv turntable increases in vfrequency dueto inaccurate gears, the output frequencies from each of the pick-up arms I9 and 20, increase proportionately. The output frequency of the unaffected arm 20 varies, of course, but the output frequency of. the! auxiliary mo V25 wif@creasesV when Yits input vfluency increases, i Thus; the ,change Acf-requeney of the Thus, the range of fre- Y Vsum is now much less affected by changes in drum or turntable speed.

An example shows this improvement. Assume that each pick-up arm was scanning 1,900 cycles and assume that the drum or record carrier rotation increased 10% in speed, then the output from each pick-up amplier would increase from 1,900 to 2,090 cycles per second, but the output from the auxiliary modulator 25 would decrease from 2,700 to 2,510 cycles per second. Upon addition the sum frequency would be 4,600 cycles (2510-2090l` as against Ll,600 cycles, had no speed increase been present (1,900 plus 2,700). Thus, a change in drum speed has produced ab solutely no change in sum frequencies. Consider the other extreme Where one pick-up arm is scanning 1,900 cycles and the other pick-up arm is scanning 2,700 cycles. Under a 10% speed in crease, the unaiected 1,900 cycle arm is delivering 2,090 cycles while the 2,700 cycles arm has increased to 2,970 cycles and output of the auxiliary modulator is 1,530 cycles (4600-2970). Thus, the result is a sum frequency of 3,720. No frequency change due to gear inaccuracies would have delivered an output frequency of 3800 cycles. Thus, a 10% change in drum speed has produced only an 80 cycle change in sum frequency, Thus, We see that this system under this extreme condition gives an improvement of 5.75 to l (460 to 80), and under the other extreme condition, it gives an innitely improved condition. Therefore, we may assume that the average improvement is at least 10 to l and the gear requirements and tolerances are therefore reduced by the same amount. it will be understood that while a record medium in the form of a phonograph disc and a turntable are disclosed that any other well-known form of record medium and pick-up may be employed, for example a photographic sound film and associated scanning equipment, or a magnetized wire or 1oand record.

It will be understood that in connection with the modification of Fig. 5 that identical means are used at the receiving end as shown.

While certain particular embodiments of the invention have been described, it will be understood that various changes and modifications may be made therein Without departing from the spirit and scope of the invention.

What is claimed is:

l. rEhe method of secret signal transmission which comprises, transposing the original frequency succession into a different frequency succession occupying a band width greater than that of the original frequencies, generating a haphazardly variable frequency carrier, modulating said carrier by said other frequencies, selecting one side band of the modulations, and remodulating each side band to restore the original band width but without restoring the original frequency succession.

2. The method o-f secret signal transmission which comprises, transposing the signal frequencies into other frequencies occupying a band approximately twice the width of the original band, modulating said transposed band on a haphazardly variable carrier whose frequency is higher than the width of the transposed band, the frequency excursion of said variable carrier being less` than the band Width of the original frequencies, selecting one side band from the modulated variable carrier, remodulating the selected side band on another carrier of lower frequency than said variable carrier to restore the original band width without restoring the original frequency successions. A

3. The method of secret signal transmission which comprises, conning the original signals to a band e. g., ,fo-fi, modulating said hand on a carrier of frequency f1, simultaneously modulating said band on another carrier of frequency nfl, selecting only the difference frequencies from each modulated carriery combining said difference frequencies, modulating a haphazardly varying frequency carrier by said combined difference frequencies, selecting from said modulated Variable frequency carrier the difference frequencies, modulating the lastmentioned selected frequencies on a carrier of nfl, and selecting from the last-mentioned modulated carrier the difference frequencies. Y Y

4. The method according to claim 3 in which said variable frequency carrier is varied at a haphazard rate but over a frequency band whose width is less than ,fof-f1.

5. The method of secret signal transmission of an original signal frequency spectrum of a predetermined band width which comprises, subjecting the signals to successive carrier modulations, one of which is of thehaphazardly variable carrier type whereby the nal signals have a frequency spectrum of approximately the same width as the original signals but with at least a portion of the. original frequency spectrum scrambled.

6. The method of secret signal transmission of an original signal frequency spectrum of a predetermined band width which comprises, equalizing the power in all frequencies of the signal band, subjecting the signal frequencies to at least three successive carrier modulation stages, the first stage producing an inverted frequency spectrum the band width of which is a multiple of the original spectrum, the second stage producing a scrambled frequency spectrum, and the third stage producing a scrambled frequency spectrum of substantially the same width as the original band.

7. The method of secret signal transmission of an original signal frequency spectrum which comprises, subjecting said spectrum at a transmitter to successive carrier modulations including modulation of a haphazardly variable carrier, one of which is a varying frequency carrier stage to produce a frequency spectrum for transmission of substantially the same band width as the original signals but with the frequencies scrambled, receiving the last-mentioned band and subjecting it to successive carrier modulations similar to those at the transmitter with the varying carrier at the receiver synchronized with the varying carrier at the transmitter to reproduce the original frequency spectrum.

8. In a secret signaling system, a source of signal frequency spectrum having a predetermined band width, means to subject said spectrum to successive frequency changing stages, one of which is of the haphazardly Varying carrier type for scrambling the frequencies, and means to select from the last modulation stage a frequency spectrum of substantially the same band width as the original spectrum but with the frequencies scrambled.

9. In a secret signaling system, a source of signal frequencies Within a predetermined spectrum, three successive frequency-changing stages for said spectrum, the first stage having means to expand the spectrum to a multiple of the original, the second stage having means to scramble the signals from the first stage in a haphazard manner, and the third stage `having means lto restore the band width ofthe scrambled frequencies to that of the original band width.

y10.. A secret signaling system according Yto claim 9 in which the .original spectrum has an upper .frequency of il and the said first stage ,comprises a pair oi modulators one of which is supplied with a carrier of approximately f1 and the other of which is supplied with a carrier approximately nfi, means to select from the .ii-rst modulator the spectrum below f1, means to select from the second modulator the .spectrum below nfl, means to combine the two .selected spectrums; the said second .stage comprises a modulator upon which said combined spectrums are impressed and a source of haphazardly varying carrier connected thereto, said carrier having a band width which is less than that of the original spectrum but with its lower frequency limit ,above nfl, and means to select from said second modulator stage the spectrum with a maximum frequency limit of approximately nii; and said third Astage comprises a modulator supplied with carrier .of fre- `quency nii, and means to select from its output a spectrum of substantially the same width as the original spectrum but with the frequencies scrambled. Y

11. A secret signaling system according to claim 9 in which the said rst stage comprises, a pair of double-balanced modulators upon which the original signals are separatelyl impressed and means to select and combine from the said pair of double-balanced modulators the difference frequencies; said second stage comprising another double-balanced modulator upon which the combined difference frequencies from the rst stage are impressed, the double-balanced modulator oi the second stage being connected to a source ci carrier of haphazardly varying frequency and means to select from the output of said variable carrier modulator a spectrum which Yis approximately the same band Width as the input hand width; said third stage comprising adouble-balanced modulator upon which the selected spectrum from the second stage is impressed and means to select from the output of said third stage a spectrum of substantially the same width as the original spectrum but with the frequencies scrambled.v

12. A system according to claim 9 in which the selected spectrum from the third stage is fre- .ceived by a receiver having substantially the same frequency-changing stages as at the transmitter.

13. vA secret signaling system comprising a source .of signals of .a predetermined limited irequency spectrum, and a Vplurality of frequencychanging stages for scrambling the `original frequencies Vwhile maintaining the original band width', the last-mentioned means including acarrier modulation stage which is Supplied with a haphazardly varying carrier.

14. .A secret signaling system according to claim 13 in which-the source of haphazardly varying carrier comprises a record of frequencies having a frequency succession which is haphazard, a pair of pick-ups cooperating with said record at different portions thereof, and means to combine the signals from said picklps to produce a single signal of variable frequency.

15. A secret signaling system according to claim i3 in which the source of variable carrier frequency comprises a record of frequency successions varying haphazardly, a plurality of pick-ups associated with different portions of said record to produce simultaneously different frequencies, a double-balanced modulator upon which the `signals from said pick-ups are impressed, and means to select one side band of the output of said double-balanced modulator.

16. Apparatus for producing a haphazardly varying frequency ior secret signaling systems comprising a record medium embodying a record of haphazardly succeeding frequencies, a plurality vof pick-ups associated with diierent portions .of said record to produce simultaneously diierent frequencies under control of said record, and means to combine the signals from said pick-ups to produce a single signal whose frequency varies haphazardly 'but as a function of the records of vthe recorded frequencies.

17. Apparatus according to claim 16 in which a frequency inversion modulator is provided between one pick-up and the said combining means to compensate for irregularities in the movement of the record.

18. Apparatus according to claim 16 in which a dilerence frequency modulator is provided between one pick-up device and said combining means whereby irregularities in the movement of the record carrier are compensated for.

19. A re-entrant band shifter for an original frequency band fil-f3, comprising means to shift the band to a different spectrum fz--fi where f2 is intermediate fi--fs and wherein the band fz-fi is wider than the band fia-f3, means to restore the portion of the band f2-f4 which does not overlap the band f1f3 to 'the non-overlapping portion of the band fl-fs but with the frequency succession of the said restored portion different from the corresponding original portion of the band f1--3, and the resultant spectrum is of the same width as fi-fa but with a differentrfrequency succession from the said original frequency band.

ROBERT M. SPRAGUE.

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