US2211132A - Method of transmitting secret messages - Google Patents

Description

Aug. 13, 1940. P.- KOTOWSKI EI'AL METHOD-OF- TRANSMII'TING SECRET MESSAGES Filed May 6, 1936 INVENTOR; Auh Kowgmsm BY m M ATTORNEY The invention will Patented Aug. 13, 1940 UNITED STATES PATENT oFFicE;

2,211,132 METHOD or 'mansm'r'rmc SECRET MESSAGES 7 Paul Kotowski and Kurt Dannehl, Berlin, Germany, assignorsto Telefnnken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application May 6, 1936, Serial No. 78,184

In Germany May 9, 1935 1 Claim. (c1. Pill-1.5)

magnetic sound reproduction of sound pictures or disks.

best be understood by referring to the accompanying drawing, in which:

Fig. l is a curve showing the proportional relationship between the speech disturbing oscillations known in the prior art;

: Fig. 2 is a curve showing the proportional relationship between the speech and disturbing oscillations of this invention;

- Fig. 3 is a schematic detail of a rotating generator which produces the disturbance noise of this invention.

Methods of secret telephony are known in the prior art in which through the superposition of any disturbing "energies on the speech energy cover up the speech and cause what may be called a ciphering. Methods of this type have not become popular especially in the field of wireless telephony, however, since according to the requirements demanded of the "ciphering," the disturbance voltage must be three to fiVe timesthe maximum, speech voltage so that the useful modulation of the transmitters becomes thereby so low that the transmitter power must be considerably increased in order to obtain the same range as in the case of undisturbed speech (an 40 increase ten to twenty five times). This is clearly shown by Fig. 1, in which curve-ST indicates the disturbance voltage and. SP the speech voltage.

The invention is based upon the teaching that a disturbance analogous to the noise voltages of talking films, record plates or tubes, is sumcient to obscure the speech to apoint of complete unintelligibility already at amplitudes which are. 1

1.2 to 1.3 times larger than the maximum speech amplitudes. an ideal disturbing band for which a. relatively small amplitude is suiiicient in order to render the speech oscillations undecipherabie to a suitable degree. Where the ideal disturbance spectwin is not fully attained, a larger disturbing amplitude is required. A generator for such noise Such a film, or disk noise has thusvoltage is suitably such as to provide a frequency spectrum of approximately the same strength throughout the limits between the zero frequency and the highest frequency used, and is shown by curve a in Fi 2. Also, in this figure, ST represents the disturbance voltage and SP the speech voltage. Smaller generators of disturbance and disturbance lines will be adequate if a narrower frequency band underlies the calculation of the disturbance generator, for instance in accordance with the curve b of Fig. 2. In thiscase, the

disturbance is distributed over the frequency band in approximately the same manner as the speech energy. It has then suitably a slightly pronounced maximum'between 500 and 1000 cycles. The disturbance producer may be designed as rotating generator for instance in that a rotatable disk S shown in Fig. 3 of irregular shape and having slots or pointed projections, produces short pulses in a corresponding scanning organ AT through electrical, mechanical or optic means, whereby the duration of an individual pulse is approximately equal or shorter than the half cycle of the highest disturbance frequency used. When narrowing the band of the disturbance frequency, then the irregularly distributed individual pulses are-so dimensioned that the majority thereof have a duration from 0.2 to 0.8 miiii-seconds. Such a disturbance generator could for instance also be obtained in that a disturbing noise, known to be suflicient (record,

plate, talkingfilm, water fall, etc.) is transformed into fluctuations of light, and these fluctuations are photographed on a rotating plate. In like manner, an endless film band or record plate with endless grooves could be utilized. As compared with a round disk with irregularly pointed projections provided mechanically thereon, this method has the advantage that the disturbance patterns produced therewithcan be easily produced at the receiver through a copying method in any desired number for the purpose of deciphering. The parts of mechanical disturbance generators by which the frequency is determined can be produred in simultaneous ways' for instance by means of machining a plurality of discs mounted on a single mandrel.

In order/to emphasize the rotation frequency of the disturbance generator in the mixture of the disturbing frequencies and which isof practi'cal usefulness for many purposes, a mark K (in Fig.3) is provided at a certain place on disc :8. (Pause in' the disturbing noise, or definite Under certain conditions, instead of extending the uniform disturbance spectrum from zero to the highest frequency as shown by curve SI of Fig. 2, it may be desirable to filter out by electrical filters totally or partially the range of for instance 200-2500 cycles at speech transmission. The particular advantage of the method described by this invention resides in that the transmitter canbe more effectively modulated for the benefit of the speech, in other words at equal speech strength, a-smaller powered transmitter can be used. e

The ciphering can be provided either by superposing by way of low-frequency the disturbing noise directly. on the speech, or else by modulating an auxiliary high-frequency with the disturbing noise and thereafter superposing this auxiliary frequency on the actual carrier frequency. 1

At the receiver side the deciphering is carried out accordingly. By-imeans of an identical disturbance generator operating in synohronism and in equal phase with that at the transmitter side, the alternating disturbance current having the proper phase position and amplitude is superposed on the arriving speech having been -ciphered by the disturbance noise, so that irregular alternating current, said alternating current forming the disturbance noise having a frequency spectrum which covers substantially the entire speech frequency spectrum, the amplitude of the frequency spectrum eicceeding but slightly the amplitude of the message frequencies, transmitting the combined message and disturbance noise on a high frequency carrier, receiving said combined message and disturbance noise'in a proper phase position with respect to the phase of the combined transmitted message to make said disturbance noise disappear, and removing 20

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