IL47692A - Method and apparatus for the camouflaged transmission of signals - Google Patents

Abstract

1512696 Scrambling systems SIEMENS AG 7 July 1975 [19 July 1974] 28486/75 Heading H4R A (speech) signal is scrambled by interchanging sub-bands according to a sequence determined by a generator Z1 controlled by a pulse sequence in the form of a cyclic variation in pulse repetition frequency (derived by multiplexing at M frequencies divided in FT1-FT8 from a carrier generator TV whose output is fed via switched interchange pattern circuits UE to frequency shift the signal sub-bands) the duration of each interchange being determined at Z2 from the same pulse sequence as employed for the interchange. [GB1512696A]

Description

η ηικ 1010 imoon^ tpnni ne*w Method and apparatus for the camoufiaged· transmission of signals SIEMENS AKTIENGESELLSCHAPT C 45345 This invention relates to the camouflaged transmission of signals, in particular speech signals. In particular this invention relates to a method of effecting camouflaged transmission of signals wherein at the transmitting end the signal band is split up into a plurality of subsidiary bands which are interchanged in accordance with a camouflage code, which is variable in a time sequence and is set up automatically and in stages in accordance with a code sequence which is determined by a code sequence generator, thereby to form a transmission band, and at the receiving end the interchange which has been effected is cancelled in the same time sequence thereby to cancel the camouflage. In such a method there is no mutual control of the code change by the end stations of the transmission path.

The widths of the individual subsidiary bands can be equal or can differ in size.

For example from German Patent Specification No. 1 273 002 it is already known for the camouflaged trans- . mission of speech signals to split the speech frequency band into a plurality of subsidiary bands of equal widths and to interchange the subsidiary bands with one another.

Various processes are known for interchanging the subsidiary bands. For example, in the so-called band switch-over process, an interchanger selects n_ from 2n_ modulators each of which is assigned a fixed carrier frequency. In the so-called carrier switch-over process 47692/2 in each subsidiary band channel a permanent switc -through is provided of two modulators for a first and second frequency — conversion via a unit filter, wherein an interchanger select^ n front 2n carrier frequencies.

When a suitable interchange is provided, with the aid of the above-mentioned processes it can b ensured that the transmitted speech is unintelligible and that unauthorised listening-in on telephone conversations is avoided. The safeguard against unauthorised listening-in increases in proportion to the frequency of code changes and also increases in the event of a constant change of the interchange code at shorter intervals. The length or duratio of elements in the code sequence is however subjec to a lower limit due to the interference effect of the unavoidable switching noise, the synchronising error between the camouflage devices of the speech circuit, and possibly also the time-dependent divergence of the transmitting-end and receiving-end switching pulse-trains.

This invention seeks to provide an improved method of effecting camouflaged transmission of signals, particularly speech signals.

According to this invention there is provided a method for effecting camouflaged transmission of signals comprising the steps of: generating a code sequence by using a code sequence generator? automatically generating a camouflaged code in stages in accordance with said code sequence; splitting up the signal band at the transmitting end into a plurality of subsidiary bands; 47692/2 interchanging the plurality of subsidiary bands in accordance with said camouflage code thereby to form a transmission band; cancelling the interchange of said plurality of subsidiary bands i accordance with said code sequence at the receiving end; controlling the elements provided in said code sequence synchronously with respect to their relation in an identical sequence at both the transmitting and receiving ends* The method in accordance with this invention arises in particular from the recognition that relatively short element lengths are associated with relatively small disturbances which are caused by unavoidable switching, crackling, and synchronisation faults whan element lengths of differing durations are used, as in the case of unequal element lengths these interferences are governed by the average element length.

Thus in the method in accordance with this invention, two programme sequences, namely a camouflage code sequence and an element alteration sequence, which ma be controlled independently of one another, are superimposed upon one another. The ratio of minimum to maximum element duration is in particular approximately in the range from 1?2 to 1:8, the control of the element duratio being effected discontinuously.

The method of the invention permits the advantage of an increase in the safeguard against unauthorised reception of the signals and a switching interference noise which is particularly low in relation, to the mini^^m element duration to be achieved.

Preferably the code sequence and the element length sequence have unequal period durations; this renders the sequence, which are in effect superimposed upon one another, particularly difficult to detect.

The element length sequence preferably contains elements lengths each of which is a non-whole-numbered multiple of the shortest element length. In this case the structure of the camouflaged signals can be made particularly difficult to determine if the element length sequence only contains element lengths each of which is a non-whole-numbered multiple of the shortest element length.

The method of the invention is particularly suited to the camouflaged transmission of speech signals.

In code sequence generators for known speech camouflage apparatus, generally the usual practice is that in the timing of a supplied pulse train said generators emit signals in a time sequence or numbers in coded form which are in each case assigned to one of the employed interchange combinations. Such a code sequence generator can be stopped for a variable length of time in a determinate irregular manner so that a temporary blockage or break in the advance is ensured.

The invention also extends to apparatus for use in carrying out the method recited above comprising a code sequence generator, a switch-over device which is arranged to effect said interchanging of said plurality of subsi^a bands in dependence upon an output signal of the code sequence generator with which said switch-over device is supplied, and an element length sequence generator which is provided for controlling the element length and is arranged to supply a pulse sequence which has a controlled variable pulse train frequency to the code sequence generator. The code sequence generator is thus stepped onwards. in a determinate, irregular time sequence.

Preferably the element length sequence generator contains a frequency divider which is arranged to be controlled in pulsed fashion and also to be controlled in respect of its division ratio by means of a number sequence. The numbers which serve to control and hence programme the frequency divider can be supplied in an equal or unequal time sequence. In particular, the pulses with which a random sequence generator connected in series with the frequency divider is controlled, can be taken from the output of the frequency divider.

In this arrangement it is advantageous to interpose the frequency divider between a pulse generator and the code sequence generator of already existing apparatus for interchanging subsidiary bands, so that a substantial improvement may be achieved with the aid of an additional device of simple construction.

: In . n embodiment of the apparatus of the ; invent ion the element length sequence generator includes a number sequence generator output signals of which serve to ^ control, said division ratio.

In another embodiment of the apparatus of the invention, the apparatus includes a number sequence generator part of which constitutes the code sequence generator and. part of which, including a part not constituting a component of the code sequence generator, constitutes a component of the element length sequence generator output signals of which serve to control said division ratio.

The invention will be further understood from the following description by way of example of two embodiments thereof with reference to the accompanying drawings, in which :- Figure 1 shows part of a transmitting component of apparatus for transmitting camouflaged speech signals and which contains two quasi-random-generators which are independent of one another; and Figure 2 shows the same part of a transmitting component of apparatus for transmitting camouflaged speech signals and in which the code sequence and the element length sequence are controlled with the aid of the same quasi-random generator.

Referring to Figure 1, the apparatus shown therein -comprises a subsidiary band interchanger which includes a switch-over device U composed from a plurality of switch-over units UE for switching over carrier frequencies supplied by a carrier generation device TV ^ output terminals shownat the right-hand side of Figure 1. A first' quasi-random generator Zl which supplies a number or code sequence in a l-from-n code to the switch-over units UE which consequently are swi ched one a t a time with the aid o"f this code sequence. Each switch-over unit UE contains switches which are controlled in - a ; predetermined combination between the multiply provided incoming and outgoing lines of the switch-over units UE .

If, for example, five carriers are to be selected from five possible carriers, each of the switch-over units UE for example contains five switching transistors which, in their actuated states, connect the interchange modulators (not shown) to the carrier generation device TV in the manner required for the particular interchange.

The code-dependent interchange of the carriers can instead be carried out differently, e.g. with. the aid of appropriately connected multiplexers or the like. The numbers of the code sequence can in particular be binary-coded items of paral lei information , instead of being in a l-from-n_ code.

The code element duration is contrived to be variable and the element length variation is controlled by a program mable pulse generator, consisting of the device TC and selectively activated frequency dividers FTl to FT8. In this way the camouflage . code is constantly changed at varying short intervals; the code sequence period and the element length sequence period are expediently selectee^ to be very long resulting in a virtually random distribution. - The code length period and element length period are preferably different so that in effect two programme sequences are superimposed upon' one another. Therefore the criteria governing the camouflage are not only the code change time and code sequence, but in addition the element length sequence and the beginning of the element length period.

The quasi-random code sequence generator Zl is advanced step-by-step with the aid of a pulse sequence which is obtained by programmable frequency division from the pulse train Tl.

The pulse train Tl supplied by the carrier generation device TV is conducted to the frequency dividers FT1 to FT8 of which only one is connected by means of a subsequent multiplexer M to the pulse train input of the quasi-random generator Zl. The frequency dividers FTl to FT8 and the multiplexer M thus constitute a programmable variable frequency divider which supplies its output pulse train not only to the first quasi-random generator Zl but also to the pulse train input of a second quasi-random generator Z2. This second quasi-random generator Z2 has three outputs via which it supplies binary number sequences, each of which represents an address for a particular one of the eight frequency dividers FTl to FT8 which is to be connected, to the multiplexer M.

The apparatus shown in Figure 2 corresponds to that shown in Figure 1 except that only the first quasi^ random .generator is provided. In Figure 2 the progreunmable variable frequency divider, which is shown merely as a block FT but can e.g. be constructed in exactly the same form as described above with reference to Figure 1, again receives the' pulse train T and supplies its output pulse train to the quasi-random generator Zl. . In this apparatus, however, the quasi-random generator Zl not only contains a part Zll which serves to control tie carrier switch-over, but also a part Z12 which is not employed for this control. This additional part Z12 supplies part of the binary number sequence for the control of the frequency divider FT, the other part of this binary number sequence being derived from connections to the part Zll of the quasi-random generator Zl , which connections obviously may also be connected to the switch-over device U.

All the speech band interchangers which engage in traffic with one another expediently obtain a quartz-controlled pulse generation, the switching phases of the individual pulse trains being adapted to one another at longer intervals. The pulse train is in this case derived with a low outlay from the quartz generator of the carrier generation device TV which is anyhow required for the speech band interchange .

The variable pulse train times can on the other hand advantageously be generated with commercially available modules, in particular TTL or C MOS modules, which possess not only a pulse train input and an output, but^a plurality of programming inputs which are connected via control' lines to the quasi-random generator.

- Modules of this kind in particular have four programming inputs "8", "4", "2", and "1". The "divider ratio of the module here corresponds to the binary number connected to the programming inputs.

In the event of a divider ratio of 1;0, the frequency divider would not emit any pulses at the output for stepping on the quasi-random generator. Since the divider ratio zero therefore cannot occur, although the quasi-random generator also supplies the number 0, one of the four inputs is expediently connected to a high or binary "1" potential. 3 bits of the quasi-random generator are connected to the other inputs. Thus, in dependence upon which input carries the high potential, four different series each with eight different divider ratios are obtained. The pulse train time at the output corresponds to the input pulse train time multiplied by the relevant divider ratio.

For example the series with an "8" input carrying the high potential has the numbers, 8, 9, 10, 11, 12, 13, 14 and 15, none of which is a whole-numbered multiple of the smallest number. The variation in the element duration thus is not based on whole-numbered multiples of the shortest element duration. A pulse train established for the shortest elements does not provide a frame for the longer elements. The minimum element Lenqth is in particular not greater than the duration of a short speech sound.

- If two inputs of the module are permanently wired, one input being able to be connected to a low or binary "0" potential, total of eighteen different series each with four divider ratios are available.

If the connection of the control lines and the application of the fixed potential are contrived to be adjustable, this provides a simple possibility of changing the element length sequence.

In practical operation, the camouflage devices at the two ends of a speech circuit do not switch precisely in synchronism. Instead a small setting-up error must be expected. This effect- becomes particularly apparent e.g. in -short-wave single-side band speech circuits. However, even in the case of exact synchronisation, after a short time switching errors occur because for various reasons the frequencies of the pulse generators of the individual camouflage devices deviate slightly from the theoretical frequency. During the time of the switching difference the speech does not lose its camouflage, but the speech voltages act in the manner of a strong interference voltage penetrating into the speech circuit from the exterior.

The interference effect of the switching errors constantly increases in proportion to the shortening of the element duration. During the decoding further nters ference influences are added to the interference action of the switching errors, and theref6re it is not possible to adopt arbitrarily short code elements.

However, the apparatus shown offers the possibility of shortening elements of the element sequence without increasing the interference influence of the switching errors during the cancellation of the camouflaging if the average element length or duration is retained. The interference pulses follow irregularly in the case of a variable element duration, and follow one another periodically in the case of a constant duration. The sum of the interference pulse times is equal in both element sequences with the same average element length. Thus their interference effect is also approximately the same.

The element length control can be permanently wired and, as shown in Figure 2, partially coupled to the code sequence period. It is particularly advantageous if code-sequence and element length period can be set up independently of one another. 47692/2

Claims (14)

1. A method for effecting camouflaged transmission of signals comprising the steps of; generating a code sequence by using a code sequence generator; automatically generating a camouflaged code in stages in accordance with said code sequence; splitting up the signal band at the transmitting and into a plurality of subsidiary bands; interchanging th plurality of subsidiary bands in accordance with said camouflage code thereby to form a transmissio band; cancelling the interchange of said plurality of subsidiary bands in accordance with said code sequence at the receiving end; controlling the elements provided in said code sequence synchronously with respect to their relation in an identical sequence at both the transmitting and receiving ends.

2. · A method according to Claim 1 wherein said set of controlling the elements in said code sequence comprises the step of : controlling the code sequence generator in pulsed fashion by a pulsed frequency whose pulse repetition frequency is controlled.

3. A method as claimed i Claim 1 or 2 wherein the code sequence and the element length sequence have unequal period durations.

4. A method as claimed in any of the preceding claims wherein the periods of the code sequence and the element length sequence may be set-up independently of one another.

5. A. method as claimed in any of the preceding claims wherein the element length sequence contains element lengths each of which is a non-whole-numbered multiple of the shortest element length.

6. A method as claimed in Claim 5 wherein the element length sequence only contains element lengths each of which is a non-whole-numbered multiple of the shortest element length.

7. A method as claimed in any of the preceding claims wherein the signals are speech signals.

8. A method as claimed in Claim 7 wherein the minimum element length is not greater than the duration of a short speech sound.

9. .. 9. A method of effecting camouflaged transmission of speech signals substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings . :

10. Apparatus for use in carrying out the method of Claim 1 comprising a code sequence generator, a switch-over device which is arranged to effect said interchanging of said plurality of subsidiary bands in dependence, upon an output signal of the code sequence generator with which said switch-over device is supplied, and an element length sequence generator which is provided for controlling the element length and is arranged to supply a pulse sequence which has a controlled variable pulse train frequency to the code sequence generator.

11.. Apparatus as claimed in Claim 10 wherein the element length sequence generator contains a frequency divider which is arranged to be controlled in pulsed fashion and also to be controlled in respect of its division ratio by means of a number sequence.

12. Apparatus as claimed in Claim 11 wherein the element length sequence generator includes a number sequence generator output signals of which serve to control said division ratio.

13. Apparatus as claimed in Claim 11 including a number sequence generator part of which constitutes the code sequence generator and part of which, including a part not constituting a component of the code sequence generator, constitutes a component of the element length sequence generator output signals of which "serve to control said division ratio.

14. Apparatus for use in carrying our the method of Claim 1 substantially as herein described with reference to. Figure -1 or Figure 2 of the accompanying drawings .