GB2186466A - Security apparatus for a video display unit - Google Patents

Abstract

This comprises a jamming apparatus including an r.f. transmitter to jam intelligence carrying stray r.f. radiation from visual display units (VDU's). The apparatus generates swamping radiation which prevents decoding of a video signal from the VDU, in a relevant frequency band. The radio transmitter has one or more antennae 3a, 3b that are supplied with energy at a plurality of frequencies by the action of digital division and pseudo-random sequence generator circuits 4. The oscillator 1 from which these frequencies are derived may be phase-locked to the line-scan frequency of the VDU to be protected by means that include a coupling loop 10 that is immersed in the magnetic field of the said VDU. Correct operation is signalled to the user by indicator 13 and its associated circuit 14. <IMAGE>

Description

SPECIFICATION Security device for a video output unit This invention relates two methods of and apparatus for preventing the interception of the intelligence carried bythe radiation that is emitted bythe video signal generating or processing circuits of a video display unitor a raster-scanned printer. Within the specification ofthis invention video display unit and visual display unit are synonymous, and are hereafter abbreviated to VDU. References to VDU should be taken to include rasterscanned printers and the claims should be interpreted accordingly. The invention also relates to VDUs or printers incorporating or modified to incorporate means whereby such security is achieved.

VDUs are widely used as components of systems which store and manipulate confidential information.

The incidental radio frequency emission from the circuits of VDUs can be received at some distance from the VDU itself, and these emissions can be decoded or deciphered upon another VDU screen. In view ofthe commercial and legal importance of maintaining the security of such information, attempts have been made eitherto reduce the radio frequency emissions, orto change them so that decoding or deciphering ofthe signals is made more difficult or impossible. Known methods for reducing radio frequency emission include the use of shielding or screening ofthe VDU, careful choice of circuit layout, and the filtering of external connections. However, none of these known methods is wholly satisfactory.

It is an object of the present invention to provide means whereby it is made more difficult or impossibleto decode ordecipherthe RF emission from a VDU art a location remote from the screen.

According to the present invention there is provided apparatus to prevent the interception of the intelligence carried by the radiation that is emitted by the video signal generating or processing circuits of a visual display unit, said apparatus comprising means adapted to emit swamping radiation whose characteristics are such asto prevent the decoding of the said video signal in any frequency band wherein interception might otherwise be possible.The invention also provides a method to preventthe interception of the intellig- ence carried by the radiation that is emitted by the video signal generating or processing circuits of a visual display unit, said method comprising the emission of swam ping radiation whose characteristics are such as to prevent the decoding ofthe said video signal in any frequency band wherein interception might otherwise be possible.

Within the description of this invention a swamping signal is one whose total structure, including itsampli- tude time and frequency characteristics, is adapted to achieve the object of the invention. Such a signal may have an overall power less than that of the incidental VDU emission.

There are described hereinafter a number of ways in which such swamping can be achieved. These ways may be used separately or together, or in different combinations within the scope ofthe present invention.

Various embodiments and methods in accordance with the present invention will now be described by way of example, with reference to the accompanying drawing, in which: Figure lisa schematic diagram of a conventional VDU system and of the circuitry associated therewith.

Figure2 is a graph which shows the relative amplitude at different frequencies of the radio-frequency emissions of a typical VDU.

Figures 3,4, 5, 6, 7, 9 and 11 are schematic diagrams of alternative embodiments of the electrical circuits of the apparatus.

Figure 8 shows in perspective apparatus according to the invention removeably attached to a VDU.

Figure 10 shows 3 antennae arranged orthogonally.

Referring to figure 1, a VDU system includes a cathode ray tu be 30, associated with which is a scan generator 31 and scanning circuits which deflect the light spot to form a raster pattern on the CRT screen. The VDU also includes digital interface circuits, including a data register32 and a divider 33 forthe character clock, which, for each position on the screen in turn, take from memory over the character data bus lines 34the information asto the identity ofthe character required (the "character code"), translate this information using information stored in the font memory 35, and using a serialiser40 re-order into a serial sequence the pixel information that constitutes the required video signal on line 36.The video signal input on line 36 is required to provide sequentially the brightness information for each element our pixel of each line in turn. The pixel sequence is timed by a pixel clock generator 37, that is usually a high-stability crystal oscillatordedicated to the timing ofthe VDU display circuits. The output from the pixel clock generator37, as well as being taken to the divider 33 and serialiser40, is also fed to a display controller38, among the outputs of which are synchronising signals on line 39which goes to the scan generator31 and which definethetimes atwhich new lines and new pictures and "fields" are to commence.

The video signal on line 36 is a precisely timed electrical signal which contains all the information displayed on the screen, and which is repeated every time the display is rescanned, typically 50 times per second. Furthermore, the video information is based on character shape, and so contains more detail than the minimum information required to identify the character. The precise frequency control, regular repetition and inherent redundancies of the video signal make it easyto extractthe information contentform the RF emission even when the signal-to-noise ratio is poor. it is also well-known taht most VDUs radiate detectable signals at the frequency ofthe pixel clock generator 37, and at the frequencies of its harmonics, and thatthese signals are all modulated with the video signal.

This can be more clearly understood from figure 2 which shows the relative amplitude at differentfrequencies ofthe radio-frequencies emissions of a typical amplitude at different frequencies ofthe radio-frequency emissions of a typical VDU. The harmonic content ofthis spectrum arises from the fast transitions between logic voltage levels in the display driver circuits of the VDU.These transitions are timed by a pixel clock or "dot frequency" generator 37, which is usually crystal-controlled at a frequency of between 10 and 20MHz.

This frequency is denoted 'f' in figure 2, and its harmonics will be seen to be the major signals emited. The action ofthe VDU character generator serialiser 40 is to amplitude modulate all these harmonics, and so produce realtively weaksidebands. In figure 2the subsidiary structure corresponds to the characterfrequency (one ninth of the dot frequency in this example as a result of division by 9 in the character divider33).

The actual intelligence is contained within and below this subsidiary structure, clustered around each harmonic. The sideband clusters closely resemble a normal TV signal and can be decoded witha modified receiver.

Raster-scanned printers are electrically similarto VDUs but generally operate at lower scan frequencies.

Therefore the proposals of this specification may be applied to their electrical circuits.

There are hereinafter described generators of swamping radio-frequency emissions intended to interfere with these emissions intended to interfere with these emissions described above. The amplitude and from quency spectrum, the time distribution, and the spatial distribution ofthe swamping emissions are to be controlled to'achieve the previously specified objectiveswhile also minimising the electromagnetic pollution that might result from the generation of such radio signals.

Since the video signal is repeated at precisely known intervals, interception apparatus may be designed which will remove the effect of randomly-timed swamp signals. The eye and brain, either unaided or with the assistance of long-persistance CRT phosphors, or digital picture processing means, may be employed by an interceptorforthis purpose.

Afirstform of countermeasure which can be adapted in accordance with the present invention is the addition of circuits to the VDU. This method, which is preferably applied at the design stage, is described in relation to figure 1, wherein the element and components shown in broken lines are added in accordance with the present invention. There is shown an additional font memory 35a, an additional serialiser 40a and an additional data path 36a, which togetherwill emit a simiiar or greater level of radiation than the true video signal, but of a meaningless or confusing character set.Care must betaken to ensure that the radiation paths and energies ofthe swamping signal are consistent with those ofthevideo signal in orderto minimisethe riskthat in certain directions or at certain distances the video signal might achieve an adequate signal-toswamp ratio to allow decoding to take place. In orderto achieve this, the swamping signal maysharethe same signal path as the video signal yet be of a waveform and duration such that the VDU dispaly is not significantly degraded. The swamping character set should preferably contain more than one characterform and yet not bear a one-to-one relationship to the displayed character set, so as to make it more difficultto design interception apparatus.

A second form, that may be preferred when the object of the invention is to be achieved with an existing design ofvideo output device which cannot easily be modified or whose internal characteristics are unknown, consists in its first implementation of means to generate a "comb" spectrum whose "teeth" are spaced for example 100KHz to 300KHz apart, apprecialbe energy being radiated at each "tooth" frequency up to, for example, 400MHz. As shown in figure 3, this might be achieved by a rectangular pulse generator 1 of appropriate pulse repetition frequency, spectrum shaping circuits 2, and antenna means 3, though it should be noted thatthe antenna design may besuch asto inherently provide any necessaryfrequency spectrum shaping.Such a rectangular-pulse generator 1 may be constructed from a higher4requency oscillator and divider chain. If the pulses are generated at a rate off pulses per second, then the swamping impulses are regularly spaced upon the VDU screen at intervals corresponding to 1/f seconds. Preferablythe pulsegenerating means may be constructed according to figure 4, in which a pseudo-random sequence generator 4 is used to increase and make more uniform the amplitude of the high-frequency components of the comb spectrum, and also to decreasethetime interval between successive impulses so asto move closertogether on the screen the swamping impulses. Such sequence generators are described in the book "Shift Register Sequences" by S. Golomb, published in 1967 by Holden-Day, San Francisco.The pseudo-random sequence generator may use a shift register and combinational logic feedback means or alternatively a counter chain whose various outputs are fed to combinational logic that forms a suitable signal for the antenna.Additionally, means may be provided to frequency modulate the pulse generator 1 and so introduce sidebands of every "tooth" in the "comb" spectrum.

In a second implementation ofthe second form of the present invention that is appropriate if it is known that decodeable VDU emission only occurs in the sidebands adjacentto the harmonics of the known dot frequencyf, the swamping emission is concentrated around these harmonic frequencies. Means according to figure 5 may be used, wherein the pulse generator 1 is set to the "dot frequency" f, and the harmonic spectrum that results after passage through the spectrum shaper 2 is modulated at 21 before being radiated by the antenna 3. The lower frequency modulating inputto 21 is derived from an auxiliary divider and/or sequence generator 4, chosen to concentrate the sideband energy into the region, for example, 100KHzto 1 MHz each side of each dotfrequency harmonic.

The effect of apparatus according to either of these two implementations of the second form is to superimpose upon the screen of an interceptor a pattern of lines, either equally spaced or at pseudo-random spacings according to the pulse generating means employed. Unless the rectangular pulse generator issynchronised to the line-scan frequency of the VDU,this pattern will continuously move across the interceptors screen.

A second aspect of the second form of the present invention seeks to hold stationary the swamping pattern generated according to the above aspect, so as to prevent the removal ofthis interfering pattern bytimeaveraging. Circuit means are to be provided to ensurethatthefrequency f, of the above-mentioned re ctangular pulse generator shall be related to the line-scan frequency oftheVDUfe bythe following equation: fp = np x fe ne where np and ne are integers.

In figure 6, it is shown how this may be achieved by use of a phase-lock loop comprising frequency dividers 5 and 6 that divide fp and fe respectively by np and nl,togetherwith a phase detector 7 and a low-passfilter8 that provide a voltage to control the frequency ofthe oscillator 1. The VDU line frequency may be obtained either by direct connection to the VDU or by a limiting amplifier 9 connected to a pick-up loop lOthat isso positioned as to couple to the stray magnetic field of the line-scan coils of the VDU. Random initialisation of the dividers 5 and 6 may advantageously alterthe position of the superimposed pattern on the interceptors screen.Preferably the divider 6 should have a division ratio equal to the numberoflinesthatform a row of characters upon the VDU screen so that the swamping pattern is unchanged ifthe VDU display is scrolled downwards, ora ratio equal to the numberoflines in a complete field sothatan interceptor cannot obtain a sample of the swamping pattern without the presence of character information.

Athird aspect of the second form of the present invention comprises a multiplicity of swamping sources, arranged so as to reduce the probability of an interceptor achieving success by finding a location in space where there is a null in the signal strength ofthe swamping signal that is not matched by a null in theVDU emission strength. To achieve this it has been taughtthatthe swamping source should be as near as possible to the VDU source; according to this form there may be provided two or more antennae spaced slightly apart and driven with swamping signals at differentfrequencies or different times so that these cannot mutually interfere. Preferably these antennae should be positioned orthogonally so that energy is radiated with all possible planes of polarisation.Either dipole or loop antennae may be used, according to the desired dimensions and frequency range of emission. Each antenna may consist of a number of elemental antennae each covering different frequency ranges.

Afirst embodiment of the second form of invention will now be described byway of example with refer ence to figu res 7 and 8, wherein parts with functions described above are correspondingly numbered.

The second antenna 3b is provided according to the third aspect of the second form of the invention and is driven by a divider stage 1 2thatchanges state on the opposite edge ofthe output oscillator 1 tothatwhich times the othercircuits, so asto provide time diversity. The "SAFE" lamp 13 is driven by a logic circuit i4that ensures that the lamp will only light when the phase-locked loop is in lock, and both the first and second antennae are receiving power.

The voltage regulator 15 is supplied buy a plug-mounted transformer 16. The connection between these two and thus to the electricity supply system forms part of the radiating antenna system and the'common' connection 17 and decoupling capacitor 18 may be arranged to provide an appropriately controlled imped ance over a wide range of frequencies. As taught abovewith referenceto thefirstform of countermeasure, the connections to transformers 16may preferably follow a path consistent with that ofthe VDU power supply cable.As shown in figure 8, the unit constructed according to this invention may be contained in a box 19 removeably attached to the VDU 20 in a position that provides good magnetic coupling between the horizontal line-scan coils of the VDU and the pick-up loop 10, together with convenientvisibility of the'SAFE' lamp 13.

A second embodiment of the second form of the invention will now be described with reference to figures 9 and 10, wherein parts with functions described above are correspondingly numbered but with an appropriate suffix where necessary.

In the schematic shown in figure 9 the frequency divider chain forthe phase-locked loop is split into sections 5a to 5f, so as to provide three additional functions. First, an output is provided atf/2 which is used as the clock signal of a 3 stage pseudo-random sequence generator 4b with a recurrence length of 7, so asto supply to the antenna 3b a signal of recurrence length 14clock periods and thus radiate from this antenna a "comb" spectrum with the "teeth" spaced by f/1 4. Secondly, outputs are provided at f/2, f/4 and f 16 connected to the and gate 21, so that this gate will supply to the antenna 3c a comb spectrum with the "teeth" spaced byf/16 and with swamping impulses occuring alternately atinvervals of 12/fand 4/f. Thirdly an output is provided atf /240 which is added into the frequency control signal forthe pulse generator 1 by the circuit 22, so as to frequency modulate all the frequencies emitted by the device. It should be noted that it may be cryptographically advantageous if the three sequences radiated from the antennae 3a, 3b and 3c all recur an integral number of times during each cycle off. This property is obtained with the division ratios here embodied, since the recurrence lengths ofthe outputs at 3a, 3b and 3c, which are respectively 15, 14 and 16, are all factors of the division ratio of the divider 5a to 5f.

The three antennae of this second embodiment may preferably be arranged orthogonally as shown in figure 10. Each antenna may be based on the known bi-conical structure, modified to allow construction from three planar conductive surfaces. Each antenna consists of two symmetrical halves, driven via one ofthe balance transformers shown at 23a, 23b and 23c in figure 9. The low-frequency limit of effectiveness ofthese transformers is matched to the cut-off frequency of the filter 24 in the power supply so that at lowfrequencies each antenna will function unsymmetrically using the power supply cable as a counterpoise element.

Athird embodiment of the second form ofthe invention will now be described with reference to figure 11, wherein parts with functions described above are similarly numbered but with an appropriate suffix where necessary.

In this embodiment up to three preferably orthogonal antennae may each employ a radiating loop for example thatformed by 3aa and 3ab, whose current distribution is adjusted by the network 41 a to provide optimum radiation over the required frequency range. The network 41 a may comprise series and/orparallel combinations of resistance, capacitance or inductance, and it may be physically distributed along a path joining the ends ofthe elemental antennae 3aa and 3ab. Each loop may be driven symetrically by com plementary outputs Q and 0 from sequence generator4a.The three pseudo-random sequence generators 4a, 4b and 4c may each employ different shift-rigister lengths or taping positions so asto provide different code sequence lengths. Thethree oscillators 1 a, 1 band 1c may operate at different nominal frequencies and may be so constructed that changes offrequency due to ambient temperature and supply voltage variation are such as to make more difficult the separation of the swamping and intelligent signals.The line-frequency signal from the pick-up loop 10 is amplified byamplifier9 and applied directlytothe divider 6, which inthis embodiment may advantageously provide three differently-timed impulse outputs each recurring at the field repetition rate ofthe VDU and which are employed to reset and so synchronise the sequence generators 4a, 4b and 4c. The output ofamplifier 9 is also rectified and applied to logic circuit 14which controls "SAFE" lamp 13to indicate proper operation as previously described.

As indicated earlier, the different forms, aspects and implementations of this invention and the features of the embodiments may be used singly or in any combination as appropriate. Furthermore, they may be used alternatively with raster-scanned printer systems instead ofVDUs.

Claims (18)

1. Apparatus to prevent the interception ofthe intelligence carried by the radiation that is emitted bythe video signal generating or processing circuits of a visual display unit, said apparatus comprising means adapted to emit swamping radiation whose characteristics are such as to prevent the decoding of the said video signal in any frequency band wherein interception might otherwise be possible.

2. Apparatus according to claim 1 wherein the characteristic pattern of the swamping radiation in the time domain is synchronised to the scanning process ofthe display device.

3. Apparatus according to claims 1 or2 in which the meansforemitting swamping radiation are placed in close proximity to those circuits that generate or process the video signal.

4. Apparatus according to claims 1 or 2 in which a common physical circuit is used to emit both swam ping radiation and also intelligence-bearing radiation.

5. Apparatus according to claims 1 or 2,from which are emitted 5 or more intentional signals located within the frequency band 1 OMHz to 400M Hz.

6. Apparatus as claimed in any ofthe preceding claims, wherein is generated a sequence of digital signals such thatthetime interval between the radiation of successive impulses is less than the reciprocal of the frequency spacing between adjacent radiated signals.

7. Apparatus as claimed in any ofthe preceding claims, wherein a plurality of signal in the frequency domain are generated from a single rectangularwaveform.

8. Apparatus as claimed in claim 7,wherein the single rectangu larwaveform is itself frequency modula- ted.

9. Apparatus as claimed in any or the preceding claims, wherein the swamping radiation is generated from circuit means that include a pseudo-random sequence generator.

10. Apparatus according to claim 2, wherein the emitted signal sequence is adapted to repeat in synchronism with each successive line-scan ofthe visual display unit.

11. Apparatus according to claim 2, wherein the emitted signal sequence is adpated to repeat in synchronism with thatsub-multiple of the line-scan frequency that corresponds to the number of lines used to form each row of characters on the screen of the visual display unit.

12. Apparatus according to claim 2, wherein the emitted signal sequence is adapted to repeat in synchronism with each successive field or picture scan of the visual display unit.

13. Apparatus as claimed in claim 2, wherein the horizontal line-scan frequency is sampled by means of a coupling loop, which in use is immersed in the magneticfield ofthe line-scan coils of a cathode raytubethat forms part ofthe visual display unit that isto be made secure.

14. Apparatus according to any of claims 1,2 or5to 13, wherein is provided a plurality of signal adapting means and a plurality of antennae.

15. Apparatus according to anyofclaims 1,2 or5to 13, wherein each antenna comprises a loop element with a series-connected matching network.

16. Apparatus according to any of the preceding claims wherein an indication is given ofthe correct function of a substantial portion ofthe device.

17. Apparatus substantially as described herein with reference to figure 1 orfigures3to 11 ofthe ac- companying drawings.

18. A method to prevent the interception of the intelligence carried by the radiation that is emitted bythe video signal generating or processing circuits of a visual display unit, said method comprising the emission of swamping radiation whose characteristics are such as to prevent the decoding of the said video signal in any frequency band wherein interception might otherwise be possible.