# US 5,351,292 A · Method and apparatus for preventing external detection of signal information > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,351,292 A - Document ID: US5351292A - Status: Granted - Kind: A - Application number: 07/944,427 - Filed: 1992-09-14 - Granted: 1994-09-27 - Earliest claimed: 1989-01-26 - Assignees: Cominvest Research AB - Inventors: Per Lindholm - Web page: https://patentgrep.com/patent/US5351292A ## CPC - H04K 3/42: {characterized by the control of the jamming frequency or wavelength} - H04K 3/825: {by jamming} - H04K 3/45: {characterized by including monitoring of the target or target signal, e.g. in reactive jammers or follower jammers for example by means of an alternation of jamming phases and monitoring phases, called "look-through mode"} - H04K 2203/14: for the transfer of light or images, e.g. for video-surveillance, for television or from a computer screen - H04K 2203/32: including a particular configuration of antennas - H04K: SECRET COMMUNICATION; JAMMING OF COMMUNICATION ## Abstract In a method and an apparatus for preventing external detection of the signal information in video signals occurring in, and being emitted from, video signal circuits in a display unit, or a similar unit, and including substantially consecutive frame or field signals, each constituted by substantially consecutive line signals, a phantom signal in the form of at least one pseudo-random bit signal sequence with properties similar to those of the video signals is emitted in addition to the video signals. A generator for generating the phantom signal is connected to an external power supply line to the unit containing the video signal circuits, for output of the phantom signal on this line. Using a control unit, the bit frequencies of the pseudo-random bit signal sequence/sequences are varied. Furthermore, the pseudo-random bit signal sequence/sequences are formed of a number of part signal sequences with a duration varying from one frame or field period to another. ## Claims ### Claim 1 A method for preventing external detection of signal information in video signals, comprising: (a) emitting video signals containing a bit signal sequence, from a video circuit via electromagnetic waves; (b) generating a phantom signal with at least one pseudo-random bit signal sequence and having properties similar to said bit signal sequence of said video signals emitted from said video circuit and emitting said phantom signal via electromagnetic waves within a temporally overlapping time interval as said video signals are being emitted; and (c) while conducting step (b), supplying said phantom signal to an external power supply line which is powering said video circuit. ### Claim 2 Apparatus for preventing external detection of signal information in video signals, comprising: (a) means for emitting video signals containing a bit signal sequence, from a video circuit via electromagnetic waves; (b) means for generating a phantom signal with at least one pseudo-random bit signal sequence and having properties similar to said bit signal sequence of said video signals emitted from said video circuit and emitting said phantom signal via electromagnetic waves within a temporally overlapping time interval and spatially overlapping region as said video signals are being emitted; said means for generating a phantom signal further including a maximum recurrence length shift register with an adjustable maximum length; and (c) means for varying bit frequencies of each said pseudo-random bit signal sequence. ### Claim 3 Apparatus for preventing external detection of signal information in video signals, comprising: (a) means for emitting video signals containing a bit signal sequence, from a video circuit via electromagnetic waves including: a first aerial having a group plane and a broadening fin projecting outwardly from said ground plane, said broadening fin having surfaces which are perpendicular to one another and to said ground plane; and a dipole aerial arranged circularly around said broadening fin, in a plane that is parallel to said ground plane; (b) means for generating a phantom signal with at least one pseudo-random bit signal sequence and having properties similar to said bit signal sequence of said video signals emitted from said video circuit and emitting said phantom signal via electromagnetic waves within a temporarily overlapping time interval and spatially overlapping region as said video signals are being emitted; and (c) means for varying bit frequencies of each said pseudo-random bit signal sequence. ### Claim 4 Apparatus for preventing external detection of signal information in video signals, comprising: (a) means for emitting video signals containing a bit signal sequence, from a video circuit via electromagnetic waves; (b) means for generating a phantom signal with at least one pseudo-random bit signal sequence and having properties similar to said bit signal sequence of said video signals emitted from said video circuit and emitting said phantom signal via electromagnetic waves within a temporally overlapping time interval and spatially overlapping region as said video signals are being emitted; (c) means for varying bit frequencies of each said pseudo-random bit signal sequence; and a conductor for effectively coupling said means for generating said phantom signal to a video display unit. ### Claim 5 Apparatus for preventing external detection of signal information in video signals, comprising: (a) means for emitting video signals containing a bit signal sequence, from a video circuit via electromagnetic waves, and including an external power supply line for powering said video circuit; (b) means for generating a phantom signal with at least one pseudo-random bit signal sequence and having properties similar to said bit signal sequence of said video signals emitted from said video circuit and emitting said phantom signal within a temporally overlapping time interval as said video signals are being emitted; and (c) means for supplying said phantom signal to said external power supply line. ### Claim 6 (depends on 2) The apparatus of claim 2, wherein: said means for varying bit frequencies comprises a frequency modulator effectively coupled to said means for generating a phantom signal. ### Claim 7 (depends on 6) The apparatus of claim 6, wherein: said means for generating a phantom signal includes means for providing at least one said pseudo-random bit signal sequence with a plurality of partial pseudo-random bit signal sequences which are varied in duration from one said frame period to another said frame period, or from one said field period to another. ### Claim 8 (depends on 7) The apparatus of claim 7, wherein: said means for generating a phantom signal further includes a maximum recurrence length shift register with an adjustable maximum length. ### Claim 9 (depends on 2) The apparatus of claim 2, wherein: said means for emitting said video signals and said means for emitting said phantom signal include: a first aerial having a ground plane and a broadening fin projecting outwardly from said ground plane, said broadening fin having surfaces which are perpendicular to one another and to said ground plane; and a dipole aerial arranged circularly around said broadening fin, in a plane that is parallel to said ground plane. ### Claim 10 (depends on 9) The apparatus of claim 9, further including: a conductor for effectively coupling said means for generating said phantom signal to a video display unit. ### Claim 11 (depends on 10) The apparatus of claim 10, wherein: said video signal has a given line frequency within said time interval, and said means for varying bit frequencies of each said pseudo-random bit signal sequence includes means for causing said bit frequencies to vary at a magnitude which is substantially equal to that of said line frequency. ### Claim 12 (depends on 11) The apparatus of claim 11, wherein: said video signal has a given line frequency within said time interval, and said means for varying bit frequencies of each said pseudo-random bit signal sequence includes means for causing said bit frequencies to vary at a magnitude which is substantially the same as that of said given line frequency. ### Claim 13 (depends on 12) The apparatus of claim 12, wherein: said video signal has a given frame period or a given field period within said time interval and an ensemble average differing from zero, and said means for varying bit frequencies of each said pseudo-random bit signal sequence forms that pseudo-random bit signal sequence from a plurality of partial pseudo-random bit signal sequences, each of which is varied within each said frame period or field period. ### Claim 14 (depends on 13) The apparatus of claim 13 further comprising: a video display unit effectively connected by said conductor, to said means for generating said phantom signal.