JPH088792A - Wiretapping prevention device for output cable signal - Google Patents

Abstract

PURPOSE:To easily and effectively eliminate radio wave leakage and to prevent wiretapping. CONSTITUTION:An interference signal line 4 is provided in parallel to an output cable 3 where the analog signal outputted from a signal converter 5 is transmitted. The clock signal that a signal converter 5 outputs is transmitted to the interference signal line 4 via a variable resistor 42. Thus, the radio wave of the analog signal leaking from the output cable 3 and the radio wave of the interference signal leaking from the interference signal line 4 are overlapped. At this time, the intensity of the radio wave leaking from the interference signal line 4 can be adjusted to a range which eliminate the radio wave leaking from the output cable 3 completely and does not exceed the reference of a radiation interference specification by the adjustment of the variable resistor 42. As a result, the radio wave by the analog signal leaking from the output cable 3 is effectively eliminated by the radio wave of the interference signal and the decoding of the contents is made impossible at the outside of the output cable 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output cable signal interception preventing device for preventing the illegal use of image information displayed on a display device such as a CRT.

[0002]

2. Description of the Related Art Generally, a CRT or the like and a main body of a personal computer or the like are connected by an output cable, but radio waves leaking from the output cable may be intercepted. In order to prevent this, a preventive device has been conventionally provided. This type of interception prevention device is not provided inside the CRT display device or the like, but is provided outside. For example, in order to prevent radio waves leaking from the output cable from being emitted to the outside, a method of shielding a room or a building in which a computer is installed, or a device for generating noise in synchronization with a CRT signal is installed in the room. There is a method such as.

[0003]

However, the above-mentioned conventional techniques have the following problems. That is, it is expensive to shield a room or building, and it is very difficult or impossible to change the installation location of a computer or the like. Further, in the method of installing the jammer generator near the display device such as the CRT, the jammer generator is required for each CRT. Therefore, the cost required for the installation environment also increases. On the other hand, if the radio waves leaking from the output cable are lowered to a level at which interception is impossible, it is necessary to take measures to remove the influence of external noise on the video signal, which increases the cost of the device itself.

[0004]

In order to solve the above-mentioned problems, the output cable signal interception preventing device of the present invention is characterized by the following points. (1) A signal converter that converts a digital signal into an analog signal and outputs the analog signal is provided. An output cable for transmitting an analog signal output from the signal converter is provided. An interference signal line that is provided in parallel with the output cable and that transmits a signal having a frequency higher than the frequency of the analog signal output from the signal converter is provided.

(2) An output circuit for outputting basic color signals of red, green and blue is provided. An output cable for transmitting each basic color signal output from the output circuit is provided. A distributor for distributing each basic color signal output from the output circuit is provided. The delay circuit delays each basic color signal output from the distributor for each predetermined time set for each basic color signal. A synthesizing circuit for synthesizing the delayed signals output from the delay circuit is provided. A signal output from the synthesis circuit is transmitted, and an interfering signal line is provided in parallel with the output cable.

[0006]

[Action]

(1) A disturbing signal line is provided in parallel with an output cable through which an analog signal output from the signal converter is transmitted, and a signal having a frequency higher than the frequency of the analog signal output from the signal converter is provided on the disturbing signal line. Is transmitted. This causes the analog signal leaking from the output cable and the disturbing signal leaking from the disturbing signal line to overlap each other. As a result, outside the output cable, the analog signal leaking from the output cable is effectively erased by the interfering signal, making it impossible to decipher the contents.

(2) An interfering signal line is provided in parallel with the output cable for transmitting the red, green, and blue basic color signals output from the output circuit. Then, each interference signal line is delayed by a predetermined time for each basic color signal, and the interference signal generated by synthesizing the delayed signals is transmitted. As a result, as in the case described above, the analog signal leaking from the output cable and the interfering signal leaking from the interfering signal line overlap with each other, and the analog signal leaking from the output cable is more effectively interfered with by the interfering signal outside the output cable. It is erased and the contents cannot be deciphered.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram of an embodiment of an output cable signal interception preventing apparatus according to the present invention. The device of FIG. 1 comprises a device body 1, a CRT unit 2 and an output cable 3 connected between them. A disturbing signal line 4 is provided along the output cable 3, and a clock signal used in the signal converter 5 is transmitted to the disturbing signal line 4.

The output cable 3 is R (red), G (green),
It transmits the B (blue) analog signal and the vertical and horizontal synchronizing signals 7 for synchronizing these signals. The disturbing signal line 4 is connected to the apparatus main body 1 via the connector 41.
Inside the device main body 1, the clock signal input to the signal converter 5 is branched and connected to the connector 41 via the variable resistor 42. The variable resistor 42 adjusts the strength of the signal output to the disturbing signal line 4, satisfies the standard value of the radiation noise, and outputs a signal of sufficient strength to disturb the interception. To do so.

The signal converter 5 converts the digital signal 6 into R (red), G (green), and B (blue) analog signals according to the clock signal, and transmits the analog signals to the output cable 3 via the connector 31. This analog signal is input to the CRT unit 2 via the connector 32 and is displayed as image information on the CRT screen while being synchronized by the vertical and horizontal synchronizing signals 7. Although the interference signal line 4 is provided outside the output cable 3 in the illustrated example, the interference signal line 4 may be incorporated inside the output cable 3. Figure 2
It is an external view of an apparatus to which the present invention is applied. In FIG. 2, the device body 1 and the CRT unit 2 are connected to the output cable 3
Connected by. An interference signal line 4 is provided along the output cable 3.

Next, the operation of the above-mentioned device will be described. FIG. 3 shows radiated noise from a conventional output cable. 3A, 3B, and 3C at the rising and falling points of each of the R, G, and B signals.
The noise shown in (d) is generated, but since this noise overlaps at the rising and falling points of each signal,
It cannot interfere with the interception of each signal.

On the other hand, in the present invention, the noise shown in FIG. 4 (e) is generated at the rising time and the falling time of the clock signal shown in FIG. 4 (d). The variable resistor 42 adjusts the amplitude of the clock signal shown in FIG. 4D, and the noise amplitude shown in FIG.
By matching the amplitudes of the R, G, and B signals shown in (b) and (c), the R, G, and B signals can be canceled. This makes it completely impossible to decode the R, G, B signals. In this way, the interception of the video signal can be effectively prevented by adding the simple configuration of only the interference signal line 4, the connector 41, and the variable resistor 42. Further, it is not necessary to lower the level of the video signal in order to prevent interception.

(Second Embodiment) FIG. 5 is a block diagram of a second embodiment of an output cable signal interception preventing apparatus according to the present invention. The apparatus shown in FIG. 5 includes an apparatus main body 11, a CRT unit 12, and an output cable 13 connected between them. Then, along with the output cable 13, the interfering signal line 1
4 are provided. The output cable 13 is R (red),
It transmits G (green) and B (blue) analog signals and vertical and horizontal synchronizing signals 17 for synchronizing these signals. The interference signal line 14 is connected to the apparatus main body 11 via the connector 41. Inside the device body 11, each signal output from the output circuit 15 is branched by the distributor 18 and input to the delay circuit 19.

The output circuit 15 includes R (red), G (green), and B.
The (blue) analog signal is output and transmitted to the output cable 13 via the connector 31. This analog signal is input to the CRT unit 12 via the connector 32 and displayed on the CRT screen as image information while synchronizing with the vertical and horizontal synchronizing signals 17. The delay circuit 19 delays each signal output from the distributor 18 by a predetermined time. For example, as shown in FIGS. 6A and 6D, R
When delaying the signal, the G signal is delayed by 2 times as shown in FIGS. 6B and 6E, and the B signal is delayed by 3 times as shown in FIGS. 6C and 6F. The synthesis circuit 20 is 3
It is composed of an input OR circuit and outputs the logical sum of the signals output from the delay circuit 19.

In the illustrated example, the interfering signal line 14 is provided outside the output cable 13 as in the first embodiment, but the interfering signal line 14 is incorporated into the output cable 13. Good. Next, the operation of the above-mentioned device will be described. FIG. 6 is a time chart for explaining radiation noise according to the second embodiment of the present invention. The operation of the apparatus shown in FIG. 5 will be described with reference to FIG. In FIG. 5, the R, G, and B signals output from the output circuit 15 are respectively divided into two by the distributor 18, one of which is sent to the CRT unit 12 via the output cable 13 and the other of which is delayed. It is sent to the circuit 19. Each signal input to the delay circuit 19 is delayed for a different predetermined time, and the combining circuit 2
0 is combined into a logical sum and sent to the disturbing signal line 14 via the connector 41.

The signal transmitted to the interfering signal line 14 is shown in FIG.
As shown in (g). By this signal, FIG. 6 (h)
As shown in FIG.
The video signals shown in (a), (b) and (c) are erased. Therefore, interception can be prevented. In this way,
By adding only the interfering signal line 14, the connector 41, the distributor 18, the delay circuit 19, and the combining circuit 20, it is possible to effectively prevent interception of the video signal. Further, it is not necessary to lower the level of the video signal. In the second embodiment, since the interference signal is generated from the video signal, the amplitude of the video signal and the amplitude of the interference signal are always equal to each other, and the adjustment as in the first embodiment is unnecessary. That is, the variable resistor 42 becomes unnecessary. Further, the output level of the video signal can be freely changed within the standard range.

The present invention is not limited to the above-mentioned embodiment, but various modifications can be made. For example, the first
In the embodiment described above, the clock signal is transmitted to the interfering signal line, but the invention is not limited to this, and another signal may be used as long as it is a signal having a frequency higher than the frequency of the video signal and being periodic with the video signal. Further, the case of an output cable to a display device such as a CRT has been described, but the present invention is not limited to this, and an input / output cable between other devices can be applied.

[0018]

As described above, according to the output cable signal interception prevention apparatus of the present invention, an interfering signal having a frequency higher than the frequency transmitted to the cable in synchronization with the signal transmitted to the output cable can be obtained. Since the signal line to be transmitted is provided along the output cable, it is not necessary to shield the room or building, the cost is low, and the installation location of the computer or the like can be easily changed.
Further, the jamming radio wave generator for each CRT is unnecessary, and the installation cost can be reduced. On the other hand, it is not necessary to reduce the electric wave leaking from the output cable to a level at which interception is not possible, and there is no need to take measures to remove the influence of external noise on the video signal, and the cost of the device itself can be prevented from rising.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of an output cable signal interception prevention device according to the present invention.

FIG. 2 is an external view of a device to which the present invention is applied.

FIG. 3 is a time chart explaining conventional radiation noise.

FIG. 4 is a time chart for explaining radiation noise according to the first embodiment of the present invention.

FIG. 5 is a block diagram of a second embodiment of an output cable signal interception prevention device according to the present invention.

FIG. 6 is a time chart illustrating radiation noise according to the second embodiment of the present invention.

[Explanation of symbols]

 1, 11 Device body 2, 12 CRT unit 3, 13 Output cable 4, 14 Interference signal line 5 Signal converter 15 Output circuit 18 Distributor 19 Delay circuit 20 Synthesizing circuit

Claims (2)

[Claims] 1. A signal converter for converting a digital signal into an analog signal and outputting the analog signal, an output cable for transmitting an analog signal output from the signal converter, and an output cable provided in parallel with the output cable. An interception device for an output cable signal, which comprises an interference signal line through which a signal having a frequency higher than the frequency of the analog signal output from the signal converter is transmitted. 2. An output circuit for outputting red, green, and blue basic color signals, an output cable for transmitting each basic color signal output from the output circuit, and each basic output from the output circuit. A distributor that distributes the color signal, a delay circuit that delays each basic color signal output from the distributor by a predetermined time set for each, and a delay signal that is output from the delay circuit An interception device for an output cable signal, comprising a synthesizing circuit and an interfering signal line which is provided in parallel with the output cable and through which a signal output from the synthesizing circuit is transmitted.