JPH04249934A - Privacy telephone signal transmission system - Google Patents

Abstract

PURPOSE:To prevent a signal on a radio transmission line from being tapped or decoded by a third party by transmitting a waveform on which an audio signal and a noise are superimposeed on the transmission line. CONSTITUTION:A band elimination filter 2 which passes the signal, a noise generator 8, a synchronizing signal generator 4, and an adder 3 which adds the signal and the noise are provided at a transmission side. Also, a noise generator 13 which reproduces the same noise, a synchronizing signal receiver 10, a subtractor 14 which subtracts the noise, and a band elimination filter 15 which takes out the signal are provided at a reception side. An input signal and the noise generated by the noise generator 8 are added by at the adder 3 at the transmission side, and are sent out to the transmission line, and the subtraction processing of a noise waveform generated by the noise generator 13 from the signal on which an inputted noise is superimposed is performed by the subtractor 14 at the reception side, and it is outputted to a signal output terminal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a secret signal transmission system that prevents signals sent to a transmission line from being easily intercepted or understood by a third party.

[Summary of the invention] The present invention includes a noise generator that outputs exactly the same waveform on the transmitting side and the receiving side, and superimposes the noise on the signal on the transmitting side, and cancels the noise on the receiving side. This is what I did.

[Background Art] In recent years, with the development of wireless communication technology, mobile phone services such as car phones, mobile phones, and cordless phones have become popular. Along with this, the convenience of telephone services has improved significantly, but on the other hand, many wideband wireless receivers aimed at some enthusiasts have also been sold, and these services using radio waves have become
The ease with which the content of such communications can be intercepted by a third party has become a kind of social problem. To deal with this situation, various methods have been considered and some have been introduced, such as those shown in Reference (1), but none of them are sufficiently effective or have poor signal quality. to degrade,
Due to the high cost and other reasons, sufficient countermeasures are not being taken.

[Object of the invention] The object of the present invention is to solve these conventional problems, so that the signal on the transmission path cannot be easily intercepted or understood by a third party, and the receiving side can receive the signal without losing quality. The objective is to provide a secret signal transmission method that can be reproduced.

[Structure of the Invention] In order to achieve the above object, the confidential signal transmission system of the present invention includes a "transmission side noise generator" and an "adder" on the transmitting side and an "adder" on the receiving side for noise superimposition and cancellation. Equipped with a "receiving side noise generator" and a "subtractor", the transmitting side adds the signal and noise and sends it out to the transmission path, and the receiving side subtracts the same noise waveform as the transmitting side from the noise-superimposed signal. It is characterized by processing and extracting only the signal. Regarding the synchronization signal transmission method, the transmission side is equipped with a "transmission side band deletion filter" and a "synchronization signal generator", and the reception side is equipped with a "reception side band deletion filter" and a "synchronization signal receiver".
The transmitting side sends the bit synchronization signal as time information and the frame synchronization signal as amplitude information to the transmission path, and the receiving side receives and reproduces this information to operate the receiving side noise generator. .

[Embodiments] Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. Based on the configuration shown in FIG. 1, the following explanation will be given by taking as an example the case where the transmission path is a currently commonly used wireless line with a voice transmission band of 0.3 to 3.0 kHz. The audio signal input from the signal input terminal 1 has a frequency f0 partially removed from the band by a transmitting side band elimination filter 2 (hereinafter referred to as BEF2), and is input to the adder 3. Although any part of the frequency range from 0.3 to 3.0 kHz can be removed, it is generally preferable from the viewpoint of audio quality to remove the high frequency part where audio energy is low. In this explanation, f0=2.9kHz as an example.
z, the high frequency part of the communication band is 2.8 to 3 by BEF2.
．． The explanation will be made assuming that 0 kHz is removed. In this case, the actual configuration of BEF2 is that the cutoff frequency fc=2.
An 8 kHz low pass filter (LPF) can be used, and its output audio signal has a waveform whose band is limited to 2.8 kHz or less. The synchronization signal generator 4 is BEF2
A synchronization signal 5 that can be transmitted in the band 2.8 to 3.0kHz, which was deleted in to drive. FIG. 2 shows the synchronization signal 5 transmitted to the receiving side through the transmission line 9 and the noise generator 8 on the transmitting side.
2 shows the relationship between the bit synchronization signal 6 and the frame synchronization signal 7 that drive the frame synchronization signal 7. The synchronization signal 5 includes the second
A sine wave signal of frequency f0 as shown in Fig. 1(a) is used.

By modulating the amplitude so that the amplitude is normally B and becomes A every frame period, the bit synchronization signal and the frame synchronization signal are simultaneously transmitted through the same transmission path 9 as the audio signal by frequency division. As the transmitting side noise generator 8, a pseudo-random noise generator whose output waveform can be reproduced by a clock (=synchronization signal) is used. As pseudo-random noise, M-sequence codes and the like are known from Reference (2) and the like. A uniquely determined noise waveform can be output by generating pseudo-random noise using the bit synchronization signal 6 as a register clock and presetting the register in the noise generator to a specific pattern every frame period. By the way, when the number of register stages n=7, the frame period Tf is Tf=1/f0*(2n-1)
=44 [msec]. The noise generated in this way is added together with the audio signal and the aforementioned synchronization signal by the adder 3 and output to the transmission line 9, but the noise waveform generated by the digital clock is usually a combination of rectangular waves. , contains up to very high frequency components, so if the usage band is limited to 3kHz or less, such as in a wireless transmission path, it is preferable to limit the noise band with an LPF before transmitting it. . In addition, if you want to emphasize and superimpose noise at a specific frequency based on human hearing, use a bandpass filter (BPF), lowpass filter (LPF), or highpass filter (HPF) to emphasize or attenuate the specific frequency component. It is also effective to input the signal to the adder as a rectangular wave, and if there are no restrictions on the transmission path, it is also possible to superimpose the signal as a rectangular wave.

The synthesized waveform of the voice, synchronization signal, and noise thus added is sent to the receiving side via the transmission path 9. Third
The figure shows the frequency relationship between voice, synchronization signal, and noise.
In the figure, the noise output from the noise generator is 2
．． A case where the band is limited to 7 kHz or less is illustrated. In the current mobile phone service, the transmission path 9 includes:
Analog PM (phase modulation) wireless transmitter/receivers are often used. On the receiving side, a synchronizing signal receiver 10 reproduces a bit synchronizing signal 11 and a frame synchronizing signal 12 that are exactly the same as those on the transmitting side. In the synchronization signal receiver 10, the input waveform is first passed through a 2.8 kHz high-pass filter (HPF), thereby extracting the same waveform as shown in FIG. 2(a). By comparing this with the average value of the waveform (=0V) and converting it into a digital signal of 0V-+5V, the bit synchronization signal shown in FIG. 3(b) is reproduced.

Further, by performing a comparison using the average value C (=(A+B)/2) of the amplitudes A and B, the frame synchronization signal shown in FIG. 2(c) is reproduced. When the reception side noise generator 13 is driven using these synchronization signals, it is possible to reproduce noise having exactly the same waveform as that superimposed on the transmission side. The subtracter 14 subtracts noise having the same waveform as that on the transmitting side from the composite waveform of "voice + synchronization signal + noise" input from the transmission path, and the output is "voice + synchronization signal". It is taken out. This signal is further passed through a receiving side band elimination filter 15 (hereinafter referred to as BEF 15) to remove the frequency component (2.8 to 3.0 kHz) of the synchronizing signal and output to the signal output terminal 16. In general, the characteristics of BEF15 can be the same as those of BEF2 on the transmitting side, but if the level of the synchronization signal is low or if the quality of the call voice is not affected much, it may be possible to use one with gentler characteristics. , and may be omitted depending on the case. In the above example, 2.9 is used as the bit synchronization signal.
Transmits kHz, and the noise generator clock is also the same 2.9k.
We have explained the case where Hz is used, but of course it is also possible to use a PLL (phase-locked loop) on the receiving side to multiply this and use a higher clock frequency, that is, to generate a more complex pattern of noise. It is possible.

[Effect of the invention] As explained above, according to the present invention,
Since a waveform in which a voice signal and noise are superimposed is transmitted on a wireless transmission path, even if a third party intercepts it, it is not possible to demodulate only the voice signal as it is, so it is difficult to understand the content of the communication. The advantage is that there is no Furthermore, in the signal transmission method of the present invention, the relative relationship between the voice level and the noise level can be freely set within the allowable range of the transmission path characteristics, so the ratio of superimposed noise can be changed depending on the importance of the content of the call. It also has this feature.

[References] (1) Published by Nikkei BP, Nikkei Communication, July 28, 1986, P31-P36, "Security measures for wireless communication that cannot keep up with policies for effective use of radio waves" (2) Published by the University of Tokyo Press, "Correlation functions and spectra", P170-P181

[Brief explanation of the drawing]

Figure 1 shows an embodiment of the present invention, Figure 2 shows an example of the waveform of a synchronization signal,
FIG. 3 shows the frequency relationship between voice, noise, and synchronization signal. 1: Signal input terminal, 2: Transmission side band deletion filter, 3:
Adder, 4: Synchronization signal generator, 5: Synchronization signal sent to transmission line, 6: Bit synchronization signal, 7: Frame synchronization signal,
8: Transmission side noise generator, 9: Transmission line, 10: Synchronization signal receiver, 11: Bit synchronization signal, 12: Frame synchronization signal, 13: Receiving side noise generator, 14: Subtractor, 15: Receiving side band Deletion filter, 16: Signal output terminal.

Claims (2)

[Claims] (1) Addition of a "signal input terminal" and a "transmission side noise generator" on the transmission side, with the signal from the signal input terminal and the noise output from the transmission side noise generator being input and the output terminal connected to the transmission line. On the receiving side, there is a signal output terminal, a receiving side noise generator that outputs exactly the same noise waveform as the transmitting side noise generator, and an input signal from the transmission line and input side noise generator. A subtracter whose output terminal is connected to a signal output terminal with the output noise as input, and on the transmitting side, the above input signal and the noise generated by the transmitting side noise generator are added together and sent to the transmission line. A confidential signal transmission method characterized in that, on the receiving side, a noise waveform generated by a receiving side noise generator is subtracted from a signal superimposed with noise input from a transmission path, and the result is output to a signal output terminal. (2) In order to transmit the synchronization signal necessary for the transmitting side noise generator and the receiving side noise generator to operate so that they output exactly the same noise waveform, the transmitting side must be connected to the The receiving side is equipped with a ``transmission-side band elimination filter that removes a specific frequency f0 in the signal band'' and a synchronous signal generator that sends its output signal to the adder input, and a synchronous signal generator connected to the transmission line. The synchronization signal generator is equipped with a synchronization signal receiver having a synchronous input terminal and a reception-side band elimination filter installed between the subtracter and the output terminal to remove the frequency f0. In addition to supplying the bit synchronization signal and frame synchronization signal necessary for operation, the bit synchronization signal is sent as time information and the frame synchronization signal as amplitude information to the transmission path, and the synchronization signal receiver uses this information to A confidential signal transmission system according to claim (1), characterized in that a bit synchronization signal and a frame synchronization signal are regenerated to operate a receiving side noise generator.