JPH0158701B2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The purpose of the present invention is to remove and reduce signal interception and interference by changing the audio signal spectrum to be transmitted from the original audio signal spectrum in frequency and time. This is about the secret method.

(Background technology) Here, wireless transmission will be explained as an example.

2. Description of the Related Art Conventionally, various confidential communication methods have been known in which a wirelessly transmitted audio signal is confidential to prevent the audio signal from being intercepted by anyone other than the sender and receiver. FIG. 1 is an example of this, and is the most common configuration example of the conventional confidential communication system. In FIG. 1, 1 is an audio signal input device, 2 is an audio signal secret device, 3 is an audio signal modulation transmitter, 4 is a transmitting antenna, 5 is a receiving antenna, 6 is an audio signal receiving/demodulating device, and 7 is an audio signal transmitting device. An audio signal secret story reproducing device, 8 is an audio signal output device. In FIG. 1, the audio signal input from the audio signal input device 1 is passed through a bandpass filter in the device 1, and the frequency _{is set to a.}
After the band is limited from [Hz] to _b [Hz] (0< _a < _b ), the audio signal is input to the confidential communication device 2. In the case of a normal wireless transmission method, the above _a and _a are: _a = 300Hz,
_b = 3kHz is often chosen. The signal input to the audio signal privacy device 2 is subjected to privacy manipulation, and an output audio signal spectrum different from the input audio signal spectrum of the device 2 is output from its output terminal. There are many known types of secret speech, but the one that is practically effective is one in which the frequency band of the signal to which the secret speech has been applied is expanded from the frequency band _a to _b [Hz] of the audio signal input to the secret speech device. There is no secret method. Examples include (i) Frequency-inverted secret stories in which the required frequency band remains unchanged; (ii) Frequency-inverted secret stories in which the required frequency band remains unchanged. (iii) Frequency-division inverted secret stories in which the required frequency band remains unchanged and the secret key changes over time. (iv) Tone signal masking secrets are mentioned.

Now, the confidential voice signal outputted from the confidential voice device 2 after being subjected to any of the confidential voice operations described above is subjected to M modulation, FM modulation, or
After receiving any one of the PM modulations, the signal is transmitted to the transmitting antenna 4 and sent out from the antenna 4 into space. On the receiving side, on the other hand, the radio waves from 4 are received by the receiving antenna 5, and amplified and demodulated by the audio signal receiving/demodulating device. Further, the demodulated secret speech audio signal is sent to the audio signal secret speech reproduction device 7. The audio signal confidential communication device 7 performs the operation opposite to any of the confidential communication operations (i) to (iv) described above. As a result, an audio signal spectrum equal to the output audio signal of the audio signal input device 1 with an occupied frequency band _a to _b [Hz] is obtained from the output terminal 7.

The reason why such a confidential communication method is used is to prevent transmitted voice information from being intercepted. That is, since there is a possibility that a person other than the signal receiver may intercept the radio signal, a secret key (secret code) is determined in advance by the sender and the receiver, and the radio signal is transmitted in accordance with the key. In this way, unless the user knows the key to the secret message in advance and also owns a device for reproducing the secret message, he or she will not be able to know the original voice information. Therefore, it is essential that the secret key (secret code) be sent from the sender to the receiver, preferably by another communication means, before the start of communication.

On the other hand, in recent wireless transmission systems, especially mobile communication systems, in order to make effective use of frequencies, the same frequency repetition technology, which reuses the same wireless transmission frequency in areas separated by a certain distance, has come to be used. Ta. According to this method, carrier waves of the same frequency can be used repeatedly within the same time period, so frequency utilization efficiency can be dramatically increased. However, due to various changes in the radio wave propagation state,
A situation may arise in which radio waves of the same frequency from another region, which should originally have a sufficiently low level, are received as interference waves. That is, in a certain region, the received power level of a signal with carrier frequency _p that is originally desired to be received is D[W], and the received power level of an interference wave with the same carrier frequency _p used in another region is V[W]. ], the ratio D/V may deteriorate. When such a development occurs, a situation arises in which, in addition to the original call voice signal, a voice signal mixed in from another area is also received at the same time, giving an unfavorable impression to the person making the call.

For such problems, it is effective to use secret technology. In other words, if you encrypt each call with a different key, even if interference waves enter the receiver, they will only be heard as noise, so you will not be able to hear audio information from other parties. It doesn't happen. Therefore, the confidential communication technique can be an effective technique in the sense of preventing so-called co-channel interference for systems that use the same frequency repetition technique, such as mobile communication systems.

Now, when using secret communication technology in this way, there is no need to agree on the secret key (secret code) between the parties involved in the call in advance; rather, when interference occurs on the same channel, It is preferable to change the key. In addition, in cases such as public mobile communication systems, where an unspecified number of customers may make calls randomly depending on time and location, it is difficult to set a specific secret code in advance. There are many cases.

(Problem to be solved by the invention) As described above, the present invention proposes a confidential communication method whose primary purpose is to prevent intelligible same-channel interference from occurring in a system where same-channel interference may occur. is characterized in that it simultaneously transmits a secret key (secret code) using a frequency band in which no audio signal spectrum exists, and changes the secret key when no audio signal exists. Hereinafter, the present invention will be explained in detail with reference to the drawings.

(Structure and operation of the invention) FIG. 2 is a block diagram of a transmitter/receiver to which the present invention is applied, in which 1 is an audio signal input device, 4 is a transmitting antenna, 5 is a receiving antenna, and 9 is an audio signal secreting device. , 10 is an adder, 11 is a delay circuit of t _D (seconds) (t _D > 0), 12 is a modulator, 13 is a frequency conversion amplifier, 14 is a secret code generator, and 15 is a secret control signal interrupt device. , 16 are various control signal generators, 1
7 is an audio signal power detection device, 18 is a frequency conversion amplifier, 19 is a demodulator, 20 is a band pass filter, 21 is a low pass filter, 22 is a secret speech reproduction device, 23 is an audio signal amplification output device, 24 is a secret speech code and various control signal code separation devices, 25
is the control device.

In FIG. 2, an audio signal input from an input terminal of an audio input device 1 is band-limited from _a [Hz] to _b [Hz] by a bandpass filter within the device 1. The band-limited audio signal output is input to the audio encrypting device 9, where it is encrypted. Types of secrets,
The time change of the key and the timing at which the type of secret information changes are determined by the device 14 depending on the situation of the secret information at that time.
It is determined based on the digital control signal input from the. The timing and the role of the device 17 that determines the timing will be discussed in detail later. The types of secrets are (i) Frequency inversion secrets where the required frequency band remains unchanged (ii) Frequency division inverted secrets where the required frequency band remains unchanged (iii) Frequency division where the required frequency band remains unchanged and the secret key changes over time Inverted secret speech (iv) There is a tone signal masking secret speech, and in the secret speech whose secret code changes depending on the time, the code of the signal sent from 14 becomes a function of time. This secret code generator 14
The signal sent from the private voice communication device 9 is input to the private voice control signal interrupt device 15 . When the private conversation control signal is present, the device 15 interrupts the digital control signal with priority over the call channel switching information, call quality monitoring information, and transmission output power control information that are normally sent out from the various control signal generators 16, The secret control signal is sent to adder 1.
This is a device that sends data to 0. Note that the private conversation control signal and call channel switching information must be band-limited to _a [Hz] or less.

Now, in the adder 10, the output audio signal of the secret device 9 and the output from the device 15, such as the secret code and call channel switching information, are added. Since these two signals have different frequency bands, they can be separated later by a filter. The output of adder 10 is sent to modulator 12 and modulated. The modulation is AM modulation, FM modulation, or PM modulation. The modulator output signal is transmitted to the frequency conversion amplifier 13
and is emitted into space from the antenna 4. The above is an explanation of the operation of the transmitter.

On the receiving side, the reverse operation is performed. First, a radio signal received by the antenna 5 is input to the frequency conversion amplifier 18 and converted into an IF signal with a relatively high power level. This signal is demodulated by a demodulator 19 and then input to a band pass filter 20 and a low pass filter 21.
The pass band of the band pass filter 20 is _a to _b [Hz],
Set the passband of the low-pass filter 21 to 0 to _a [Hz]
If set to , the output of the bandpass filter will be the audio signal with the secret code of the occupied bands _a to _b , and the output of the low pass filter 21 will be the secret code, call channel switching information, etc. Various control information will appear. The control information outputted from 21 is divided into confidential codes and various control signal code separators 24
In the step, information regarding the confidential information and other control information are separated. Call channel switching information, call quality monitoring information, etc. are sent to the control device 23. Further, the information regarding the secret story is converted into a digital signal that is operated by the secret story reproduction circuit 20, and the secret story reproduction device 2
2 is sent out. In the secret speech reproducing circuit 22, the audio signal sent from the band pass filter 20 is reproduced in accordance with the digital control signal from 24.
The signal reproduced from the secret message is sent to the audio signal amplification output device 2.
After being amplified at step 3, the signal is emitted from the speaker.

Here, an inverted privacy circuit is shown as an example of an audio signal privacy device. Figure 3 shows an example of the configuration of a temporally controllable inverted secret speech circuit, where 26 indicates frequencies _a to _b [Hz].
27 is a first analog switch, 28 is a second analog switch, 29 is an oscillator that generates a sine wave of _a + _b [Hz], 30 is _a to _b [Hz] ); 31 is an inverted secret circuit output terminal; 3
2 is a control signal input terminal, 33 is an inverter, 34
is a mixer. In this circuit, either the analog switch 27 or 28 is closed depending on the polarity of the signal applied from the control signal input terminal 32, and the "inverted confidential mode" and "non-confidential mode" can be controlled accordingly. It's getting old. That is, the audio signal input from the input terminal 26 is
If 7 is “closed” and 28 is “open”, mixer 3
4, it is mixed with the sine wave of the oscillator 29. As a result, an inverted spectrum appears in the frequency band _a to _b [Hz], and a non-inverted spectrum appears in the band (2 _a + _b ) to ( _a + 2 _b ) [Hz]. The bandpass filter 30 selects _a to _b of the above two spectra.
This is a filter for extracting the inverted spectrum of [Hz]. Finally, an inverted spectrum is obtained from the terminal 31. On the other hand, 27 is “open” and 28 is “closed”
In this case, the input audio signal is output from the terminal 31 without being subjected to any secret operation. The selection of these two modes is determined by the signal input from terminal 32.
Depends on whether it is HIGH or LOW.
In the present invention, the polarity of the signal applied from the terminal 32 is set according to the lower band (0 to _a [Hz]) of the audio signal to be transmitted.
band), and is transmitted simultaneously with the audio signal. Therefore, on the receiving side, the inverted confidential mode and the non-secure mode can be determined based on the signal sent from 0 to _a [Hz].

Next, the configuration of the audio signal power detection device 17 necessary for switching the secret mode when the audio signal is interrupted will be described. FIG. 4 shows an example of the configuration of the audio signal power detection device 17. In FIG. 4, 35 is a full-wave rectifier circuit, 36 is a low-pass filter, 37 is a comparator, 38 is an audio signal input terminal, and 3
9 is a reference voltage input terminal, and 40 is an output signal terminal. The amplitude envelope of the audio signal input from 38 is detected by a full-wave rectifier circuit 35 and a low-pass filter 36. When the detected envelope and the reference voltage from the terminal 39 are compared by the comparator 37, the output of the output terminal 40 is determined. In other words, if the reference voltage is 0V and the envelope voltage is higher than the reference voltage, the voltage will change from 40 to
If it is determined that a LOW level signal is output and a HIGH level signal is output when the envelope voltage is lower than the reference voltage, the output voltage will be HIGH level only when no audio signal is present. The voltage at terminal 40 is supplied to secret code generator 14 in FIG. If the device 14 is made operational only when the output of the device 17 is HIGH, the confidential mode change will occur when no audio signal is present. Note that the device 14 always operates when the output of the device 17 is HIGH, and there is no need to change the secret mode. The secret mode may be changed only when there is a command from the control signal generator 16 and a HIGH signal is input from the device 17. Note that if the secret conversation is switched during a call, the naturalness of the conversation will be disturbed, so it is preferable to switch the confidential conversation when the audio signal is interrupted.

Here, the sending timing of the transmission audio signal and the control signal will be described. Figure 5 shows the transmitted audio signal,
This is a diagram showing the timing of the transmitted digital control signal in the case of inverted secret speech. In FIG. 5, 41 is a timing diagram showing whether or not an input audio signal exists, 42 is a timing diagram showing whether or not an output signal of the delay circuit 11 is present, and 43 is a timing diagram showing whether or not an output signal of the delay circuit 11 is present. A timing diagram showing a control signal flow, 44 is a time domain where an audio signal exists and there is no need to invert the audio signal, 45 is a time domain where an audio signal exists and it is necessary to invert the audio signal. , 46 is a time domain in which no audio signal exists, 47 is a digital control signal such as call channel switching information, call quality monitoring information, etc., 48 is a control signal indicating to apply reverse confidentiality, and 49 is a control indicating to restore reverse confidentiality. It's a signal. As shown in FIG. 5, the timing 42 is delayed from the timing 41 by the delay time _tD of the delay circuit 11.
As shown in the timing chart 41, the signal 45 is subjected to inversion secrecy according to a command from the control section.
In response to the fact that the inverted secret message has been applied, a control signal 48 indicating that the secret message is to be applied is included in the transmission control signal. This signal 48 is a signal that is inserted into the digital control signal control signal 47 according to a command from the control unit after it is confirmed that the transmitted audio signal has been interrupted in the timing diagram 41. . The signal actually sent out from the transmitter is 4
2 audio signals and 43 control signals. Therefore,
On the receiving side, before receiving the signal 45 to which the inverted secret speech has been applied, it can be known from the secret speech signal 48 that the next signal to be sent has the inverted secret speech. That is, according to this method, it is possible to know in advance whether the next audio signal to be sent is in confidential mode or non-confidential mode. The above is the case when changing from the non-confidential mode to the confidential mode, but the same applies when changing from the confidential mode to the non-confidential mode, as shown in FIG. A time region 46 in which no audio signal exists is detected, and a signal 49 indicating that the inverted secret story is to be restored is inputted therein. On the receiving side, if the secret operation is not performed from the next voice signal after receiving 49, the voice signal will be transmitted correctly.

(Effect of the invention) As explained above, if the audio signal to be transmitted is encrypted and at the same time the secret key (secret code) is transmitted using the lower band of the audio signal frequency spectrum, when the secret code is to be changed. Code changes can be made instantly. Therefore, even when intelligible same-channel interference occurs, the confidential mode can be changed immediately, and crosstalk of the audio signal can be prevented. Also,
Since the confidential conversation mode is switched at a timing when no audio signal is present, there is also the advantage that no noise is introduced into the audio signal.

[Brief explanation of drawings]

FIG. 1 is an example of the configuration of a conventional confidential communication system, FIGS. 2 a and b are examples of the configuration of a transmitter and receiver to which the present invention is applied, and FIG. 3 is a configuration of a confidential communication circuit in a transmitter/receiver to which the present invention is applied. For example, FIG. 4 is a configuration example of an audio signal power detection device, and FIG. 5 is a diagram illustrating the timing of an input audio signal, a transmitted audio signal, and a transmitted digital control signal. DESCRIPTION OF SYMBOLS 1...Audio signal input device, 2...Audio signal secret device, 3...Audio signal modulation transmission device, 4...Antenna for transmission, 5...Antenna for reception, 6...Audio signal reception/
Demodulator, 7...Audio signal secret playback device, 8...Audio signal output device, 9...Audio signal secret device, 10...Adder, 11...t _D seconds delay circuit, 12...Modulator, 1
3... Frequency conversion amplifier, 14... Secret code generator, 15... Secret control signal interrupt device, 16... Various control signal generators, 17 is audio signal power detection device, 18... Frequency conversion amplifier, 19... Demodulator ,
20...Band pass filter, 21...Low pass filter, 22...Secret message playback device, 23...Audio signal amplification output device, 24...Secret code and various control signal code separation device, 25...Control device, 26...Frequency _a
~ _b Audio signal input terminal with band limit limited to [Hz],
27... first analog switch, 28... second analog switch, 29... oscillator that generates a sine wave of _a + _b [Hz], 30... bandpass filter that passes only signals of _a to _b [Hz], 31... Inverted private speech circuit output terminal, 32... Control signal input terminal, 33... Inverter, 34... Mixer, 35... Full wave rectifier circuit, 36...
Low-pass filter, 37...Comparator, 38...Audio signal input terminal, 39...Reference voltage input terminal, 40...Output signal terminal, 41...Timing diagram showing whether or not an input audio signal is present, 42...Delay circuit Timing diagram showing whether or not output signals of 11 are present, 4
3... Timing diagram showing the control signal flow in the secret control signal interrupt device output, 44... Time domain where an audio signal exists and there is no need to invert the audio signal, 45... Time domain where an audio signal exists and the audio signal does not need to be inverted. Time domain in which the signal needs to be inverted, 46... Time domain in which no audio signal exists, 47... Digital control signal, 48... Control signal indicative of applying inversion secrecy, 49... Control signal indicative of restoring inversion secrecy.

Claims (1)

[Claims] 1. Frequency is from _a [Hz] to _b [Hz] (0< _a < _b )
A predetermined confidential operation is applied to the band-limited analog audio signal, and a digital control signal that occupies an arbitrary band between frequencies 0 and _a [Hz] is added to the confidential audio signal and transmitted along with the audio signal. The type of secret message and the time change of the key of the secret message are transmitted to the receiving side by the digital control signal, and the type of confidential message and the change of the key are transmitted to the receiving side when the audio signal is interrupted by the digital control signal. A confidential communication method that is characterized by