JPH0332127A - Privacy call communication system - Google Patents

Abstract

PURPOSE:To improve the performance of privacy call processing by varying hourly a frequency band inserting a synchronous signal carrying the information such as frequency replacement for privacy call. CONSTITUTION:A voice input signal is inputted to (n) band pass filter groups 1, where the signal is subjected to frequency division and a switch circuit 2 is changed-over according to a pattern for the frequency replacement decided by a transmission controller 11 and (n) outputs are inputted respectively to the frequency converters F1-Fn of a frequency converter group 3. The synchronous signal is generated by a modulator receiving control information from the transmission controller 11 and synchronization information and is inputted to a frequency converter Fn+1 and a narrow band pseudo noise signal generated by a narrow band noise generator 8 is inputted to a frequency converter Fn+2 as a dummy signal. (n+2) outputs from the frequency converter group 3 are (n) outputs by a switch circuit 4 and inputted to an output synthesizer 6 via (n) band pass filter group 5. Thus, the performance of privacy call is improved.

Description

[Detailed Description of the Invention] (Technical Field to which the Invention Pertains) The present invention provides a system that makes it difficult to decipher the contents even if a third party other than the parties who are conducting voice communication with each other in analog signal format intercepts the information. The present invention relates to a confidential communication system that allows confidential voice communication.

(Prior art and its problems) A confidential device for transmitting a confidential signal to an analog voice communication line divides the transmission frequency band of the analog signal into a plurality of bands and mutually replaces the divided band signals. There are frequency permutation methods, time-domain permutation methods that divide a signal on the time axis and replace the divided signal blocks, or a combination of these two methods, and these methods are known to be highly confidential. . In these systems, in order to further increase confidentiality, it is effective to change the arrangement of the substitutions from time to time.To do this, a synchronization signal is required as substitution information indicating the arrangement order of the substitutions, and the synchronization signal is It was a signal with a specific frequency component and was placed at a predetermined frequency. However, if noise is superimposed on the frequency component of this synchronization signal or the frequency characteristics deteriorate, it becomes difficult to extract the synchronization signal, causing problems such as difficulty in appropriately restoring confidential speech.

(Object of the Invention) In order to solve these problems, it is an object of the present invention to provide a confidential communication system that eliminates the weaknesses associated with synchronization signals as replacement information and further improves confidentiality.

(Structure and operation of the invention) The confidential communication system according to the present invention does not transmit a synchronization signal as replacement information in the confidential communication system by frequency substitution at a constant frequency as in the conventional method, but changes its center frequency from time to time. This eliminates the weaknesses related to the synchronization signal, and the synchronization signal contains not only information on the permutation arrangement of the divided band signals and information on the insertion band of the dummy signal, but also
It is characterized by further enhancing confidentiality by providing information such as conversion coefficients of signal levels in designated frequency bands.

FIG. 1 is a circuit diagram of a device implementing the present invention.

Shown in Figure 2. FIG. 1 shows a circuit diagram of a transmitting section, and FIG. 2 shows a circuit diagram of a receiving section. The present invention will be explained in detail below according to this system diagram.

First, the configuration of the transmitter shown in FIG. 1 will be explained.

The human input signal is input to input terminal A of the transmitter.

This input signal is passed through a bandpass filter group (BPF) having n bandpass filters (B P F, -B P F,), each having a different center frequency and collectively forming a transmission band of audio frequencies. )1 and is frequency-divided. The frequency-divided band signals are input to a level detector 7 and a first switch circuit 2 that measure their respective energies. The first switch circuit 2 is switched according to a pattern for frequency replacement determined by the transmission controller 11. The n outputs of the first switch circuit 2 are respectively input to the frequency converters Ft''F- of the frequency converter group 3 having n+m frequency converters (F, ~Fa+F++1.~Fi1.) In this example, m=2.

The n+m outputs of the frequency converter group 3 are converted into n outputs by the second switch circuit 4, and are input to the output combiner 6 via the n bandpass filter group 5, which is the same as the BPF 1, where each output is output as a confidential signal to signal terminal B.
is output from.

The first switch circuit 2 is a switch group that performs frequency replacement of a band signal divided into n pieces, and n inputs are switched and connected to n outputs. The second switch circuit 4 is n
+m inputs (m-2 in the example of FIG. 1) are switched and connected to n outputs. This is because a synchronization signal and a dummy signal, which has no meaning as information but is used to increase the strength of the secret message, are mixed into the output secret message signal in the audio signal. In the example of FIG. 1, one channel each of the synchronization signal and the dummy signal is used, so m=2. Therefore, in this case, two of the n bands of the audio signal are replaced with a synchronization signal and a dummy signal, and are therefore not transmitted. This explanation will be given later using FIG. 3 and the like.

The synchronization signal is generated by the modulator 9 by inputting control information and synchronization information from the transmission controller 11, and is generated by the frequency converter (F
n. 1) is input. On the other hand, when using a narrowband pseudo-noise signal as the dummy signal as in the embodiment, the narrowband pseudo-noise signal generated by the narrowband noise generator 8 is input to the frequency converter (FI. 2). Frequency converter group 3
In addition to the above-mentioned signals, a local signal is inputted from the local signal generating section 10 and is frequency-converted into a signal of a desired band. The frequency of the local signal is specified by the transmission controller 11.

This designation is set so that the switch position of the first switch circuit 2 and the switch position of the second switch circuit 4 have an arbitrary relationship.

Next, the configuration of the receiving section shown in FIG. 2 will be described.

A confidential signal from the communication line is input to the receiving terminal C and is frequency-divided by a bandpass filter group (BPF) 21, which is the same as the BPFI and 5 in the transmitting section. The n outputs are input to the third switch circuit 22. The third switch circuit 22 performs the reverse switching connection of the third switch circuit 2 of the transmitting section, whereby the frequency-replaced and manually input signal returns to its original position. This signal is returned to the original frequency band by the frequency converter group 23, passes through the same bandpass filter group (BPF) 25 as the above-mentioned BPF 21, and is synthesized by the output synthesizer 26 to obtain the original audio signal in which the secret signal is restored. become. The synchronization signal added by the transmitter is sent to the third switch circuit 22,
The n+1st frequency converter in the frequency converter group 23 (
), and its output is demodulated into synchronization information and control information by a demodulator 29 via a synchronization signal bandpass filter (BPF) 2B, and then inputted to a reception controller 30. The reception controller 30 controls the third switch circuit 22 and the local signal generation circuit 27 based on this information. The dummy signal is controlled by the third switch circuit 22 so as not to pass its signal components, and the restored audio signal has a spectrum in which the signal band in which the dummy signal was inserted is missing. In the transmitting section and the receiving section, the insertion position of the synchronization signal changes from time to time according to mutually known rules, so it is possible for the receiving section to know in which frequency band the synchronization signal is inserted, and it is possible to perform initial synchronization. Can be pulled in.

Next, we will explain how the secret processing is executed with the above configuration.
This will be explained by expressing the audio signal on the frequency axis.

In the following description, the number of band divisions is assumed to be n=8, one synchronization signal and one dummy signal, and a total of m=2. In addition, the transmission frequency bandwidth of the audio signal is set to 300Hz to 270Hz.
It is assumed that the frequency is 0 Hz, and the bandwidth of eight band-pass filters that frequency-divide it are each 300 Hz.

FIG. 3 is a frequency spectrum for explaining confidential speech processing according to the present invention. FIG. 3(a) shows the spectrum of a human voice signal. Figure 3 (B) shows the divisions (1) to (8) and band signals ■ to ■ when the frequency is divided by the transmitter bandpass filter group 1. Figure 3 (C) shows the first An example of switching the switch circuit 2 is shown. Here, ■
An example is shown in which the band signals are swapped in the order of →■, ■→■, and so on. FIG. 3(d) shows the frequency spectrum of the output signal of the switch circuit 2. In this way, frequency permutation is performed. Therefore, it can be seen that in the third switch circuit 22 of the receiving section, the original signal can be obtained by performing the conversion from bottom to top in the reverse direction of the arrow shown in FIG. 3(C). The above is the principle of secret speech processing using frequency replacement.

Next, the insertion of the synchronization signal (2) and the dummy signal (2), etc., which are features of the present invention, will be explained.

For example, change the insertion position of the synchronization signal ■ to (7) in Figure 3 (b).
), if the energy of the band signal ■ in (7) is very large as shown in the diagram, removing it and inserting the synchronization signal ■ may cause a significant deterioration in the clarity of the restored voice. Conceivable. Therefore, in this case, the band signal (2) in a band with low signal energy, that is, the band (4) in FIG. 3(d), is removed and the band signal (2) is transferred, and at the same time, the synchronization signal (2) is made to carry that information.

On the other hand, there is a phenomenon in which envelope information is preserved, which is a factor that prevents the frequency replacement method from increasing the secret speech strength. As a countermeasure against this, the present invention adds insertion of a dummy signal and conversion of band signal energy.

First, the signal (2) in a band whose energy is small and does not significantly affect the intelligibility of the restored signal is removed, and the pseudonym signal (2) is inserted in its place. In this case, the receiving side requires information indicating in which band the dummy signal has been inserted, as well as synchronization information indicating the above-mentioned replacement information.

Next, regarding signal energy conversion, for example, if we set a rule that the signal level of the band with the maximum energy is always lowered by 10 dB, the band signal ■ in Figure 3 (a) becomes ■゛ in (e). . In this case, on the receiving side, information regarding the level indicating in which band the signal is lowered by 10 dB is required.

The information required on the receiving side is organized as follows.

B. Information indicating the band to which the original band signal of the band in which the synchronization signal ■ is inserted is to be changed.

Information indicating the signal band in which the b, da ξ-signal (ii) is inserted.

Ha, it was determined to have the maximum energy and the level was 10d.
B Information indicating the lowered signal band.

These are value-added information according to the present invention that is added to conventional subband replacement information. A above.

Since it is only necessary to specify the number of the signal band for each of the first and second bands, in this embodiment, it is one of the bands (1) to (8).
=8, so 3 bits each is sufficient as the amount of information. Therefore, if only I is added, 3 bits,
If any two of A, B, and C are added, it will be 6 bits, and if all of A, B, and C are added, it will be 9 bits.

In addition, the total number of subband replacement types (combinations) is 8.
! = 40,320 types, but it is said that the effective number of them is about 100 types empirically, so if 100 types are used, the amount of information of 2' = 128, that is, 7 bits is sufficient.

Therefore, a total of 16 bits of replacement information, including 7 bits as conventional subband replacement information and a maximum of 9 bits as additional replacement information added according to the present invention, may be transmitted as a synchronization signal.

The spectrum of the signal after the secret signal processing, in which the divided frequency bands are replaced, the dummy signal and the synchronization signal are inserted, the signal level is converted, etc., that is, the secret signal that appears at the output terminal B of the transmitter, is shown in Figure 3(e). It becomes as shown in . That is, the band signals are arranged as follows.

Further, the information that the synchronization signal (2) should carry is as follows.

B. Band signal replacement information ``For example, ■→■, ■→■...
・■→■” Information indicating the band (4) to which the signal ■ should be moved, which should exist at the insertion position of the synchronization signal.

C. Information indicating the dummy signal insertion band (1).

2. Information indicating the band (6) whose level is being converted.

Therefore, the audio signal spectrum restored by obtaining this information on the receiving side has a synchronization signal added with this information as replacement information, and a dummy signal inserted, as shown in Figure 3(f). This results in a spectrum in which the band signals of bands (5) and (8) are removed.

Since the energy of the original signals (2) and (3) in this cut band is small, the effect of deterioration in clarity is small.

Signal level conversion can be easily achieved by providing an attenuator or amplifier in the switch circuit.

If multilevel modulation is used as the modulation format of the synchronization signal, lHz
Since information can be transmitted at 2 bps or more per signal, if the bandwidth is 300 Hz and multilevel modulation of 2 bps/Hz is used, data at 600 bps can be transmitted. In the example described here, the total amount of information is 16 bits per frame, and if redundancy is added to make it about 40 bits per frame, one frame becomes 66 m5ec, which is about 1 syllable of speech (about 100 sec for vowels). The length is suitable for processing confidential information.

If the number of band divisions is further increased and the number of dummy signals to be inserted is increased, this can be handled by increasing the degree of multilevel modulation or by increasing the number of synchronization signals.

In addition, although the dummy signal used here is a narrowband noise signal, it is also possible to use a dummy signal that has the same characteristics as the synchronization signal, or a dummy signal that reproduces a part of the band of the audio signal. be.

(Effects of the Invention) According to the present invention, in a frequency division replacement secure speech device, by constantly changing the frequency band for inserting an important synchronization signal that carries information such as frequency replacement for confidential communications, an eavesdropper can use the synchronization signal. At the same time as increasing the difficulty of trying to detect and decipher the
A similar effect can be obtained by selecting the attributes of the dummy signal. Furthermore, by converting the level of a part of the frequency-divided band signal and making the synchronization signal carry information about the conversion, it is possible to make it even more difficult for an eavesdropper to decipher the confidential signal. Extremely large.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a secret device transmitter according to the present invention, FIG. 2 is a block diagram of a secret device receiver according to the present invention, and FIG.
The figure is a frequency spectrum explaining the invention in detail. 1, 5.21.25... BPF group, 2.4.22.
...Switch circuit, 3,23...Frequency converter, 6
．． 26... Output synthesizer, 7... Level detector, 8
... Narrowband noise generator, -9 ... Modulator, 10, 2
7... Local signal generation circuit, 11... Transmission controller,
28... BPF, 29... Demodulator, 30...
Receiving controller.

Claims (1)

[Scope of Claims] A plurality of band signals in which an analog voice signal is divided into a plurality of bands and arranged in mutual substitution on an analog voice communication line;
In the frequency division permutation secret communication method for transmitting replacement information indicating the contents of the permutation array and one of the plurality of band signals and the contents of changing the band to be replaced and inserted in a predetermined order, The replacement information includes a first item indicating that when the signal level of the band signal to be replaced in the band into which the replacement information is inserted exceeds a predetermined value, the band signal is replaced with a band of a band signal detected as another minimum level. additional replacement information, and a second band indicating a band in which at least one dummy signal is to be removed and replaced with a band signal detected as having a minimum signal level among the plurality of band signals.
and level conversion information indicating that the level of at least one of the plurality of band signals has been lowered or amplified compared to a predetermined value. A confidential communication system characterized by increasing confidentiality in the analog voice communication line.