RU2167497C2 - Method for masking of acoustic communication lines - Google Patents

Abstract

FIELD: electric communication systems, in particular, systems employing acoustical waves, applicable in masking of information acoustic signal against unauthorized recording of it, for example, with the aid of a dictating of it, for example, with the aid of a dictating machine. SUBSTANCE: method consists in generation of an amplitude-manipulated radio-frequency signal, its reception and forming of masking electric signal out of it. The received information acoustic signal is converted in an information electric signal out of it. The received information acoustic signal is converted in an information electric signal that is mixed with the masking electric signal and isolated in the terminal load. EFFECT: facilitated procedure. 2 cl, 5 dwg

Description

 The claimed technical solutions are united by a single inventive concept, relate to the field of electrical communication systems, namely, communication systems using freely propagating elastic waves (acoustic communication lines) and, in particular, can be used to build acoustic communication lines that exclude unauthorized listening (for example using microphone amplifiers) or documenting transmitted information (for example, using a voice recorder).

 A known method of masking communication lines, in which the source of the information signal, not subject to unauthorized listening, is spurious radiation of digital equipment. (see Application PCT (WO) N A1 90/00840, publ. 25.01.90,).

 The known method consists in generating an interference signal by pre-recording a set of characters that do not correspond to information symbols processed in digital equipment, randomly selecting them, modulating them with a high-frequency generator signal and emitting an electromagnetic interference signal. The interference signal “masks” the information signal, which complicates its unauthorized interception and listening (documentation). However, the known method is acceptable only for "masking" the information transmitted using electromagnetic waves and is unacceptable for acoustic communication lines.

 There is also a method of masking a sound information signal in the interior according to US Pat. USA N 4.052.564, IPC H 04 K 3/00, publ. 10/12/1977, which consists in the formation of an acoustic masking signal, its generation using a central generator installed above the source of the masked acoustic information signal and the creation of additional sources of the acoustic masking signal, which are stationary at several points in the room.

 The disadvantage of this method is the resulting uneven level of the masking signal in the room due to the presence of obstacles (partitions, cabinets, equipment, etc.), the inability to adapt the amplitude-frequency characteristics of the masking signal and, as a consequence, a higher probability of unauthorized listening ( documentation) information acoustic signal (IAS).

 Closest to the claimed is a known method of masking information acoustic signal according to US patent N 4.319.088 from 03/09/1982, IPC H 04 K 3/00.

 The known prototype method consists in preliminary monitoring of the noise level created in the premises of the IAS to be masked; the definition of indoor areas within which the level of IAS is the same; the formation of a masking acoustic signal (MAC), its emission in the centers of previously defined zones and, in addition, in the MAC radiation at additional points of each zone, the subsequent adjustment of the amplitude-frequency characteristics of the emitted MACs, the reception of IAS and MAC, their mixing and the allocation of the mixed signal in load.

 The IAS source can be a loud-speaking telephone, an intercom speaker or an individual (employee). In general, a mixed signal can be extracted through a microphone amplifier into a load, for example, for recording on a tape recorder.

 This method allows, due to the optimal placement of MAC sources and the possibility of regulating their amplitude-frequency characteristics, to adapt the masking system of acoustic communication lines with possible changes in their noise background, to exclude the effect of interfering objects in the room.

However, the prototype method has disadvantages: the fact of noise of the acoustic communication line has unmasking signs due to the fact that the masking signal is also acoustic;
relatively high energy costs for creating a masking signal, because its level is chosen based on the worst masking conditions without taking into account the actual situation in the room;
the implementation of the method requires the construction of expensive, bulky equipment and significant costs for its installation.

 A device for masking communication lines according to the application PCT (WO), IPC H 04 K 3/00, N A1 90/00840, publ. 01/25/1990, which includes a synchronizer that controls the operation of the device using signals generated in digital equipment protected from unauthorized access, a storage device, a microprocessor, a digital-to-analog converter, the output of which is formed by modulating signals for a transmitter emitting masking signals through an antenna signals.

 However, the known device is unacceptable for masking an information signal transmitted using elastic waves.

 A masking device for an acoustic communication line according to US Pat. USA N 4.052.564, IPC H 04 K 3/00, opul. 10/12/1977, containing a MAC generator, an amplifier, an equalizer and one or more MAC emitters (speakers) installed above the sources of acoustic information signals that are subject to masking.

 However, the known device is not effective enough, because the predetermined amplitude-frequency parameters of the MAC cannot be adapted to possible changes in the levels of masked signals and, in addition, the efficiency of the device is reduced due to the interfering action of objects in the room.

 Closest in its technical essence to the claimed is a known device for masking an acoustic communication line located in rooms, according to US Pat. USA N 4.319.088, IPC H 04 K 3/00, publ. March 9, 1982

 The prototype device consists of several main blocks of a masking acoustic signal, each of which is connected to several additional blocks of a masking acoustic signal (MAC). Each MAC main unit with additional MAC units connected to it is installed in the subceiling space of the room within the area with approximately the same level of acoustic information signals (IMS) to be masked. The MAC main unit consists of a MAC oscillator, an amplifier, an equalizer, and an output speaker. Each additional MAC unit includes an output speaker driven from the main MAC unit by connecting a cable to it. The main and additional units are equipped with MAC parameter controls with ports for connecting alternative MAC sources.

 In general, a device also includes a MAC and IC mixer connected to a terminal device, which can be a microphone connected to a sound recorder.

 The known device provides the ability to adapt the amplitude-frequency characteristics of the MAC to the changing operating conditions of masked acoustic communication lines, less susceptible to the shielding effect of the equipment located in the room.

However, the prototype device has disadvantages:
possesses unmasking signs of masking the acoustic communication line, because sound signals are used for this purpose;
relatively high energy costs to create the required MAC level;
the device is cumbersome and requires significant material costs for its installation in the room.

 The aim of the claimed invention is to develop a method and device for masking an acoustic communication line, eliminating unmasking signs of the fact of masking, requiring less material and energy costs when they are implemented and used, and also able to flexibly adapt to possible changes in the operating conditions of the masked acoustic communication line.

 In the claimed method, the goal is achieved by the fact that in the known method of masking an acoustic communication line, which consists in receiving an information acoustic signal, generating a masking signal, mixing the information and masking signals and extracting the mixed signal into a final load, the received information acoustic signal is converted into an information electric signal (IES). At the same time, an amplitude-manipulated high-frequency (AM HF) signal is additionally generated and emitted and received. Then, the received AM RF signal is converted into a masking electrical signal (MES) by preliminary selection, detection and differentiation. Then the IES and MES are mixed.

 To generate an AM RF signal, a continuous harmonic signal is generated, which is manipulated by a random train of pulses.

 The listed new set of essential features provides masking in the final load of the received informational acoustic signal, which is then converted into an informational electric signal without showing unmasking signs of masking due to the formation of a masking signal from the amplitude-manipulated RF signal in the frequency range of the speech spectrum of the information signal. The implementation of this method does not require any additional equipment premises.

 In the claimed device, the goal is achieved by the fact that in the known device for masking an acoustic communication line containing a source of information acoustic signal and a mixer connected to the final load, an electromagnetic converter installed at the receiving end of the acoustic communication line, a nonlinear conversion unit, an amplitude manipulated RF signal, the output of which is connected to a transmitting antenna. The output of the electro-acoustic transducer is connected to the input of the nonlinear conversion unit, the output of which is connected to the input of the mixer.

 The amplitude-manipulated RF signal generator consists of a random pulse train generator, a harmonic high-frequency signal generator, mixer and amplifier, the output of which is the output of the amplitude-manipulated RF signal generator. The outputs of the generators of a random pulse sequence and a harmonic high-frequency signal are connected respectively to the first and second inputs of the mixer, the output of which is connected to the input of the amplifier.

 The nonlinear conversion unit consists of a cascade-connected pre-selector, detector and differentiating circuit. The input of the preliminary selector and the output of the differentiating circuit are respectively the input and output of the nonlinear conversion unit.

 The indicated new set of essential features provides effective masking of the acoustic communication line due to the possibility of generating, using the introduced generator, an amplitude-manipulated RF signal and a nonlinear conversion unit in which the received AM RF signal is converted into a masking electric signal in the frequency range of the speech spectrum.

 The analysis of the prior art made it possible to establish that analogues that are characterized by a combination of features identical to all the features of the claimed technical solutions are absent, which indicates the compliance of the claimed inventions with the condition of patentability "novelty".

 Search results for known solutions in this and related fields of technology in order to identify features that match the distinctive features of the claimed invention transformations to achieve the specified technical result. Therefore, the claimed invention meets the condition of patentability "inventive step".

The claimed objects of the invention are illustrated by drawings, which show:
in FIG. 1 - diagrams explaining the principle of implementation of the claimed method;
in FIG. 2 is a general block diagram of a masked acoustic communication line;
in FIG. 3, 4 - the results of experimental studies that determine the requirements for the parameters of the elements of the device.

 The possibility of implementing the claimed method is explained as follows.

 When a source excites in a room, for example, an electro-acoustic transducer (loudspeaker) or a person’s voice, sound in the spectrum of audible frequencies (from 16 Hz to 20 kHz), which is an information acoustic signal, elastic waves arise that are perceived by an electro-acoustic receiver (for example, a microphone ), at the output of which an electrical signal is formed, which is the sum of the harmonic components characterized by frequency, phase and amplitude. The shape of such a signal is shown in FIG. 1 a. Unauthorized documentation of this information signal can be eliminated by distorting its frequency-amplitude spectrum.

 In the claimed method, this is achieved by forming an additional masking electrical signal, the frequency spectrum from which covers the frequency spectrum of the information signal. The formation of a masking signal is achieved as follows.

A continuous continuous RF harmonic signal is generated at a frequency f ₀ (Fig. 1, b), and a random sequence of pulses, the duration of which and the time intervals t between them vary in time randomly (Fig. 1, c). Then, an RF harmonic signal is manipulated with a random pulse sequence, thus forming an amplitude-manipulated RF signal (Fig. 1d), which is emitted in the direction of receiving the acoustic information signal.

 Next, the received AM RF signal after preliminary selection is detected, restoring a random sequence of pulses (Fig. 1, e). The result of differentiating this sequence is a collection of short pulses, the position of which on the time axis corresponds to the position of the leading and trailing edges of the pulses of a random sequence (Fig. 1, e). The repetition rate of short pulses in time varies randomly. Consequently, the amplitude-frequency structure of the spectrum of this sequence also randomly changes. If the repetition rate of short pulses varies in the frequency range spanning the frequency spectrum of the information signal, then after mixing the information signal (Fig. 1a) and a random sequence of short pulses (Fig. 1e), which is actually a masking signal, the mixed load the signal will be a disguised information signal (Fig. 1, g).

The experimental studies of the masking of the acoustic line (Fig. 2), in which the source of the information acoustic signal (IAS) creates a sound pressure at the receiving point equivalent to the pressure created by a person’s conversation, and the acoustic wave receiver (portable voice recorder) is located at a distance R = 1-5 m, the distinguishability index (a parameter determined on the basis of expert estimates by the ratio of the number of correctly and incorrectly received words of the transmitted information) of the information signal at the final load ( Recording on the magnetic tape) is provided in the range of 0.1. . . 0.2 when reaching those shown in FIG. 3 values of the equivalent isotropically radiated power P _EIRP = P _A G _{i of the} amplitude-manipulated RF signal. P _A is the power supplied to the antenna; G _i is the isotropic gain of this antenna. Removal D was selected within 3-5 m. In FIG. Figure 3 also shows the P _EIRP values when using linear and rotating polarization antennas. Due to the equally probable orientation of the receiver at the same values of P _EIRP, masking will be more effective when using electromagnetic waves of rotating polarization. According to the graph in FIG. 3, it is possible to determine specific requirements for the parameters of technical devices that implement the method, depending on the conditions for organizing the acoustic communication line in the room.

In FIG. Figure 4 also shows the experimentally obtained dependence of the distinguishability index on the carrier frequency f ₀ AM of the RF signal (Fig. 1b, d).

The studies were performed using a wide class of voice recorders produced by leading companies of the world as receivers of an acoustic communication line. From the data obtained it follows that the minimum level of the distinguishability index is achieved at f ₀ = 400-600 MHz. So, at P _A G _i = 50 W; D = 3 m, R = 2 m and sound pressure at the point of reception of the acoustic signal corresponding to the pressure created by the person’s conversation, the distinguishability index is 0.1 - 0.2 at f ₀ = 400 ... 600 MHz, which corresponds to guaranteed masking signal. Moreover, the specified interval f ₀ minimizes the distinguishability index for any values of P _A G _i ; D; R.

 To achieve the best conditions for masking the information signal, the clock frequency of the pulse sequence should be selected in the range of 10-20 kHz.

Thus, an acceptable level of the distinguishability index is achieved with relatively low levels of P _EIRP and the complete absence of unmasking signs of the masking of the information acoustic signal.

 The claimed device shown in FIG. 5, consists of a source 1 of an acoustic information signal remote from the electro-acoustic transducer (receiver) 2 by a distance R, a nonlinear conversion unit (BNP) 3, a mixer 4, a final load 5, a generator 6 of an amplitude-manipulated RF signal, to the output of which a transmitting antenna 7, located at a distance D from the electro-acoustic transducer (EAP) 2.

 The output of the EAA 2 is connected to the input of the BNP 3, the output of which is connected to the input of the mixer 4, the output of which is connected to the final load 5.

 The amplitude-manipulated RF signal generator 6 can be implemented in various ways, in particular, its block diagram shown in FIG. 5, consists of a generator 6.1 harmonic high-frequency signal and a generator 6.2. random sequence of pulses, the outputs of which are connected respectively to the first and second inputs of the high-frequency mixer 6.3. whose output is connected to the input of the amplifier 6.4. Amplifier output 6.4. is the output of the amplitude-manipulated RF signal generator.

 BNP 3 can also be performed in various ways, in particular its structural diagram shown in FIG. 5, consists of a cascade-activated pre-selector 3.1, detector 3.2 and differentiating circuit 3.3. Preselector input 3.1. and the output of the differentiating circuit 3.3. are respectively the input and output of the BNP 3.

 As the source 1 of the information acoustic signal, any electro-acoustic transducer that converts electromagnetic energy into elastic wave energy, for example, a loudspeaker, can be used. Also, the source of information acoustic signal can be considered information that comes verbally from a person.

 As an EAP 2, a converter can be used, the action of which is based on the principle of electromagnetic induction, for example, any microphone.

 A feature of the claimed device is that as the BNP 5, the electronic unit of the recorder itself can be used, containing respectively the equivalent preliminary selector 3.1, detector 3.2 and differentiating circuit 3.3. The mixer 4 can be considered the load of the output stage of the recorder, which forms all the output signals that have passed BNP 3, including information and masking.

 As the final load can be used to document the output signal, for example, on a tape.

 The elements included in the generator 6 of the amplitude-manipulated RF signal are known and described in the corresponding information sources. So as a generator 6.2 of a random sequence of pulses can be used a well-known random generator of pulses according to ed. testimonial. USSR N 1116524 A, IPC H 03 B 29/00, publ. 09/30/84, the clock frequency of the pulse sequence of such a generator should be selected in the range of 10-20 kHz.

 The scheme and principle of operation of the generator 6.1. harmonic oscillations is known and, in particular, is described in the book: Goroshkov B.I. Elements of electronic devices. Directory. - M.: Radio and Communications, 1989, p. 107, Fig. 7.27.

 The scheme of the high-frequency mixer 6.3, in which the amplitude manipulation of the HF harmonic oscillation is also known, and, in particular, is described in the book: Kachanov V.I. Transistor Transmitters. Ed. 2. - M.: Energy, 1976, p.397, Fig. 10-14b.

The parameters of the amplifier 6.4 (power at its output P _A ) and transmitting antenna 7 (polarization and gain - G _i ) are selected based on the specific placement conditions of the acoustic communication line, i.e. from the values of R, D using the graph data in FIG. 3.

 As the amplifier 6.4 and the transmitting antenna 7, any known technical solutions can be used, and their specific choice is determined by the operating conditions of their application.

 The claimed device operates as follows.

 When an acoustic signal is received by a radio-electronic means (RES) containing an EAP (for example, a microphone) and a low-frequency (LF) amplification path, the acoustic signal is converted into an electric signal with subsequent amplification. Moreover, for acoustic and electrical signals having a frequency spectrum in the range of 16 Hz - 29 kHz, both the EAP and the amplification path are linear, i.e. transform them without distortion. At the same time, for an RF signal, for example, in the region of 200-800 MHz, such a RES represents a nonlinear path. Therefore, the RF signal induced in it will undergo nonlinear distortion. Those. in this case, the RES can be considered for the RF signal as a block of nonlinear conversion.

 If you select the structure of the RF signal such that, after its nonlinear conversion, the frequency components in the spectrum of the speech signal prevail (in energy), then the converted RF signal will be a masking signal of the information LF signal.

This principle underlies the operation of the claimed device. When organizing the operation of a masked acoustic communication line, in which the source 1 of the acoustic information signal is located at a distance R from the EAP 2 connected through the BNP 3 and the mixer 4 to the terminal load 5, the necessary level P _{EIRP is} preliminarily estimated for reliable masking of the information signal. Based on the results of the assessment, the required level P _EIRP is established based on the data shown in FIG. 3 and 4. The value of P _EIRP can be controlled by the following parameters: output power P _A , transmit antenna gain G _i and its polarization. In the considered acoustic communication line, the equipment at the receiving end is installed covertly or explicitly for the purpose of unauthorized documentation of the information signal. The equipment for the emission of the amplitude-manipulated RF signal is installed discreetly from the user of unauthorized documentation equipment. Simultaneously with the excitation of elastic waves of the acoustic signal (the beginning of negotiations), the generator 6 of the amplitude-manipulated RF signal is turned on, the transmitting antenna 7 is guided by the maximum of the radiation pattern (DN) 8 to the installation site of the EAP 2.

 For any electronic means of documenting the signal from the output of the EAP 2, this means contains the equivalent ones: a preliminary selector 3.1, a detector 3.2, and a differentiating circuit 3.3, which form the BNP 3. In BNP 3, physical processes occur with an induced amplitude-manipulated RF signal, as shown in the description way. The result of these physical processes is the formation of a masking signal at the output of the BNP 3. At the same time, the received acoustic information signal and converted into an informational electronic converter 2 into an informational electric signal without distortion is amplified in the BNP 3 path, because its circuit has linear gain characteristics for the frequency spectrum of the information electric signal. A linear converter for this signal is EAP 2.

 The use of an amplitude-manipulated RF signal with a random sequence of pulses for generating a masking signal provides, with minimal energy costs and the simultaneous use of unauthorized documenting equipment (for example, a voice recorder), for generating a masking signal a high degree of information masking without unmasking signs of protection of this information. The device is cheap to manufacture, does not require any modifications to the room in which it is used.

Claims (2)

 1. The method of masking the acoustic communication line, which consists in receiving an information acoustic signal, generating a masking signal, mixing the information acoustic signal and the masking signal and extracting the mixed signal in the final load, characterized in that before the information acoustic signal and the masking signal are received, the information acoustic signal at the receiving point, they are converted into an information electric signal in the spectrum of audible frequencies, and the masking signal forms by generating and emitting in the direction of the receiving point of the amplitude-manipulated high-frequency signal, which is received at the receiving point and after preliminary selection is detected, restoring a random sequence of pulses, the differentiation of which is a random sequence of short pulses, which is a masking signal, the frequency spectrum of which covers the frequency spectrum of information electrical signal.  2. The method according to claim 1, characterized in that for the formation of the amplitude-manipulated high-frequency signal, a continuous harmonic high-frequency signal is generated, which is manipulated by a random sequence of pulses.

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