Suppression of voice recorders – possibilities and practical application.

#Suppression of voice recorders

Suppression of voice recorders — possibilities and practical application.

Iskhakov Boris Simukovich
Kargashin Viktor Leonidovich, Candidate of Technical Sciences
Yudin Leonid Mikhailovich, Candidate of Technical Sciences

At present, despite the prohibitions in force in accordance with Russian legislation, the probability of leakage of confidential information organized by means of technical means is very high.

Therefore, the problem of preventing unauthorized removal of confidential speech information is still very acute.

This article examines methods of counteracting unauthorized removal of speech information using voice recorders, which, from the consumer's point of view, are the simplest devices that do not require special training to work with them.

Thanks to the rapid development of electronics, small-sized voice recorders have high performance characteristics, allowing you to record information with high quality in the most difficult acoustic conditions in automatic mode.

The variety of types of voice recorders from analog devices for general use to professional digital ones allows you to meet any consumer requirements for the planned events.

The following basic methods of protection against unauthorized recording of speech information using a voice recorder are possible:

• organizational measures that allow the voice recorder to be confiscated if an attempt is made to bring it into the premises;
• detecting it during “work”;
• precluding the possibility of recording speech information.

Organizational measures that prevent an attempt to bring a voice recorder into the premises can be used in very limited cases, and the effectiveness of their use can be very low.

Given technological advances, this type of technical equipment can be camouflaged as almost any object.

In addition, while respecting current legislation, the methods for detecting a dictaphone on a visitor are very limited.

Detecting a working dictaphone is also not an easy task.

To this day, discussions between experts about the choice of effective criteria for detecting modern dictaphones do not subside.

An analysis of devices recommended by the modern market for this purpose showed that the probability of detecting a working voice recorder differs significantly from 100%.

The possibility of recording speech information can be eliminated in several ways:

• use special intercoms;
• equip a room for negotiations;
• form a screen in the form of an electromagnetic interference field.

In our opinion, from the point of view of quality and price, a more effective way to prevent information leakage at present is an electromagnetic interference field created by a special device — a dictaphone suppressor.

A dictaphone, as an electronic device, can be represented as two subsystems:

• a subsystem for converting an acoustic signal into an electrical one;
• a subsystem for storing information.

A subsystem for converting an acoustic signal into an electrical one usually consists of:

• Microphone, most often an electret type.
• A low-frequency signal amplifier. Modern voice recorders usually use amplifiers with a logarithmic characteristic or with automatic gain control (AGC). They have a large dynamic range in terms of input signal level, which is one of the indicators of the class of the device.
• Analog-to-digital converter (ADC). It is used only for digital voice recorders that use electronic devices or magnetic tape as a memory block.

The acoustic signal conversion subsystem is most susceptible to external electromagnetic interference.

Almost every element of this subsystem can be taken out of normal operation by induced interference.

Electrical signals are sent from the microphone output to the amplifier, the level of which, at best, is measured in fractions of a millivolt.

Interference with additive superposition can suppress the useful signal.

Amplifiers with AGC or logarithmic characteristics are usually implemented with strong feedback, characterized by constant response and recovery times.

Therefore, the effect of interference exceeding the level of the useful signal can suppress it, loading the input stages of the amplifier.

That is, the useful signal can be below the threshold of the real sensitivity of the amplifier. For this purpose, pulse interference correlated with the speech signal is most effective.

The ADC is also subject to interference in two ways:

• ADCs used in voice recorders are usually designed for an input signal level significantly exceeding the noise level. At a signal/noise ratio of ? 1, the converter switches to a nonlinear mode, significantly worsening this ratio at the output. Therefore, interference entering the speech signal band causes a failure in the ADC.
• The ADC requires high-quality synchronization by clock frequency. Interference induced in the clock frequency generator circuit will take the ADC out of synchronization mode. In this case, the voice recorder will not work as an information storage device.

It should be noted that professional digital voice recorders have very serious design and circuit methods for protecting units subject to interference. Therefore, when studying voice recorders, it was noted that the effect of interference on some digital voice recorders was weakened compared to analog ones.

Factors Affecting the Susceptibility of a Voice Recorder to Electronic Suppression

A dictaphone is a rather complex object of suppression from the point of view of electrodynamics.

The magnitude of the interference signal induced on the electrical circuits of the dictaphone's recording path is determined by the intensity of the interference electric field in the area of ​​maximum susceptibility of the dictaphone's electronic circuit.

This area is located in the area of ​​the input of the low-frequency amplifier of the recording path.

The losses associated with the conversion of the electrical field strength of the interference signal into the induced EMF of the interference can be estimated by representing the receptivity zone as a conductor with a certain effective height.

Considering the small physical dimensions of the circuit elements and the conductors connecting them, the equivalent expected effective height of the receptivity zone is much less than the wavelength of the interference irradiating the dictaphone and is estimated to be approximately 0.2…1 cm.

Further conversion of the interference signal into a low-frequency AM signal occurs as a result of direct detection on nonlinearities inherent in any electronic circuit.

The induced low-frequency interference signal is amplified together with the speech signal and recorded on the storage medium.

Depending on the interference/signal ratio at the output of the recording amplifier, the following is observed:

• distortion of the speech signal, leading to deterioration in its intelligibility when listening to the recording;
• complete suppression of the speech signal, for example, due to overloading of the amplifier stages with interference.

Speech signal distortion can occur at a noise/signal ratio close to unity and depends on the frequency-time structure of the low-frequency noise signal and its correlation with the speech signal.

For complete suppression of the speech signal, the level of induced noise must significantly exceed the voltage of the speech signal.

Dictaphones in a plastic case without special shielding measures are most susceptible to interference.

The interference signal directly penetrates the circuit elements of the voice recorder with virtually no additional attenuation in the case material.

Voice recorders in metal cases are better protected from interference due to the shielding of the interference signal by the metal case.

The electromagnetic wave of interference, when interacting with the metal case of the voice recorder, is practically reflected from its surface.

However, part of the energy is induced on the wall of the case and penetrates into the internal area of ​​the voice recorder.

The level of high-frequency electromagnetic interference field reaching the electronic components of the voice recorder depends on the frequency of the field, the elements of the case design and the properties of the material from which it is made.

In practice, it is quite rare to find a case in the form of a solid screen.

The dictaphone must have the ability to replace internal elements and connect external power supply, input and output information, connect an external microphone, monitor operating modes, change microcassettes or electronic memory devices.

All this leads to the presence of holes, cracks, covers in the walls of the case, which are its electrical inhomogeneities.

The presence of breaks in the electrical homogeneity of the walls of the dictaphone case leads to a significant disruption of the structure of induced surface currents and to the flow of these currents into the case.

As a result, zones with an increased level of electromagnetic field intensity appear inside the dictaphone case.

The size and shape of the casing of the voice recorder significantly affect the level of penetrating interference signal. The larger the area of ​​the voice recorder, the higher the level of interference signal pickup.

In order to assess the effectiveness of shielding the external field by different types of casings, comparative experimental studies were conducted on the susceptibility of voice recorders with plastic, combined and metal casings to external interference.

The results of the experimental studies are presented in Table 1.

The comparison of the shielding efficiency was carried out by the value of the power flux density of the electromagnetic field of the interference signal required for complete suppression of the dictaphone. The radiated power of the suppressor was the same.

The Casio TR-15 dictaphone in a plastic case was taken as a reference dictaphone, as the most susceptible to the action of external electromagnetic interference.

The presented results show that the susceptibility to interference depends on both the design and the material of the recorder case.

The compact Sony M909 recorder with a metal case and analog recording on magnetic tape requires 28 dB more interference power for its suppression than the reference Casio TR-15 recorder in a plastic case.

The U 7201 digital recorder, in the design of which increased shielding and protection measures are taken, is better protected from interference.

For its complete suppression, the interference power must be increased by 32 dB compared to the reference recorder.

Voice recorders with combined plastic and metal housings occupy an intermediate position.

Table 1.
Comparative characteristics of the efficiency of shielding external interference by the housing of different types of voice recorders

Effect of the carrier frequency of electromagnetic radiation of a voice recorder on its susceptibility to interference

The complex structure of the case and the dimensions of the voice recorder, the features of the element base used in it, the method and quality of installation determine the resonant frequency nature of the effect of interference on the voice recorder.

As experimental studies of the effectiveness of interference on voice recorders in various parts of the radio frequency range show, there are clearly defined resonant areas where the effect of interference is maximum.

Fig. 1 shows the characteristic dependencies of the relative sound intensity on the induced interference signal for some types of voice recorders.

The given dependencies were obtained by changing the interference frequency in a wide frequency range from hundreds of MHz to several GHz and were constructed taking into account the normalization of the intensity of the induced sound interference signal relative to the maximum level, which occurs in the low-frequency region for each voice recorder.

As follows from Fig. 1, the studied voice recorders have different resonant regions of susceptibility. The largest number of such resonant regions is observed in the compact Sony M909 voice recorder in a metal case.

At the same time, there is no increase in the efficiency of the voice recorder's suppression when moving to higher-frequency resonant regions.

This is due to the fact that, despite the presence of higher-frequency resonant regions with less attenuation of interference in the casing of the voice recorder, with increasing frequency, interference losses on the propagation path increase.

Common to all the voice recorders considered is the presence of resonant regions of close frequency in the low-frequency part of the range.

Fig. 1. Dependence of the relative level of the recorded acoustic tone on the carrier frequency of the amplitude-manipulated interference for:

The frequency dependence of the influence of interference for different types of voice recorders manifests itself differently.

The effect of interference depends on two main factors:

1) the degree of shielding of external electromagnetic signals by the case;
2) the resonant properties of the voice recorder design.

For voice recorders whose cases have high shielding properties, the effect of interference is determined mainly by the second factor.

The effect of interference on voice recorders with poor shielding is determined primarily by the level of currents induced on the conductive elements of the voice recorders.

In all cases, this dependence has a sharply individual character, which is confirmed by the results of experimental studies (Fig. 1).

The most typical dependence of the effect of interference on the voice recorder in a metal case Sony M909. In this case, the first resonance, characterized by the highest susceptibility to the effect of interference, occurs at a wavelength corresponding to the perimeter of the side wall of the voice recorder.

All other resonances have a lower relative level and appear at frequencies that are multiples of the frequency of the first resonance.

For the Sanyo TRC-6200 voice recorder, the dimensions of which are somewhat larger than those of the Sony M909 voice recorder, the first resonance shifts toward lower frequencies. Resonances at multiples, higher frequencies, are expressed less strongly than for the Sony M909 voice recorder.

In Sanyo TRC-520M dictaphones with a plastic case and Perlcorder S302 with a combined plastic and metal case, the frequency dependence of the interference signal is formed by resonances of induced currents on the set of internal conductive elements of the dictaphones.

For the Sanyo TRC-520M dictaphone, the greatest resonance in intensity appears at a frequency slightly lower than 2 f0. The second, but slightly weaker — at a frequency of 2.6 f0, the third, stronger than the second — at a frequency of 5 f0.

For the Perlcorder S-302 voice recorder, the frequency dependence of the interference is manifested in a wide frequency band (1…2) f0. A weak resonance region is observed at frequencies (6.5…7) f0.

If in the low-frequency range, resonances of different voice recorders have overlapping frequency regions, then in the high-frequency region there are much fewer such coincidences or they are simply absent.

Therefore, to ensure suppression of voice recorders in the absence of a priori information about the type of voice recorder used, the operating range of emitted interference signals must overlap the regions of maximum susceptibility of all voice recorders.

Such interference can be implemented using a multi-frequency interference signal or by frequency sweeping in a given frequency range.

An important characteristic of a voice recorder jammer is the spatial suppression zone, limited by the width of the antenna beam in azimuth and elevation, as well as the maximum suppression range.

If the voice recorder is within the suppression zone, then it cannot be used to carry out unauthorized recording of speech information.

Due to the fact that the shielding level of different models of voice recorders differs from each other, the maximum range of their suppression is different.

Usually, the maximum range of a voice recorder suppression is determined experimentally for a specific suppressor.

Since this value depends on the design features of the voice recorders, then with constant characteristics of the suppressor (radiated power and interference structure), voice recorders with a plastic case, as expected, will be suppressed at greater distances compared to voice recorders in metal cases.

Table 2 shows the experimentally obtained relative values ​​of the maximum suppression range of analog voice recorders with different types of cases, normalized to the maximum suppression range of a voice recorder in a plastic case.

As the shielding efficiency of the case increases, the relative suppression range of voice recorders decreases significantly.

A similar picture is observed for digital voice recorders (Table 3).

Using an external microphone or remote control increases the range of the voice recorder suppression, which is especially noticeable when suppressing digital voice recorders with a highly effective shielding case.

Table 2.
Relative suppression range of analog voice recorders with different cases

Note: * with remote microphone.

Table 3.
Relative suppression range of digital voice recorders with different housings

Note: * with a remote microphone.

Features of the construction of dictaphone jammers.

For electronic jamming of dictaphones, portable equipment is usually used, placed inside an attaché case or briefcase, or stationary equipment built into interior items (table, chair, etc.).

Currently, two main schemes are used for constructing equipment for electronic jamming of dictaphones.

In the first scheme, the interference signal is generated at a fixed frequency directly at a high power level using a powerful noise generator.

The width of the interference spectrum in such suppressors is determined by the ability of the output semiconductor device to generate powerful noise signals.

The disadvantage of such suppressors is the limited ability to generate noise in a wide frequency band.

In addition, the expansion of the interference spectrum in order to cover the full frequency range of maximum susceptibility to interference of known types of dictaphones leads to a decrease in the spectral density of the interference power and, as a result, to a decrease in the efficiency of suppression of dictaphones.

In the second scheme, the interference signal is formed at a low power level using a special interference signal generator with subsequent amplification to the required power level in a broadband power amplifier (Fig. 2).

The advantage of this design of the equipment is its high versatility in terms of interference types and modes, ensuring high spectral density of interference power in a given frequency range, and the ability to ensure high suppression efficiency using low-power amplifiers by constructing an active phased antenna array on them.

Fig. 2 Structural diagram of a portable voice recorder suppressor with a power amplifier.

In general, a portable voice recorder jammer consists of an antenna system, a power amplifier, a noise generator, and a power source.

In the simplest case, the voice recorder jammer antenna system can be a single emitter of the corresponding frequency range, forming an electromagnetic field in the required spatial sector.

The solid angle of the suppression zone is usually not less than 60°.

In other cases, the antenna system can be dual-channel, for example, for emitting elliptically polarized interference signals.

The most promising is the use of an active phased antenna array on low-power amplifiers, which allows for the most effective solution to the problem of interference radiation in a given spatial sector and ensuring the required energy potential, including with a conformal array design, ensuring the best thermal conditions for the equipment and, as a result, having high reliability in suppressing voice recorders.

In order to ensure long-term continuous operation in a stationary position or in a car, a portable suppressor usually has the ability to connect to an external AC power source or to a car battery.

The interference generator generates an amplitude-shifted high-frequency signal in a given frequency range.

Amplitude manipulation is set by a microprocessor pre-programmed to form a pulse sequence of the required structure.