Suppression of voice recorders — possibilities and practical application..

Апр 27, 2024
podavlenie diktofonov vozmojnosti iprakticheskoe primene

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


Currently, despite the prohibitions in force in accordance with Russian legislation, the likelihood of leakage of confidential information organized using technical means is very high. Therefore, the problem of preventing unauthorized recording of confidential speech information is still very acute. This article discusses ways to counter unauthorized recording 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 that allow you to record information with high quality in the most difficult acoustic conditions in automatic mode. The variety of types of voice recorders from widely used analogue devices to professional digital ones allows you to satisfy any consumer requirements for planned events.

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

  • organizational measures that make it possible to remove the recorder when trying to bring it into the premises;
  • detecting it during “work”;
  • excluding the possibility of recording speech information.

Organizational measures that prevent an attempt to bring a voice recorder into a room can be used in very limited cases, and the effectiveness of the application can be very low. Taking into account technological advances, this type of technical means can be camouflaged to look like almost any object. In addition, while respecting current legislation, the methods for detecting a voice recorder on a visitor are very limited.

Detecting a working voice recorder is also not an easy task. To this day, discussions among experts about the choice of effective criteria for detecting modern voice recorders continue. An analysis of devices recommended by the modern market for this purpose has shown that the probability of detecting a working voice recorder is significantly different from 100%.

Eliminating the possibility of recording speech information can be done in several ways:

  • use special intercom devices;
  • equip a meeting room;
  • form a screen in the form of an electromagnetic interference field.

In our opinion, in terms of quality and prices, a more effective way to prevent information leakage is currently an electromagnetic interference field created by a special device — a voice recorder suppressor.

A voice recorder, as an electronic device, can be represented in the form of two subsystems:

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

The subsystem for converting an acoustic signal into an electrical signal, as a rule, consists of:

  • Microphone, most often — electret type.
  • LF 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 the input signal level, which is one of the indicators of the class of the device.
  • An analog-to-digital converter (ADC). It is used only for digital voice recorders that use electronic devices or magnetic tape as a memory unit.

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 using 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. Additive interference can suppress the desired signal.

Amplifiers with AGC or logarithmic characteristics are usually made with strong feedback, characterized by constant response and recovery times. Therefore, exposure to noise that exceeds the level of the desired signal can suppress it, loading the amplifier's input stages. Those. the useful signal may be below the real sensitivity threshold of the amplifier. For this purpose, pulsed noise correlated with the speech signal is most effective.

The ADC is also susceptible to interference in two ways:

  • ADCs used in voice recorders are usually designed for an input signal level significantly higher than the noise level. With a signal/noise ratio of ? 1, the converter goes into a nonlinear mode, which significantly worsens this output ratio. Therefore, interference entering the speech signal band causes a malfunction of the ADC.
  • The ADC requires high-quality synchronization at the clock frequency. Interference induced on the clock 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 circuitry methods for protecting components susceptible to interference. Therefore, when studying voice recorders, a weakening of the effect of interference on some digital voice recorders compared to analog ones was noted.

Factors influencing the susceptibility of a voice recorder to electronic jamming

A voice recorder is a rather complex suppression object from the point of view of electrodynamics. The magnitude of the interference signal induced on the electrical circuits of the recorder's recording path is determined by the strength of the interference electric field in the region of maximum susceptibility of the recorder's electronic circuit. This area is located near 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 interference can be estimated by representing the receptive zone in the form of a conductor with a certain effective height. Taking into account the small physical dimensions of the circuit elements and the conductors connecting them, the equivalent expected effective height of the receptive zone is much less than the wavelength of the interference irradiating the recorder and is estimated to be approximately 0.2…1 cm.

Further conversion of the interference signal into low-frequency AM -the signal occurs as a result of direct detection of nonlinearities inherent in any electronic circuit. The induced low-frequency interference signal is amplified together with the speech signal and recorded on the 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 of its intelligibility when listening to the recording;
  • complete suppression of the speech signal, for example, due to overloading the amplifier stages with interference.

Distortion of the speech signal can occur when the interference/signal ratio is close to unity and depends on the time-frequency structure of the low-frequency interference signal and its correlation with the speech signal. To completely suppress the speech signal, the level of induced interference must significantly exceed the voltage of the speech signal.

Voice recorders 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 housing material. Voice recorders in metal cases are more 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 body of the voice recorder is practically reflected from its surface. However, some of the energy is induced at the housing wall and penetrates into the internal area of ​​the voice recorder. The level of high-frequency electromagnetic interference field that reaches the electronic components of the voice recorder depends on the frequency of the field, the structural elements of the housing 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 continuous screen. The recorder must be able to replace internal and external power supply elements, input and output information, connect an external microphone, control operating modes, change microcassettes or electronic memory devices. All this leads to the presence of holes, slots, and covers in the walls of the housing, which are its electrical inhomogeneities. The presence of breaks in its electrical homogeneity in the walls of the voice recorder body leads to a significant disruption of the structure of induced surface currents and to the flow of these currents into the body. As a result, zones with an increased level of electromagnetic field strength appear inside the voice recorder body.

The size and shape of the voice recorder body significantly influence the level of the penetrating interference signal. The larger the area of ​​the voice recorder, the greater the level of interference signal. In order to evaluate the effectiveness of shielding of the external field by various types of housings, comparative experimental studies were carried out on the susceptibility of voice recorders with plastic, combined and metal housings to the effects of external interference. The results of the experimental studies are shown in Table 1.

The shielding efficiency was compared based on the power flux density of the electromagnetic field of the interference signal required to completely suppress the voice recorder. The emitted power of the suppressor was the same. The Casio TR-15 voice recorder in a plastic case was taken as a reference voice recorder, as it is the most susceptible to external electromagnetic interference.

The results presented show that susceptibility to interference depends on both the design and the material of the voice recorder body. The compact Sony M909 voice recorder with a metal case and an analog recording method on magnetic tape requires 28 dB more interference power to suppress than the reference Casio TR-15 voice recorder in a plastic case. The digital voice recorder U 7201, which is designed with enhanced shielding and protection measures, is better protected from interference. To completely suppress it, the interference power should be increased by 32 dB compared to the reference voice recorder. Voice recorders with combined housings made of plastic and metal occupy an intermediate position.

Table 1.
Comparative characteristics of the effectiveness of shielding external interference by the housing of various types of voice recorders

Dictaphone type Casio
L400 Sony
Relative increase in density
interference power flow, dB
0 3 10 28 32

The influence of the carrier frequency of the electromagnetic radiation of the 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 pronounced resonant regions where the effect of interference is maximum. Figure 1 shows the characteristic dependences 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 audio interference signal relative to the maximum level that occurs in the low-frequency region for each voice recorder. As follows from Fig. 1, the studied voice recorders have different resonant susceptibility areas. The largest number of such resonant areas is observed in the small-sized Sony M909 voice recorder in a metal case. At the same time, there is no increase in the efficiency of voice recorder 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 voice recorder body, the loss of interference along the propagation path increases with increasing frequency. Common to all considered voice recorders is the presence of resonant regions close in 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 amplitude-keyed 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 housing;
2) the resonant properties of the voice recorder design.

For voice recorders whose housings 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 is highly individual in nature, which is confirmed by the results of experimental studies (Fig. 1).

The most characteristic dependence of the influence of interference on the voice recorder in a metal case is Sony M909. In this case, the first resonance, characterized by the highest susceptibility to interference, occurs at a wavelength corresponding to the perimeter of the side wall of the 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, whose dimensions are slightly larger than the Sony M909 voice recorder, the first resonance shifts towards lower frequencies. Resonances at multiple, higher frequencies are less pronounced than for the Sony M909 voice recorder.

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

For the voice recorder Sanyo TRC-520M the greatest intensity, resonance appears at a frequency slightly lower than 2 f0. The second, but somewhat 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 manifests itself in a wide frequency band (1…2) f0 . A weak resonant region is observed at frequencies (6.5…7) f0.

If in the low-frequency range different voice recorders have resonances that overlap in frequency areas, then in the high-frequency range 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 cover the areas of maximum susceptibility of all voice recorders. Such interference can be implemented using a multi-frequency interference signal or by frequency sweeping over a given frequency range.

An important characteristic of a voice recorder suppressor 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 located within the suppression zone, then it cannot be used to record voice information without permission. Due to the fact that the level of shielding of different models of voice recorders differs from each other, the maximum range of their suppression turns out to be different. Typically, the maximum suppression range of a voice recorder is determined experimentally for a specific suppressor. Since this value depends on the design features of the voice recorders, with constant suppressor characteristics (radiated power and interference structure), voice recorders with a plastic body, as one would expect, will be suppressed at longer ranges compared to voice recorders with metal cases.

Table 2 shows the experimentally obtained relative values ​​of the maximum suppression range of analog voice recorders with different types of housings, normalized to the maximum suppression range of a voice recorder in a plastic case. As the effectiveness of housing shielding increases, the relative suppression range of voice recorders decreases significantly. A similar picture is observed for digital voice recorders (Table 3). The use of a remote microphone or remote control increases the suppression range of the voice recorder, which is especially noticeable when suppressing digital voice recorders with highly effective housing shielding.

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

No. Model Case type

Relative max. range of complete suppression zone, m

1 S-801 Pearlcorder Olimpus
TP-35 Casio


2 TRC 520M Sanyo
S 724 Pearlcorder Olimpus

S 725 Pearlcorder Olimpus

S 302 Pearlcorder Olimpus

Metal weakly shielded
Metal weakly shielded


3 L-400 Pearlcorder Olimpus Metal


4 TRC 6200 Sanyo Talkbook Metal with shielded measures


5 RN-Z30 National Panasonic

M 909 Sony

Metal with increased shielding



  RN-Z10 National Panasonic  


Note: * with remote microphone.

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

Model Enclosure type and recording method

Relative max. full suppression zone range, m

DMP 260 X Toshiba
Olimpus D-1000
Plastic base with metal front and back walls. Electronic memory


SVR –240 Samsung Plastic base with metal front and back walls. Electronic memory


Voice recorder company
General vertertung
Electronic memory


TCD-D7 Sony Metal with increased magnetic tape shielding


SVR S820/S410 Samsung Electronic memory  


TCM 359V Sony Electronic memory


U 7102 Metal with increased shielding and protection, small-sized.
Electronic memory


Note: * with remote microphone.

Features of the construction of voice recorder jammers

For electronic jamming of voice recorders, wearable 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 radio-electronic suppression of voice recorders. 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 their limited ability to generate noise over a wide frequency band. In addition, expanding the interference spectrum in order to cover the full frequency range of maximum susceptibility to interference of known types of voice recorders leads to a decrease in the spectral power density of the interference and, therefore, to a decrease in the suppression efficiency of voice recorders.

In the second scheme, the interference signal is generated at a low power level using a special interference signal shaper, followed by amplification to the required power level in a wideband power amplifier (Fig. 2). The advantage of this equipment design scheme is its high versatility in types and modes of interference, ensuring a 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 array antenna on them.

podavlenie diktofonov vozmojnosti iprakticheskoe primene 2
Fig. 2 Block diagram of a wearable voice recorder suppressor with a power amplifier.

In general, a voice recorder suppressor in a wearable version consists of an antenna system, a power amplifier, a noise generator and a power source. In the simplest case, the antenna system of a voice recorder suppressor can be a single emitter of the appropriate frequency range, forming an electromagnetic field in the required spatial sector. The solid angle of the suppression zone is usually at least 60°. In other cases, the antenna system may be two-channel, for example, to emit elliptically polarized interference signals. The most promising is the use of an active phased array antenna on low-power amplifiers, which makes it possible to most effectively solve the problem of interference emission in a given spatial sector and provide the required energy potential, including with a conformal array design, ensure the best thermal conditions of the equipment and, therefore, have high reliability suppression of voice recorders. In order to ensure long-term continuous operation in a stationary position or in a car, a portable jammer usually has the ability to connect to an external AC power source or to a car battery.

The jammer generates an amplitude-keyed high-frequency signal in a given frequency range. Amplitude manipulation is set by a microprocessor, pre-programmed to generate a pulse sequence of the required structure.

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