Comparative analysis of digital voice recorders.
Comparative analysis of digital voice recorders
Small-sized sound recording equipment for covert use must meet a number of additional requirements in relation to typical (reporter) dictaphones. In addition to overall dimensions, weight, camouflage, the presence of a remote control (RC), such a specific unmasking feature as electromagnetic radiation can play a significant role. For classic analog sound recording equipment on a magnetic carrier (tape, wire), the characteristic unmasking radiation is a monochrome magnetization signal located in the frequency range of 25 – 100 kHz, and signals in the frequency range below 1 kHz, caused by the operation of a DC motor. Recording these signals is the basis for the operation of existing equipment for detecting working dictaphones.
Currently, small-sized digital audio recording equipment is beginning to be used for covert audio recording, including those recording a digitized audio signal in semiconductor memory – the so-called kinematic-free storage devices. Recording digital signals on a magnetic medium does not require a magnetization signal, and kinematic-free storage devices do not require an electric motor with its radiation. Nevertheless, digital audio recording equipment has its own unmasking electromagnetic radiation, which allows not only to detect, but also to identify this equipment by specific signs of radiation modulation.
Below are the results of an examination of several samples of small-sized digital audio recording equipment.
A set of portable digital tape recorder with a magneto-optical disk drive (Portable minidisk recorder) type MZ-R3 manufactured by SONY.
The set includes:
— stereo tape recorder;
— headphones with remote control;
— network power adapter;
— additional battery pack;
— stereo microphone.
Measurements of electromagnetic radiation parameters were carried out in the tape recorder operating mode “recording” and “playback” with the tape recorder powered by internal power elements. During the measurements, the mutual arrangement of the components of the kit (microphone, telephones with remote control) was ensured, similar to the arrangement on the human body.
| Electromagnetic radiation signals from the tape recorder are detected at frequencies up to 200 MHz. The structure of the radiation spectrum and the subjective perception of demodulated signals are typical for microprocessor technology products. The spectrum is based on a set of harmonic and subharmonic components of the clock frequency of 2.82 MHz. In the region of the 1st, 2nd, 3rd harmonics of this frequency, broadband electromagnetic radiation signals caused by the operation of the microprocessor are grouped (Fig. 1).
In the “recording” mode, there is pulse modulation of individual components of the emission spectrum by a signal with a duration of 500 ms and a repetition period of 2 s (Fig. 2), similar to the recording signal by sectors in floppy disk drives in a computer, as well as modulation of individual high-frequency components by an unconverted speech signal, especially noticeable at input signal levels not exceeding the AGC response threshold. The above features can be used to detect the fact of operation and identification of the above set of equipment, as well as to intercept the information being recorded. |
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| Fig. 1 |
Fig. 2 |
The levels of electromagnetic radiation in the entire specified frequency range are largely determined by the connection of headphones with a remote control, a microphone and their mutual arrangement to the tape recorder.
At a frequency of 2.82 MHz at a distance of 1 meter from the equipment set, the electric component of the field strength reaches 600 μV/m, the magnetic component 0.1 μA/m.
At frequencies above 30 MHz at a distance of 1 meter from the set, the field strength of side radiation reaches 5 μV/m.
Disabling the headphones with the remote control reduces the radiation level by about 10 dB.
The main component that determines the level of electromagnetic radiation and the secrecy of the set's operation are the headphones with the remote control.
A set of portable stereophonic digital tape recorder (Digital Micro Recorder) type NT-2 manufactured by SONY with a data storage device on a microcassette and the use of the principle of transverse recording by a rotating block of magnetic heads (NT-SONY format).
The kit includes:
— stereo tape recorder;
— headphones with remote control;
— stereo microphone;
— power adapter.
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Measurements of electromagnetic radiation parameters were carried out under the same conditions. Electromagnetic radiation from the NT-2 tape recorder is detected at frequencies up to 350 MHz (Fig. 3). The spectrum of electromagnetic radiation is a set of harmonic and subharmonic components, multiples of the clock generator frequency (presumably 4.1 MHz), modulated by a periodic signal with a sampling frequency (128 kHz), a wideband digital signal and a periodic pulse sequence (duration 7 ms, repetition period of about 20 ms), the parameters of which depend on the operating mode of the tape recorder. |
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Fig. 3 |
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| Individual components of the electromagnetic radiation spectrum in the “playback” mode are modulated by the unconverted speech signal (Fig. 4).
The distribution of the amplitudes of the spectrum components has maxima at frequencies that are multiples of 61.5; 24.6; 4.1 MHz. Electromagnetic radiation signals that will determine secrecy depend on the method of powering the tape recorder, the presence and mutual arrangement of headphones and a microphone, the wires of which form a system of radio frequency emitters with random parameters. For example, at a frequency of 61.5 MHz at a distance of 1 meter from the equipment set, the field strength is 38 μV/m, and at a frequency of 208 MHz 45 μV/m. |
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Fig. 4 |
In addition, the equipment manufactured by SONY MZ-R3 and NT-2 is characterized by electromagnetic radiation caused by the operation of the electric motor.
A set of portable stereophonic kinematic-free digital storage device of speech information JBIRD manufactured by ADS Inc.
The drive is designed to record speech information coming from microphones in electronic memory, store it and then play it back (rewrite) using a PC equipped with a special expansion board containing a DAC and having an analog output.
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The kit includes: — drive; — microphones; — remote mode switch; — cable for external power source; — expansion board for PC; — interface cable. |
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Fig.5 |
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| Electromagnetic radiation generated during reproduction of information is not considered in this context. Electromagnetic radiation from the JBIRD drive in the “recording” (REC) mode is detected at frequencies up to 400 MHz (Fig. 5). The spectrum of electromagnetic radiation is a set of harmonic and subharmonic components, multiples of the clock generator frequency (presumably 1129 kHz), modulated (index 0.01-0.2) by a signal with a sampling frequency of 8.82 kHz (Fig. 6). The distribution of the amplitudes of the spectrum components has maxima at frequencies multiples of 40 MHz, 9.032 MHz, 1.129 MHz. |  |
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Fig.6 |
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| The levels of electromagnetic radiation and their distribution over the frequency range are determined by the connection to the drive and the mutual arrangement of microphones, a remote mode switch and an external power source, forming a system of radio frequency emitters with random parameters, since the circuit and design of the drive do not provide for measures to prevent the “leakage” of high-frequency signals from the metal case.
In the “recording” mode, individual components of the spectrum are modulated by a pulse sequence with a pulse train repetition period of about 30 ms (Fig. 7), and in the low-frequency region in this mode, a magnetic field of side radiation of the pulse signal arises (Fig. 8). |
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Fig. 7 |
Fig.8 |
In the “storage” (STOP) mode, the drive continues to emit radio signals. In this case, the radiation level of individual components decreases by 5 – 10 dB, but the maximum values of the field strength remain virtually unchanged. At a distance of 1 meter from the drive kit, the electromagnetic field strength reaches 1.62 mV/m at a frequency of 146 MHz, which is comparable to the signal levels of broadcasting stations. Due to the multiplicity of electromagnetic radiation signals and their modulation by a pulse sequence with a repetition rate of pulse packets in the audio range, the operating drive is detected by household receivers at a distance of about 10 meters.
The design of the drive does not provide for measures aimed at increasing the stealth of operation.
The main components of the kit that determine the stealth parameters are the wires of the remote mode switch and microphones.
A set of portable kinematic storage WREN 4 manufactured by Winkelmann with FLESH electronic memory chips. The design and composition of the set are similar to the JBIRD equipment.
Electromagnetic radiation signals are detected at frequencies up to 300 MHz. The radiation levels in the radio frequency range are the lowest of the considered types of digital audio recording equipment. The main components of the electromagnetic radiation spectrum are the harmonic components of the sampling frequency (presumably 22.75 kHz, Fig. 9) and the pulse sequence of the recording signal with a pulse train period of 28 ms (Fig. 10). The components of the electromagnetic radiation spectrum with the highest levels are in the frequency range of about 100 kHz. The field strength at a distance of 1 meter from the equipment is about 50 μV/m for the electric field component and 0.06 μA/m for the magnetic field component.
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| Fig. 9 | Fig. 10 |
The signal with a maximum field strength of 0.34 μA/m at a distance of 1 m from the WREN 4 equipment was recorded at a frequency of 22.75 kHz, which corresponds to the minimum values of the field strength of the bias signal of analog voice recorders.