A Simulation Study on Clandestine Audio Recording

Transcription

1 , pp A Simulation Study on Clandestine Audio Recording Ik-Soo Ahn 1, Myung-Sook Kim 2 and Myung-Jin Bae 3* 1 Department of Cultural Contents, Soongsil University, Korea 2 Department of English Language and Literature, Soongsil University, Korea 3 Information and Telecommunication of Department, Soongsil University, Korea aisbestman@naver.com, kimm@ssu.ac.kr, mjbae@ssu.ac.kr Abstract In this paper, the effectiveness of clandestine audio recording with respect to different placements of recording devices was evaluated through a simulation study. Audio samples from prospective placements and methods of clandestine audio recording were analyzed, and finally the optimal configuration was suggested in this paper based on waveform analysis. Existing methods of clandestine audio recording were first investigated and analyzed in order to identify their problems, and then to suggest an improved method to solve these problems. The result of this research shows that there is an optimal position of recording devices for clandestine audio recording and that we may use an improved recording method for better performance. The result can be applied to areas including the use of personal audio blackboxes for keeping records for legal disputes and the protection of national security from potential espionage. Keywords: clandestine audio recording, simulation of audio recording, recording quality 1. Introduction For clandestine audio recording, recording devices must be concealed and camouflaged so that the subject whose voice is being recorded does not notice it. The manner of concealment is important not only because there needs to be a compromise between the recording quality good enough to be useful and the degree of concealment adequate enough to stay hidden, but also because the detection of the recording itself may lead to a larger problem. Clandestine audio recordings must be carefully proceeded and collected if anyone intends to use them as evidence in legal disputes, provided that the original recordings must be preserved. The result of this research may be useful in initiating the developement and commercialization of personal audio blackboxes. A personal audio blackbox is a commerical device that records the voice of the individual as well as his/her surroundings to be used as legal evidence if a situation arises. In addition, the result of this research may be used as a standard procedure in cases of espionage concerning national security. Therefore, the objective of this paper is to find the best placement of clandestine audio recording devices which ensures the audio quality through a simulation study and to explore a better recording method to produce the best quality of the audio recording. This paper consists of 5 sections. Section I is the introduction. In Section II, the optimal position of a recording device is determined through conducting waveform analysis on the recording samples collected from various placements of recording * Corresponding author : Myung Jin Bae(mjbae@ssu.ac.kr) ISSN: IJSEIA Copyright c 2014 SERSC

2 devices. In Section III, the results of the experiments are analyzed to identifiy the problems of existing recording methods and to suggest an improved method for clandestine audio recording by comparing and reviewing existing methods. Section IV, the last section of this paper, evaluates the result of the research and proposes direction for future improvements. 2. A Simulation Study on Placements of Audio Recording Devices The primary requirement for clandestine audio recording is to be discrete without losing recording quality. In order to satisfy this requirement and to function properly as an audio recorder we should come up with a new idea. Existing methods of clandestine audio recordings are only to utilize miniaturized recorders to be installed inside of pens or placed in pockets and other personal belongings. Accordingly, audio recorders have been developed for lightweight and smaller size so that it may be useful to conceal them. However, their lightweight and smaller size could not guarantee either the audio recording quality or the length of time for recording. In order to overcome these limitations, an integral type of audio recording device was used for this study and the audio samples were recorded indoors for precise measurement. The audio sample recorded from an unobstructed and unconcealed placement was chosen as the control audio sample for the experiment. The recordings were done in four different placements for prospective clandestine audio recording based on past instances. 1. On the table (control sample) 2. Under the table 3. Inner jacket pocket (Fixed), 3-1. Inner jacket pocket (Not fixed) 4. Behind the neck Figure 1. Placements of Clandestine Audio Recording devices The experiment was conducted with the two subjects, the recorder (subject 1) and the recordee (subject 2) facing one another across a wooden table, and the audio recorder was placed at various prospective placements as illustrated in Figure 1. An integral typed digital audio recorder was used for the experiment, and the sample sentence, You have left., was taken from the first line of the poem Your silence written by Yongwoon Han. The first placement chosen as a control sample was placing the audio recorder on top of the table without any kind of concealment. The second placement was placing the recorder under the table and on top of the recorder s lap. The third placement was placing the recorder in the inner jacket pocket of the subject 1 who was calmly sitting down on the chair. An additional recording was done in which the subject moved around with the audio recorder in the inner jacket pocket in order to simulate the situation of possible friction caused by any noise from cloths and to identify its effects on the recording quality. The fourth placement was placing 298 Copyright c 2014 SERSC

3 the recorder behind the neck of the subject 1. The audio samples attained through the experiment were compared to analyze the recording quality with respect to 4 different placements of the recorder. Figure 2. Frequency Spectrogram of the Sentence You have left of the Control Sample In general, a human speech consists of a base harmonic frequency and 4~5 formant component frequencies. In this study, the changes in frequency distribution and the amplitude of the formants of the speech signal of the recorded audio samples were analyzed according to the placement of the audio recorder. Fig.2 shows the frequency spectrogram of the control sample recorded in the audio recorder placed on top of the table. We can see that in Figure 2, the low frequency component and the high frequency components are evenly distributed. Information pertaining to personality and unique features of human voice can be found in the pitch characteristics and the formants which are clearly present across the frequency range. The control sample for the experiment has a good quality since the bandwidth of the sound was not reduced. We will compare the recordings from all other placements in the experiment to this control sample as follows. Figure 3. Frequency Spectrogram of You have left Recorded from Placement 2 Figure 3 shows the frequency spectrogram of the audio sample from the recorder placed under of the table with the table acting as a covering. Comparison to the control sample in Figure 2 shows that the low frequency component is decreased by a small amount but still the recording maintains a fair degree of information delivery capabilities. On the other hand, the high frequency component is gradually decreased due to the filtering effect caused by the thickness of covering (table). The table used in the experiment is 4cm thick and 90cm wide in Copyright c 2014 SERSC 299

4 its size, with the filtering effect proportional to its size. Figure 2 shows the result of the transmittance and diffraction of the sound filtered through the table affecting the bandwidth. a) Spectrograms of Placement 3: the subject sitting down on the chair b) Spectrograms of Placement 3-1: the subject moving around Figure 4. Frequency Spectrograms of You have left Recorded from Placements 3 and 3-1 Figure 4 illustrates the spectrograms of the audio samples recorded from the recorder placed in the inner jacket pocket, with and without movement during recording. In this recording the pocket itself acts like a filter and therefore, the spectrogram shows dampened high frequency components while the low frequency components remaied relatively the same. This phenomenon suggests that the pocket acts like a low pass filter. The pitch and formants measured relatively the same but one can see that the energy of high frequency components are significantly reduced. The spectrogram of audio sample recorded from the subject moving around shows the filtering effects of friction caused by noise from pocket with the microphone affecting the recording simultaneously affected the entire bandwidth. The recording quality of this sample was so poor that no one could understand the recorded speech. 300 Copyright c 2014 SERSC

5 Figure 5. Frequency Spectrogram of You have left recorded from Placement 4 Figure 5 shows the frequency spectrogram of the sample recorded from the recorder placed behind the neck of the subject 1. The microphone was not covered by anything and, therefore, the recorded sample was not affected by low pass filtering effect of the clothes. Although somewhat inferior to that of the control sample, the bandwidth of the recorded sound was evenly distributed with the pitch and the formant information being identifiable. 3. Results The audio samples collected for this paper were interfaced through a AD/DA converter which sampled the audio at 11 khz at 16-bits and evaluated for their performances based on the algorithm illustrated below. The algorithm processed each frame as 256 samples. Figure 6. Flow Diagram of Speech Signal Analysis for this Study Figur 6 shows the process of the frequency analysis in which the speech is converted into the frequency domain and then into formant forms in order to evaluate the pitch and formant. First, the speech is passed through a low pass filter (LPF) to isolate the voice band. Then it is passed through a fast fourier transform (FFT) to evaluate the measured pitch and formant values. These values were used when evaluating the 8kHz frequency components of each voice sample. The voiceprint of the voice-band and the emission spectrum were analyzed using the fixed spectrum compensation as shown below. The transfer function between the vocal tract and the voice sounds were simulated using the following function. (1) Copyright c 2014 SERSC 301

6 (2) And the transfer function between the voiceless sounds and the vocal tract was simulated using the following transfer function. Formant is the resonance frequency of the vocal tract when a voice is being generated. Therefore, it specifies the shape of vocal tract for each person and shows the changes in voice over time. Considering these features of human voice, the spectrums of the voiceless and the voiced sounds are compared to each other in frequency domain. (3) 1. Spectrum of the audio from Placement 1 (Control sample) 2. Spectrum of the audio from Placement 2 3. Spectrum of the audio from Placement 4 Figure 7. Spectrum Comparison for Audio Samples Recorded from Different Placements Figure 7 shows the comparison of spectrums between the control sample and the two other samples recorded from Placement 2 and Placement 4. In order to enhance the validity of this experiment, the audio qualities of these samples were analyzed in detail. The lower frequency range of the voice band was distributed around 1 khz and all three samples showed relatively good sound quality. However, in the high frequency range, the quality decreased in the order of Placement 2 and Placement Copyright c 2014 SERSC

7 1. Spectrum of the audio from Placement 2 2. Spectrum of the audio from Placement 4 Figure 8. Spectrum Comparison between the Audio Samples from Placement 2 and Placement 3 Figure 8 shows the spectrum comparison of the audio samples from Placement 2 with the recorder placed under the table and Placement 4 with the recorder placed behind the neck. Overall, the two had audio quality similar to each other but in the lower frequency range, the audio from Placement 2 showed a higher quality. Results of the experiments showed that in clandestine audio recording, the audio quality descended in order of under the table, behind the neck, and the inner jacket pocket. Table 1. Audio Quality Rank and the Placement of the Recordings Recorder Placement Frequency Characteristics Audio Quality Rank On the table Good across entire bandwidth Good (control sample) Under the table In a jacket pocket (stable) Hi freq. : Large reduction Mi freq. : Small reduction Lo freq. : Good Hi freq. : Large reduction Mi freq. : Large reduction Lo freq. : Small reduction 1 3 In a jacket pocket (moving) Bad (all bandwidths) 4 Behind the neck Hi freq. : Large reduction Mi freq. : Small reduction Lo freq. : Good 2 Although the audio quality of the sample recorded from Placement 2 is inferior to that of the control sample, it showed a strong retention in quality over the entire bandwidth when compared to the control sample. The sample recorded from Placement 4 showed a similar Copyright c 2014 SERSC 303

8 performance to the one recorded from Placement 2, However, out of the 4 formants, the middle frequency range of 2~3KHz was more well-defined in the control sample. As far as Placement 3 is concerned, the inner jacket pocket was not a useless placement because it has an advantage having a higher degree of concealment. The quality of the audio recording from Placement 3, however, depended on the level of movement. This is due to the fact that the microphone may cause friction when it contacts the clothes and thus the audio can carry distorted or destroyed information concerning pitch and formants. As proved in this experiment, the performance of the microphone may be a crucial element to determine the audio quality from the recorder placed inside of jacket pockets. Or, the person should be warned against excessive movements when recording the conversation with recorder placed inside of his/her jacket pocket. Existing methods require the microphone to be on the main body of the recorder for use in clandestine audio recording an recorders are usually installed invisibly inside of pens or other small objects. Therefore, these recording devices are limited in their performance in a sense that the microphone would not function properly in certain situations, thus making the audio quality dependent on the placement of the device. Morever, miniaturization of recording devices resulted in lower sound quality as well as shorter recording time. In case of clandestine audio recording, the microphone should stay with the person who is recording. The concern for misplacing the components as well as maintaining a proper degree of audio quality of recording should be considered crucial in clandestine audio recording. In order to secure recording for a long time, the quality of battery must be also improved. A small and lightweight battery with minimal powerloss as well as convenient and quick recharging system must be developed for better performance of the clandestine audio recording devices. 4. Conclusion Due the characteristics of clandestine audio recording, the recording device must be concealed appropriately while maintaining certain level of recording quality. People may assume that a concealed audio recording device has a lower quality than that of an open and uncovered recording device. In this research, we found the optimal placement for a recording device among several prospective placements, clandestine audio recording in particular. Based on the results of the simulation study conducted for this paper, it should be concluded that a high quality recording can be obtained from Placement 2 with the recorder placed under the table and Placement 4 with the recorder placed behind the neck. From these placements, speech covering the bandwidth of 1~4kHz, the bandwidth below 3kHz in particular, is sufficently recorded converying proper degree of information concerning pitch and formants. However, if the clandestine recording takes place outdoors, the ambient noise may become a factor affecting the recording quality. A large noise absorbs and eliminates smaller noises in case of recording in places with ambient noise. Therefore, in case of outdoor recording, the recorder should not be placed in pockets since the LPF effects of the clothing materials and the HPF effects of the ambient noise will affect the quality of recording. Still, a novel idea is required to solve the mechanical limitations of audio recording devices for better performance and a better battery system. Acknowledgements This article is a revised and expanded version of the paper entitled [Recording Qualities and Placements for an Audio Surveillance Device] presented at international Symposium on Advanced and Applied Convergence held on November 14-16, 2013 at Seoul Korea. 304 Copyright c 2014 SERSC

9 References [1] I.-S. Ahn, Recording Qualities and Placements for an Audio Surveillance Device, event of the First International Symposium on Advanced and Applied Convergence (ISAAC 2013), and the First International Conference on Advanced Culture Technology (ICACT 2013), Gangnam, Seoul, Korea, (2013) November 15. [2] T.-G. Kim, Evidence ability of the tape which is recorded by a private individual and Entrapment by a private individual, Legal Institute of Kyung Sang University Legal Institute, vol. 19, no. 2,, (2011) August, pp [3] J.-G. Kim, H.-B. Park and M.-J. Bae, A Study on Real Time Pitch Alteration of Speech Signal, The Journal of the Acoustical Society of Korea, vol. 23, no. 1, (2004) January, pp [4] J.-G. Kim, M.-J. Bae, W.-L. Cho and M.-J. Bae, A Study on the Pitch Detection of Speech Harmonics by the Peak-Fitting, Speech Science. The Korea Association of Speech Science, vol. 10, no. 2, (2003) June, pp [5] Y.-K. Kim and M.-J. Bae, A Study on the Pitch Alteration Technique by Subband Scaling in Speech Signal, The Korea Association of Speech Science, vol. 10, no. 4, (2003) December, pp Authors Ik-Soo Ahn, he received M.S. in Cultural Contents from Chung- Ang University in He is currently working toward Ph.D. at Soongsil University. aisbestman@naver.com Myung-Sook Kim, she received Ph.D. in English from Wisconsin- Madison in She is currently Professor of Department of English Language and Linguistics at Soongsil University. kimm@ssu.ac.kr Myung-Jin Bae, he received Ph.D. in Electronic Engineering from Seoul National University in He is currently Professor of Dept. of Information & Telecommunication at Soongsil University. mjbae@ssu.ac.kr Copyright c 2014 SERSC 305

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