Localization of the acoustic source position using microphone array.
Key words: acoustic source localization, microphone array, time-delay estimation, cross-correlation, signal processing. [DELTA]d
Beginning of audio localization is dated to the year 1880 when the first device for this purpose was designed. Its inventor Professor Mayer used it for navigation improvement in fog. This instrument was called by its author Mayer's topophone. The biggest interest in audio location systems occurs in the period between World War 1 and World War 2 where they were primarily used for detection a localization of the aircraft engine sound. Constructions and dimensions of these systems were very various but the basic concept is based on Mayer's topophone improved with next two horns oriented in vertical plane. Due to state of electronics then minimally two people were required for sound analysis originated from horn system. Main problem with these systems was small sensitivity due to limitations in horn dimensions. For better gain static dishes and walls based on spherical reflection surface was developed. After radio locator invention in 1934 audio location devices were not further developed because they were completely replaced (Self, 2004).
Nowadays very dynamical development in electronics and computer science enables applying of the sound localization systems in areas where it was impossible due to technical and economical aspects several years ago. These areas include applications in security, teleconferencing, robotic systems and other else where information is coded in audio signal source position.
Paper deals with acoustic source localization using sensory system based on microphone array. It describes time-delay estimation method of the direction of arrival of the sound wave on the microphone pair which can in cooperation with other microphone pairs provide information about audio source position. In experimental part is proposed experimental evaluation system consisting of microphone sensory system and standard personal computer equipped with Advantech PCI-1716 data acquisition card.
2. TIME-DELAY ESTIMATION METHOD
This method is very often used in microphone sensory systems due to its simple implementation in hardware and software. It is based on samples analysis acquired from each microphone unit in an array. Analysis takes place usually in two basic steps--time-delay determining of each data samples acquired from different microphones following by computation of the sound source position on the basis of microphone array geometrical organization (Rajmic, 2002).
[FIGURE 1 OMITTED]
On assumption that sound source is in much larger distance than is each sensor spacing dS, spherical surface of the sound wave can be considered as a flat surface. It considerably simplifies evaluation of the direction of arrival of the sound wave. This assumption and resulting simplification is obvious from figure 1. Then sound wave arrival angle [alpha] can be determined by equation (1).
[alpha] = arcsin ([DELTA]d/[d.sub.s]) (1)
Difference of the sound wave trajectory [DELTA]d can be computed as product of estimated time-delay [DELTA]t and sound wave speed c. Resulting formula for arrival angle computation then is:
[alpha] = arcsin ([DELTA]t x c/[d.sub.s]). (2)
For time-delay estimation [DELTA]t can be very advantageously used cross-correlation analysis which is defined by equation (3). Where [x.sub.1] is acquired signal from microphone 1, x2 from microphone 2 and N number of acquired data samples.
[[??].sub.x1x2][k] = 1/N [N-k-1.summation over (n=0)] [x.sub.1][n][x.sub.2][n+k] (3)
During the computation [x.sub.1] is reference signal and [x.sub.2] compared signal. Cross-correlation of two same shifted signals resulting in coefficients representing conformity of these two signals. Maximum value of correlation coefficient and corresponding shift of k samples indicates relative shift of these signals. Time-delay [DELTA]t can be computed using formula:
[DELTA]t = 1/[f.sub.vz] argmax ([[??].sub.x1x2][k]) (4)
where [f.sub.vz] is sampling frequency.
3. EVALUATION SYSTEM DESIGN
Design of the evaluation system is conformed to requirements to portability, configurability and simple connection with personal computer. These requirements best fulfills modular system which block schematics is depicted in figure 2. It consists of the following main components: three microphone units with integrated preamplifier, 3-channel automatic gain control amplifier with output anti-aliasing filter and evaluation unit--in this case standard personal computer equipped with multifunction Advantech PCI-1716 data acquisition card. Components are connected together with shielded cables to avoid interference leakage to acquired signal.
3.1 Sensory system
Surrounding sound field is captured with three microphone units each equipped with three omnidirectional electret microphones. They are installed on the triangular base with the side size of 20mm. This construction improves sensitivity and signal-to-noise ratio. Each microphone is connected with summing preamplifier with the gain of 40dB which is mounted in the base of the microphone unit.
From the microphone units is signal passed to the three channel automatic gain control amplifier and low-pass filter. The purpose of this last amplification stage is to adapt signal voltage levels to the appropriate level suitable for analog inputs of the data acquisition card. In the case that input signal has too high voltage level the gain of the amplifier is automatically lowered to prevent overdriving of the card inputs. After amplifier stage is connected 4th order low-pass filter designed using Bessel approximation which function as anti-aliasing filter with cutoff frequency of 20000Hz. This type of the filter has linear curve of the phase characteristic in wide frequency range and advantageous step response with small overshot. On the other hand its drawback is smaller slope of the stop-band part of the frequency characteristic.
3.2 Evaluation unit
For laboratory measurements is evaluation unit based on standard personal computer with processor AMD Athlon64 equipped with multifunction data acquisition card Advantech PCI-1716. This card is dedicated for PCI bus interface with bus master data transfer capability. Integrated FIFO memory with capacity of 1K samples enables efficient data transfer from the card to the system memory without excessive CPU utilization. Card provides sixteen analog inputs in single-ended or eight analog inputs in differential mode. Each input is via analog multiplexor connected to analog-to-digital converter with 16-bit resolution and maximum sampling rate equal to 250 kHz. It is also equipped with two analog outputs, sixteen digital inputs and outputs with TTL compatible logic and finally with 16-bit timer with reference frequency of 10 MHz (Advantech, 2001).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
4. EXPERIMENTAL VERIFICATION
Evaluation system was verified with microphone array consisting of 3 microphone units in equilateral triangle configuration with size of [d.sub.S] = 1m. As a test sound signal was chosen linear chirp which can be generated using "chirp" function of the Matlab's DSP toolbox. For audio data acquisition and analysis software application which is able to visualize measured sound waves and perform frequency analysis was created. It was developed in MS Visual C++ 6.0 software development studio as a win32 application with utilization of MFC library. Frequency analysis is performed by FFTW library which is distributed under GPL license (Frigo & Johnson, 2006). Result of the cross-correlation of the captured sound wave is depicted in figure 3.
This paper deals with acoustic source localization using sensory system based on microphone array. It describes time-delay estimation method of the direction of arrival of the sound wave which is in various modifications commonly used in practical applications. Experimental part proposes design of the whole evaluation system. Designed sensory system has modular architecture which enables easy portability, configurability and connectivity with evaluation unit. Properties of designed sensory system were verified by laboratory experiments in which standard personal computer equipped with Advantech PCI-1716 card represented the evaluation unit.
The work was performed with financial support of research project MSM 7088352102. This support is very gratefully acknowledged.
Advantech. (2001). PCI-1716 250kS/s, 16-bit High-Resolution Multifunction Card. Available from WWW: http://www.advantech.com. Accessed: 2007-02-10.
Frigo, M., Johnson, S. G. FFTW3. (2006). Available from WWW: http://www.fftw.org/#documentation. Accessed 2007-02-15.
Rajmic, P. (2002). Simple time-shift method and modified time-shift method for incoming sound direction detection. (In Czech: Prosta metoda casovych posunu a modifikovana metoda casovych posunu pro detekci smeru prichazejiciho zvuku.) Available from WWW:
http://www.elektrorevue.cz. Accessed: 2007-02-10. Self. D. (2004). Acoustic Location and Sound Mirrors. Available from WWW: http://www.dself.dsl.pipex.com/museum/comms/ear/ear.htm#steer. Accessed: 2007-02-10.
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|Author:||Dostalek, Petr; Vasek, Vladimir; Dolinay, Jan|
|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2007|
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