Dinh-Quy Nguyen

Time-Reversal Approach to the Stereophonic Acoustic Echo Cancellation Problem

Stereophonic acoustic echo cancellation (SAEC) plays an important role in delivering realistic teleconferencing experience. The fundamental problem of SAEC system is that stereophonic channels are linearly related and this results in slow convergence of the adaptive filters. In this paper, we present a novel algorithm by employing a selective time-reversal block to solve the SAEC problem which results in a significant increase in the convergence performance of adaptive filters such that the stereophonic image as well as quality are preserved. The proposed algorithm employs time-reversal operation on selective blocks of input data samples for one of the two channels to decorrelate stereophonic channels in the SAEC system. To achieve good stereophonic perception, time-reversal operation is only applied to the selective blocks whose magnitudes fall below a pre-determined threshold. Theoretical and numerical simulation results are also studied and investigated to show that the proposed algorithm achieves faster convergence in terms of normalized misalignment and better stereophonic perception with less audio distortion compared to the well-known nonlinear transformation algorithm for the SAEC system.

Novel DORT Method in Non-Well-Resolved Scatterer Case

The decomposition of time reversal operator (DORT) method is an efficient technique to select focusing signal on the target in well-resolved scatterer case. The DORT method requires the measurement of the inter-element impulse responses and diagonalizes these responses to find eigenstructure of the medium. However, the DORT method is not able to produce satisfactory result for non-well-resolved scatterer case (i.e., two scatterers that are placed closely about 0.2lambda). Therefore, in this letter, a weighted least squares (WLS) algorithm is introduced in the DORT method to perform selective focusing in non-well-resolved scatterer case. This technique called the WLS-DORT method performs better spatial focusing resolution than the conventional DORT method and results in simpler computational complexity than the minimum variance beamforming method under practical implementation.

Selective focusing with numerical technique of time reversal operator decomposition

Recently, a time reversal operator method that is applied to ultrasonic detection and focusing with arrays of transducers has been proposed. This method is known as the DORT (Decomposition de lpsilaOperateur de Retournement Temporel). The DORT method requires the measurement of the inter-element impulse responses of the propagating medium for all pairs of elements in the array. Time reversal operator (TRO) is deduced from these responses and diagonalized. The eigenvector and eigenvalue of TRO is analyzed to perform selective focusing on different scatterers. We develop the numerical technique of DORT method by using the Greenpsilas function estimation in homogeneous and multiple scattering medium. This technique can set up the feasibility study of DORT method to perform selective focusing on multiple scatterers.

Weighted least squares DORT method in selective focusing

The DORT (decomposition de lpsilaoperateur de retournement temporel) method is an efficient technique to focus signal on the target. The DORT method requires the measurement of the inter-element impulse responses of the propagating medium for all pairs of elements in the array. The time reversal operator (TRO) is deduced and diagonalized from these responses. But DORT method has worse results in non-ideally resolved scatterer case that two scatterers are placed close with each other. Therefore, in this paper, weighted least-squares (WLS) combining with the DORT method are proposed to perform selective focusing in non-ideally resolved scatterer case. This technique can also perform target focusing with higher spatial resolution than original DORT method and faster computation than adaptive beamforming method.

Decomposition of time reversal operator method in a small elastic scatterer

Decomposition of time reversal operator (DORT) method is developed based on iterative time reversal technique and eigenvector decomposition for scattering analysis. This method involves the determination of the invariant of the time-reversal operator (TRO) obtained by singular value decomposition of the array response matrix. A recent work has used DORT method to characterize an elastic cylindrical or spherical object in an anisotropic scattering medium. In this paper, the DORT method is extended to localize and characterize the physical properties of a small elastic scatterer in anisotropic scattering case. The proposed approach not only estimates the position of the scatterer but also determines the shape (cylinder or sphere), the material (density), and the size (radius) of a small elastic scatterer. The numerical technique is also developed to reduce computational load and storage requirement compared to experimental technique. Thus, our proposed approach can be a simple and potential solution to solve an acoustic inverse scattering problem of a small elastic scatterer.