Inverse Filtering Technique Research Articles

Focused Vibrotactile Stimuli From a Wearable Sparse Array of Actuators.

Wearable vibrotactile actuators are non-intrusive and inexpensive means to provide haptic feedback directly to the user's skin. Complex spatiotemporal stimuli can be achieved by combining multiple of these actuators, using the funneling illusion. This illusion can funnel the sensation to a particular position between the actuators, thereby creating virtual actuators. However, using the funneling illusion to create virtual actuation points is not robust and leads to sensations that are difficult to locate. We postulate that poor localization can be improved by considering the dispersion and attenuation of the wave propagation on the skin. We used the inverse filter technique to compute the delays and amplification of each frequency to correct the distortion and create sharp sensations that are easier to detect. We developed a wearable device stimulating the volar surface of the forearm composed of four independently controlled actuators. A psychophysical study involving twenty participants showed that the focused sensation improves confidence in the localization by 20% compared to the non-corrected funneling illusion. We anticipate our results to improve the control of wearable vibrotactile devices used for emotional touch or tactile communication.

Female adolescent singing voice characteristics: an exploratory study using LTAS and inverse filtering

Background and Aim: To date, little research is available that objectively quantifies female adolescent singing-voice characteristics in light of the physiological and functional developments that occur from puberty to adulthood. This exploratory study sought to augment the pool of data available that offers objective voice analysis of female singers in late adolescence. Methods: Using long-term average spectra (LTAS) and inverse filtering techniques, dynamic range and voice-source characteristics were determined in a cohort of vocally healthy cis-gender female adolescent singers (17 to 19 years) from high-school choirs in Aotearoa New Zealand. Non-parametric statistics were used to determine associations and significant differences. Results: Wide intersubject variation was seen between dynamic range, spectral measures of harmonic organisation (formant cluster prominence, FCP), noise components in the spectrum (high-frequency energy ratio, HFER), and the normalised amplitude quotient (NAQ) suggesting great variability in ability to control phonatory mechanisms such as subglottal pressure (Psub), glottal configuration and adduction, and vocal tract shaping. A strong association between the HFER and NAQ suggest that these non-invasive measures may offer complimentary insights into vocal function, specifically with regard to glottal adduction and turbulent noise in the voice signal. Conclusion: Knowledge of the range of variation within healthy adolescent singers is necessary for the development of effective and inclusive pedagogical practices, and for vocal-health professionals working with singers of this age. LTAS and inverse filtering are useful non-invasive tools for determining such characteristics.

Active noise control using remotely placed sources: Application to magnetic resonance imaging noise and equivalence to the time reversal inverse filter

This paper explores the feasibility of active control of noise from magnetic resonance imaging (MRI) systems using control sources placed outside of the region that MRI scanners are sensitive to ferrous materials. Active noise control (ANC) is shown here to be equivalent to the time reversal (TR) inverse filter technique with a 180° phase shift. Since TR is capable of remotely focusing sound energy to a point in space, it follows that ANC may also be remotely focused. This paper explores the remote delivery of a noise-canceling signal to a desired location (e.g. a patient’s ears). A parameterization study testing frequency dependence and signal length is conducted to determine the effectiveness of the remote delivery. The reduction of MRI noise using ANC is demonstrated using recordings of MRI noise. The parameterization and MRI noise reduction studies are conducted in a reverberation chamber and in a laboratory room with a lower reverberation time. Frequencies below 1 kHz and narrowband signals are easier to control, with reductions up to 20 dB. MRI noise is reduced by up to 18 dB in overall sound pressure level. These results utilized a single control loudspeaker; further reductions should be possible with multiple control sources.

Jittered Sampling in Linear Canonical Domain

Linear canonical transform (LCT) based sampling has been attracted much attention in signal processing community. A variety of sampling theorems associated with the LCT are therefore derived. However, the random jittered sampling in linear canonical domain still remains unresolved. Starting from the Shannon's result for the LCT, a biased interpolated estimate for bandlimited signals in linear canonical domain from jittered samples is proposed. The inverse filter technique is then applied to correct the bias, giving rise to an unbiased estimator whose interpolation noise variance approaches zero as the jitter becomes less and less pronounced. To be specific, for bandlimited stationary stochastic signals in linear canonical domain the interpolation noise variance is increased, and this result is also verified through some examples and simulations.

Influence of cutting parameters on force coefficients and stability in plunge milling

Fixed dynamometer is often used to collect cutting force data in plunge milling. The cutting force test signal is distorted because of the dynamic characteristics of the test system consisting of the workpiece and the dynamometer. Then the accuracy of cutting force coefficients based on experimental data analysis is affected. The inverse filtering technique is utilized to compensate for the measurement signal dynamically and verified by experiments. The influence of cutting parameters on cutting force coefficients is investigated by means of the nonlinear instantaneous milling force method. The results demonstrate that the cutting force coefficient has a nonlinear relationship with the feed per tooth and the spindle speed. An improved semi-discrete method is proposed for the machining feature of the plunge milling and utilized to predict the stability of the plunge milling. The measurement signal is analyzed by using the nonlinear method such as phase plane and Poincare section; the accuracy of the prediction of the stability lobe diagram is verified. The results demonstrate that the stability boundary of instantaneous cutting force coefficients is higher than that of the average cutting force coefficients. The research results provide theoretical guidance for the optimization of cutting parameters in the actual machining process.

Algorithm and Constraints for Exact Non-blind Deconvolution

Image filtering is generally an irreversible process. Image restoration methods, such as inverse filtering or other deconvolution techniques, cannot precisely recover the original image and often introduce some level of artifacts. In the current paper, we formulate the filtering as a blending of several transformed replications of the original image. We assume zero noise after the filtering. Using this convention, we propose a method that precisely restores the original image from its filtered version. The method is applicable for a family of filters including average box filters and approximated Gaussian filters.

SAW Synthesis With IDTs Array and the Inverse Filter: Toward a Versatile SAW Toolbox for Microfluidics and Biological Applications.

Surface acoustic waves (SAWs) are versatile tools to manipulate fluids at small scales for microfluidics and biological applications. A nonexhaustive list of operations that can be performed with SAW includes sessile droplet displacement, atomization, division, and merging but also the actuation of fluids embedded in microchannels or the manipulation of suspended particles. However, each of these operations requires a specific design of the wave generation system, the so-called interdigitated transducers (IDTs). Depending on the application, it might indeed be necessary to generate focused or plane, propagating or standing, and aligned or shifted waves. Furthermore, the possibilities offered by more complex wave fields such as acoustical vortices for particle tweezing and liquid twisting cannot be explored with classical IDTs. In this paper, we show that the inverse filter technique coupled with an IDTs array enables us to synthesize all classical wave fields used in microfluidics and biological applications with a single multifunctional platform. It also enables us to generate swirling SAWs, whose potential for the on-chip synthesis of tailored acoustical vortices has been demonstrated lately. The possibilities offered by this platform are illustrated by performing many operations successively on sessile droplets with the same system.

On-chip generation of acoustical vortices with swirling surface acoustic waves for single particle manipulation and vorticity control

Surface acoustic waves (SAWs) are versatile tools for the manipulation of fluids at small scales. These waves can be used to displace, divide, merge, and atomize sessile droplets, but also actuate fluids embedded in microchannels. In this presentation, we will show that IDTs array and inverse filter technique enable on-chip synthesis of a new type of SAWs, called swirling SAWs, which degenerate into bulk acoustical vortices when transmitted to a liquid. These acoustical vortices can be tailored to create a 3D particle trap and thus selective single particle acoustical tweezers, with digital control of the trap position. They can also induce controlled vortical flows, whose topology essentially relies on the topology of the underlying acoustical vortex.

Taming the degeneration of Bessel beams at an anisotropic-isotropic interface: Toward three-dimensional control of confined vortical waves.

Despite their self-reconstruction properties in heterogeneous media, Bessel beams are known to degenerate when they are refracted from an isotropic to an anisotropic medium. In this paper, we study the converse situation wherein an anisotropic Bessel beam is refracted into an isotropic medium. It is shown that these anisotropic Bessel beams also degenerate, leading to confined vortical waves that may serve as localized particle trap for acoustical tweezers. The linear nature of this degeneration allows the three-dimensional control of this trap position by wavefront correction. Theory is confronted to experiments performed in the field of acoustics. A swirling surface acoustic wave is synthesized at the surface of a piezoelectric crystal by a microelectromechanical integrated system and radiated inside a miniature liquid vessel. The wavefront correction is operated with inverse filter technique. This work opens perspectives for contactless on-chip manipulation devices.

Anisotropic Swirling Surface Acoustic Waves from Inverse Filtering for On-Chip Generation of Acoustic Vortices

From radio-electronics signal analysis to biological samples actuation, surface acoustic waves (SAW) are involved in a multitude of modern devices. Despite this versatility, SAW transducers developed up to date only authorize the synthesis of the most simple standing or progressive waves such as plane and focused waves. In particular, acoustical integrated sources able to generate acoustical vortices (the analogue of optical vortices) are missing. In this work, we propose a flexible tool based on inverse filter technique and arrays of SAW transducers enabling the synthesis of prescribed complex wave patterns at the surface of anisotropic media. The potential of this setup is illustrated by the synthesis of a 2D analog of 3D acoustical vortices, namely "swirling surface acoustic waves". Similarly to their 3D counterpart, they appear as concentric structures of bright rings with a phase singularity in their center resulting in a central dark spot. Swirling SAW can be useful in fragile sensors whose neighborhood needs vigorous actuation, and may also serve as integrated transducers for acoustical vortices. Since these waves are essential to fine acoustical tweezing, swirling SAW may become the cornerstone of future micrometric devices for contactless manipulation.

Estimation and Statistical Analysis of Human Voice Parameters to Investigate the Influence of Psychological Stress and to Determine the Vocal Tract Transfer Function of an Individual

In this study the principal focus is to examine the influence of psychological stress (both positive and negative stress) on the human articulation and to determine the vocal tract transfer function of an individual using inverse filtering technique. Both of these analyses are carried out by estimating various voice parameters. The outcomes of the analysis of psychological stress indicate that all the voice parameters are affected due to the influence of stress on humans. About 35 out of 51 parameters follow a unique course of variation from normal to positive and negative stress in 32% of the total analyzed signals. The upshot of the analysis is to determine the vocal tract transfer function for each vowel for an individual. The analysis indicates that it can be computed by estimating the mean of the pole zero plots of that individual’s vocal tract estimated for the whole day. Besides this, an analysis is presented to find the relationship between the LPC coefficients of the vocal tract and the vocal tract cavities. The results of the analysis indicate that all the LPC coefficients of the vocal tract are affected due to change in the position of any cavity.

Strain sensitivity model for guided waves in plates using the time-reversal technique

The use of the time-reversal technique to detect variations in the external applied traction in a strip of aluminum plate is presented. Experiments have been performed in transmission-reception mode, using two fixed ultrasonic transducers near the ends of the plate. Two different parameters of the time-reversed signal are used for monitoring the strain state, the peak amplitude and the focusing time. The inverse filter technique was used in the transmitted signal to equalize the amplitude spectrum, enhancing the sensitivity in the focus amplitude by 5 times. The external traction sensitivity was experimentally verified by using an aluminum plate (800 × 100 × 3 mm) and two 5-MHz ultrasonic transducers spaced 700 mm apart. At the maximum strain state (180 μstrain), the peak value was reduced by about 10% in the conventional process and by 50% using the inverse filter. To evaluate the effects of the strain in the time-reversal signal, a theoretical model was constructed. This model is successful in predicting the changes in the group delay and, consequently, the focusing time using a linear equation. This relationship can also be used to determine the strain level quantitatively. The experimental results show that the time-reversal signal technique can be used in practical monitoring of changes at the strain level in mechanical structures.

Dereverberation of a closed test section of a wind tunnel with a mult imicrophones cesptral method

Today, the manufacturers of aircraft want to perform at the same time aeroacoustic and classical aerodynamic tests in the wind tunnels with closed test sections in order to reduce test duration and costs involved in new aircraft developments. However, this kind of wind tunnels is not optimal for acoustics testing because the measurements are often masked by background noise (i.e., noise generated by the wind tunnel flow mechanism) and reverberations on the walls without acoustic liner within the test section. This is a serious problem because these unwanted phenomena limit the accuracy of identification, and quantification of acoustic sources. The reduction of background noise can be obtained with subspace techniques or noise subtraction based on a noise reference measurement. Concerning the reduction of the contamination of the echos, it can be achieved with an inverse filtering technique or with a single microphone cepstral method. A new approach is proposed in this study to remove the spurious effects of the echos. It is based on multi microphones cesptral technique. This new technique has been successfully applied on numerical simulations, and experimental data. The theoretical developments of this method and the obtained results will be presented in the full paper.

Dynamic time reversal acoustic focusing of ultrasound for biomedical applications

Time Reversal Acoustic (TRA) system provides effective focusing in inhomogeneous media that can be used in various biomedical applications including high intensity ultrasound treatment, ultrasound-assisted drug delivery, ultrasonic battery charging of implants, etc. In many cases, the TRA focusing is conducted in tissues with time varied properties and variation in propagating media degrades the TRA focused signal. Dynamic focusing is required to maintain the same acoustic intensity in the focus. We suggest adjustment to the radiated signal in order to maintain the maximal amplitude in the focus using the inverse filtering technique with Tikhonov regularization. The suggested algorithm enables to determine the radiated signal based on the changes in the focused signal received by a beacon. Developed algorithm of TRA refocusing was tested in the experiments with a TRA system with an aluminum reverberator comprising ten piezotransducers with resonance around 1 MHz, DAC control system and a multi-channel binary amplifier. This system was built in Artann Laboratories for investigation of time reversal acoustic focusing for hyperthermia therapy and drug delivery. The dynamic TRA refocusing is shown to be effective even in the case of significant lateral shifts in the media and strong focused signal variation.

Blind deconvolution by a Newton method on the non‐unitary hypersphere

SUMMARYBlind deconvolution is an inverse filtering technique that has received increasing attention from academia as well as industry because of its theoretical implications and practical applications, such as in speech dereverberation, nondestructive testing and seismic exploration. An effective blind deconvolution technique is known as ‘Bussgang’, which relies on the iterative Bayesian estimation of the source sequence. Automatic gain control in blind deconvolution keeps constant the energy of the inverse filter impulse response and controls the magnitude of the estimated source sequence. The aim of the present paper is to introduce a class of Newton‐type algorithms to optimize the Bussgang cost function on the inverse‐filter parameter space whose geometrical structure is induced by the automatic‐gain‐control constraint. As the parameter space is a differentiable manifold, the Newton‐like optimization method is formulated in terms of differential geometrical concepts. The present paper also discusses convergence issues related to the introduced Newton‐type optimization algorithms and illustrates their performance on a comparative basis.Copyright © 2012 John Wiley & Sons, Ltd.

Time reversed acoustics techniques for elastic imaging in reverberant and nonreverberant media: An experimental study of the chaotic cavity transducer concept

In view of emerging imaging technologies based on the combination of Time Reversed Acoustics (TRA) with Nonlinear Elastic Wave Spectroscopy (NEWS) for the detection and localization of micro-damage in solids, we have investigated the benefits of chirped source signal excitation, inverse filtering techniques, and the implementation of chaotic cavity transducers to improve the quality of energy focusing, especially for weakly reverberant media. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal Time Reversal (or Inverse Filter) technique on the other hand. Experimental data for reverberant and nonreverberant composite plates show that the use of chirps, inverse filtering and chaotic cavity transducers significantly enhances the focusing process, and enables focusing in a nonreverberant medium using only one transducer. As a potential exploitation, the application of the chaotic cavity transducer concept for synthetic imaging is examined, revealing several properties similar to phased arrays.

Inverse filtering based method for estimation of noisy autoregressive signals

In this paper we present a new method for estimating the parameters of an autoregressive (AR) signal from observations corrupted with white noise. The least-squares (LS) estimate of the AR parameters is biased when the observation noise is added to the AR signal. This bias is related to observation noise variance. The proposed method uses inverse filtering technique and Yule–Walker equations for estimating observation noise variance to yield unbiased LS estimate of the AR parameters. The performance of the proposed unbiased algorithm is illustrated by simulation results and they show that the performance of the proposed method is better than the other estimation methods.

Application of the chaotic cavity transducer concept for imaging in non-reverberant media.

The concept of chaotic cavity transducer utilizes a combination of a piezoelectric ceramic disk glued on a cavity of chaotic shape on the hardware side, with the time reversal (or inverse filter) technique on the software side. Using a two step procedure of direct excitation-reception and time reversed excitation-reception, it is possible to focus energy anywhere inside a non-reverberating sample, thanks to the ergodicity of the chaotic cavity. Moreover, the same basic concept can be used to create a virtual phased array using a single channel device. Both experimental data and simulations will be provided to illustrate the concepts. The goal is to use the chaotic cavity transducer concept to enhance the localization of micro-damage coupled to nonlinear elastic wave spectroscopy methods.

On the use of a chaotic cavity transducer in nonlinear elastic imaging

We discuss the advantages of “chaotic cavity transducer focusing” to enhance the localization of microdamage in conjunction with nonlinear elastic wave spectroscopy methods. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal time reversal (or inverse filter) technique on the other hand. Additional optimization through the use of sweeps and inverse filtering techniques are discussed as well. The technique is applied to image a crack in a steel sample.

Experimental observation of azimuthal shock waves on nonlinear acoustical vortices

Thanks to a new focused array of piezoelectric transducers, experimental results are reported here to evidence helical acoustical shock waves resulting from the nonlinear propagation of acoustical vortices (AVs). These shock waves have a three-dimensional spiral shape, from which both the longitudinal and azimuthal components are studied. The inverse filter technique used to synthesize AVs allows various parameters to be varied, especially the topological charge which is the key parameter describing screw dislocations. Firstly, an analysis of the longitudinal modes in the frequency domain reveals a wide cascade of harmonics (up to the 60th order) leading to the formation of the shock waves. Then, an original measurement in the transverse plane exhibits azimuthal behaviour which has never been observed until now for acoustical shock waves. Finally, these new experimental results suggest interesting potential applications of nonlinear effects in terms of acoustics spanners in order to manipulate small objects.