Impulse Response Time Research Articles

Refining the Perfusion–Diffusion Mismatch Hypothesis

The Echoplanar Imaging Thrombolysis Evaluation Trial (EPITHET) tests the hypothesis that perfusion-weighted imaging (PWI)-diffusion-weighted imaging (DWI) mismatch predicts the response to thrombolysis. There is no accepted standardized definition of PWI-DWI mismatch. We compared common mismatch definitions in the initial 40 EPITHET patients. Raw perfusion images were used to generate maps of time to peak (TTP), mean transit time (MTT), time to peak of the impulse response (Tmax) and first moment transit time (FMT). DWI, apparent diffusion coefficient (ADC), and PWI volumes were measured with planimetric and thresholding techniques. Correlations between mismatch volume (PWIvol-DWIvol) and DWI expansion (T2(Day 90-vol)-DWI(Acute-vol)) were also assessed. Mean age was 68+/-11, time to MRI 4.5+/-0.7 hours, and median National Institutes of Health Stroke Scale (NIHSS) score 11 (range 4 to 23). Tmax and MTT hypoperfusion volumes were significantly lower than those calculated with TTP and FMT maps (P<0.001). Mismatch > or =20% was observed in 89% (Tmax) to 92% (TTP/FMT/MTT) of patients. Application of a +4s (relative to the contralateral hemisphere) PWI threshold reduced the frequency of positive mismatch volumes (TTP 73%/FMT 68%/Tmax 54%/MTT 43%). Mismatch was not significantly different when assessed with ADC maps. Mismatch volume, calculated with all parameters and thresholds, was not significantly correlated with DWI expansion. In contrast, reperfusion was correlated inversely with infarct growth (R=-0.51; P=0.009). Deconvolution and application of PWI thresholds provide more conservative estimates of tissue at risk and decrease the frequency of mismatch accordingly. The precise definition may not be critical; however, because reperfusion alters tissue fate irrespective of mismatch.

System identification of linear structures based on Hilbert–Huang spectral analysis. Part 2: Complex modes

AbstractA method, based on the Hilbert–Huang spectral analysis, has been proposed by the authors to identify linear structures in which normal modes exist (i.e., real eigenvalues and eigenvectors). Frequently, all the eigenvalues and eigenvectors of linear structures are complex. In this paper, the method is extended further to identify general linear structures with complex modes using the free vibration response data polluted by noise. Measured response signals are first decomposed into modal responses using the method of Empirical Mode Decomposition with intermittency criteria. Each modal response contains the contribution of a complex conjugate pair of modes with a unique frequency and a damping ratio. Then, each modal response is decomposed in the frequency–time domain to yield instantaneous phase angle and amplitude using the Hilbert transform. Based on a single measurement of the impulse response time history at one appropriate location, the complex eigenvalues of the linear structure can be identified using a simple analysis procedure. When the response time histories are measured at all locations, the proposed methodology is capable of identifying the complex mode shapes as well as the mass, damping and stiffness matrices of the structure. The effectiveness and accuracy of the method presented are illustrated through numerical simulations. It is demonstrated that dynamic characteristics of linear structures with complex modes can be identified effectively using the proposed method. Copyright © 2003 John Wiley &amp; Sons, Ltd.

System identification of linear structures based on Hilbert–Huang spectral analysis. Part 1: normal modes

AbstractBased on the Hilbert–Huang spectral analysis, a method is proposed to identify multi‐degree‐of‐freedom (MDOF) linear systems using measured free vibration time histories. For MDOF systems, the normal modes have been assumed to exist. In this method, the measured response data, which are polluted by noises, are first decomposed into modal responses using the empirical mode decomposition (EMD) approach with intermittency criteria. Then, the Hilbert transform is applied to each modal response to obtain the instantaneous amplitude and phase angle time histories. A linear least‐square fit procedure is proposed to identify the natural frequency and damping ratio from the instantaneous amplitude and phase angle for each modal response. Based on a single measurement of the free vibration time history at one appropriate location, natural frequencies and damping ratios can be identified. When the responses at all degrees of freedom are measured, the mode shapes and the physical mass, damping and stiffness matrices of the structure can be determined. The applications of the proposed method are illustrated using three linear systems with different dynamic characteristics. Numerical simulation results demonstrate that the proposed system identification method yields quite accurate results, and it offers a new and effective tool for the system identification of linear structures in which normal modes exist. Copyright © 2003 John Wiley &amp; Sons, Ltd.

ZnSSe-based ultra-violet photodiodes with extremely high detectivity

A noise study on a set of Zns 0.58 Se 0.42 UV photodiodes with 1 μm thick Al bonding pads selected from a 2 wafer was carried out. Based on their 1/f noise behaviors, they were divided into three groups. The noise spectral density, impulse response time, bandwidth, responsivity, dark current at reverse bias up to 12 V and zero bias resistance were determined for three typical detectors, i.e. one from each of the three groups. The uneven performance of these detectors is mostly likely due to the different damage level induced in the wire-bonding process. With thicker Al bonding pads of 1.5 μm, a detectivity as high as 7.1 x 10 14 cm Hz 1/2 W -1 has been achieved.

Rapid identification of a sparse impulse response using an adaptive algorithm in the Haar domain

This paper proposes a fast convergence adaptive algorithm for identifying a sparse impulse response that is rich in spectral content. A sparse impulse response is referred here as a discrete time impulse response that has a large number of zero or near zero coefficients. The basic idea for rapid identification is to automatically determine the locations of the nonzero impulse response coefficients for their adaptation and eliminate the unnecessary adaptation of zero coefficients. The proposed method, which is called the Haar-Basis algorithm, employs a transform approach by modeling the sparse impulse response in the Haar domain. The Haar transform has many basis sets and each of them contains basis vectors that span the entire time domain range. This special nature of the Haar transform allows for the selection of one small subset of adaptive filter coefficients whose basis vectors span the entire range of the impulse response. These coefficients are adapted at the beginning and are then used subsequently to identify, from the hierarchical structure of the Haar transform, the rest of the filter coefficients that must be adapted to correctly model the unknown sparse impulse response. The consequence is avoiding adaptation of many zero coefficients, leading to a dramatic improvement in either convergence speed or steady state excess mean-square error (EASE), while requiring no a priori knowledge such as the number of nonzero coefficients in the unknown sparse impulse response. The proposed algorithm has been tested with a variety of unknown sparse systems using both white noise input and colored input whose spectrum closely resembles that of speech. Simulation results show that the new approach provides promising results.

Rapid identification of a sparse impulse response using an adaptive algorithm in the Haar domain

This paper proposes a fast convergence adaptive algorithm for identifying a sparse impulse response that is rich in spectral content. A sparse impulse response is referred here as a discrete time impulse response that has a large number of zero or near zero coefficients. The basic idea for rapid identification is to automatically determine the locations of the nonzero impulse response coefficients for their adaptation and eliminate the unnecessary adaptation of zero coefficients. The proposed method, which is called the Haar-Basis algorithm, employs a transform approach by modeling the sparse impulse response in the Haar domain. The Haar transform has many basis sets and each of them contains basis vectors that span the entire time domain range. This special nature of the Haar transform allows for the selection of one small subset of adaptive filter coefficients whose basis vectors span the entire range of the impulse response. These coefficients are adapted at the beginning and are then used subsequently to identify, from the hierarchical structure of the Haar transform, the rest of the filter coefficients that must be adapted to correctly model the unknown sparse impulse response. The consequence is avoiding adaptation of many zero coefficients, leading to a dramatic improvement in either convergence speed or steady state excess mean-square error (EASE), while requiring no a priori knowledge such as the number of nonzero coefficients in the unknown sparse impulse response. The proposed algorithm has been tested with a variety of unknown sparse systems using both white noise input and colored input whose spectrum closely resembles that of speech. Simulation results show that the new approach provides promising results.

Blind adaptive receiver for coded multicarrier DS-CDMA systems in multipath fading channels

The authors present a multicarrier asynchronous direct sequence code division multiple access (DS-CDMA) system which uses an adaptive blind algorithm for the optimum combining of the desired signal contributions from different carriers. The output of the transmitter convolutional encoder modulates multiple band-limited DS-CDMA waveforms which are parallel transmitted with different carrier frequencies. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver and a RAKE receiver with a sliding window decorrelator filter. In this latter case, the RAKE receiver uses pilot symbols to track the channel impulse response time variations. Results demonstrate a performance similar to that of a decorrelating detector in the context of a severe near–far situation while exhibiting lower complexity.

A generalized modal impulse response and Fourier transform approach to investigate acoustic transient Bessel beams and Bessel bullets

A generalized modal space–time impulse response and Fourier transform approach is developed to investigate the general properties of transient Bessel beam (TBB) fields which are generated using planar circular apertures. The approach is based on a modal decomposition of the aperture source distributions which are required to generate the space–time TBB fields. Numerical results are presented to illustrate the general space–time properties of the modal impulse responses and the modal source functions. The on-axis far field for the finite aperture is shown to be simply related to the lowest order term in the modal expansion. The space–time properties of the acoustic field for a particular type of band-limited TBB field which is designated as an acoustic Bessel bullet (BB) are then investigated. Some numerical results are presented to illustrate the space–time properties of acoustic BB fields generated from an infinite aperture. Analogous on-axis harmonic results from a finite aperture are presented. These latter results clearly indicate that the higher-order modes exhibit spatial rates of decay which are greater than the inverse range decay for the lowest order mode.

PMD second-order effects on pulse propagation in single-mode optical fibers

The time-space varying birefringence in single-mode optical fibers causes the polarization mode dispersion (PMD) to be a serious problem in high bit-rate transmissions. The PMD first- and second-order statistics are well known in the literature, but second-order PMD-induced pulse distortions have still to be clarified for sequences of pulses of arbitrary shape. We give, for the first time, the exact PMD time impulse response, up to second order. We show, both numerically and experimentally, that the effect of the rotation of the principal states of polarization (PSP) is to generate power overshoots on the "1" and "0" bit sequences.

High-speed low-temperature-grown GaAs p-i-n traveling-wave photodetector

We report a novel type of p-i-n traveling-wave photodetector utilizing low-temperature-grown GaAs (LTG-GaAs). The devices show a record impulse response time (530-fs fullwidth at half-maximum, /spl sim/560 GHz -3-dB bandwidth) which agrees with theoretical estimates. The effects of various limiting factors on the device performance were analyzed theoretically and compared with measurements obtained by electrooptic characterization of our devices. Calculations indicate that the device speed is dominated by the short carrier lifetime. DC external quantum efficiencies as high as 8% were obtained.

THERMAL NON-LINEAR EFFECTS ON UNDERWATER SOUND GENERATION BY LASER RADIATION

In the present paper, the authors study the thermal non-linear effects on the excitation of sound generation which is applicable to the determination of the pressure in the direction of the laser beam and at an angle with the laser beam. An additional pressure term appears due to this non-linear effect. Considering the small amplitude waves, the impulse responses of the system corresponding to both the pressure terms are determined. The pressure corresponding to the main term is expressed as convolution between the impulse response and the time derivative of the laser intensity and that corresponding to the additional term is given as a convolution between the system's impulse response and double time derivative of the square of the energy released in timet. The effects of the thermal non-linearity on the sound pressure are studied, considering the different time profiles of the laser pulse. It is found that besides the thermal non-linearity, the pressure profile depends on laser parameters and the shape. It is found that the pressure profiles are drastically different for the small and long thermoacoustic arrays. The thermal non-linearity affects the pressure waveform and the effect is pronounced for a short source. The results obtained are compared with those of previous studies. A reasonable explanation of the experimental observations of anomalous behaviour of underwater sound generation is also provided, on the basis of thermal non-linearity.

Near and mid infrared silicon/germanium based photodetection

Short-period silicon/germanium (Si m Ge n ) superlattice and SiGe quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on (100)Si substrates for near (1.3 μm) and mid-infrared (3–5 μm; 8–12 μm) detection. For the near IR detection – suitable for fibre optical communication within a Si integrated circuit (IC) chip – short-period Si m Ge n superlattices with period lengths of several atomic monolayers (ML; e.g. m= n=5 ML; 1 ML~1.4 Å) and 2–4 monolayers wide p-doped Ge quantum well layers separated by 20 MLs of Si and embedded in two 10-nm thick Si 1− x Ge x layers have been grown. In the superlattice structure the zone folding effect (U. Gnutzmann, K. Clausecker, Appl. Phys. 3 (1974) 9) has been predicted to produce strong interband transitions near 0.8 eV (≈1.3 μm), in the latter one the sharp Si/Ge hetero-interfaces break the k-selection rules and strong localisation of electron and hole wave function favour a strong interband excitonic transition at 1.3 μm. This results in a rather efficient room temperature photo- and electroluminescence and in sufficient absorption. An integrated waveguide/photodetector deposited on a SIMOX (Si substrate with separation by implantation of oxygen) substrate has been fabricated and an external quantum efficiency of 11% with an impulse response time of 400 ps has been observed. For the mid IR range (3–5 μm) highly p-doped Si/SiGe quantum well detectors have been deposited on an undoped, double-sided polished Si substrate based on hetero-internal photoemission (HIP) over the Si/SiGe barrier. The absorption and photocurrent spectra have been measured from fabricated mesa detectors at 77 K. The photoresponse spectrum of the HIP detectors is shown to be widely tunable in the technological important wavelength band 3–5 μm by choice of Ge-content, well thickness and doping level. Quantum efficiencies of ~1% at 4 μm and 77 K have been achieved from SiGe HIP structures, dark currents as low as 10 −8 A/cm 2 can be obtained by modulation doping. Detectivity values of D* of 10 9 cm √Hz/W have been achieved, the quantum efficiency spectrum is considerably broader and up to a factor of 4 higher than Pt:Si at 4 μm.

Conference system with automatic speaker detection and speaker unit

A conference system comprises speaker units (12) coupled to a central unit (14). The speaker unit (12) comprises an echo canceller (20) with an adaptive filter (38) having an impulse response of comparatively short duration. In a speech pause of the user of the speaker unit (12) the common listening signal (LS) of the conference system is applied to the loudspeaker (18). Of the sounds then picked up by the microphone (16) only those sounds are compensated which are produced by the loudspeaker (18) and which reach the microphone (16) directly and which reach the microphone (16) indirectly via reflections from nearby objects. These sounds have a comparatively short impulse response time and enable a comparatively simple adaptive filter (38) to be used. The other sounds picked up by the microphone (16) reach the microphone (16) via reflections from the walls of the conference space (10) in which the conference system is situated. The other sounds have a comparatively long impulse response time. For the purpose of automatic speech detection these other sounds do not require echo cancellation because they are the same for all the speaker units. Automatic speech detection is now accomplished if the signal strength of the compensated microphone signal (MCCS) produced by the echo canceller (20) exceeds the average signal level of the speaker units.

Precise navigation using adaptive FIR filtering and time domain spectral estimation

This paper introduces a new low cost, short range, positioning system based on adaptive finite impulse response (FIR) filtering and time domain spectral estimation. The system can determine absolute positions with a high degree of accuracy and is well suited for real time navigation. The approach is based upon signal processing techniques and a priori knowledge of the system transfer function. The first step is to measure the phase response of the linear transfer function and then using a FIR filter the time response of the system can be determined. The FIR filter computes the time response by performing a deconvolution between the measured phase response, and the complex conjugate of the transfer function. By correlating the known input impulse response with the output of the FIR filter, an error term is generated. The time delay of the system is determined by adjusting the FIR filter coefficients to minimize the error term. Simulated analysis of the system indicates a worst case error of /spl plusmn/16 cm. >

Broadband geoacoustic inversion in shallow Arctic water

A broadband technique used successfully to localize acoustic sources and determine water depth and array position at a deep Pacific Ocean site [P. C. Mignerey, Oceans ’90 Proc., 121–124] is used to determine geoacoustic parameters at a shallow water site in the Arctic. The technique minimizes differences between calculated impulse response times and pulse arrival times measured on a vertical array. Initially the method is used to localize the source (SUS) and determine bottom depth and array orientation. The method is then applied to pulse arrivals that penetrate the upper sedimentary layers of the shallow bottom. The inversion provides compressional speed profiles and sediment layer thicknesses of the bottom near the array site. Differences between calculated arrival times and data show a significant decrease, i.e., improvement, when using the geoacoustic profile as determined by the inversion technique. Broadband matched‐field processing is used to demonstrate inversion effects on processor performance. The results demonstrate the broadband method to be useful for geoacoustic surveys of shallow water environments.

Determination of gap-state distributions in amorphous semiconductors from transient photocurrents using a fourier transform technique

Abstract We present a novel spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay exhibiting either anomalous or conventional dispersion. A numerical Fourier Transform procedure is used to convert the impulse response time decay data to phase and a]plitude frequency spectra. The DOS is then computed using a procedure developed for analysis of modulated photocurrent (MPC) data in a multiple-trapping context. The method avoids many of the distortions and practical difficulties associated with other TPC analytical techniques. Computer generated TPC and MPC data are used to evaluate the transform technique in comparison with a ‘direct’ interpretive method for model systems of discrete and distributed states.

Lowpass filtering method for heart rate variability analysis

Various methods have been proposed for the spectral analysis of heart rate variability (HRV). Such methods always require constant period data in time series. However, since the R-R interval of HRV is, in general, irregular, somehow the experimental data must be converted to constant period data. Assuming an integral pulse frequency modulation (IPFM) model, constant period data can be obtained by passing the heartbeat impulse train through a digital filter using the alias-free method proposed by French and Holden. However, the forementioned method has two problems; that is: (1) the frequency characteristics of the lowpass filter are assumed to be ideal, hence the impulse response function of the filter spreads infinitely with respect to time; and (2) only pulses inside the analyzing section are considered while those outside the analyzing section in the convolution integral are ignored. To solve those problems, the method of French and Holden has been modified in the following three points: (1) such a finite impulse response digital filter was used that the sidelobe of the filter decays rapidly in its impulse response function; (2) the analyzing section as well as its impulse response time were lengthened, thus the pulses outside the analyzing section were considered; and (3) an appropriate window was used. With these improvements, the truncation error and the computation time could be reduced. Furthermore, as a result of simulation, a much higher accuracy in the demodulation of the heartbeat signal was obtained.

The relationship between the impulse response and angular spectrum methods to evaluate acoustic transient fields

Impulse response time domain and angular spectrum or wave vector time-domain methods can be used to investigate the spatial temporal characteristics of acoustic fields from planar ultrasonic transducers which are subjected to pulsed wideband excitations. Both methods are developed here from the time-dependent Green’s function solution of the initial-boundary value problem. The close relationship of the methods is made apparent by the use of spatial-temporal Fourier transform techniques of analysis. Axisymmetric fields are then addressed as a special case via the use of Hankel and Fourier transform techniques. Finally, classical solutions to the associated harmonic boundary value problems are then shown to result from temporal Fourier transforms of the time-dependent field solutions.

Application of the generalized time-frequency representation cone-shaped kernel to structural acoustics data.

In the field of target structural acoustics, the backscattered signal is determined by the structural properties of the object and acoustic interaction phenomena. Traditionally, information about the target is obtained by observing the impulse response (time domain) or the frequency response (transfer function). For a detailed study of the scatterer’s structure, a joint time-frequency representation can reveal some inherent features that do not stand out in a single domain representation. The generalized time-frequency representations (GTFR) can provide a good estimate of the spectral components in the signal, and resolve the starting time of these spectral peaks in the time-frequency plane. The GTFR is characterized by a kernel, and the properties of a particular representation are determined by the constraints imposed on its kernel. Here, the cone-shaped kernel is compared to the spectrogram and Wigner distribution, and shown to provide and improved tool for understanding the backscattering phenomena. The method is illustrated with data from a cylindrical shell in water.

Anomalous geomagnetic fields of internal origin in Czechoslovakia

The magnetovariational data of 143 stations distributed over the eastern margin of the Bohemian Massif, over the Brunovistulicum and the West Carpathian sector were analysed to obtain transfer functions of the geomagnetic field. Methods of multivariate coherence analysis (spectral domain approach) and of impulse response (time domain approach) were employed. Complex induction vectors were estimated and contour maps of transfer functions were computer generated. Analysing their spatial distribution, we mapped the zones of anomalous induction and interpreted them in terms of electrical conductivity structure with its tectonic implications.