Damage Detection in Composite Plates by Using an Enhanced Time Reversal Method

A damage detection technique, which does not rely on any past baseline signals, is proposed to assess damage in composite plates by using an enhanced time reversal method. A time reversal concept of modern acoustics has been adapted to guided-wave propagation to improve the detectability of local defects in composite structures. In particular, wavelet-based signal processing techniques have been developed to enhance the time reversibility of Lamb waves in thin composite laminates. In the enhanced time reversal method, an input signal at an excitation point can be reconstructed if a response signal measured at another point is reemitted to the original excitation point after being reversed in a time domain. This time reversibility is based on linear reciprocity of elastic waves, and it is violated when nonlinearity is caused by a defect along a direct wave path. Examining the deviation of the reconstructed signal from the known initial input signal allows instantaneous identification of damage without requiring the baseline signal for comparison. The validity of the proposed method has been exemplified through experimental studies on a quasi-isotropic laminate with delamination.     

Effects of lossy compression on lesion detection: predictions of the nonprewhitening matched filter

The nonprewhitening matched filter (NPWMF) was used to study the effect of wavelet-based lossy image compression on lesion detectability. Two classes of images, representing lesion-present and lesion-absent cases, were generated by simulation techniques. The performance index, da, of the NPWMF was calculated from the reconstructed images of the two classes and was used as an objective measure for quantifying signal detectability. The effect of compression on detectability was analyzed by defining signal amplitude, signal size, and noise amplitude at a constant da. Since human observer studies directly correlate with the performance index da, the effect of signal parameters on compression ratios could be evaluated. For example, a signal with 7 pixels in diameter and an amplitude of 10 that was compressed at a ratio of 4.6:1, and a signal of the same size with an amplitude of 15 but compressed at a ratio of 24.3:1, both had a da of 4.1, implying identical detectability. Similar results for other combinations of signal and noise parameters, but at constant da, can be used to analyze the effect of compression on detectability without requiring human observer studies.     

Percutaneous myocardial revascularization procedures in Brazil in 1996-1997 compared to the 1992-1993 period. A report of the National Registry--National Center for Cardiovascular Interventions (CENIC)

Electrogastrography (EGG) is the recording of gastric electrical activity (GEA) from the body surface. The cutaneous signal is low in amplitude and consequently must be amplified considerably. The resultant signal is heavily contaminated with noise, and visual analysis alone of an EGG signal is inadequate. Consequently, EGG recordings require special methodology for acquisition, processing and analysis. Essential components of this methodology involve an adequate system of digital filtering, amplification and analysis, along with minimization of the sources of external noise (random motions of the patient, electrode-skin interface impedance, electrode bending, obesity, etc) and a quantitative interpretation of the recordings. There is a close relationship between GEA and gastric motility. Although it has been demonstrated that EGG satisfactorily reflects internal GEA frequency, there is not acceptable correlation with gastric contractions or gastric emptying. Many attempts have been made to relate EGG 'abnormalities' with clinical syndromes and diseases; however, the diagnostic and clinical value of EGG is still very much in question.     

Characterization of the spatial-frequency spectrum in the perception of shape from texture

The major cue to shape from texture is the compression of texture as a function of surface curvature [J. Exp. Psychol. 13, 242 (1987); Vision Res. 33, 827 (1993)]. A number of computational models have been proposed in which compression is measured by detection of changes in the spatial-frequency spectrum [Comput. Graphics Image Process. 5, (1976)]. We propose that the visual system uses a strategy of characterizing the frequency spectrum by a simple set of measures and of tracking the changes in this characterization rather than determining changes in the shape of the actual spectra. Our evidence is based on a number of psychophysical demonstrations that use stimuli with specifically tailored frequency spectra, constructed from white noise filtered in the frequency domain. Our evidence suggests that the visual system determines the average peak frequency of the spectrum and uses this measure as its characterization. Changes in fp are strongly correlated with the degree of surface curvature, and, over a range of stimuli, fp takes account of the variance in local estimates of the frequency spectrum. One computes fp by determining the peak frequency at each spatial location and then averaging these frequency values over a local spatial region. We show that fp is related to the second-order moment but is more biologically plausible and shows superior ability to function in the presence of noise. As a test of this model, we have constructed a neural network architecture for computing shape from texture. Our model is limited to orthographically projected, homogeneous textures without in-surface rotation. The early stages of the model consist of multiple simple-cell units tuned to different orientations and spatial frequencies. We show that these simple cells are inadequate for the determination of compression but that the outputs of complex-cell-like units after normalization generate estimates of surface slant and tilt. The network shows qualitative agreement with human perception of shape from texture over a wide range of real and artificial stimuli.     

Computer signal processing of long duration biotelemetric brain data

The EEG represents brain processing under diverse physiological conditions. A complete system involving acquisition and quantitation of this important information about brain function is described. The time-domain EEG and other biological signals are obtained using a multichannel PAM/FM biotelemeter mounted on the head of the experimental animal. This data is transmitted, demodulated and recorded by electronic recording techniques. A computer-based EEG analysis system is described for acquiring the primary data and transforming it into the frequency domain using Fourier methods. The computing system is developed to semi-automatically signal process about 4 h of eight channel EEG records. Data compression by plotting in a quasi-three-dimensional spectral profile allows visual correlations of pattern features to drug manipulations, etc. The software programs are briefly described for each step in signal processing. The feasibility of the complete system approach is demonstrated using biotelemetry to acquire low voltage EEG signals without behavioral distortions or introduction of artifacts by cables.     

Acoustic correlates of dysphonia: type and severity

The purpose of this study was to explore the acoustic discrimination and graded severity of three clinical voice types. Listeners classified 102 samples of dysphonic vowels /a/ and /i/ on the basis of voice types: breathy, hoarse, and strained. The vowels were analyzed acoustically with two measures of perturbation and 2 measures of spectral noise. Discriminant analysis showed that apriori, acoustic classifications of voice type were made with 92% accuracy using four acoustic parameters: (a) cepstral peak prominence (CPP), (b) jitter standard deviation (SD-J), (c) fundamental frequency (F0), and (d) standard deviation of signal-to-noise ratio (SD-SNR). Findings suggest that voice type is associated with the interaction of spectral noise, fundamental frequency, and signal irregularity, and that dysphonic severity is associated with similar parameters, regardless of voice type.     

Role of histamine in the regulation of coronary circulation

AIMS/BACKGROUND: An objective method for detecting hemifield and quadrantic visual field defects has been developed using steady state visual evoked cortical potentials (VECPs), an adaptive noise canceller (ANC), and Hotelling's t2 statistic. The purpose of this study was to determine the sensitivity and specificity of the technique. METHODS: Nine subjects (mean age 44 years) were investigated with field loss due to a variety of causes including both anterior and posterior visual pathway lesions. Dynamic perimetry was performed by means of a Goldmann or Tübingen perimeter. VECP recordings were made from each visual field quadrant (23 degrees X 23 degrees) by means of a steady state reversing checkerboard (7.7 rev/s). The central 5 degrees of the visual field and the vertical and horizontal meridians were masked during these measurements. Recordings were made from three electrode sites, positioned over the visual cortex, relative to a mid frontal electrode. Each recording lasted 2 minutes, during which time fixation was monitored. The data from each recording were divided into 4 second segments, and the amplitude and phase of the VECP signal measured using the ANC. Hotelling's t2 statistic was applied to determine the probability of signal detection. Receiver operating characteristic curves were used to find the optimum signal detection threshold for identification of the visual field defects. RESULTS: The results of the study confirmed patterns of subjective visual field loss. The technique had a sensitivity and a specificity of 81% and 85%, respectively, for detecting 'non-seeing' areas in the inferior visual field, and 82% and 89%, respectively, for detecting 'non-seeing' areas in the superior visual field. CONCLUSION: These results demonstrate that the technique is of potential clinical value to ophthalmologists and neurologists when subjective perimetry is not possible.     

基于移频技术的短时傅里叶变换阶比分析

提出了一种基于移频技术的短时傅里叶变换阶比分析算法.该算法利用傅里叶变换在频域的卷积性质,对原始信号在时域乘以e-j2πf_it使f_i的频谱能量搬迁到零频处,按一定的频率间隔改变f_i就可以在零频处得到其他频率的频谱能量,以此来提高短时傅里叶变换在时频分析中的频率分辨率.然后在时频面上进行局部阈值降噪,同时跟踪转速的变化,最终应用到变速机械的阶比分析中.与短时傅里叶变换分析结果对比表明,本文方法可以更加准确地跟踪到实际的转速.实际降速过程中轴承信号利用本文方法进行阶比分析,成功提取到轴承的故障特征频率.      

Numerical signal treatment for optimized alanine/ESR dosimetry in the therapy-level dose range

Electron spin resonance (ESR) spectra of alanine detectors irradiated to absorbed doses below 5 Gy are affected by a varying non-linear background which mainly influences the lower limit of detection in alanine/ESR dosimetry. A mathematical method based on fast Fourier transform is described capable of filtering simultaneously background and noise in the frequency domain of ESR spectra. It provides clearer alanine/ESR signals down to 50 mGy. Even in non-irradiated but long-term stored alanine detectors an ESR signal could be observed similar to irradiated alanine (pre-signal). A linear ESR signal vs absorbed dose relationship was found above 200 mGy, after correction for background and pre-signal. The number of repeated ESR read-out cycles and hence the time required for a precise and reliable low-dose evaluation have significantly been reduced. The method has been worked out for the therapy-level dosimetry range and tested on a Bruker ESP 300 and for comparison Bruker EMS 104 ESR spectrometer.     

Resolving capacity of the human auditory analyzer

Resolution of human auditory analyser to tine-impulse signals in the noise has been studied AX method was put in the basis of the experiment. Signals were the sinusoid intercepts. Relationships between probability of erroneous distinguishing of the signals and untuning, signal/noise and number of periods in the frequency range from 150 to 4800 Hz. It has been shown that the accepting auditory properties are invariants to the time transformations, the wave number of the sinusoid being the determining characteristics of the stimulus. A concept of frequency--time band of the auditory system is introduced. Its width according to the experimental data depends on the signal/noise ratio.     

Auditory frequency discrimination in the white rat

Frequency discrimination was investigated in the albino rat using a modified go/no-go positive reinforcement procedure in which subjects reported frequency increments in an ongoing series of pure tone bursts. Weber ratios (frequency difference limen/frequency) were measured from 5 to 32 kHz at 50 dB sound pressure level. A signal detection analysis of the procedure enabled a direct comparison to be made with the rat's performance in a discrete trial go/no-go task. A mean Weber ratio of 3.06+/-0.44% was measured in the frequency range 5-32 kHz. This indicates that the rat has better frequency discrimination acuity than has previously been thought. The result is discussed in the context of factors affecting performance. Among the factors that were explored we found that long training times and the specific training paradigm played important roles. In comparison to discrete trial go/no-go paradigms, rats performed much better when detecting signals from a repeating background. Frequency discrimination performance decreased linearly for tones less than 50 ms in duration. For longer tone duration performance was unaffected. The means and variability of reaction times for threshold changes of frequency were greater in comparison with supra-threshold frequency changes.     

Why we live and why we die

Several different techniques for urodynamic signal compression have been proposed in the last few years. Using these techniques it is possible to reduce the requirements for digital storage or transmission. There are a number of applications where it is essential to use such techniques in diagnostic and ambulatory urodynamics. The purpose of this study is to compare different techniques of urodynamic data compression. The so-called FAN, voltage triggered, two point projection and second difference methods. The comparison between the methods is based on 65 pressure, 46 uroflow and 18 surface electromyogram signals. The reduction ratio achieved for different allowable errors between the original and compressed signals is calculated and compared for the different techniques. Results show that it is possible to store urodynamic signals accurately at a low sampling rate, where FAN and voltage triggered methods seem to be superior to the rest.     

Speech intelligibility in noise with fast compression hearing aids

The effect of fast (syllabic) compression with overshoot reduction was studied in moderately hearing-impaired and in severely hearing-impaired listeners in quiet and in noisy situations. A test battery of daily masking noises was selected using multidimensional scaling techniques. Four relevant noises were selected: a multi-talker babble, the noise in an industrial plant, in a printing office and a city-noise background. The speech measurements show that only selected patients benefit from syllabic compression, i.e. listeners with a poor speech discrimination score. The effect in noisy surroundings was tested at the critical signal-to-noise ratio of each patient, showing whether they benefited from compression in the most critical condition or not. It turns out that the effect depends largely on the speech discrimination score and the modulation of the noise signal. When the speech discrimination score is good, compression tends to impair the results. When the speech discrimination score is poor, compression helps if the noise is modulated.     

Recognition of multiply degraded speech by young and elderly listeners

This study investigated the hypothesis that age effects exert an increased influence on speech recognition performance as the number of acoustic degradations of the speech signal increases. Four groups participated: young listeners with normal hearing, elderly listeners with normal hearing, young listeners with hearing loss, and elderly listeners with hearing loss. Recognition was assessed for sentence materials degraded by noise, reverberation, or time compression, either in isolation or in binary combinations. Performance scores were converted to an equivalent signal-to-noise ratio index to facilitate direct comparison of the effects of different forms of stimulus degradation. Age effects were observed primarily in multiple degradation conditions featuring time compression of the stimuli. These results are discussed in terms of a postulated change in functional signal-to-noise ratio with increasing age.     

Upper displacement limits for spatially broadband patterns containing bandpass noise

How is the spatial-frequency content of a moving broadband pattern analysed by the visual system? Observers were asked to discriminate the direction of motion in random-noise patterns containing equal energy in each two-dimensional octave band. Uncorrelated noise could be introduced into either low- or high-frequency bands in order to force the visual system to rely on the outputs of putative mechanisms tuned to a narrow frequency range of the stimulus. In two experiments the dependent measure was the magnitude of dmax, the largest discrete displacement whose direction could be discriminated reliably. It was found that dmax was unaffected by the presence of high-frequency noise reaching down to 0.67 c/deg, but that the task became impossible thereafter. In the case of low-frequency noise, dmax fell as the noise was moved up towards about 2 c/deg, at which point the task became impossible at any displacement. This pattern of results would be expected if the system were using information from the lowest signal frequencies in all conditions. In experiment 2, dmax was measured for stimuli in which the spectral position and quantity of high-frequency noise were manipulated. It was found that only noise spectrally-adjacent to the signal band has a detrimental effect on dmax. Three different single-filter models of motion detection each failed to provide a satisfactory account of the spatial-frequency range of good direction discrimination performance. Rather, the modelling shows that the visual system can access the outputs of a low-frequency channel when the noise is high and a high-frequency channel when the noise is low.     

Time-frequency scaling transformation of the phonocardiogram based of the matching pursuit method

A time-frequency scaling transformation based on the matching pursuit (MP) method is developed for the phonocardiogram (PCG). The MP method decomposes a signal into a series of time-frequency atoms by using an iterative process. The modification of the time scale of the PCG can be performed without perceptible change in its spectral characteristics. It is also possible to modify the frequency scale without changing the temporal properties. The technique has been tested on 11 PCG's containing heart sounds and different murmurs. A scaling/inverse-scaling procedure was used for quantitative evaluation of the scaling performance. Both the spectrogram and a MP-based Wigner distribution were used for visual comparison in the time-frequency domain. The results showed that the technique is suitable and effective for the time-frequency scale transformation of both the transient property of the heart sounds and the more complex random property of the murmurs. It is also shown that the effectiveness of the method is strongly related to the optimization of the parameters used for the decomposition of the signals.     

Randomized trial of an oral platelet glycoprotein IIb/IIIa antagonist, sibrafiban, in patients after an acute coronary syndrome: results of the TIMI 12 trial. Thrombolysis in Myocardial Infarction

In fMRI studies, Gaussian filtering is usually applied to improve the detection of activated areas. Such lowpass filtering enhances the signal to noise ratio. However, undesirable secondary effects are a bias on the signal shape and a blurring in the spatial domain. Neighboring activated areas may be merged and the high resolution of the fMRI data compromised. In the temporal domain, activation and deactivation slopes are also blurred. We propose an alternative to Gaussian filtering by restoring the signal using a spatiotemporal Markov Random Field which preserves the shape of the transitions. We define some interaction between neighboring voxels which allows us to reduce the noise while preserving the signal characteristics. An energy function is defined as the sum of the interaction potentials and is minimized using a simulated annealing algorithm. The shape of the hemodynamic response is preserved leading to a better characterization of its properties. We demonstrate the use of this approach by applying it to simulated data and to data obtained from a typical fMRI study.     

Non-Fourier information in bandpass noise patterns

Random dot patterns and white-noise luminance textures are widely used in psychophysical experiments to study low-level visual processes. Because these noise patterns are broadband, bandpass filtered versions are employed to limit their frequency content. It is not recognized, however, that bandpass noise patterns have amplitude-modulation (AM) components. The AM signal is not present in the Fourier spectrum, nonetheless, it is a valid signal for second-order mechanisms. We characterize the properties of the AM signal in bandpass noise textures: the relevant periodicities of the AM signal are always much lower than the actual passband; the upper frequency limit of the AM signal increases with the linear bandwidth. We present psychophysical data to demonstrate the perceptual significance of the AM signal in bandpass noise. We provide a method for obtaining AM-free bandpass patterns, and compare psychophysical performance in experiments employing AM-present and AM-free bandpass noise patterns as stereoscopic stimuli. The results show that the AM component contributes to stereoscopic discrimination performance at large disparities. We suggest that the low-frequency AM signal is a possible confounding factor in experiments employing bandpass noise textures, and that linear filtering can isolate spatial scales effectively only for linear systems.     

基于小波包的某矿爆破震动信号分析

为了解某矿爆破震动波传播规律,在去噪的基础上对信号进行4层db8小波包分解,提取噪声和频段分信号.分析表明,噪声易与真实信号混叠,使PPV增大或减小,降低原始信号的准确性与可靠性;真实信号在0~256 Hz均有分布,各频段未见明显集中现象,但在同一爆破中,随着传播距离的增加,速度有向低频集中的趋势.最后,将数据进行线性拟合,获取了爆破震动波的衰减规律公式,为该矿爆破设计提供了依据.     

Independent processing across spatial frequency in moving broadband patterns

The aim of the experiments was to discover whether the visual system has independent access to motion information at different spatial scales when presented with a broadband stimulus. Subjects were required to discriminate between a pair of two-frame motion sequences, one containing a coherently displacing pattern and the other containing a pattern with high-frequency noise. The stimuli were either narrowband (1 octave) or broadband (6 octaves spanning 0.23-15.0 cycles deg-1) and their power spectra were either flat or followed a 1/f2 function. For the broadband stimuli, noise was introduced cumulatively into increasingly lower frequencies. For the narrowband stimuli, noise was introduced into the same frequency band as the signal. All stimuli could be defined by the lowest noise frequency (nl) they contained. For each stimulus, the largest spatial displacement across the two frames at which the task could be performed was measured (dmax). For the narrowband stimuli, dmax increased as nl was lowered. This was true over the entire frequency range for the 1/f2 stimuli, though the task became impossible for the flat-spectrum stimuli at the lowest frequencies. This is attributed to the very low contrast of these latter stimuli. The dmax values for the broadband stimuli tended to shadow those of the narrowband stimuli with the equivalent values of nl being around 25% lower. The data were modelled by spatiotemporally filtering the stimuli and considering the amount of directional power in the signal and noise sequences. The results suggest that there must be multiple spatial-frequency channels in operation, and that for broadband patterns the visual system has perceptual access to these individual channel outputs, utilising different filters depending on the task requirements.