Successive Time Samples Research Articles

Dense velocity reconstruction with VIC-based time-segment assimilation

The vortex-in-cell time-segment assimilation (VIC-TSA) method is introduced. A particle track is obtained from a finite number of successive time samples of the tracer’s position and velocity can be used for reconstruction on a Cartesian grid. Similar to the VIC + technique, the method makes use of the vortex-in-cell paradigm to produce estimates of the flow state at locations and times other than the measured ones. The working principle requires time-resolved measurements of the particles’ velocity during a finite time interval. The work investigates the effects of the assimilated length on the spatial resolution of the velocity field reconstruction. The working hypotheses of the VIC-TSA method are presented here along with the numerical algorithm for its application to particle tracks datasets. The novel parameter governing the reconstruction is the length of the time-segment chosen for the data assimilation. Three regimes of operation are identified, based on the track length and the geometrical distance between neighbouring tracks. The regime of adjacent tracks arguably provides the optimal trade-off between spatial resolution and computational effort. The VIC-TSA spatial resolution is evaluated first by a numerical exercise; a 3D sine wave lattice is reconstructed at different values of the particles concentration. The modulation appears to reduce (cut-off delay) when the time-segment length is increased. Large-scale PIV experiments in the wake of a circular cylinder at Red = 27,000 are used to evaluate the method’s suitability to real data, including noise and data outliers. Both primary vortex structures in the Kármán wake as well as interconnecting ribs are present in this complex flow field, with a typical diameter close to the average inter-particle distance. When the time-segment is increased to adjacent tracks and beyond, a more regular time dependence of local and Lagrangian properties is observed, confirming the suitability of the time-segment assimilation for accurate reconstruction of sparse velocity data.Graphical abstract

A spectral synthesis method to suppress aliasing and calibrate for delay errors in Fourier transform correlators

Context. Fourier transform (or lag) correlators in radio interferometers can serve as an efficient means of synthesising spectral channels. However aliasing corrupts the edge channels so they usually have to be excluded from the data set. In systems with around 10 channels, the loss in sensitivity can be significant. In addition, the low level of residual aliasing in the remaining channels may cause systematic errors. Moreover, delay errors have been widely reported in implementations of broadband analogue correlators and simulations have shown that delay errors exasperate the effects of aliasing.

Quantifying intracardiac organization of atrial arrhythmias using temporospatial phase of the electrocardiogram.

Separating nonisthmus-dependent atrial flutter (AFL) from "organized" atrial fibrillation (AF), or isthmus-dependent AFL, may be difficult using ECG characteristics alone. We hypothesized that temporal and spatial phase analysis of ECG atrial waveforms could effectively separate these rhythms by quantifying subtle variations in ECG atrial activation during supraventricular tachycardias (SVT). We studied 52 patients at electrophysiologic study (EPS) who demonstrated isthmus-dependent (n = 15) and nonisthmus-dependent (n = 9) AFL, atrial tachycardia (n = 6), AV nodal reentry (n = 9), orthodromic reciprocating tachycardia (n = 6), and AF (n = 7). Atrial activity was represented as a series of correlations of an atrial template to successive time samples of the arrhythmia ECG. Spatial phase was analyzed as a reproducible relationship of this atrial activity between leads over time; temporal regularity was measured from power spectra. Spatial phase was maintained (coherent) in lead planes V5/aVF (XY), V5/V1 (XZ), and aVF/V1 (YZ) in 15 of 15 cases of isthmus-dependent AFL, but in only 1 of 9 cases of nonisthmus-dependent AFL (P < 0.01; chi2). Temporally, all cases of AFL showed one dominant peak on correlation spectra (magnitude >6 dB), suggesting one activation wavefront, although this was smeared in nonisthmus-dependent cases. In contrast, AF showed inconsistent spatial phase in all planes and broad band spectra, consistent with multiple and/or variable activation paths. All other SVTs showed spatial coherence and one dominant spectral peak. Coherence of temporal and spatial phase is a powerful approach to measure the spatial organization of intracardiac activation from the ECG that reveals a spectrum from SVT to isthmus-dependent and nonisthmus-dependent AFL, to AF.

Spatial spectral analysis of human electrocorticograms including the alpha and gamma bands

Spatial spectral analysis is essential for deriving spatial patterns from simultaneous recordings of electrocorticograms (ECoG), in order to determine the optimal interval between electrodes in arrays, and to design spatial filters, particularly for extraction of information about the dynamics of human gamma activity. ECoG were recorded from up to 64 electrodes 0.5 mm apart in a linear array 3.2 cm long, which was placed on the exposed superior temporal gyrus or motor cortex of volunteers undergoing diagnostic surgery. Visual displays of multiple traces revealed broad spectrum oscillations in episodic bursts having a common aperiodic wave form with recurring patterns of spatial amplitude modulation (AM patterns) on selected portions of the array. The one-dimensional spatial spectrum of the human ECoG was calculated at successive time samples and averaged over periods of up to 20 s. Log power decreased monotonically with increasing log spatial frequency in cycles/mm (c/mm) to the noise level ∼2 log units below maximal power at minimal frequency (0.039±0.002 c/mm). The inflection point at 0.40±0.05 c/mm specified an optimal value for a low pass spatial filter to remove noise, and an optimal interelectrode spacing of 1.25 mm to avoid undersampling and aliasing. An 8×8 array with that spacing would be 10×10 mm.

An efficient implementation of forward-backward LMS adaptive filters

Without changing the characteristics of the forward-backward LMS (FBLMS) adaptive line enhancer, an efficient implementation of FBLMS adaptive filters is presented in this paper. This implementation technique reduces 25% of multiplications and 12.5% of additions in two successive time samples in comparison to that of the direct implementation of the FBLMS algorithm in both prediction and weight control sections.

Orthogonal transformations of stacked feature vectors applied to HMM speech recognition

The paper reports improvements in speech recognition accuracy by using more sophisticated time analysis as part of the feature selection process. The recognition methodology utilises hidden Markov modelling with continuous density functions. The authors propose using, as speech features, linear transformations of the vector consisting of successive time samples of the cepstrum. Taylor series, the Legendre polynomial transform and the discrete cosine transform share several properties with principal components analysis. These transforms are expected to improve speech recognition accuracy by incorporating higher-order time derivatives (such as the second time derivative) of spectral information while at the same time producing an essentially diagonal covariance. In an experimental evaluation of these ideas, accuracy in speakerindependent recognition of the E-set of the alphabet improved from 55%, with no time varying information, to 68%, with first-order time varying information, and 74%, by including second-order time varying information.

Constrained disparity and motion estimators for 3DTV image sequence coding

This paper presents two-dimensional motion estimation methods which take advantage of the intrinsic redundancies inside 3DTV stereoscopic image sequences. Most of the previous studies extract, either disparity vector fields if they are involved in stereovision, or apparent motion vector fields to be applied to motion compensation coding schemes. For 3DTV image sequence analysis and transmission, we can jointly estimate these two feature fields. Locally, initial image data are grouped within two views (the left and right ones) at two successive time samples and spatio-temporal coherence has to be used to enhance motion vector field estimation. Three different levels of ‘coherence’ have been experimented subject to the crucial assumption that an initial calibration of the stereoscopic sensors provides us with geometric change of coordinates for two matched features.

Passive synthetic arrays

Passive‐synthetic apertures [W. Carey and N. Yen, J. Acoust. Soc. Am. Suppl. 1 75, S62 (1984)] were formed with experimental towed hydrophone data in a sound channel that supported RR and RSR transmission. These apertures were formed with lengths up to 95λ with coherent temporal processing gains approaching 0.75 of theoretical. These results and those of previous investigators [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752–753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] indicate that synthetic apertures can be formed by the coherent summation of the phase‐corrected summation of either hydrophone or subaperture beams over successive time samples when the synthetic aperture length is less than the spatial coherence length and the processing time is less than the temporal coherence length. The evaluation of synthetic apertures requires comparisons with conventional and other high‐resolution techniques. Comparisons between conventional array processing and high‐resolution techniques [maximum entropy (ME) and maximum likelihood (ML) methods] are performed by use of the analytical expressions developed by A. T. Parsons (AUWE, TN 700/83) for the determination of the array, integration, and the net processing gains. Analytical comparisons between conventional and synthetic aperture arrays formed with either the same number of hydrophones or with the same effective length but a different number of hydrophones, show that, when the spatial processing gain exceeds the loss in integration gain, then the use of synthetic apertures is advantageous. [Work performed while at NORDA.]

The formation of a synthetic aperture with towed hydrophones

Passive‐synthetic apertures were formed with experimental data from the coherent summation of subapertures formed from 1–4 hydrophone groups at successive time samples. The results show that the coherent signal gain for synthetic apertures with lengths up to 95 λ was a function of the number of successive time samples (Nts) described by α 10 log(Nts) with α∼0.7. The hydrophones were towed with a speed‐to‐wavelength ratio of 0.168 s−1 at a depth of 26 λ. A calibrated moored source was at a depth of 37 λ in 3200 m of water at distance‐to‐wavelength ratio of between 20 and 30 × 103 from the receiver. The stable environment was characterized by a sound‐speed profile with a negative gradient of − 0.130 s−1 to a sound channel axis of 150 m and a positive gradient of + 0.016 s−1 to a critical depth of 1825 m. The resulting propagation was RR and RSR with losses of between 90 and 100 dB. These results show that for these environmental‐acoustic conditions the synthetic apertures can be formed by the coherent summation of phase corrected subaperture beams over successive time samples as long as the synthetic aperture length is less than the single path coherence length and the synthetic aperture processing time is less than the signal's temporal coherence length. The results presented represent an extension of earlier work [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752‐753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] by the demonstration that coherent gain can actually be achieved with resolution such that multipath vertical arrival angle differences can be resolved.

Alteration in Frequencies of Different Classes of Chromosome Aberrations Throughout the First Cell Cycle Following X Irradiation

The frequencies of different classes of aberrations induced by 250 rad of X rays following pulse-labeling with tritiated thymidine were scored in successive time samples of metaphase cells over one cell cycle in a cell line with a very simple karyotype. The experimental conditions precluded the possibility that cells were in their second division following irradiation. It was found that chromosome-type aberrations were common in cells apparently in early S at the time of irradiation, that is, in labeled metaphases found at later collection times, and gradually declined in frequency at earlier collection times. The most frequent class of aberrations in early S cells was sister unions. Sister unions were also found in cells irradiated in G/sub 1/. The data suggest that those classes of aberrations usually described as of the isochromatid type are more probably of the chromosome type, or are chromosome-chromatid interchanges.

A simple adaptive algorithm for real-time processing in antenna arrays

A new adaptation algorithm designed for real-time data processing in large antenna arrays is presented. The algorithm is used to determine the set of filter coefficients (weights) which minimizes the mean-square error in a multidimensional linear filter. The algorithm forms an estimate of the target signal, which is assumed to be of interest, in the presence of interfering noises. It is assumed that the direction of arrival and spectral density of the target signal are known a priori. No such information is assumed to be available regarding the structure of the interfering noise field. The a priori target information is incorporated directly into the adaptation procedure using a modified gradient descent technique. The mathematical convergence properties of the algorithm are presented and a computer simulation experiment is used as an illustration. It is shown that as the number of iterations becomes large, the expected value of the adaptive solution converges to the minimum mean-square-error solution. It is further shown that the variance of the adapted filter about the optimum solution can be made arbitrarily small by appropriate choice of a scalar constant in the algorithm. These results are based on the assumption that the array signals are Gaussian and that successive time samples are statistically uncorrelated. Thus, the new algorithm is shown to converge to the optimum processor in the limit as the number of adaptations becomes large. Any disadvantage which may arise in the use of such an asymptotically optimum system is offset by the extreme simplicity of the adaptive procedure. This simplicity should prove to be particularly useful in many of the practical array processing problems recently encountered in seismic and sonar data processing.

Temporal Effects in Speech Analysis and Synthesis

Experiments in adjusting the time dimension of natural speech have been carried out with a hybrid computing facility. The distribution of spectral energy for successive time samples was derived by scanning the output of a channel-vocoder analyzer, and the relationship between the analytic and synthetic sampling rate was modified in various ways. Time normalization of utterances was done by both linear and nonlinear methods; the nonlinear, arc-length method may be better for automatic speech recognition. The perceptual effects of time adjustment were investigated in an intelligibility test. A dichotic method for time compression of speech was investigated in which odd-numbered time samples were presented to one ear and even-numbered samples to the other ear. Potential applications for time-adjusted speech and possibilities for further research are suggested.

Methamphetamine and pentobarbital effects on human motor performance.

Speed in three types of repetitive motor performance, and manual reaction time was tested at different levels of task complexity, on 36 students who served as their own control over repeated sessions, in which they received intravenous doses of methamphetamine, pentobarbital, and placebo adjusted to their body weight. The simpler the task the more likely was methamphetamine to speed up and pentobarbital to slow down performance rate. Compared with placebo, pentobarbital significantly affected rate in counter pressing, and tended to retard performance in a manual skill of finer co-ordination as well as in a cancellation test. Methamphetamine significantly speeded counter pressing with the dominant hand, but exercised that effect on the other hand only over the first minute of performance. Acitvity rate in the manual dexterity test was significantly faster on methamphetamine than on pentobarbital. Individual variability in work rate over successive time samples, and decrement over time, tended to be highest with methamphetamine. The favorable effect of methamphetamine on performance rate may thus stem primarily from an initial spurt that cannot be sustained, although it may revive after intervals longer than those investigated in the present study. No methamphetamine effect could be determined in several tests of reaction time, but pentobarbital slowed down responses, whether warning signals were provided or not. A slow rate in repetitive motor performance with barbiturates may, therefore, be partly attributable to a delay in monitoring cues for response.