Relative Time Shifts Research Articles

Local wavelet correlation: applicationto timing analysis of multi-satellite CLUSTER data

Abstract. Multi-spacecraft space observations, such as those of CLUSTER, can be used to infer information about local plasma structures by exploiting the timing differences between subsequent encounters of these structures by individual satellites. We introduce a novel wavelet-based technique, the Local Wavelet Correlation (LWC), which allows one to match the corresponding signatures of large-scale structures in the data from multiple spacecraft and determine the relative time shifts between the crossings. The LWC is especially suitable for analysis of strongly non-stationary time series, where it enables one to estimate the time lags in a more robust and systematic way than ordinary cross-correlation techniques. The technique, together with its properties and some examples of its application to timing analysis of bow shock and magnetopause crossing observed by CLUSTER, are presented. We also compare the performance and reliability of the technique with classical discontinuity analysis methods. Key words. Radio science (signal processing) – Space plasma physics (discontinuities; instruments and techniques)

Study on phase perception in speech

The perceptual effect of the phase information in speech has been studied by auditory subjective tests. On the condition that the phase spectrum in speech is changed while amplitude spectrum is unchanged, the tests show that: (1) If the envelop of the reconstructed speech signal is unchanged, there is indistinctive auditory perception between the original speech and the reconstructed speech; (2) The auditory perception effect of the reconstructed speech mainly lies on the amplitude of the derivative of the additive phase; (3) t d is the maximum relative time shift between different frequency components of the reconstructed speech signal. The speech quality is excellent while t d 35ms.

Collision-induced timing-jitter analysis in a wavelength-division-multiplexed optical soliton-transmission system with dispersion management

Collision-induced timing jitter is theoretically and numerically studied for two-channel wavelength-division-multiplexed optical soliton transmission systems with both weak and strong (practical) dispersion management. For a weakly managed case the timing jitter induced by the residual frequency shift strongly depends on bit rate per channel owing to the correlation in the relative time shift in neighboring pulses. We also discuss the effect of dispersion management for reducing the timing jitter. For a strongly managed case the timing jitter is mainly induced either by the accumulated position shift or by an incomplete-collision-induced residual frequency shift. It becomes significant for nearly zero path-averaged dispersion. We optimize the path-averaged dispersion of the line composed of 1.3-µm zero-dispersion single-mode fiber and its reversed dispersion fiber by minimizing the jitter caused by both collision and amplifier noise.

Zero moveout (ZMO) stacking

This work describes a simple stacking procedure based on summing the traces of a common shot or common seismic section without the usual normal moveout (NMO) relative time shifts. We refer to this procedure as zero moveout (ZMO) stacking. A simple horizontal summation with no shift in time results in a stacked trace that is the one‐dimensional seismogram for horizontally layered media. We explore the practical implementation of ZMO stacks for source estimation or wavelet extraction in deconvolution and/or identification of source‐reflector signatures necessary for performing inversion.

Phase aberration correction using near-field signal redundancy. I. Principles [Ultrasound medical imaging

The signal redundancy principle in the near field is analyzed quantitatively. It is found that common midpoint signals are not identical (or redundant) for echoes coming from arbitrary target distributions in the near field. A dynamic near-field correction is proposed to reduce the difference between common midpoint signals for echoes coming from the region of interest. When phase aberrations are present, it is shown that the dynamic correction can generally be done assuming no phase aberration, and the relative time-shift between common midpoint signals can be used to measure phase-aberration profiles. A phase-aberration correction algorithm based on that principle is proposed. In this algorithm, after common midpoint signals are collected they are dynamically corrected for near-field effects and cross-correlated with one another. In a related way, the phase errors are measured from peak positions of these cross-correlation functions. The phase-aberration profile across the array is derived from these measurements. The relationship between the errors in the derived phase aberration profile and the errors in the measured relative time-shift between common midpoint signals is derived. A method for treating the situation of different transmission and reception phase-aberration profiles is also proposed. This algorithm works for general target distributions, iteration is not required, and it can be used in other near-field, pulse-echo, imaging systems.

Measurement of the parameters of all-pole transfer functions using shifted hermite modulating functions

Abstract A method is presented for investigating the errors incurred when the low-order parameters of an all pole system are measured by the Hermite modulating function technique. The correlation form of the basic linear system equations is used. It has been found that measurement errors are a function of the relative time shift between the cross-correlation function and the modulating function. When the Hermite modulating functions are aligned with the peak of the measured cross-correlation function actual system parameters can be determined with low error even when a low-order model of the system is assumed.

Timing in the regulation of neural crest cell migration: Retarded “maturation” of regional extracellular matrix inhibits pigment cell migration in embryos of the white axolotl mutant

In larvae of the white axolotl mutant (Ambystoma mexicanum), contrary to normal dark ones, trunk pigmentation is restricted because the epidermis is unable to support subepidermal migration of pigment cells from the neural crest (NC). This study examines whether the subepidermal extracellular matrix (ECM) is the defective component which prevents pigment cell migration in the white embryo. We transplanted subepidermal ECM, adsorbed in vivo on membrane microcarriers, from and to white and dark embryos in various combinations. White embryos have demonstrated normal NC cell migration along the medioventral pathway, and in order to test the effects of medial ECM on subepidermal migration, this ECM was similarly transplanted. Carriers with ECM attached were inserted subepidermally in host embryos at a premigratory NC stage. Control carriers without ECM and carriers with subepidermal ECM from white donors did not affect NC cell migration in white or dark embryos. In contrast, subepidermal ECM from dark donors triggered NC cell migration in the subepidermal space of both white and dark hosts. Remarkably, subepidermal ECM from white donors which were older than those normally used also stimulated migration in embryos of both strains. Likewise, medial ECM from white donors elicited migration in white as well as dark hosts. Pigment cells occurred among those NC cells that were stimulated to migrate in response to contact with ECM on carriers. These results indicate that the subepidermal ECM of the white embryo is transiently defective as a substrate for pigment cell migration, implying that "maturation" of the ECM is retarded beyond the times during which pigment cells are able to respond. In contrast, the medial ECM of the white embryo appears to mature normally. These findings suggest that the effect of the d gene is expressed regionally through the subepidermal ECM during a limited period of development. Hence, the action of the d gene seems to retard ECM maturation, bringing it out of phase with the migratory capability of the pigment cells. We propose that such a shift in relative timing of the developmental phenomena involved inhibits pigment cell migration in embryos of the white axolotl mutant and, accordingly, that the restricted pigmentation of the mutant larva is generated through heterochrony.

Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean

SH and sSH seismograms are modeled to determine the shear velocity structure in the D″ region beneath India and the Indian Ocean. The signals show waveform complexities similar to those observed in data sampling the D″ region beneath Alaska, the Caribbean, and Eurasia (Lay and Helmberger), which have been attributed to a 2.7% shear velocity discontinuity ∼ 280 km above the core-mantle boundary. The new data set consists of long-period tangential component recordings at WWSSN stations in Africa, the Middle East, and Europe for 11 intermediate and deep focus Indonesian earthquakes. In the distance range 70–82° the waveforms show an arrival between SH and ScSH with systematic moveout. From 89 to 94° there is a strong distortion of the SH waveforms, indicating the arrival of several phases closely spaced in time. The relative time shifts of similar complexity in the corresponding sSH phases requires a deep mantle origin. The depth dependence and moveout of the interference effects are well-predicted for both SH and sSH phases by a model with a lower mantle discontinuity. Alternative explanations of the interference as resulting from receiver reverberations, SKS contamination, multiple source complexity, or near source multipathing are ruled out by systematic tests. While it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data, synthetic waveform modeling (using generalized ray theory and reflectivity) shows that the data can be well-fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions (Lay and Helmberger), but with a negative velocity gradient within the D″ layer.

LONG PERIOD STATICS DETERMINATION BY INVERSE FILTERING*

AbstractThe relative time shift of CDP reflections, caused by long period statics, is treated as the result of a filtering process. The shape of the filter is that of a comb having the magnitude and spacing of its teeth determined by the CDP spread geometry. Convolution of a near surface anomaly with a comb filter representing the CDP spread geometry produces the same time fluctuations at CDP reflections (CDP statics) as the stacked traces across the anomaly. Conversely, the near surface time anomaly is given by the convolution of the CDP statics with the inverse of the comb filter. Provided that CDP statics can be separated from noise, dip, and residual normal moveout, it is pcissible to determine long period statics with a relative wavelength of between half a spreadlength and five spreadlengths.The object of this paper is to present the theory and practical applications of the method. Several examples based on synthetic and real data will also be discussed.

Correlational handling of broadband processes (up to 1.5 MHz) having relative time and frequency shifts

Correlational handling of broadband processes (up to 1.5 MHz) having relative time and frequency shifts

A new set of orthogonal periodic sequences

A group of periodic sequences that remain orthogonal for all values of relative time shifts between themselves is shown to be obtainable from sampling of suitable sine or cosine waveforms. It is also established that the number of sequences that can be accommodated in a group is maximum in such a case and is limited to N/2+1 for even valued N and (N-1)/2+1 for odd value of N, where N is the number of elements in each sequence.

A Zero-Crossing Discriminator with Picosecond Time Slewing

A zero-crossing type of discriminator with a very wide dynamic range and a short dead-time is described. When used with a 56 AVP photomultiplier with a linear dynode resistor chain, a light amplitude variation of at least 125:1 may be accommodated. The relative time shift of the output discriminator pulse is then only 200 p.s.