Showing papers by "Patrick J. Loughlin published in 2006"

Recognition of propagating vibrations and invariant features for classification

Abstract: The vibrations produced by objects, for example by a plate or cylinder insonified by a sonar wave, exhibit characteristics unique to the particular structure, which can be used to distinguish among different objects The situation is complicated, however, by many factors, a particularly important one being propagation through media As a vibration propagates, its characteristics can change simply due to the propagation channel; for example, in a dispersive channel, the duration of the vibration will increase with propagation distance These channel effects are clearly detrimental to automatic recognition because they do not represent the object of interest and they increase the variability of the measured responses, especially if measurements are obtained from targets at different locations Our principal aim is to identify characteristics of propagating vibrations and waves that may be used as features for classification We discuss various moment-like features of a propagating vibration In the first set of moments, namely temporal moments such as mean and duration at a given location, we give explicit formulations that quantify the effects of dispersion Accordingly, one can then compensate for the effects of dispersion on these moments We then consider another new class of moments, which are invariant to dispersion and hence may be useful as features for dispersive propagation We present classification results comparing these invariant features to related non-invariant features, for classification of simulated backscatter from different steel shells in a dispersive environment

Time-Varying Spectral Approximation of Filtered Signals

Abstract: An approximation of the time-varying (Wigner) spectrum of a filtered signal is presented. The approximation is simple to apply, yet insightful, in that it shows the effects of the magnitude and phase of the frequency response of the filter on the time-frequency properties of the signal. In addition, the approximation yields the correct Fourier spectrum of the filtered signal, to within a complex constant

Wigner distribution approximation for filtered signals and waves

Abstract: An approximation of the Wigner distribution of a filtered signal is presented. The approximation is simple to apply, yet insightful, in that it shows the effects of the magnitude and phase of the frequency response of the filter on the Wigner distribution of the signal. Also given is an approximation for the Wigner distribution of an amplitude modulated signal. Examples are given to illustrate the approach, including application of the approximation to wave propagation.

Time-frequency approximations with applications to filtering, modulation, and propagation

Abstract: Signals with time-varying spectral content arise in a number of situations, such as in shallow water sound propagation, biomedical signals, machine and structural vibrations, and seismic signals, among others The Wigner distribution and its generalization have become standard methods for analyzing such time-varying signals We derive approximations of the Wigner distribution that can be applied to gain insights into the effects of filtering, amplitude modulation, frequency modulation, and dispersive propagation on the time-varying spectral content of signals

Effect of convolution and modulation on the time-varying spectrum of a signal, with application to target recognition

Abstract: In active sonar or radar, the received signal can often be modeled as a convolution of the transmitted signal with the channel impulse response and the target impulse response. Because the received signal may have a time-varying spectrum, due for example to target motion or to changes in the channel impulse response, time-frequency methods have been used to characterize propagation effects and target effects, and to extract features for classification. In this paper, we consider the time-varying spectrum, in particular the Wigner time-frequency representation, of a received signal modeled as the convolution of the transmitted signal with the channel and target responses. We derive a simple but insightful approximation that shows the effects of the magnitude and phase of the frequency response of the target and of the channel on the Wigner representation of the transmitted signal. We also consider time-varying effects on the Wigner representation, such as changes in reflected energy, which we model by amplitude modulation.

A model-based approach to attention and sensory integration in postural control of older adults.

Abstract: We conducted a dual-task experiment that in- volved information processing (IP) tasks concurrent with pos- tural perturbations to explore the interaction between attention and sensory integration in postural control in young and older adults. Data were fit to a postural control model incorporating sensory integration and the influence of attention. This model hypothesizes that the cognitive processing and integration of sensory inputs for balance requires time, and that attention influences this processing time, as well as sensory selection by facilitating specific sensory channels. Differences in the time delay of the postural control model were found for age and IP task, suggesting enhanced vulnerability of balance processes in older adults to interference from interfering cognitive IP tasks.