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Using ray theory the combined effects of time-dependent changes in source depth and receiver-source range are examined for a cw signal transmitted over relatively short ranges. Approximating deep-ocean sound speed with a bilinear profile, general results are obtained when the receiver is taken fixed on the surface, while the source moves on an arbitrary constant-velocity path above the SOFAR axis. Time-dependent expressions for the amplitude and phase of the received multipath signal are used to present numerical data for suitably restricted time intervals. Then, the effects of source path and speed are analyzed using convenient formulas which closely approximate numerical results. For strictly horizontal motion, total-field phase rate remains approximately proportional to time, source frequency, speed, and horizontal receiver-source range, but virtually independent of source depth. However, when source depth varies with time, overall linear phase patterns are interrupted by regularly spaced, brief changes in phase rate. The periodicity of these changes, and accompanying amplitude fades of up to 35 dB, are virtually proportional to vertical speed, source frequency, and range, but invariant with changes in horizontal speed, direction, and initial source depth. [ABSTRACT FROM AUTHOR]

Deep-water sound transmission from a moving source to a fixed receiver is studied, where the source emits a random broadband signal whose expected distribution is normal. A bilinear sound-speed profile is employed, the source is located above the SOFAR axis, and the receiver below. Long ranges are assumed, so that only SOFAR rays need be considered. Basic propagation equations are given, and travel time and spreading loss expressions are approximated in stability regions within which all four SOFAR rays exist for each number of SOFAR-axis crossings. General equations are derived for the received average power spectrum and power in terms of the spectrum at the source. Then, the broadband signal is taken to be bandlimited white noise. The received spectrum is simplified and examined for nonmultipath, multipath, and Doppler contributions. The influences of range, source speed, frequency, and observation time are considered. Average power at the receiver is studied similarly. Received spectra for a stationary source are investigated, and exhibit greater variations than those arising from a moving source. However, received average power for stationary and moving broadband sources are about equal. Average received power from cw and broadband sources, both moving and stationary, are compared. Power variations in the cw case are found to be much larger than in the broadband case. [ABSTRACT FROM AUTHOR]

Analytical methods are employed to obtain general results for the effect of cw source motion on the total acoustic field at a fixed receiver in the deep ocean. A bilinear sound-speed profile is used, a long range is assumed, and SOFAR rays are considered. Equations are developed which give amplitude and phase as functions of receiver time. Then, an investigation is made of stability regions, defined as range intervals for which all rays for given numbers of SOFAR-axis crossings arrive at the receiver. Three types of such regions are identified and examined. Amplitude and phase are linearized for a short source run, and novel numerical results for these quantities are presented. Approximate formulas for cumulative phase and Doppler-shift frequency are derived and discussed. Finally, an analytical description is presented for two mechanisms responsible for amplitude fades. [ABSTRACT FROM AUTHOR]

A treatment of the effects of arbitrary motion of a cw source and depth-dependent sound speed on the total acoustic field at a fixed receiving point is considered for an ocean with horizontal boundaries. Application of our general method is made to a constant sound-speed channel in which the range-to-depth ratio is large, when the source follows a short straight-line path with constant velocity. Total-field phase is investigated as a function of receiver time for various source trajectories and phase rate is examined in terms of an arbitrary, but fixed, acoustic frequency. It is shown that source motion may be accounted for by assuming the sound source to be stationary, and by replacing its frequency by approximate Doppler frequency. For long source trajectories, cumulative phase can be approximated as a hyperbolic function of time. The outputs of two uniform colinear arrays, together with power spectra there, are employed to illustrate one method for determing source speed, location, bearing, and frequency. Subject Classification: [43]30.20; [43]20.20.. [ABSTRACT FROM AUTHOR]

Objective: A chirp is a brief signal within which the frequency content changes rapidly. Spectrographic chirps are found in signals produced from many biological and physical phenomena. In radar and sonar engineering, signals with chirps are used to localize direction and range to the signal source. Although characteristic frequency changes during epileptic seizures have long been observed, the correlation with chirps and chirp technology seems never to have been made., Methods: We analyzed 19404 s (1870 s of which were from 43 seizures) of intracranially (subdural and depth electrode) recorded digital EEG from 6 patients for the presence of spectral chirps. Matched filters were constructed from methods in routine use in non-medical signal processing applications., Results: We found that chirps are very sensitive detectors of seizures (83%), and highly specific as markers (no false positive detections). The feasibility of using spectral chirps as matched filters was demonstrated., Conclusions: Chirps are highly specific and sensitive spectrographic signatures of epileptic seizure activity. In addition, chirps may serve as templates for matched filter design to detect seizures, and as such, can demonstrate localization and propagation of seizures from an epileptic focus.

This paper concerns the dynamical behavior, in probabilistic sense, of a simple perceptron network with sigmoidal output units performing autoassociation for novelty filtering. Networks of retinotopic topology having a one-to-one correspondence between input and output units can be readily trained using the delta learning rule, to perform autoassociative mappings. A novelty filter is obtained by subtracting the network output from the input vector. Then the presentation of a "familiar" pattern tends to evoke a null response; but any anomalous component is enhanced. Such a behavior exhibits a promising feature for enhancement of weak signals in additive noise. As an analysis of the novelty filtering, this paper shows that the probability density function of the weight converges to Gaussian when the input time series is statistically characterized by nonsymmetrical probability density functions. After output units are locally linearized, the recursive relation for updating the weight of the neural network is converted into a first-order random differential equation. Based on this equation it is shown that the probability density function of the weight satisfies the Fokker-Planck equation. By solving the Fokker-Planck equation, it is found that the weight is Gaussian distributed with time dependent mean and variance.