R.G. Stockwell

This forum has been flooded by bots lately, and it results in hundreds of email failure messages per day. Temporarily, the sign-up feature has been disabled (and in the future an entire new software suite will be employed for this webpage). If you wish to sign up, just send an email to me, or to webmaster@cora.nwra.com. Sorry for the inconvenience.

Or, one can use the Comment section below. I will monitor that regularly.

-bob

Digital Imaging publication

Sylvia Drabycz et al. published a paper employing the orthonormal ST in the Journal of Digital Imaging. It is available online (link below)

Abstract:

We present a new efficient approach for characterizing image texture based on a recently published discrete, orthonormal space-frequency transform known as the DOST. We develop a frequency-domain implementation of the DOST in two dimensions for the case of dyadic frequency sampling. Then, we describe a rapid and efficient approach to obtain local spatial frequency information for an image and show that this information can be used to characterize the horizontal and vertical frequency patterns in synthetic images. Finally, we demonstrate that DOST components can be combined to obtain a rotationally invariant set of texture features that can accurately classify a series of texture patterns. The DOST provides the computational efficiency and multi-scale information of wavelet transforms, while providing texture features in terms of Fourier frequencies. It outperforms leading wavelet-based texture analysis methods.

http://www.springerlink.com/content/t5j2p45h00482727/

Recent Publications- November 2007

Recently PUBLISHED:

1) Stockwell, R. G., D. M. Riggin, W. J. R. French, G. B. Burns, and D. J. Murphy (2007), Planetary waves and intraseasonal oscillations at Davis, Antarctica, from undersampled time series, J. Geophys. Res., 112, D21107, doi:10.1029/2006JD008034, 13 November 2007

2) (chapter in a book) "Why use the ST?" by Robert Stockwell,
Pseudo-Differential Operators: Partial Differential Equations and Time-Frequency Analysis
Edited by: Luigi Rodino, UniversitÃ di Torino, Italy, Bert-Wolfgang Schulze, UniversitÃ¤t Potsdam, Germany, and M. W. Wong, York University, Toronto, ON, Canada
(link: http://www.ams.org/bookstore?fn=20&arg1=ficseries&item=FIC-52)

Orthogonal Basis for the S-Transform

This paper is available for download:
PDF file download

Download

The citation is:
"A basis for efficient representation of the S-transform"
R.G. Stockwell, Digital Signal Processing
Volume 17, Issue 1, January 2007, Pages 371-393

A recent paper describes an orthogonal basis for the S-Transform.
Currently, the ST of a N-point time series returns a matrix with N^2 points. This can get quite large as N increases, and limits the utility of the ST. The new orthogonal representation allows one to take an N-point time series and return an N_point local spectrum representation.
Thus allowing the application of the ST to almost arbitrarily large data sets.

Stockwell, R.G. A Basis for Efficient Representation of the S-Transform, Digital Signal Processing, doi:10.1016/j.dsp.2006.04.006 (Article in Press) (2006a)
It is available online at:

http://doi.org/
past in the doi number above:
doi:10.1016/j.dsp.2006.04.006

ABSTRACT:
The S-transform is a time-frequency representation known for its local spectral phase properties. A key feature of the S-transform is that it uniquely combines a frequency dependent resolution of the time-frequency space and absolutely referenced local phase information. This allows one to define the meaning of phase in a local spectrum setting, and results in many desirable characteristics. One drawback to the S-transform is the redundant representation of the time-frequency space and the consumption of computing resources this requires (a characteristic it shares with the continuous wavelet transform, the short time Fourier transform, and Cohen's class of generalized time-frequency distributions). The cost of this redundancy is amplified in multidimensional applications such as image analysis. A more efficient representation is introduced here as a orthogonal set of basis functions that localizes the spectrum and retains the advantageous phase properties of the S-transform. These basis functions are defined to have phase characteristics that are directly related to the phase of the Fourier transform spectrum, and are both compact in frequency and localized in time. Distinct from a wavelet approach, this approach allows one to directly collapse the orthogonal local spectral representation over time to the complex-valued Fourier transform spectrum. Because it maintains the phase properties of the S-transform, one can perform localized cross spectral analysis to measure phase shifts between each of multiple components of two time series as a function of both time and frequency. In addition, one can define a generalized instantaneous frequency (IF) applicable to broadband nonstationary signals. This is the first time a channel IF has been integrated in an orthogonal local spectral representation. A direct comparison between these basis functions and complex wavelets is performed, highlighting the advantages of this approach. The relationship between this basis set and the fully redundant S-transform is demonstrated highlighting the ability to arbitrarily sample the time-frequency space. The introduction of this basis set leads to efficient analysis routines that may find use in a wide range of fields.

Recent Publications - November 2006

A couple of recent papers have been published:

Fritts, D.C., D. Janches, D.M. Riggin, R.G. Stockwell, M.P. Sulzer, and S. Gonzalez, Gravity waves and momentum fluxes in the mesosphere and lower thermosphere using 430 MHz dual-beam measurements at Arecibo: 2. Frequency spectra, momentum fluxes, and variability. J. Geophys. Res., 111, D18108, doi:10.1029/2005JD006883, 2006.

Abstract:
Janches et al. (2006) described a new dual-beam use of the 430 MHz incoherent scatter radar at the Arecibo Observatory in Puerto Rico. We found the technique to define the radial wind field in the mesosphere and lower thermosphere with sufficient accuracy to characterize gravity waves occurring at high frequencies and small spatial scales over an extended altitude range. The coplanar, dual-beam experiment was also designed to test the ability of the system to measure gravity wave momentum fluxes and their frequency distributions, and we report here on those results. Initial measurements were of limited duration and necessarily represent a case study, but they demonstrate the value of such measurements for studies of GW variability and large-scale interactions. Radial velocity variances reveal preferential eastward propagation for most intervals and altitudes, with the greatest propagation bias at lower altitudes and later times on 11 September when strong westward mean winds favor strong gravity filtering. The momentum fluxes observed during this experiment had âˆ¼50-min averages that were often near zero, occasionally achieved amplitudes of âˆ¼20 to 50 m2sâˆ’2, displayed significant consistency in altitude, and exhibited an approximate anticorrelation with the zonal wind field in cases with significant momentum fluxes. Frequency spectra defined the major contributions to the momentum fluxes, while S transforms were employed to examine the temporal variability of the GWs and momentum fluxes in greater detail.

Laser Doppler Flowmetry

Here is the reference of Said Assous' paper published this month.
"S-Transform Applied to Laser Doppler Flowmetry Reactive Hyperemia
Signals"
Assous, S.; Humeau, A.; Tartas, M.; Abraham, P.; L'Huillier, J.P.
IEEE Trans Biomed Eng. 2006 Jun;53(6):1032-7
Digital Object Identifier 10.1109/TBME.2005.863843

PDF file download

LINK

Recent Publications

A paper employing two dimensional S-Transform analysis
of gravity (buoyancy) waves seen in atmospheric airglow
layers has been published.

The reference is:
Stockwell, R. G., M. J. Taylor, K. Nielsen, and M. J. Jarvis (2006),
A novel joint space-wavenumber analysis of an unusual Antarctic gravity
wave event, Geophys. Res. Lett., 33, L08805, doi:10.1029/2005GL025660.

a related paper is:
Nielsen, K.; Taylor, M. J.; Stockwell, R. G.; Jarvis, M. J.
An unusual mesospheric bore event observed at high latitudes
over Antarctica, Geophys. Res. Lett., Vol. 33, No. 7, L0780310.1029/2005GL025649 05 April 2006

Comments - Make a comment

The comments are owned by the poster. We are not responsible for its content.

Forums

Latest Forum Posts