Introduction to Wavelet

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Introduction to Wavelet : 

Introduction to Wavelet Bhushan D Patil PhD Research Scholar Department of Electrical Engineering Indian Institute of Technology, Bombay Powai, Mumbai. 400076

Outline of Talk : 

Outline of Talk Overview Historical Development Time vs Frequency Domain Analysis Fourier Analysis Fourier vs Wavelet Transforms Wavelet Analysis Typical Applications References


OVERVIEW Wavelet A small wave Wavelet Transforms Convert a signal into a series of wavelets Provide a way for analyzing waveforms, bounded in both frequency and duration Allow signals to be stored more efficiently than by Fourier transform Be able to better approximate real-world signals Well-suited for approximating data with sharp discontinuities “The Forest & the Trees” Notice gross features with a large "window“ Notice small features with a small

Historical Development : 

Historical Development Pre-1930 Joseph Fourier (1807) with his theories of frequency analysis The 1930s Using scale-varying basis functions; computing the energy of a function 1960-1980 Guido Weiss and Ronald R. Coifman; Grossman and Morlet Post-1980 Stephane Mallat; Y. Meyer; Ingrid Daubechies; wavelet applications today

Mathematical Transformation : 

Mathematical Transformation Why To obtain a further information from the signal that is not readily available in the raw signal. Raw Signal Normally the time-domain signal Processed Signal A signal that has been "transformed" by any of the available mathematical transformations Fourier Transformation The most popular transformation


FREQUENCY ANALYSIS Frequency Spectrum Be basically the frequency components (spectral components) of that signal Show what frequencies exists in the signal Fourier Transform (FT) One way to find the frequency content Tells how much of each frequency exists in a signal


STATIONARITY OF SIGNAL Stationary Signal Signals with frequency content unchanged in time All frequency components exist at all times Non-stationary Signal Frequency changes in time One example: the “Chirp Signal”




CHIRP SIGNALS Frequency: 2 Hz to 20 Hz Frequency: 20 Hz to 2 Hz Same in Frequency Domain At what time the frequency components occur? FT can not tell!


NOTHING MORE, NOTHING LESS FT Only Gives what Frequency Components Exist in the Signal The Time and Frequency Information can not be Seen at the Same Time Time-frequency Representation of the Signal is Needed Most of Transportation Signals are Non-stationary. (We need to know whether and also when an incident was happened.) ONE EARLIER SOLUTION: SHORT-TIME FOURIER TRANSFORM (STFT)


SFORT TIME FOURIER TRANSFORM (STFT) Dennis Gabor (1946) Used STFT To analyze only a small section of the signal at a time -- a technique called Windowing the Signal. The Segment of Signal is Assumed Stationary A 3D transform A function of time and frequency


DRAWBACKS OF STFT Unchanged Window Dilemma of Resolution Narrow window -> poor frequency resolution Wide window -> poor time resolution Heisenberg Uncertainty Principle Cannot know what frequency exists at what time intervals Via Narrow Window Via Wide Window


MULTIRESOLUTION ANALYSIS (MRA) Wavelet Transform An alternative approach to the short time Fourier transform to overcome the resolution problem Similar to STFT: signal is multiplied with a function Multiresolution Analysis Analyze the signal at different frequencies with different resolutions Good time resolution and poor frequency resolution at high frequencies Good frequency resolution and poor time resolution at low frequencies More suitable for short duration of higher frequency; and longer duration of lower frequency components


PRINCIPLES OF WAELET TRANSFORM Split Up the Signal into a Bunch of Signals Representing the Same Signal, but all Corresponding to Different Frequency Bands Only Providing What Frequency Bands Exists at What Time Intervals


DEFINITION OF CONTINUOUS WAVELET TRANSFORM Wavelet Small wave Means the window function is of finite length Mother Wavelet A prototype for generating the other window functions All the used windows are its dilated or compressed and shifted versions


SCALE Scale S>1: dilate the signal S<1: compress the signal Low Frequency -> High Scale -> Non-detailed Global View of Signal -> Span Entire Signal High Frequency -> Low Scale -> Detailed View Last in Short Time Only Limited Interval of Scales is Necessary


COMPUTATION OF CWT Step 1: The wavelet is placed at the beginning of the signal, and set s=1 (the most compressed wavelet); Step 2: The wavelet function at scale “1” is multiplied by the signal, and integrated over all times; then multiplied by ; Step 3: Shift the wavelet to t= , and get the transform value at t= and s=1; Step 4: Repeat the procedure until the wavelet reaches the end of the signal; Step 5: Scale s is increased by a sufficiently small value, the above procedure is repeated for all s; Step 6: Each computation for a given s fills the single row of the time-scale plane; Step 7: CWT is obtained if all s are calculated.






DISCRETIZATION OF CWT It is Necessary to Sample the Time-Frequency (scale) Plane. At High Scale s (Lower Frequency f ), the Sampling Rate N can be Decreased. The Scale Parameter s is Normally Discretized on a Logarithmic Grid. The most Common Value is 2. The Discretized CWT is not a True Discrete Transform Discrete Wavelet Transform (DWT) Provides sufficient information both for analysis and synthesis Reduce the computation time sufficiently Easier to implement Analyze the signal at different frequency bands with different resolutions Decompose the signal into a coarse approximation and detail information

Multi Resolution Analysis : 

Multi Resolution Analysis Analyzing a signal both in time domain and frequency domain is needed many a times But resolutions in both domains is limited by Heisenberg uncertainty principle Analysis (MRA) overcomes this , how? Gives good time resolution and poor frequency resolution at high frequencies and good frequency resolution and poor time resolution at low frequencies This helps as most natural signals have low frequency content spread over long duration and high frequency content for short durations


SUBBABD CODING ALGORITHM Halves the Time Resolution Only half number of samples resulted Doubles the Frequency Resolution The spanned frequency band halved


RECONSTRUCTION What How those components can be assembled back into the original signal without loss of information? A Process After decomposition or analysis. Also called synthesis How Reconstruct the signal from the wavelet coefficients Where wavelet analysis involves filtering and down sampling, the wavelet reconstruction process consists of up sampling and filtering


WAVELET APPLICATIONS Typical Application Fields Astronomy, acoustics, nuclear engineering, sub-band coding, signal and image processing, neurophysiology, music, magnetic resonance imaging, speech discrimination, optics, fractals, turbulence, earthquake-prediction, radar, human vision, and pure mathematics applications Sample Applications Identifying pure frequencies De-noising signals Detecting discontinuities and breakdown points Detecting self-similarity Compressing images


REFERENCES Mathworks, Inc. Matlab Toolbox Robi Polikar, The Wavelet Tutorial, Robi Polikar, Multiresolution Wavelet Analysis of Event Related Potentials for the Detection of Alzheimer's Disease, Iowa State University, 06/06/1995 Amara Graps, An Introduction to Wavelets, IEEE Computational Sciences and Engineering, Vol. 2, No 2, Summer 1995, pp 50-61. Resonance Publications, Inc. Wavelets. R. Crandall, Projects in Scientific Computation, Springer-Verlag, New York, 1994, pp. 197-198, 211-212. Y. Meyer, Wavelets: Algorithms and Applications, Society for Industrial and Applied Mathematics, Philadelphia, 1993, pp. 13-31, 101-105. G. Kaiser, A Friendly Guide to Wavelets, Birkhauser, Boston, 1994, pp. 44-45. W. Press et al., Numerical Recipes in Fortran, Cambridge University Press, New York, 1992, pp. 498-499, 584-602. M. Vetterli and C. Herley, "Wavelets and Filter Banks: Theory and Design," IEEE Transactions on Signal Processing, Vol. 40, 1992, pp. 2207-2232. I. Daubechies, "Orthonormal Bases of Compactly Supported Wavelets," Comm. Pure Appl. Math., Vol 41, 1988, pp. 906-966. V. Wickerhauser, Adapted Wavelet Analysis from Theory to Software, AK Peters, Boston, 1994, pp. 213-214, 237, 273-274, 387. M.A. Cody, "The Wavelet Packet Transform," Dr. Dobb's Journal, Vol 19, Apr. 1994, pp. 44-46, 50-54. J. Bradley, C. Brislawn, and T. Hopper, "The FBI Wavelet/Scalar Quantization Standard for Gray-scale Fingerprint Image Compression," Tech. Report LA-UR-93-1659, Los Alamos Nat'l Lab, Los Alamos, N.M. 1993. D. Donoho, "Nonlinear Wavelet Methods for Recovery of Signals, Densities, and Spectra from Indirect and Noisy Data," Different Perspectives on Wavelets, Proceeding of Symposia in Applied Mathematics, Vol 47, I. Daubechies ed. Amer. Math. Soc., Providence, R.I., 1993, pp. 173-205. B. Vidakovic and P. Muller, "Wavelets for Kids," 1994, unpublished. Part One, and Part Two.

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