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# Tag Info

18

If you're using Matlab, you may be interested in the Chebfun project. Chebfun takes a function, samples it and tries to represent it as a polynomial interpolant. If your function has discontinuities, Chebfun should be able to detect them with the splitting on command. You can find some examples here. If you're interested in the underlying algorithms, a good ...

11

I suspect that chebfun algorithm must appear to be more practical, but it is neccesary to mention one more way to detect discontinuities, namely the discrete wavelet transform. You can get an idea how it works by looking at this Mathematica documentation page, see section > Applications > Detect Discontinuities and Edges. Briefly speaking, you can take DWT ...

8

Weighted Essentially Non-Oscillatory (WENO) methods use "smoothness indicators" to detect discontinuities in finite volume and difference methods. From the description of Chebfun that Pedro gave, it seems as though the general idea is the same: construct a set of interpolating polynomials and use them to compute some measure of smoothness. See G.S. Jiang,...

5

Along with @Pedro, I would look at edge detection algorithms. A discontinuity is an infinity on the derivative, so consider looking at an increasingly fine mesh and targeting regions of interest. The finite difference approximation to the derivative of a continuous function should reduce as the mesh is refined. Comparing the finite difference result for the ...

4

Frequency separations The easiest way to estimate the large separation (without fitting the individual frequencies) is to take the autocorrelation of the power spectrum and find the maximum. That's a start. To find the small separation, you'd be looking for a second peak. Anyway, have a look at the autocorrelation of the pwer spectrum and see if you can see ...

3

I asked this question on the Signal Processing exchange as well. Eventually I found the answer myself and posted it there. Here I copy/paste my answer as posted there. After many weeks I give the answer to my own question. There is a limit in which we can solve this problem in a reasonably simple way. Suppose we sum enough points in our DFT that the ...

2

You start at the wrong end. Your question is about how to do it with a vector of length $N$ in Matlab but your question states that you are not clear about what periodic noise actually means in this case. You can't ask how to do something you don't yet understand. You first need to understand what it is you want to do, and then actually doing it will become ...

2

The reason that Python code is slower than the equivalent Matlab code is usually that in Matlab, more computations are carried out by libraries that are written in a lower-level language, and heavily optimized. Luckily, you can do the same in Python. For example, Matlab is linked against (and bundles) Intel's hand-tuned MKL library, while Python (that is,...

2

For an operator $R$ to be linear, it has to satisfy two conditions: $R(f+g) = Rf + Rg$ for any two operands $f,g$; $R(\alpha f) = \alpha Rf$ for any operand $f$ and (real or complex) number $\alpha$. This is true for the Radon transform, as one easily verifies. Whether compressed sensing can be applied to it is something beyond my realm of knowledge.

2

This is due to an implicit time shift, which corresponds to a phase shift, perceived here as a sign inversion. The signal you are Fourier transforming is symmetric around t=0 and this is why you should expect a positive power spectrum. However, the discrete Fourier transform (DFT) of a time-series $x_n$, which is what the FFT alorithm implements, is ...

2

Fitting the peaks of gamma spectra is a typical task in non-destructive analysis of spent fuel or neutron activation analysis. Since these applications are already "quite old", there is some standard software available, like Genie 2000. A paper Evaluation of Peak-Fitting Software for Gamma Spectrum Analysis from 2015 compares a number of these tools. However,...

1

If I understand you correctly, you have a periodic reference signal with pulses, and a list of the local maxima. From every local max, search for the nearest reference signal pulse and measure the distance (shift). If the frequency of your reference signal and the measured one is the same, than this shift should always point in the same direction. If you ...

1

Since convolution can be written as a matrix-vector product $Ax=b$ of a circulant or Toeplitz matrix $A$ acting on a vector $x$, you can invert or pseudoinvert via SVD $A$ to obtain $x=A^{-1}b$. That said, FFT deconvolution will always be dramatically faster than this approach and should be preferred unless the kernel function ($h$ in your example above) has ...

1

You may start with median or Gaussian filters. There are many libraries that implement them and they are simple to use. That said, I think this approach may be not enough because from what I've seen this noise is not an image noise in the classical sense, i.e. it's not randomly distributed 'dots', but rather periodic 'waves' and their presence is connected ...

1

This is not my field any more, but when I was a student, I remember people advocating the use of Markov Chain for this kind of things. Maybe something to search for, although I expect that things have moved on since. Indeed the paper you mention seems to be an evolution of it. I don't really understand why the "Texas method" is not working, not having read ...

1

How to make that process faster? Well, firstly if you feel like having the time you could check the implementation, which I'm doing just right now. (Warning: I need to truncate links as stackexchange does not let me have more than 2 links) Code is here: assembla . com /code/PySpectrum/subversion/nodes/37/trunk/src/spectrum Giving a look at the package it ...

1

This site gives formulas for butterworth and various common filters. And here's some C code to try.

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