# wentropy

Wavelet entropy

## Syntax

``ent = wentropy(X)``
``ent = wentropy(X,Name=Value)``
``[ent,re] = wentropy(___)``

## Description

````ent = wentropy(X)` returns the normalized Shannon wavelet entropy of `X`.```

example

````ent = wentropy(X,Name=Value)` specifies options using one or more name-value arguments.```
````[ent,re] = wentropy(___)` also returns the wavelet relative energies.```

## Examples

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Shannon Entropy

Create a signal whose samples are alternating values of 0 and 2.

```n = 0:499; x = 1+(-1).^n; stem(x) axis tight title("Signal") xlim([0 50])```

Obtain the scaled Shannon entropy of the signal. Specify a one-level wavelet transform, use the default wavelet and wavelet transform.

```ent = wentropy(x,Level=1); ent```
```ent = 2×1 1.0000 1.0000 ```

Obtain the unscaled Shannon entropy. Divide the entropy by `log(n)`, where `n` is the length of the signal. Confirm the result equals the scaled entropy.

```ent2 = wentropy(x,Level=1,Scaled=false); ent2/log(length(x))```
```ans = 2×1 1.0000 1.0000 ```

Create a zero-mean signal from the first signal. Obtain the scaled Shannon entropy of the new signal using a one-level wavelet transform.

```x = x-1; ent = wentropy(x,Level=1); ent```
```ent = 2×1 1.0000 0 ```

Renyi Entropy

Load the Kobe earthquake data. Obtain the level 4 tunable Q-factor wavelet transform of the data with a quality factor equal to 2.

```load kobe wt = tqwt(kobe,Level=4,QualityFactor=2);```

Obtain the Renyi entropy estimates for the tunable Q-factor transform.

```ent = wentropy(wt,Entropy="Renyi"); ent```
```ent = 5×1 0.8288 0.8506 0.8582 0.8536 0.7300 ```

Load the ECG data. Obtain the level 5 discrete wavelet transform of the signal using the `"db4"` wavelet.

```load wecg wv = "db4"; [C,L] = wavedec(wecg,5,wv);```

Package the wavelet and approximation coefficients into a cell array suitable for computing the wavelet entropy.

```X = detcoef(C,L,"cells"); X{end+1} = appcoef(C,L,wv);```

Obtain the Renyi entropy by scale.

```ent = wentropy(X,Entropy="Renyi"); ent```
```ent = 6×1 0.2412 0.5239 0.5459 0.6520 0.7661 0.8547 ```

Tsallis Entropy

Create a Kronecker delta sequence.

```N = 512; seq = zeros(1,N); seq(N/2) = 1;```

Obtain the scaled Shannon entropy of the signal. Specify a level 3 wavelet transform.

`ShannonEntropy = wentropy(seq,Level=3);`

Obtain the scaled Tsallis entropy of the signal for different values of exponents. Confirm that as the exponent goes to 1, the Tsallis entropy approaches the Shannon entropy.

```exps = 3:-1/4:1; TsallisExponent = zeros(length(exps),1); TsallisEntropy = zeros(length(exps),4); ctr = 1; for k=exps ent2 = wentropy(seq,Level=3,Entropy="Tsallis",Exponent=k); TsallisExponent(ctr) = k; TsallisEntropy(ctr,:) = ent2'; ctr = ctr+1; end TsallisTable = table(TsallisExponent,TsallisEntropy)```
```TsallisTable=9×2 table TsallisExponent TsallisEntropy _______________ ________________________________________ 3 0.71454 0.87888 0.97069 0.98285 2.75 0.67651 0.84955 0.95685 0.97233 2.5 0.63178 0.81187 0.93596 0.9552 2.25 0.57852 0.7628 0.90407 0.92718 2 0.51437 0.69812 0.85499 0.88149 1.75 0.43679 0.61258 0.77985 0.80825 1.5 0.34491 0.50213 0.66897 0.69658 1.25 0.24402 0.37071 0.52076 0.54417 1 0.1495 0.23839 0.356 0.37278 ```
`ShannonEntropy'`
```ans = 1×4 0.1495 0.2384 0.3560 0.3728 ```

## Input Arguments

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Input data, specified as a real-valued row or column vector, a cell array of real-valued row or column vectors, or a real-valued matrix with at least two rows.

• If `X` is a row or column vector, `X` must have at least four samples, and the function assumes `X` represents time data.

• If `X` is a cell array, the function assumes `X` to be a decimated wavelet or wavelet packet transform of a real-valued row or column vector.

• If `X` is a matrix with at least two rows, the function assumes `X` to be the maximal overlap discrete wavelet or wavelet packet transform of a real-valued row or column vector.

Example: `ent = wentropy(randn(1,1024))` returns the normalized Shannon wavelet entropy. `wentropy` computes the wavelet coefficients using the default options of `modwt`.

Data Types: `single` | `double`

### Name-Value Arguments

Specify optional pairs of arguments as `Name1=Value1,...,NameN=ValueN`, where `Name` is the argument name and `Value` is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: `ent = wentropy(X,Wavelet="coif4")` uses the `"coif4"` wavelet to obtain the wavelet transform.

Entropy returned by `wentropy`, specified as one of `"Shannon"`, `"Renyi"`, and `"Tsallis"`. For more information, see Wavelet Entropy.

Exponent to use in the Renyi and Tsallis entropy, specified as a real scalar.

• For the Renyi entropy, the exponent must be nonnegative.

• For the Tsallis entropy, the exponent must be greater than or equal to `–1/2`.

• For the Renyi and Tsallis entropies, specifying `Exponent=1` is a limiting case and produces the Shannon entropy.

Specifying `Exponent` is valid only when `Entropy` is `"Renyi"` or `"Tsallis"`.

Note

When you specify a negative exponent for the Tsallis entropy, entropy computations may become unstable with small changes in the wavelet coefficient energies, resulting in significant changes in the entropy values.

Data Types: `single` | `double`

Transform used to obtain the wavelet or wavelet packet coefficients for the real-valued row or column vector `X`, specified as one of these:

• `"dwt"` — Discrete wavelet transform

• `"dwpt"` — Discrete wavelet packet transform

• `"modwt"` — Maximal overlap discrete wavelet transform

• `"modwpt"` — Maximal overlap discrete wavelet packet transform

The default wavelet depends on the value of `Transform`.

• If `Transform` is `"dwt"` or `"modwt"`, `wentropy` uses the `"sym4"` wavelet.

• If `Transform` is `"dwpt"` or `"modwpt"`, `wentropy` uses the `"fk18"` wavelet.

Periodic extension is used for all transforms.

Wavelet decomposition level if the input `X` is time data, specified as a positive integer. The `wentropy` function obtains the wavelet transform down to the specified level. If unspecified, the default level depends on the type of transform and the signal length N.

• If `Transform` is `"dwt"` or `"modwt"`, `Level` defaults to `floor(log2(N))-1`.

• If `Transform` is `"dwpt"` or `"modwpt"`, `Level` defaults to `min(4,floor(log2(N))-1)`.

Specifying a level is invalid if the input data are wavelet or wavelet packet coefficients.

Data Types: `single` | `double`

Wavelet used to obtain the wavelet or wavelet packet transform of a real-valued row or column vector, specified as a character vector or string scalar. If `Transform` is `"modwt"` or `"modwpt"`, the wavelet must be orthogonal. For a list of supported orthogonal or biorthogonal wavelets, see `wfilters`.

Specifying a wavelet name is invalid if the input data are wavelet or wavelet packet coefficients.

Data Types: `char` | `string`

Normalization method to use to obtain the empirical probability distribution for the wavelet transform coefficients, specified as `"scale"` or `"global"`.

• `"global"` — The function normalizes the squared magnitudes of the coefficients by the total sum of squared magnitudes of all coefficients. Each scale in the wavelet transform yields a scalar and the vector of these values forms a probability vector. The function performs entropy calculations on this vector and the overall entropy is a scalar.

• `"scale"` — The function normalizes the wavelet coefficients at each scale separately and calculates the entropy by scale.

• If the input is time series data, the output `ent` is of size (Ns+1)-by-1, where Ns is the number of scales.

• If the input is a cell array or matrix, `ent` is of size M-by-1, where M is the length of the cell array or number of rows in the matrix.

Scale wavelet entropy logical, specified as a numeric or logical `1` (`true`) or `0` (`false`). If specified as `true`, the `wentropy` function scales the wavelet entropy by the factor corresponding to a uniform distribution for the specified entropy.

• For the Shannon and Renyi entropies, the factor is `1/log(Nj)`, where Nj is the length of the data in samples by scale if `Distribution` is `"scale"`, or the number of scales if `Distribution` is `"global"`.

• For the Tsallis entropy, the factor is `(Exponent-1)/(1-Nj^(1-Exponent))`.

Setting `Scaled=false` does not scale the wavelet entropy.

Data Types: `logical`

Energy threshold, specified as a nonnegative scalar. The function replaces all coefficients with energy by scale below `EnergyThreshold` with 0. A positive `EnergyThreshold` prevents the function from treating wavelet or wavelet packet coefficients with nonsignificant energy as a sequence with high entropy.

Data Types: `single` | `double`

## Output Arguments

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Entropy of `X`, returned as a scalar or vector.

• If `X` is time data, `ent` is a real-valued (Ns+1)-by-1 vector of entropy estimates by scale, where Ns is the number of scales.

• If `X` is a wavelet or wavelet packet transform input, `ent` is a real-valued column vector with length equal to the length of `X` if `X` is a cell array or the row dimension of `X` if `X` is a matrix.

See `Distribution` to obtain global estimates of the wavelet entropy. The `wentropy` function uses the natural logarithm to compute the entropy.

Data Types: `single` | `double`

Relative wavelet energy, returned as a vector or matrix.

• If `Distribution="scale"`, the function returns the relative wavelet energies by coefficient and scale.

• If `Distribution="global"`, the function returns the relative wavelet energies by scale.

Scales where the coefficient energy is below the value of `EnergyThreshold` are equal to 0.

Data Types: `single` | `double`

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### Wavelet Entropy

Wavelet entropy (WE) is often used to analyze nonstationary signals. WE combines a wavelet or wavelet decomposition with a measure of order within the wavelet coefficients by scale. These measures of order are referred to as entropy measures. WE treats the normalized wavelet coefficients as an empirical probability distribution and calculates its entropy.

You can normalize the wavelet coefficients wt in one of two ways.

• The function normalizes all the coefficients by the total sum of their squared magnitudes: $E=\sum _{i}\sum _{j}|w{t}_{ij}{|}^{2},$where j corresponds to time, and i corresponds to scale. The probability mass function is: $ℙ\left(w{t}_{ij}\right)=|w{t}_{ij}{|}^{2}/E.$

• The function normalizes the coefficients at each scale separately by the sum of their squared magnitudes: ${E}_{i}=\sum _{j}|w{t}_{ij}{|}^{2}.$ The probability mass function is: $ℙ\left(w{t}_{ij}\right)=|w{t}_{ij}{|}^{2}/{E}_{i}.$

The `wentropy` function supports three entropy measures.

• Shannon Entropy

For a discrete random variable `X`, the Shannon entropy is defined as:

`$H\left(X\right)=-\sum _{i}ℙ\left(X={x}_{i}\right)\mathrm{ln}\left(ℙ\left(X={x}_{i}\right)\right),$`

where the sum is taken over all values that the random variable can take. By convention, 0 ln(0) = 0.

• Renyi Entropy

The Renyi entropy is defined as:

In the limit, the Renyi entropy becomes the Shannon entropy: $\underset{\alpha \to 1}{\mathrm{lim}}{H}_{r}\left(X\right)=H\left(X\right).$

• Tsallis Entropy

The Tsallis entropy is defined as:

`${H}_{t}\left(X\right)=\frac{1}{q-1}\left(1-\sum _{i}{\left(ℙ\left(X={x}_{i}\right)\right)}^{q}\right),\text{ }q\in ℝ,\text{ }q\ne 1.$`

Similar to the Renyi entropy, in the limit, the Tsallis entropy becomes the Shannon entropy: $\underset{q\to 1}{\mathrm{lim}}{H}_{t}\left(X\right)=H\left(X\right).$

## References

[1] Zunino, L., D.G. Pérez, M. Garavaglia, and O.A. Rosso. “Wavelet Entropy of Stochastic Processes.” Physica A: Statistical Mechanics and Its Applications 379, no. 2 (June 2007): 503–12. https://doi.org/10.1016/j.physa.2006.12.057.

[2] Rosso, Osvaldo A., Susana Blanco, Juliana Yordanova, Vasil Kolev, Alejandra Figliola, Martin Schürmann, and Erol Başar. “Wavelet Entropy: A New Tool for Analysis of Short Duration Brain Electrical Signals.” Journal of Neuroscience Methods 105, no. 1 (January 2001): 65–75. https://doi.org/10.1016/S0165-0270(00)00356-3.

[3] Alcaraz, Raúl, ed. "Wavelet Entropy: Computation and Applications." Special issue, Entropy 17 (2015). https://www.mdpi.com/journal/entropy/special_issues/wavelet-entropy.

## Version History

Introduced before R2006a

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