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wavedec

1-D wavelet decomposition

Syntax

``[c,l] = wavedec(x,n,wname)``
``[c,l] = wavedec(x,n,LoD,HiD)``

Description

example

````[c,l] = wavedec(x,n,wname)` returns the wavelet decomposition of the 1-D signal `x` at level `n` using the wavelet `wname`. The output decomposition structure consists of the wavelet decomposition vector `c` and the bookkeeping vector `l`, which contains the number of coefficients by level. The structure is organized as in this level-3 decomposition diagram.```
````[c,l] = wavedec(x,n,LoD,HiD)` returns the wavelet decomposition using the specified lowpass and highpass wavelet decomposition filters `LoD` and `HiD`, respectively.```

Examples

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Load and plot a one-dimensional signal.

```load sumsin plot(sumsin) title('Signal')```

Perform a 3-level wavelet decomposition of the signal using the order 2 Daubechies wavelet. Extract the coarse scale approximation coefficients and the detail coefficients from the decomposition.

```[c,l] = wavedec(sumsin,3,'db2'); approx = appcoef(c,l,'db2'); [cd1,cd2,cd3] = detcoef(c,l,[1 2 3]);```

Plot the coefficients.

```subplot(4,1,1) plot(approx) title('Approximation Coefficients') subplot(4,1,2) plot(cd3) title('Level 3 Detail Coefficients') subplot(4,1,3) plot(cd2) title('Level 2 Detail Coefficients') subplot(4,1,4) plot(cd1) title('Level 1 Detail Coefficients')```

Input Arguments

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Input signal, specified as a real-valued vector.

Data Types: `double`

Level of decomposition, specified as a positive integer. `wavedec` does not enforce a maximum level restriction. Use `wmaxlev` to ensure that the wavelet coefficients are free from boundary effects. If boundary effects are not a concern in your application, a good rule is to set `n` less than or equal to `fix(log2(length(x)))`.

Data Types: `double`

Analyzing wavelet, specified as a character vector or string scalar.

Note

`wavedec` supports only Type 1 (orthogonal) or Type 2 (biorthogonal) wavelets. See `wfilters` for a list of orthogonal and biorthogonal wavelets.

Wavelet decomposition filters, specified as a pair of even-length real-valued vectors. `LoD` is the lowpass decomposition filter, and `HiD` is the highpass decomposition filter. The lengths of `LoD` and `HiD` must be equal. See `wfilters` for additional information.

Output Arguments

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Wavelet decomposition vector, returned as a real-valued vector. The bookkeeping vector `l` contains the number of coefficients by level.

Bookkeeping vector, returned as a vector of positive integers. The bookkeeping vector is used to parse the coefficients in the wavelet decomposition vector `c` by level.

Algorithms

Given a signal s of length N, the DWT consists of at most log2 N steps. Starting from s, the first step produces two sets of coefficients: approximation coefficients cA1 and detail coefficients cD1. Convolving s with the lowpass filter `LoD` and the highpass filter `HiD`, followed by dyadic decimation (downsampling), results in the approximation and detail coefficients respectively.

where

• — Convolve with filter X

• $\begin{array}{||}\hline ↓2\\ \hline\end{array}$ — Downsample (keep the even-indexed elements)

The length of each filter is equal to 2n. If N = length(s), the signals F and G are of length N + 2n −1 and the coefficients cA1 and cD1 are of length

floor$\left(\frac{N-1}{2}\right)+n$.

The next step splits the approximation coefficients cA1 in two parts using the same scheme, replacing s by cA1, and producing cA2 and cD2, and so on.

The wavelet decomposition of the signal s analyzed at level j has the following structure: [cAj, cDj, ..., cD1].

This structure contains, for j = 3, the terminal nodes of the following tree:

References

[1] Daubechies, I. Ten Lectures on Wavelets, CBMS-NSF Regional Conference Series in Applied Mathematics. Philadelphia, PA: SIAM Ed, 1992.

[2] Mallat, S. G. “A Theory for Multiresolution Signal Decomposition: The Wavelet Representation,” IEEE Transactions on Pattern Analysis and Machine Intelligence. Vol. 11, Issue 7, July 1989, pp. 674–693.

[3] Meyer, Y. Wavelets and Operators. Translated by D. H. Salinger. Cambridge, UK: Cambridge University Press, 1995.

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