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modwptdetails

Maximal overlap discrete wavelet packet transform details

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Syntax

w = modwptdetails(x)
w = modwptdetails(x,wname)
w = modwptdetails(x,lo,hi)
w = modwptdetails(___,lev)
[w,packetlevs] = modwptdetails(___)
[w,packetlevs,cfreq] = modwptdetails(___)
[___] = modwptdetails(___,FullTree=tf)

Description

w = modwptdetails(x) returns the maximal overlap discrete wavelet packet transform (MODWPT) details for the 1-D signal, x. The MODWPT details provide zero-phase filtering of the signal. By default, modwptdetails returns only the terminal nodes, which are at level 4 or at level floor(log2(numel(x))), whichever is smaller.

Note

To decide whether to use modwptdetails or modwpt, consider the type of data analysis you need to perform. For applications that require time alignment, such as nonparametric regression analysis, use modwptdetails. For applications where you want to analyze the energy levels in different packets, use modwpt. For more information, see Algorithms.

example

w = modwptdetails(x,wname) uses the orthogonal wavelet specified by wname.

example

w = modwptdetails(x,lo,hi) uses the orthogonal scaling filter, lo, and wavelet filter, hi.

example

w = modwptdetails(___,lev) returns the terminal nodes of the wavelet packet tree at positive integer level lev.

[w,packetlevs] = modwptdetails(___) returns a vector of transform levels corresponding to the rows of w.

example

[w,packetlevs,cfreq] = modwptdetails(___) returns cfreq, the center frequencies of the approximate passbands corresponding to the MODWPT details in w.

example

[___] = modwptdetails(___,FullTree=tf), where tf is false, returns details about only the terminal (final-level) wavelet packet nodes. If you specify true, then modwptdetails returns details about the full wavelet packet tree down to the default or specified level. The default value for tf is false.

example

Examples

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Open Live Script

Obtain the MODWPT of an electrocardiogram (ECG) signal using the default length 18 Fejér-Korovkin ('fk18') wavelet and the default level, 4.

load wecg
wptdetails = modwptdetails(wecg);

Demonstrate that summing the MODWPT details over each sample reconstructs the signal.

xrec = sum(wptdetails,1);
max(abs(wecg-xrec'))
ans = 
1.7903e-11

This example uses:

Open Live Script

Obtain the MODWPT details for a signal consisting of 100 Hz and 450 Hz sine waves. The signal is sampled at 1000 Hz for one second. Each row of the modwptdetails output corresponds to a separate frequency band.

fs = 1000;
dt = 1/fs;
t = 0:dt:1;
x = (sin(2*pi*100*t)+sin(2*pi*450*t));
[lo,hi] = wfilters("fk22");
wptdetails = modwptdetails(x,lo,hi);

Use modwpt to obtain the energy and center frequencies of the signal. Plot the energy in the wavelet packets. The fourth and fifteenth frequency bands contain most of the energy. Other frequency bands have significantly less energy. The frequency ranges of fourth and fifteenth bands are approximately 94-125 Hz and 438-469 Hz, respectively. 

[wpt,~,cfreqs,energy] = modwpt(x,lo,hi);
bar(1:16,energy);
xlabel("Packet")
ylabel("Packet Energy")
title("Energy by Wavelet Packet")

Figure contains an axes object. The axes object with title Energy by Wavelet Packet, xlabel Packet, ylabel Packet Energy contains an object of type bar.

Plot the power spectral density of the input signal.

pwelch(x,[],[],[],fs,"onesided")
title("Power Spectral Density of Input Signal")

Figure contains an axes object. The axes object with title Power Spectral Density of Input Signal, xlabel Frequency (Hz), ylabel Power/frequency (dB/Hz) contains an object of type line.

Show that the MODWPT details have zero-phase shift from the 100 Hz input sine.

p4 = wptdetails(4,:);
tiledlayout(2,1)
nexttile
plot(t,sin(2*pi*100*t).*(t>0.3 & t<0.7))
title("100 Hz Sine Wave")
ylabel("Amplitude")
nexttile
plot(t,p4.*(t>0.3 & t<0.7),"r")
title("MODWPT Details")
ylabel("Amplitude")
xlabel("Time (s)")

Figure contains 2 axes objects. Axes object 1 with title 100 Hz Sine Wave, ylabel Amplitude contains an object of type line. Axes object 2 with title MODWPT Details, xlabel Time (s), ylabel Amplitude contains an object of type line.

Open Live Script

Create a 100 Hz time-localized sinusoid in noise. The sampling rate is 1000 Hz. Obtain the MODWPT at level 4 using the length 22 Fejér-Korovkin ('fk22') wavelet. Also obtain the center frequencies of the approximate passbands corresponding to the MODWPT details. 

Fs = 1000;  
t = 0:1/Fs:1;
x = cos(2*pi*100*t).*(t>0.3 & t<0.7)+0.25*randn(size(t));
[wptdetails,~,cf] = modwptdetails(x,"fk22");

The center frequencies are in cycles per sample. Convert the center frequencies to hertz. 

Fs*cf
ans = 16×1

   15.6250
   46.8750
   78.1250
  109.3750
  140.6250
  171.8750
  203.1250
  234.3750
  265.6250
  296.8750
      ⋮

The fourth center frequency is closest to the frequency of the sinusoid. Extract the details from the corresponding node.

p4 = wptdetails(4,:);

Compare the extracted details with the sinusoid. The MODWPT details represent zero-phase filtering of the input signal with an approximate passband of [3Fs/25,4Fs/25), where Fs is the sampling frequency.

plot(t,cos(2*pi*100*t).*(t>0.3 & t<0.7));
hold on
plot(t,p4,"r")
legend("Sinusoid","MODWPT Details")
hold off

Figure contains an axes object. The axes object contains 2 objects of type line. These objects represent Sinusoid, MODWPT Details.

Open Live Script

Obtain the MODWPT details of an ECG waveform using the length 22 Fejér-Korovkin scaling and wavelet filters. 

load wecg
[lo,hi] = wfilters("fk22");
wpt = modwptdetails(wecg,lo,hi);
Open Live Script

Load the ECG signal, which has 2048 samples. Obtain the MODWPT details of the full wavelet packet tree for the signal. Use the default length 18 Fejér-Korovkin ('fk18') wavelet.

load wecg   
[w,packetlevels] = modwptdetails(wecg,FullTree=true);

Confirm the terminal nodes of the packet tree are at level 4.

max(packetlevels)
ans = 
4

Extract and plot the node coefficients at level 3, node 2.

p3 = w(packetlevels==3,:);
plot(p3(3,:))
axis tight
title("Node (3,2) MODWPT Details")

Figure contains an axes object. The axes object with title Node (3,2) MODWPT Details contains an object of type line.

Input Arguments

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Input signal, specified as a row or column vector. x must have at least two elements.

Data Types: single | double
Complex Number Support: Yes

Orthogonal wavelet, specified as a character vector or string scalar. Orthogonal wavelets are designated as type 1 wavelets in the wavelet manager, wavemngr.

Valid built-in orthogonal wavelet families are: Best-localized Daubechies ("bl"), Beylkin ("beyl"), Coiflets ("coif"), Daubechies ("db"), Fejér-Korovkin ("fk"), Haar ("haar"), Han linear-phase moments ("han"), Morris minimum-bandwidth ("mb"), Symlets ("sym"), and Vaidyanathan ("vaid").

For a list of wavelets in each family, see wfilters. You can also use waveinfo with the wavelet family short name. For example, waveinfo("db"). Use wavemngr("type",wn) to determine if the wavelet wn is orthogonal (returns 1). For example, wavemngr("type","db6") returns 1.

Orthogonal wavelet filters, specified as a pair of even-length real-valued vectors. lo is the scaling (lowpass) filter and hi is the wavelet (highpass) filter. For more information, see wfilters and isorthwfb. You cannot specify both wname and a filter pair lo,hi.

Note

By default, the wfilters function returns two pairs of filters associated with an orthogonal or biorthogonal wavelet you specify. To agree with the usual convention in the implementation of MODWPT in numerical packages, when you specify an orthogonal wavelet wname, the modwptdetails function internally uses the second pair of filters returned by wfilters. For example,

wptdetails = modwptdetails(x,"db2");

is equivalent to

[~,~,lo,hi] = wfilters("db2"); wptdetails = modwptdetails(x,lo,hi);

This convention is different from the one followed by most Wavelet Toolbox™ discrete wavelet transform functions when decomposing a signal. Most functions internally use the first pair of filters.

Data Types: single | double

Transform level, specified as a positive integer less than or equal to floor(log2(numel(x))).

Return tree option, specified as a numeric or logical 0 (false) or 1 (true). If tf is false, then modwptdetails returns details about only the terminal (final-level) wavelet packet nodes. If you specify true, then modwptdetails returns details about the full wavelet packet tree down to the default or specified level.

For the full wavelet packet tree, w is a 2j+1-2-by-numel(x) matrix. Each level j has 2j wavelet packet details.

Output Arguments

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Wavelet packet tree details, returned as a matrix with each row containing the sequency-ordered wavelet packet details for the terminal nodes. The terminal nodes are at level 4 or at level floor(log2(numel(x))), whichever is smaller. The MODWPT details are zero-phase-filtered projections of the signal onto the subspaces corresponding to the wavelet packet nodes. The sum of the MODWPT details over each sample reconstructs the original signal.

For the default terminal nodes, w is a 2j-by-numel(x) matrix. For the full packet table, at level j, w is a 2j+1-2-by-numel(x) matrix of sequency-ordered wavelet packet coefficients by level and index. The approximate passband for the nth row of w at level j is [n−12(j+1),n2(j+1)) cycles per sample, where n = 1,2,...,2j.

Transform levels, returned as a vector. The levels correspond to the rows of w. If w contains only the terminal level coefficients, packetlevs is a vector of constants equal to the terminal level. If w contains the full wavelet packet tree of details, packetlevs is a vector with 2j-1 elements for each level, j. To select all the MODWPT details at a particular level, use packetlevs with logical indexing.

Center frequencies of the approximate passbands in the w rows, returned as a vector. The center frequencies are in cycles per sample. To convert the units to cycles per unit time, multiply cfreq by the sampling frequency.

Algorithms

The MODWPT details (modwptdetails) are the result of zero-phase filtering of the signal. The features in the MODWPT details align exactly with features in the input signal. For a given level, summing the details for each sample returns the exact original signal.

The output of the MODWPT (modwpt) is time delayed compared to the input signal. Most filters used to obtain the MODWPT have a nonlinear phase response, which makes compensating for the time delay difficult. All orthogonal scaling and wavelet filters have this response, except the Haar wavelet. It is possible to time align the coefficients with the signal features, but the result is an approximation, not an exact alignment with the original signal. The MODWPT partitions the energy among the wavelet packets at each level. The sum of the energy over all the packets equals the total energy of the input signal.

References

[1] Percival, Donald B., and Andrew T. Walden. Wavelet Methods for Time Series Analysis. Cambridge Series in Statistical and Probabilistic Mathematics. Cambridge ; New York: Cambridge University Press, 2000.

[2] Walden, A. T., and A. Contreras Cristan. “The Phase–Corrected Undecimated Discrete Wavelet Packet Transform and Its Application to Interpreting the Timing of Events.” Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences 454, no. 1976 (August 8, 1998): 2243–66. https://doi.org/10.1098/rspa.1998.0257.

Extended Capabilities

Version History

Introduced in R2016a

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See Also

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