Continuous Wavelet Transform (CWT)¶
This section describes functions used to perform single continuous wavelet transforms.
Single level - cwt
¶
- pywt.cwt(data, scales, wavelet)¶
One dimensional Continuous Wavelet Transform.
- Parameters
- dataarray_like
Input signal
- scalesarray_like
The wavelet scales to use. One can use
f = scale2frequency(wavelet, scale)/sampling_period
to determine what physical frequency,f
. Here,f
is in hertz when thesampling_period
is given in seconds.- waveletWavelet object or name
Wavelet to use
- sampling_periodfloat
Sampling period for the frequencies output (optional). The values computed for
coefs
are independent of the choice ofsampling_period
(i.e.scales
is not scaled by the sampling period).- method{‘conv’, ‘fft’}, optional
- The method used to compute the CWT. Can be any of:
conv
usesnumpy.convolve
.fft
uses frequency domain convolution.auto
uses automatic selection based on an estimate of the computational complexity at each scale.
The
conv
method complexity isO(len(scale) * len(data))
. Thefft
method isO(N * log2(N))
withN = len(scale) + len(data) - 1
. It is well suited for large size signals but slightly slower thanconv
on small ones.- axis: int, optional
Axis over which to compute the CWT. If not given, the last axis is used.
- Returns
- coefsarray_like
Continuous wavelet transform of the input signal for the given scales and wavelet. The first axis of
coefs
corresponds to the scales. The remaining axes match the shape ofdata
.- frequenciesarray_like
If the unit of sampling period are seconds and given, then frequencies are in hertz. Otherwise, a sampling period of 1 is assumed.
Notes
Size of coefficients arrays depends on the length of the input array and the length of given scales.
Examples
>>> import pywt >>> import numpy as np >>> import matplotlib.pyplot as plt >>> x = np.arange(512) >>> y = np.sin(2*np.pi*x/32) >>> coef, freqs=pywt.cwt(y,np.arange(1,129),'gaus1') >>> plt.matshow(coef) >>> plt.show() ---------- >>> import pywt >>> import numpy as np >>> import matplotlib.pyplot as plt >>> t = np.linspace(-1, 1, 200, endpoint=False) >>> sig = np.cos(2 * np.pi * 7 * t) + np.real(np.exp(-7*(t-0.4)**2)*np.exp(1j*2*np.pi*2*(t-0.4))) >>> widths = np.arange(1, 31) >>> cwtmatr, freqs = pywt.cwt(sig, widths, 'mexh') >>> plt.imshow(cwtmatr, extent=[-1, 1, 1, 31], cmap='PRGn', aspect='auto', ... vmax=abs(cwtmatr).max(), vmin=-abs(cwtmatr).max()) >>> plt.show()
Continuous Wavelet Families¶
A variety of continuous wavelets have been implemented. A list of the available
wavelet names compatible with cwt
can be obtained by:
wavlist = pywt.wavelist(kind='continuous')
Mexican Hat Wavelet¶
The mexican hat wavelet "mexh"
is given by:
where the constant out front is a normalization factor so that the wavelet has unit energy.
Morlet Wavelet¶
The Morlet wavelet "morl"
is given by:
Complex Morlet Wavelets¶
The complex Morlet wavelet ("cmorB-C"
with floating point values B, C) is
given by:
where \(B\) is the bandwidth and \(C\) is the center frequency.
Gaussian Derivative Wavelets¶
The Gaussian wavelets ("gausP"
where P is an integer between 1 and and 8)
correspond to the Pth order derivatives of the function:
where \(C\) is an order-dependent normalization constant.
Complex Gaussian Derivative Wavelets¶
The complex Gaussian wavelets ("cgauP"
where P is an integer between 1 and
8) correspond to the Pth order derivatives of the function:
where \(C\) is an order-dependent normalization constant.
Shannon Wavelets¶
The Shannon wavelets ("shanB-C"
with floating point values B and C)
correspond to the following wavelets:
where \(B\) is the bandwidth and \(C\) is the center frequency.
Frequency B-Spline Wavelets¶
The frequency B-spline wavelets ("fpspM-B-C"
with integer M and floating
point B, C) correspond to the following wavelets:
where \(M\) is the spline order, \(B\) is the bandwidth and \(C\) is the center frequency.
Choosing the scales for cwt
¶
For each of the wavelets described below, the implementation in PyWavelets
evaluates the wavelet function for \(t\) over the range
[wavelet.lower_bound, wavelet.upper_bound]
(with default range
\([-8, 8]\)). scale = 1
corresponds to the case where the extent of the
wavelet is (wavelet.upper_bound - wavelet.lower_bound + 1)
samples of the
digital signal being analyzed. Larger scales correspond to stretching of the
wavelet. For example, at scale=10
the wavelet is stretched by a factor of
10, making it sensitive to lower frequencies in the signal.
To relate a given scale to a specific signal frequency, the sampling period
of the signal must be known. pywt.scale2frequency()
can be used to
convert a list of scales to their corresponding frequencies. The proper choice
of scales depends on the chosen wavelet, so pywt.scale2frequency()
should
be used to get an idea of an appropriate range for the signal of interest.
For the cmor
, fbsp
and shan
wavelets, the user can specify a
specific a normalized center frequency. A value of 1.0 corresponds to 1/dt
where dt is the sampling period. In other words, when analyzing a signal
sampled at 100 Hz, a center frequency of 1.0 corresponds to ~100 Hz at
scale = 1
. This is above the Nyquist rate of 50 Hz, so for this
particular wavelet, one would analyze a signal using scales >= 2
.
>>> import numpy as np
>>> import pywt
>>> dt = 0.01 # 100 Hz sampling
>>> frequencies = pywt.scale2frequency('cmor1.5-1.0', [1, 2, 3, 4]) / dt
>>> frequencies
array([ 100. , 50. , 33.33333333, 25. ])
The CWT in PyWavelets is applied to discrete data by convolution with samples
of the integral of the wavelet. If scale
is too low, this will result in
a discrete filter that is inadequately sampled leading to aliasing as shown
in the example below. Here the wavelet is 'cmor1.5-1.0'
. The left column of
the figure shows the discrete filters used in the convolution at various
scales. The right column are the corresponding Fourier power spectra of each
filter.. For scales 1 and 2 it can be seen that aliasing due to violation of
the Nyquist limit occurs.
Converting frequency to scale for cwt
¶
To convert frequency to scale for use in the wavelet transform the function
pywt.frequency2scale()
can be used. This is the complement of the
pywt.scale2frequency()
function as seen in the previous section. Note that
the input frequency in this function is normalized by 1/dt, or the sampling
frequency fs. This function is useful for specifying the transform as a function
of frequency directly.
>>> import numpy as np
>>> import pywt
>>> dt = 0.01 # 100 Hz sampling
>>> fs = 1 / dt
>>> frequencies = np.array([100, 50, 33.33333333, 25]) / fs # normalize
>>> scale = pywt.frequency2scale('cmor1.5-1.0', frequencies)
>>> scale
array([ 1., 2., 3., 4.])
import numpy as np
import pywt
import matplotlib.pyplot as plt
wav = pywt.ContinuousWavelet('cmor1.5-1.0')
# print the range over which the wavelet will be evaluated
print("Continuous wavelet will be evaluated over the range [{}, {}]".format(
wav.lower_bound, wav.upper_bound))
width = wav.upper_bound - wav.lower_bound
scales = [1, 2, 3, 4, 10, 15]
max_len = int(np.max(scales)*width + 1)
t = np.arange(max_len)
fig, axes = plt.subplots(len(scales), 2, figsize=(12, 6))
for n, scale in enumerate(scales):
# The following code is adapted from the internals of cwt
int_psi, x = pywt.integrate_wavelet(wav, precision=10)
step = x[1] - x[0]
j = np.floor(
np.arange(scale * width + 1) / (scale * step))
if np.max(j) >= np.size(int_psi):
j = np.delete(j, np.where((j >= np.size(int_psi)))[0])
j = j.astype(np.int_)
# normalize int_psi for easier plotting
int_psi /= np.abs(int_psi).max()
# discrete samples of the integrated wavelet
filt = int_psi[j][::-1]
# The CWT consists of convolution of filt with the signal at this scale
# Here we plot this discrete convolution kernel at each scale.
nt = len(filt)
t = np.linspace(-nt//2, nt//2, nt)
axes[n, 0].plot(t, filt.real, t, filt.imag)
axes[n, 0].set_xlim([-max_len//2, max_len//2])
axes[n, 0].set_ylim([-1, 1])
axes[n, 0].text(50, 0.35, 'scale = {}'.format(scale))
f = np.linspace(-np.pi, np.pi, max_len)
filt_fft = np.fft.fftshift(np.fft.fft(filt, n=max_len))
filt_fft /= np.abs(filt_fft).max()
axes[n, 1].plot(f, np.abs(filt_fft)**2)
axes[n, 1].set_xlim([-np.pi, np.pi])
axes[n, 1].set_ylim([0, 1])
axes[n, 1].set_xticks([-np.pi, 0, np.pi])
axes[n, 1].set_xticklabels([r'$-\pi$', '0', r'$\pi$'])
axes[n, 1].grid(True, axis='x')
axes[n, 1].text(np.pi/2, 0.5, 'scale = {}'.format(scale))
axes[n, 0].set_xlabel('time (samples)')
axes[n, 1].set_xlabel('frequency (radians)')
axes[0, 0].legend(['real', 'imaginary'], loc='upper left')
axes[0, 1].legend(['Power'], loc='upper left')
axes[0, 0].set_title('filter')
axes[0, 1].set_title(r'|FFT(filter)|$^2$')