185 lines
6 KiB
Markdown
185 lines
6 KiB
Markdown
# [PyWavelets](https://pywavelets.readthedocs.io/en/latest/) -- Wavelet Transforms in Python
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## The goal
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How do we do wavelet transforms under Python?
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Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
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You might want to read: [A Practical Guide to Wavelet Analysis](https://paos.colorado.edu/research/wavelets/) -> [PDF](https://paos.colorado.edu/research/wavelets/bams_79_01_0061.pdf)
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```shell
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pip install PyWavelets
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```
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## Which [continuous mother wavelets](https://pywavelets.readthedocs.io/en/latest/ref/cwt.html#continuous-wavelet-families) are available?
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```python
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import pywt
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wavelet_list = pywt.wavelist(kind="continuous")
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print(wavelet_list)
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```
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```Python console
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['cgau1', 'cgau2', 'cgau3', 'cgau4', 'cgau5', 'cgau6', 'cgau7', 'cgau8', 'cmor', 'fbsp', 'gaus1', 'gaus2', 'gaus3', 'gaus4', 'gaus5', 'gaus6', 'gaus7', 'gaus8', 'mexh', 'morl', 'shan']
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```
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* The mexican hat wavelet "mexh"
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* The Morlet wavelet "morl"
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* The complex Morlet wavelet ("cmorB-C" with floating point values B, C)
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* The Gaussian wavelets ("gausP" where P is an integer between 1 and and 8)
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* The complex Gaussian wavelets ("cgauP" where P is an integer between 1 and 8)
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* The Shannon wavelets ("shanB-C" with floating point values B and C)
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* The frequency B-spline wavelets ("fpspM-B-C" with integer M and floating point B, C)
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see [Choosing the scales for cwt](https://pywavelets.readthedocs.io/en/latest/ref/cwt.html#choosing-the-scales-for-cwt)
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## Visualizing wavelets
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```python
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import numpy as np
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import matplotlib.pyplot as plt
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import pywt
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wavelet_name: str = "cmor1.5-1.0"
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# Invoking the complex morlet wavelet object
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wav = pywt.ContinuousWavelet(wavelet_name)
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# Integrate psi wavelet function from -Inf to x
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# using the rectangle integration method.
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int_psi, x = pywt.integrate_wavelet(wav, precision=10)
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int_psi /= np.abs(int_psi).max()
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wav_filter: np.ndarray = int_psi[::-1]
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nt: int = len(wav_filter)
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t: np.ndarray = np.linspace(-nt // 2, nt // 2, nt)
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plt.plot(t, wav_filter.real, label="real")
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plt.plot(t, wav_filter.imag, label="imaginary")
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plt.ylim([-1, 1])
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plt.legend(loc="upper left")
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plt.xlabel("time (samples)")
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plt.title(f"filter {wavelet_name}")
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```
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## Building a frequency scale for the complex Morlet wavelet
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We don't want to waste computations power. Thus we want to put the frequency band for higher frequencies further away than for smaller frequencies. Thus we will use a $2^{N \cdot Scale}$ scale.
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```python
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import numpy as np
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import matplotlib.pyplot as plt
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import pywt
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number_of_frequences: int = 20 # Number of frequency bands
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frequency_range: tuple[float, float] = (2, 200) # Hz
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dt: float = 1 / 1000 # sec
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frequency_range_np: np.ndarray = np.array(frequency_range)
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s_spacing = (1.0 / (number_of_frequences - 1)) * np.log2(
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frequency_range_np.max() / frequency_range_np.min()
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)
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scale = np.power(2, np.arange(0, number_of_frequences) * s_spacing)
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frequency_axis_np = frequency_range_np.min() * np.flip(scale)
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plt.plot(frequency_axis_np, "--*", label="Frequency we want")
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wave_scales = 1.0 / (frequency_axis_np * dt)
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frequency_axis = pywt.scale2frequency("cmor1.5-1.0", wave_scales) / dt
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plt.plot(frequency_axis, ".", label="Frequency we got")
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plt.legend()
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plt.xlim([0, number_of_frequences - 1])
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plt.xticks(np.arange(0, number_of_frequences))
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plt.ylabel("Frequency [Hz]")
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plt.xlabel("Frequency band")
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plt.show()
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```
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## Cone of influence for the complex Morlet wavelet
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At the edges of the time series, the wavelet is dangling out of the allowed time axis. Thus these values are nonsense and need to be removed. The size of the wavelet is connected to its scale, hence for different scales the bad zone has different sizes. For the complex Morlet wavelet the number of samples are defined by the equation $\sqrt(2) \cdot scale$ ( [A Practical Guide to Wavelet Analysis](https://paos.colorado.edu/research/wavelets/) -> [PDF](https://paos.colorado.edu/research/wavelets/bams_79_01_0061.pdf) ). Which looks like this:
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```python
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cone_of_influence = np.ceil(np.sqrt(2) * wave_scales).astype(dtype=np.int64)
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print(cone_of_influence)
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plt.plot(frequency_axis, cone_of_influence, "*")
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plt.ylabel("Number of invalid data samples")
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plt.xlabel("Frequency [Hz]")
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plt.show()
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```
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```Python console
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[ 8 10 12 15 19 24 31 39 50 63 80 102 130 166 211 269 342 436 555 708]
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```
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## Analyzing a test signal
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First we need a test signal. We will use a 50Hz sinus for that
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```python
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import numpy as np
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import matplotlib.pyplot as plt
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f_test: float = 50 # Hz
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number_of_test_samples: int = 1000
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dt: float = 1.0 / 1000 # sec
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t_test: np.ndarray = np.arange(0, number_of_test_samples) * dt
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test_data: np.ndarray = np.sin(2 * np.pi * f_test * t_test)
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plt.plot(t_test, test_data)
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plt.xlabel("time [sec]")
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plt.ylabel("time series")
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```
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```python
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import numpy as np
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import matplotlib.pyplot as plt
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import pywt
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f_test: float = 50 # Hz
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number_of_test_samples: int = 1000
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# The wavelet we want to use
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mother = pywt.ContinuousWavelet("cmor1.5-1.0")
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# Parameters for the wavelet transform
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number_of_frequences: int = 25 # frequency bands
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frequency_range_min: float = 15 # Hz
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frequency_range_max: float = 200 # Hz
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dt: float = 1.0 / 1000 # sec
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t_test: np.ndarray = np.arange(0, number_of_test_samples) * dt
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test_data: np.ndarray = np.sin(2 * np.pi * f_test * t_test)
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# Calculate the wavelet scales we requested
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s_spacing: np.ndarray = (1.0 / (number_of_frequences - 1)) * np.log2(
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frequency_range_max / frequency_range_min
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)
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scale: np.ndarray = np.power(2, np.arange(0, number_of_frequences) * s_spacing)
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frequency_axis_request: np.ndarray = frequency_range_min * np.flip(scale)
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wave_scales: np.ndarray = 1.0 / (frequency_axis_request * dt)
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complex_spectrum, frequency_axis = pywt.cwt(test_data, wave_scales, mother, dt)
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plt.imshow(abs(complex_spectrum) ** 2, cmap="hot", aspect="auto")
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plt.colorbar()
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plt.yticks(np.arange(0, frequency_axis.shape[0]), frequency_axis)
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plt.xticks(np.arange(0, t_test.shape[0]), t_test)
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plt.xlabel("Time [sec]")
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plt.ylabel("Frequency [Hz]")
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plt.show()
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```
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