Butterwort filter

filters.frequency.butter(data, f_low, f_high, fs, order, zero_phase=True)

Creates a butterworth filter and applies it to the input data.

Parameters:

• data – Single vector of input data

• f_low – Start of filters’s pass band.

• f_high – End of the filters’s pass band.

• fs – Sampling frequency

• order – Order of the filters.

• zero_phase – If ‘true’ the filters is applied forwards and backwards resulting in zero phase distortion.

Returns:

Filtered data.

The following code example shows how to apply the butterwort filter.

import numpy as np
import random

import matplotlib
matplotlib.use("Qt5agg")
import matplotlib.pyplot as plt

import finn.filters.frequency as ff

#Configure sample data
channel_count = 1
frequency = [random.randint(5, 50) for _ in range(channel_count)]
data_range = np.arange(0, 10000)
frequency_sampling = 200

#Configure noise data
frequency_noise = 50
shared_noise_strength = 10
random_noise_strength = 1

#Generate some sample data
raw_data = [None for _ in range(channel_count)]
for ch_idx in range(channel_count):
    genuine_signal = np.sin(2 * np.pi * frequency[ch_idx] * data_range / frequency_sampling)
    shared_noise_signal = np.sin(2 * np.pi * frequency_noise * data_range / frequency_sampling) * shared_noise_strength
    random_noise_signal = np.random.random(len(data_range)) * random_noise_strength

    raw_data[ch_idx] = genuine_signal + shared_noise_signal + random_noise_signal
raw_data = np.asarray(raw_data)

#Filter data - butter
filtered_butter_data = [None for _ in range(channel_count)]
for ch_idx in range(channel_count):
    filtered_butter_data[ch_idx] = ff.butter(raw_data[ch_idx], 1, 40, frequency_sampling, order = 7, zero_phase = True)
filtered_butter_data = np.asarray(filtered_butter_data)

#Filter data - fir
filtered_fir_data = [None for _ in range(channel_count)]
for ch_idx in range(channel_count):
    filtered_fir_data[ch_idx] = ff.fir(raw_data[ch_idx], 52, 48, 0.1, frequency_sampling, ripple_pass_band = 1e-5, stop_band_suppression = 1e-7, fft_win_sz = frequency_sampling, pad_type = "zero")
filtered_fir_data = np.asarray(filtered_fir_data)

#visualize result
plt.title("Peak at %i and noise peak removed at %i" % (frequency[0], frequency_noise))
plt.psd(raw_data[0, :], NFFT = frequency_sampling, Fs = frequency_sampling, color = "blue", label = "original signal")
plt.psd(filtered_butter_data[0, :], NFFT = frequency_sampling, Fs = frequency_sampling, color = "green", label = "butter filtered signal")
plt.psd(filtered_fir_data[0, :], NFFT = frequency_sampling, Fs = frequency_sampling, color = "red", label = "fir filtered signal")
plt.legend()

plt.show(block = True)

Applying the butterwort filter (green) notched the noise at 50Hz.