decimation_fir.py

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#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""!
Author: J.A. de Jong - ASCEE
 
Description: Decimation filter design.
"""
import numpy as np
import matplotlib.pyplot as plt
from fir_design import freqResponse, lowpass_fir_design
 
L = 128  # Filter order
fs = 48000.  # Sampling frequency
 
d = 4 # Decimation factor
 
fd = fs/d  # Decimated sampling frequency
fc = fd/2/1.5  # Filter cut-off frequency
 
fir = lowpass_fir_design(L, fs, fc)
 
fig = plt.figure()
ax = fig.add_subplot(111)
 
freq = np.logspace(1, np.log10(fs/2), 1000)
 
H = freqResponse(fs, freq, fir)
dBH = 20*np.log10(np.abs(H))
ax.plot(freq, dBH)
ax.axvline(fs/2, color='green')
ax.axvline(fd/2, color='red')
 
# Index of Nyquist freq
fn_index = np.where(freq <= fd/2)[0][-1]
 
dBHmax_above_Nyq = np.max(dBH[fn_index:])
 
print(f"Reduction above Nyquist: {dBHmax_above_Nyq} dB")
plt.show()