What I do
- Design digital filters and signal processing algorithms
- Analyze signals in time and frequency domains
- Implement Fourier transforms and spectral analysis
- Develop signal conditioning and preprocessing
- Create data compression and feature extraction
When to use me
- When filtering noisy signals
- When analyzing frequency content
- When designing digital filters
- When extracting features from signals
- When implementing DSP algorithms
Key Concepts
Discrete Signals
import numpy as np
from scipy import signal, fft
# Signal generation
def generate_sine(freq, amplitude, duration, fs):
"""Generate sinusoidal signal"""
t = np.arange(0, duration, 1/fs)
return t, amplitude * np.sin(2 * np.pi * freq * t)
def generate_impulse(N):
"""Unit impulse signal"""
impulse = np.zeros(N)
impulse[0] = 1
return impulse
def generate_step(N):
"""Unit step signal"""
return np.ones(N)
# Basic operations
def convolution(x, h):
"""Linear convolution"""
return np.convolve(x, h, mode='full')
def correlation(x, y):
"""Cross-correlation"""
return np.correlate(x, y, mode='full')
Fourier Transform
# DFT/FFT
def compute_fft(x, fs):
"""Compute frequency spectrum"""
N = len(x)
X = fft.fft(x)
freq = fft.fftfreq(N, 1/fs)
# Get positive frequencies only
positive = slice(0, N//2)
return freq[positive], np.abs(X[positive])
def compute_stft(x, fs, nperseg=256, noverlap=None):
"""Short-Time Fourier Transform"""
if noverlap is None:
noverlap = nperseg // 2
f, t, Sxx = signal.stft(x, fs, nperseg=nperseg, noverlap=noverlap)
return f, t, np.abs(Sxx)
# Window functions
WINDOWS = {
"hamming": lambda N: signal.windows.hamming(N),
"hann": lambda N: signal.windows.hann(N),
"blackman": lambda N: signal.windows.blackman(N),
"kaiser": lambda N, beta=0: signal.windows.kaiser(N, beta)
}
Digital Filters
# IIR Filter design
def design_butterworth_lowpass(cutoff, fs, order=4):
"""Design Butterworth lowpass filter"""
nyquist = fs / 2
normalized = cutoff / nyquist
b, a = signal.butter(order, normalized, btype='low')
return b, a
def design_cheby1_lowpass(cutoff, fs, order=4, ripple=0.5):
"""Chebyshev Type I lowpass filter"""
nyquist = fs / 2
normalized = cutoff / nyquist
b, a = signal.cheby1(order, ripple, normalized, btype='low')
return b, a
def apply_filter(b, a, x):
"""Apply IIR filter using difference equation"""
return signal.filtfilt(b, a, x)
# FIR Filter design
def design_fir_lowpass(cutoff, fs, num_taps=51):
"""Design FIR lowpass using window method"""
nyquist = fs / 2
normalized = cutoff / nyquist
b = signal.firwin(num_taps, normalized, window='hamming')
return b
# Filter structures
def direct_form_1(b, a, x):
"""Direct Form I IIR filter"""
N = len(b)
M = len(a)
y = np.zeros(len(x))
for n in range(len(x)):
for i in range(min(n+1, N)):
y[n] += b[i] * x[n-i]
for i in range(1, min(n+1, M)):
y[n] -= a[i] * y[n-i]
return y
Filter Response Analysis
def frequency_response(b, a, fs):
"""Compute frequency response"""
w, h = signal.freqz(b, a, worN=204 freq = w8)
* fs / (2 * np.pi)
return freq, 20 * np.log10(np.abs(h))
def phase_response(b, a):
"""Compute phase response"""
w, h = signal.freqz(b, a, worN=2048)
phase = np.unwrap(np.angle(h))
return w, phase
def group_delay(b, a):
"""Compute group delay"""
w, gd = signal.group_delay((b, a))
return w, gd
Spectral Analysis
def periodogram(x, fs):
"""Compute periodogram (power spectral density)"""
f, Pxx = signal.periodogram(x, fs)
return f, Pxx
def welch_psd(x, fs, nperseg=256):
"""Compute PSD using Welch's method"""
f, Pxx = signal.welch(x, fs, nperseg=nperseg)
return f, Pxx
def spectrogram(x, fs, nperseg=256):
"""Compute spectrogram"""
f, t, Sxx = signal.spectrogram(x, fs, nperseg=nperseg)
return f, t, 10 * np.log10(Sxx)
Adaptive Filters
class LMSFilter:
"""Least Mean Squares adaptive filter"""
def __init__(self, filter_length, mu):
self.length = filter_length
self.mu = mu # Step size
self.weights = np.zeros(filter_length)
def filter(self, x, d):
"""Process input signal"""
y = np.zeros_like(d)
for n in range(len(d)):
x_window = np.flip(x[max(0, n-self.length+1):n+1])
if len(x_window) < self.length:
x_window = np.pad(x_window, (self.length - len(x_window), 0))
y[n] = np.dot(self.weights, x_window)
error = d[n] - y[n]
self.weights += self.mu * error * x_window
return y
class RLSFilter:
"""Recursive Least Squares filter"""
def __init__(self, filter_length, delta=0.01, lam=0.99):
self.length = filter_length
self.lam = lam # Forgetting factor
self.delta = delta
self.weights = np.zeros(filter_length)
self.P = np.eye(filter_length) / delta
def filter(self, x, d):
"""Process input signal"""
y = np.zeros_like(d)
for n in range(len(d)):
x_window = np.flip(x[max(0, n-self.length+1):n+1])
if len(x_window) < self.length:
x_window = np.pad(x_window, (self.length - len(x_window), 0))
y[n] = np.dot(self.weights, x_window)
error = d[n] - y[n]
k = self.P @ x_window / (self.lam + x_window @ self.P @ x_window)
self.weights += k * error
self.P = (self.P - np.outer(k, x_window @ self.P)) / self.lam
return y
Feature Extraction
def extract_features(signal_data, fs):
"""Extract common time-domain features"""
features = {
"mean": np.mean(signal_data),
"std": np.std(signal_data),
"rms": np.sqrt(np.mean(signal_data**2)),
"peak": np.max(np.abs(signal_data)),
"crest_factor": np.max(np.abs(signal_data)) / np.sqrt(np.mean(signal_data**2)),
"skewness": stats.skew(signal_data),
"kurtosis": stats.kurtosis(signal_data)
}
# Zero crossing rate
features["zcr"] = np.sum(np.diff(np.sign(signal_data)) != 0) / len(signal_data)
return features
ffsshhttiikk