tape_recorder/core/signal_generator.py

"""
Signal Generator — Generate test signals for audio calibration.

Supports:
- Logarithmic sine sweep (chirp)
- Pink noise (1/f)
- White noise
- Sine tone
"""

import numpy as np
from scipy.signal import chirp

def generate_log_sweep(
    start_freq: float = 20.0,
    end_freq: float = 20000.0,
    duration: float = 10.0,
    sample_rate: int = 44100,
    amplitude: float = 0.8
) -> np.ndarray:
    """
    Generate a logarithmic sine sweep (chirp).
    
    This is the preferred method for measuring frequency response
    as it provides a high signal-to-noise ratio across the spectrum.
    """
    t = np.linspace(0, duration, int(sample_rate * duration), endpoint=False)
    # Generate the sweep
    sweep = chirp(t, f0=start_freq, f1=end_freq, t1=duration, method='logarithmic')
    
    # Apply a short fade-in and fade-out to prevent clicks
    fade_len = int(0.05 * sample_rate)  # 50ms fade
    window = np.ones_like(sweep)
    if len(window) > 2 * fade_len:
        fade_in = np.linspace(0, 1, fade_len)
        fade_out = np.linspace(1, 0, fade_len)
        window[:fade_len] = fade_in
        window[-fade_len:] = fade_out
        
    return (sweep * window * amplitude).astype(np.float32)

def generate_white_noise(
    duration: float = 10.0,
    sample_rate: int = 44100,
    amplitude: float = 0.8
) -> np.ndarray:
    """Generate white noise (equal energy per Hz)."""
    num_samples = int(sample_rate * duration)
    # np.random.randn generates Gaussian white noise (mean 0, variance 1)
    noise = np.random.randn(num_samples)
    # Normalize and scale
    noise = (noise / np.max(np.abs(noise))) * amplitude
    return noise.astype(np.float32)

def generate_pink_noise(
    duration: float = 10.0,
    sample_rate: int = 44100,
    amplitude: float = 0.8
) -> np.ndarray:
    """
    Generate pink noise (equal energy per octave, 1/f spectrum).
    
    Uses a standard IIR filter approximation (Voss-McCartney algorithm or similar filter bank)
    applied to white noise.
    """
    import scipy.signal as signal
    
    white = np.random.randn(int(sample_rate * duration))
    
    # Filter coefficients for a rough pinking filter
    # (Paul Kellet's refined filter)
    b = [0.049922035, -0.095993537, 0.050612699, -0.004408786]
    a = [1.000000000, -2.494956002, 2.017265875, -0.522189400]
    
    pink = signal.lfilter(b, a, white)
    
    # Normalize and scale
    pink = (pink / np.max(np.abs(pink))) * amplitude
    return pink.astype(np.float32)

def generate_sine_tone(
    frequency: float = 1000.0,
    duration: float = 5.0,
    sample_rate: int = 44100,
    amplitude: float = 0.8
) -> np.ndarray:
    """Generate a pure sine wave tone."""
    t = np.linspace(0, duration, int(sample_rate * duration), endpoint=False)
    tone = np.sin(2 * np.pi * frequency * t)
    
    # Apply fade in/out
    fade_len = int(0.05 * sample_rate)
    window = np.ones_like(tone)
    if len(window) > 2 * fade_len:
        window[:fade_len] = np.linspace(0, 1, fade_len)
        window[-fade_len:] = np.linspace(1, 0, fade_len)
        
    return (tone * window * amplitude).astype(np.float32)