fadeout.py

"""
# Usage
Prepare some wav file to fade-out. The point of cut is better not to be already faded-out.

Change variables:
- `filename` to set input file path.
- `duration` to set fade-out length in seconds.

Result will be written into `snd` directory.

Note that this script will fail to run if input audio length is shorter than `duration`.
"""

import numpy as np
import scipy.signal as signal
import matplotlib.pyplot as plt
import soundfile
from pathlib import Path

def convertoToInverseStepResponse(win):
    win /= np.sum(win)
    step = np.ones_like(win)
    return 1 - signal.convolve(win, step)[:len(win)]

def linear(frames):
    return np.linspace(1, 0, frames)

def decibel(frames):
    eps = np.finfo(np.float32).eps
    target = 20 * np.log10(eps)
    return 10**(np.linspace(0, target, frames) / 20)

def expDecay(frames):
    # Same as `decibel()`.
    eps = np.finfo(np.float32).eps
    return np.geomspace(eps, 1, frames)[::-1]

def expDecayInverse(frames):
    eps = np.finfo(np.float32).eps
    return 1 - np.geomspace(eps, 1, frames)

def quarterCosine(frames):
    # Equal power curve for cross-fade.
    return np.cos(np.linspace(0, np.pi / 2, frames))

def quarterCircle(frames):
    x = np.linspace(0, 1, frames)
    return np.sqrt(1 - x * x)

def triangleFIR(frames):
    win = signal.get_window("bartlett", frames)
    return convertoToInverseStepResponse(win)

def halfCosineFIR(frames):
    win = signal.get_window("cosine", frames)
    return convertoToInverseStepResponse(win)

def fullCosineFIR(frames):
    win = 1 + np.cos(np.linspace(-np.pi, np.pi, frames))
    return convertoToInverseStepResponse(win)

def exponentialFIR(frames):
    # Better to add some normalization to tau.
    # Current one is `fs * 100 / 48000`.
    win = signal.get_window(("exponential", None, 100), frames)
    win -= np.min(win)
    return convertoToInverseStepResponse(win)

def checkDecibelAndExpDecayEquality():
    # Unused test.
    db = decibel(frames)
    exp = expDecay(frames)

    plt.plot(db)
    plt.plot(exp)
    plt.grid()
    plt.show()

filename = "fadeout_short.wav"
duration = 2

out_dir = Path("snd")
if not out_dir.exists():
    out_dir.mkdir(parents=True, exist_ok=True)

source, samplerate = soundfile.read(filename)
sourceT = source.T

frames = int(samplerate * duration)
frames += frames % 2  # Align to even number.

gainFuncs = [
    linear,
    expDecay,
    expDecayInverse,
    quarterCosine,
    quarterCircle,
    triangleFIR,
    halfCosineFIR,
    fullCosineFIR,
    exponentialFIR,
]

start = len(sourceT[0]) - frames
for gainFunc in gainFuncs:
    gain = gainFunc(frames)
    data = sourceT.copy()
    for i in range(len(data)):
        data[i][start:] *= gain

    soundfile.write(out_dir / Path(f"{gainFunc.__name__}.wav"), data.T, samplerate)