halkyon/apps/meditate/tool/bells.py

#!/usr/bin/env python3
"""Writes Halkyon Meditate's bells (ADR 0038, D5) into the app's assets.

Each bell is synthesised here from sine partials and decays, with nothing
recorded or downloaded, so the sounds are the project's own and under its
licence. Python's standard library alone; run from apps/meditate:

    python3 -I tool/bells.py

It writes assets/bells/bowl.wav, small_bell.wav and wood_block.wav into
halkyon_meditate_flutter, mono 16-bit PCM at 22,050 Hz. The output is the
same on every run (the wooden block's noise has a fixed seed).
"""

import math
import random
import struct
import wave
from pathlib import Path

RATE = 22050
OUT = Path(__file__).resolve().parent.parent / "halkyon_meditate_flutter" / "assets" / "bells"


def partials(seconds, parts, attack=0.008):
    """Sums sine partials (frequency in Hz, amplitude, decay time in s)."""
    samples = []
    for n in range(int(seconds * RATE)):
        t = n / RATE
        rise = min(1.0, t / attack)
        value = sum(amp * math.exp(-t / decay) * math.sin(2 * math.pi * freq * t) for freq, amp, decay in parts)
        samples.append(rise * value)
    return samples


def bowl():
    """A singing bowl: a low hum with inharmonic partials, two of them close
    together so that it beats slowly, dying away over several seconds."""
    base = 196.0
    return partials(
        9.0,
        [
            (base, 1.0, 3.2),
            (base * 1.007, 0.6, 3.0),
            (base * 2.71, 0.45, 2.0),
            (base * 2.73, 0.3, 1.8),
            (base * 5.13, 0.18, 1.1),
            (base * 8.27, 0.08, 0.6),
        ],
        attack=0.02,
    )


def small_bell():
    """A small bell: brighter, a bell's minor-third partial, shorter."""
    base = 784.0
    return partials(
        4.0,
        [
            (base * 0.5, 0.35, 1.6),
            (base, 1.0, 1.3),
            (base * 1.19, 0.45, 1.0),
            (base * 1.5, 0.35, 0.8),
            (base * 2.0, 0.3, 0.6),
            (base * 2.51, 0.12, 0.35),
        ],
        attack=0.003,
    )


def wood_block():
    """A wooden block: a hollow knock, a short tone and a click of noise."""
    noise = random.Random(38)
    samples = []
    for n in range(int(0.45 * RATE)):
        t = n / RATE
        tone = math.exp(-t / 0.045) * (math.sin(2 * math.pi * 1180 * t) + 0.5 * math.sin(2 * math.pi * 2410 * t))
        click = math.exp(-t / 0.004) * (noise.random() * 2 - 1)
        samples.append(min(1.0, t / 0.001) * (tone + 0.6 * click))
    return samples


def write(name, samples, peak=0.7):
    """Writes [samples] normalised to [peak], faded out over the last 50 ms."""
    top = max(abs(sample) for sample in samples) or 1.0
    fade = int(0.05 * RATE)
    frames = bytearray()
    for n, sample in enumerate(samples):
        tail = min(1.0, (len(samples) - n) / fade)
        frames += struct.pack("<h", round(sample / top * peak * tail * 32767))
    OUT.mkdir(parents=True, exist_ok=True)
    with wave.open(str(OUT / name), "wb") as file:
        file.setnchannels(1)
        file.setsampwidth(2)
        file.setframerate(RATE)
        file.writeframes(bytes(frames))


if __name__ == "__main__":
    write("bowl.wav", bowl())
    write("small_bell.wav", small_bell())
    write("wood_block.wav", wood_block())