Skill: Design your own sound

Written for blit386 1.8.0. The rest of blit386.dev documents the newest engine, so an API you find there may be missing from a game pinned to 1.8.0.

The engine can build a sound out of nothing - no files, no recording. The six presets in BT.synthPreset are the quick way (see the play-a-sound skill). This skill is the slow way: describing a sound yourself, knob by knob, with AudioClip.synth().

When to use

Use when the presets are close but not right, when the user wants a sound of their own, when a sound is too long, too high, too harsh, or too plain, or when they ask how to make a sound effect without a sound file.

The four things every sound needs

import { AudioClip, BT } from 'blit386';

async init() {
    this.zap = await AudioClip.synth({
        waveform: 'square', // the shape of the sound: its character
        frequency: 440,     // how high it is, in hertz. 440 is the A above middle C
        duration: 0.2,      // how long it lasts, in seconds
        seed: 1,            // any number - see "the seed" below
    });
    return true;
}

update() {
    if (BT.isPressed(BT.BTN_A, 0)) {
        BT.soundPlay(this.zap);
    }
}

Those four are required. Everything below is optional, and each one is a knob you can leave alone.

The knobs

waveform - the personality of the sound. 'square' is the classic 8-bit buzz. 'sine' is soft and pure, like a whistle. 'triangle' is soft but with a bit of an edge. 'sawtooth' is bright and rasping. 'noise' is a hiss, with no pitch at all - it is what explosions and footsteps are made of.

envelope - the shape of the sound over time, in four parts. Imagine plucking a guitar string versus slowly pushing an organ key:

envelope: {
    attack: 0.01,  // seconds to fade in. Small = a sharp hit. Large = a slow swell.
    decay: 0.1,    // seconds to fall from the peak down to the sustain level.
    sustain: 0.7,  // the level it holds at, from 0 to 1.
    release: 0.1,  // seconds to fade out at the end.
}

Those are also the defaults, so leaving envelope out gives you exactly that. For a percussive hit, use a tiny attack and a low sustain. For a soft pad, use a big attack.

pitchSweep - makes the pitch slide while the sound plays. Sliding up reads as a jump or a power-up. Sliding down reads as falling, or dying.

frequency: 200,
pitchSweep: { toFrequency: 800 }, // starts at 200 Hz, ends at 800 Hz: a rising bounce

Leaving pitchSweep out means "stay at one pitch the whole time".

vibrato - a wobble in the pitch, like an opera singer. rate is how fast it wobbles (default 5 per second), depth is how far (default 0, meaning off).

noiseMix - stirs hiss into a pitched sound, from 0 (none) to 1 (all hiss). A little bit adds grit and dirt. This is how you get an engine rumble rather than a clean tone.

dutyCycle - only matters for 'square'. It is the "thickness" of the buzz, from 0 to 1, default 0.5. Move it towards 0.1 and the square wave gets thin and nasal, which is a very NES sound.

volume - 0 to 1, default 1.

A worked example: a laser that is yours

Start from a shape you understand, then turn one knob at a time and listen.

this.laser = await AudioClip.synth({
  waveform: 'sawtooth', // bright and rasping
  frequency: 900, // starts high...
  pitchSweep: { toFrequency: 120 }, // ...and dives, which reads as "firing"
  duration: 0.25,
  envelope: { attack: 0.001, decay: 0.05, sustain: 0.3, release: 0.15 },
  noiseMix: 0.15, // a little grit so it is not too clean
  seed: 3,
});

The seed

seed is required, and it is not a volume or a pitch - it is the number the engine uses to make its random choices. The same parameters plus the same seed always produce exactly the same sound, every run, on every machine. Pick any number and forget about it.

Careful: seed means something slightly different in the two places you will see it.

• In AudioClip.synth({ ... seed: 1 }) it is required, and it just makes the sound repeatable.
• In BT.synthPreset.jump(7) it is optional, and it nudges a couple of the preset's values a little, so repeated jumps do not sound robotically identical.

Key calls

• AudioClip.synth(params) (static, async) - turns a description into a playable clip. Needs await.
• Required fields: waveform, frequency, duration, seed.
• Optional fields: volume, envelope (attack, decay, sustain, release), pitchSweep (toFrequency), vibrato (rate, depth), noiseMix, dutyCycle.
• Waveforms: 'sine', 'square', 'triangle', 'sawtooth', 'noise'.
• BT.soundPlay(clip, options?) (method) - play the clip you built. See the play-a-sound skill.

Notes

• Build sounds in init() with await, not in update(). Synthesizing takes real work, and doing it mid-game will hitch the frame.
• The description is plain data - numbers, strings, and small objects. That means a sound can live in a data file and be loaded like a level, instead of being written into your code.
• One knob at a time. Change waveform, listen, then change envelope, listen. Changing four things at once and disliking the result tells you nothing.
• A synthesized sound still will not play before the player's first click or key press. That is a browser rule, not a bug - the play-a-sound skill explains it.

See docs/audio.md.