char-slop/chordpad.vibed.software

git clone https://git.t4t.associates/char-slop/chordpad.vibed.software

Charlotte Somtrack controlsfcab9202026-09-30

import { midiToFreq } from "../theory/pitch.ts";

let engine: { ctx: AudioContext; out: AudioNode } | undefined;

export function audio(): { ctx: AudioContext; out: AudioNode } {
  if (!engine) {
    const ctx = new AudioContext({ latencyHint: "interactive" });
    const compressor = ctx.createDynamicsCompressor();
    compressor.threshold.value = -16;
    compressor.ratio.value = 3;
    compressor.connect(ctx.destination);
    const out = ctx.createGain();
    out.gain.value = 0.22;
    out.connect(compressor);
    engine = { ctx, out };
  }
  if (engine.ctx.state === "suspended") void engine.ctx.resume();
  return engine;
}

/** a sounding (or scheduled) note, which can be cut short */
export type Voice = { stop(at?: number): void; readonly done: boolean };

const voices = new Set<Voice>();

/** seconds: a note's fade when released, and when cut short (about as quick as a damper) */
const RELEASE = 0.08;
const CHOKE = 0.03;

/** a fade from 1 to 0 without corners: half a cosine */
const CHOKE_CURVE = Float32Array.from(
  { length: 64 },
  (_, i) => (1 + Math.cos((Math.PI * i) / 63)) / 2,
);

/**
 * a two-operator-pair electric piano, after the DX7 E.PIANO patches: a 1:1 pair whose
 * brightness decays into a near-sine body, and a 1:14 pair for the struck-tine transient
 */
export function playNote(
  midi: number,
  when: number,
  dur: number,
  vel = 0.7,
  out = audio().out,
): Voice {
  const { ctx } = audio();
  const t = Math.max(when, ctx.currentTime);
  const end = t + Math.max(dur, 0.05);
  const f = midiToFreq(midi);
  // key tracking: higher notes are darker and shorter, like the real thing
  const kt = Math.min(1, Math.max(-1, (midi - 60) / 36));

  const release = ctx.createGain();
  // -3dB per octave around middle C, so the top end doesn't shout
  const level = 10 ** ((-3 / 20) * ((midi - 60) / 12));
  release.gain.setValueAtTime(level, t);
  release.gain.setTargetAtTime(0, end, RELEASE);
  // sits at 1 until the note is cut short, so the fade out needn't know where the envelopes are
  const choke = ctx.createGain();
  release.connect(choke).connect(out);

  let stoppedAt = Infinity;
  const voice = {
    done: false,
    stop(at = ctx.currentTime) {
      if (at >= stoppedAt) return;
      stoppedAt = at;
      // this also cancels a start still to come, leaving it silent
      choke.gain.cancelScheduledValues(at);
      choke.gain.setValueCurveAtTime(CHOKE_CURVE, at, CHOKE);
    },
  };
  voices.add(voice);

  const osc = (freq: number, detune = 0) => {
    const o = ctx.createOscillator();
    o.frequency.value = freq;
    o.detune.value = detune;
    o.start(t);
    o.stop(end + 0.8);
    return o;
  };
  const env = (peak: number, tau: number, floor = 0) => {
    const g = ctx.createGain();
    g.gain.setValueAtTime(0, t);
    g.gain.linearRampToValueAtTime(peak, t + 0.002);
    g.gain.setTargetAtTime(peak * floor, t + 0.002, tau);
    return g;
  };
  const pan = (x: number) => {
    const p = ctx.createStereoPanner();
    p.pan.value = x;
    p.connect(release);
    return p;
  };

  const brightness = (0.35 + 0.65 * vel) * (1 - 0.4 * kt);
  const bodyIndex = env(f * 2 * brightness, 0.4, 0.18);
  osc(f).connect(bodyIndex);
  const bodyLevel = 0.4 * (0.45 + 0.55 * vel);
  for (const [detune, side] of [
    [-4, -0.3],
    [4, 0.3],
  ]) {
    const carrier = osc(f, detune);
    bodyIndex.connect(carrier.frequency);
    carrier.connect(env(bodyLevel, 2.4 * (1 - 0.5 * kt))).connect(pan(side));
  }

  const tine = osc(f);
  // up top the modulator would be past what the output can carry, so it's left out
  if (f * 14 < ctx.sampleRate / 2) {
    const tineIndex = env(f * 14 * 0.3 * vel * (1 - 0.3 * kt), 0.035);
    osc(f * 14).connect(tineIndex);
    tineIndex.connect(tine.frequency);
  }
  tine.connect(env(0.28 * vel, 0.3 * (1 - 0.5 * kt))).connect(pan(0));

  tine.onended = () => {
    choke.disconnect();
    voice.done = true;
    voices.delete(voice);
  };
  return voice;
}

export function silence(): void {
  for (const voice of voices) voice.stop();
}