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fix: use integer tick counter to eliminate floating-point beat drift

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Root cause: floor(elapsed * rateA) vs floor(elapsed * rateB) where
rateB = 2*rateA doesn't maintain exact 2:1 ratio due to floating-point
multiplication errors. This creates a beat/aliasing pattern where
sequencers at 80 and 160 BPM periodically go in and out of phase.

Fix: Master clock now uses an integer tick counter (_masterTicks++)
instead of floating-point elapsed time. Sequencers derive steps via:
  stepIdx = floor(ticks / ticksPerStep) % numSteps
where ticks is an integer — no floating-point accumulation possible.

Also bumped master clock to 480 Hz for cleaner division at common BPMs:
  80 BPM: 480*60/320 = 90 ticks/step (exact)
  120 BPM: 480*60/480 = 60 ticks/step (exact)
  160 BPM: 480*60/640 = 45 ticks/step (exact)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
This commit is contained in:
Jose Luis
2026-03-21 18:01:56 +01:00
parent 8bdb953b52
commit 7d3a19ec35
3 changed files with 23 additions and 26 deletions
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28
src/engine/audioEngine.js
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@@ -13,16 +13,16 @@ const audioNodes = {};
const keyboardState = { frequency: 440, gate: false };
// ==================== Global Master Clock ====================
// Single high-resolution clock (960 ticks/sec ≈ 1ms precision).
// All sequencers/piano rolls derive their timing from this.
const MASTER_TICK_RATE = 240; // Hz — enough for 300 BPM sixteenths (20 Hz) with 12x headroom
// Single clock with integer tick counter. All sequencers/piano rolls
// derive their step positions from this shared tick count.
// Using integers avoids floating-point drift entirely.
export const MASTER_TICK_RATE = 480; // Hz — must be high enough for fastest BPM
let _masterClock = null;
let _masterTime = 0; // audio-context seconds at clock start
const _tickListeners = new Map(); // id → callback(audioTime, elapsed)
let _masterTicks = 0;
const _tickListeners = new Map(); // id → callback(audioTime, ticks)
export function getMasterTime() {
if (!_masterClock) return 0;
return Tone.now() - _masterTime;
export function getMasterTicks() {
return _masterTicks;
}
export function subscribeTick(id, callback) {
@@ -35,17 +35,11 @@ export function unsubscribeTick(id) {
function startMasterClock() {
if (_masterClock) return;
_masterTime = 0; // Will be set from first tick
let _started = false;
_masterTicks = 0;
_masterClock = new Tone.Clock((time) => {
// Capture start time from the FIRST callback — guarantees same clock source
if (!_started) {
_masterTime = time;
_started = true;
}
const elapsed = time - _masterTime;
_masterTicks++;
for (const cb of _tickListeners.values()) {
cb(time, elapsed);
cb(time, _masterTicks);
}
}, MASTER_TICK_RATE);
_masterClock.start();