Autocorrelation names the accompaniment's low notes, so it cannot check a melody played over chords

Fact

An autocorrelation pitch detector finds the strongest repeating period in the whole signal. Over chords that is the bass, a chord's common period or an earlier note still ringing, not the tune on top. Rendered with a quiet accompaniment, one 25-note melody read correctly on only a handful of its notes; the rest came back an octave or more low.

Measuring the energy at each candidate note's frequency over a short window from the note's start, and taking the loudest in the melody's own range, read all 25.

Why it matters

The wrong readings look like a bug in the synthesis or the note table, and a balance fix made to satisfy the detector changes what is heard for no reason.

How to apply

Render offline (OfflineAudioContext on the web) so timing is exact, then for each note the score says starts at time t:

const w = (2 * Math.PI * freq) / sampleRate;
let re = 0, im = 0;
for (let i = 0; i < n; i++) { re += d[a + i] * Math.cos(w * i); im += d[a + i] * Math.sin(w * i); }
const energy = re * re + im * im;

• Start just after the note's attack, and keep the window short enough to end before the next note: the tail of the last note is otherwise what you measure.
• Compare semitones from the melody's lowest note up, so the bass cannot win.
• A note that loses here is a real balance problem: the tune is not on top.