tuner/tuner.js

const NOTE_NAMES = [
  "A",
  "A#",
  "B",
  "C",
  "C#",
  "D",
  "D#",
  "E",
  "F",
  "F#",
  "G",
  "G#"
];

// We don't care about fundamentals above 4kHz, so setting a lower sample rate
// gives us finer-grained FFT buckets
const TARGET_SAMPLE_RATE = 8000;

let dom_frequency;
let dom_rate;
let dom_note;
let dom_tune;

const setup = () => {
  dom_frequency = document.getElementById("frequency");
  dom_rate = document.getElementById("rate");
  dom_note = document.getElementById("note");
  dom_tune = document.getElementById("tune");

  if (navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
    navigator.mediaDevices
      .getUserMedia({
        audio: true
      })
      .then(handleStream, err => {
        console.error("Error calling getUserMedia", err);
      });
  }

  if (navigator.wakeLock && navigator.wakeLock.request) {
    try {
      navigator.wakeLock
        .request("screen")
        .then(wakeLock => setTimeout(() => wakeLock.release(), 60000));
    } catch (err) {}
  }
};

const handleStream = stream => {
  const audioContext = new AudioContext({
    sampleRate: TARGET_SAMPLE_RATE
  });

  const analyser = audioContext.createAnalyser();
  analyser.fftSize = 32768;
  analyser.minDecibels = -90;
  analyser.maxDecibels = -10;
  analyser.smoothingTimeConstant = 0;
  const bufferLength = analyser.frequencyBinCount;
  const data = new Uint8Array(bufferLength);

  const source = audioContext.createMediaStreamSource(stream);
  source.connect(analyser);

  setInterval(tune(analyser, data), 500);
};

const tune = (analyser, data) => () => {
  analyser.getByteFrequencyData(data);

  const rate = analyser.context.sampleRate;
  dom_rate.innerText = rate / 1000;

  const bucketWidth = rate / analyser.fftSize;
  let max = 0;
  let maxBucket = -1;

  data.forEach((value, bucket) => {
    if (value > max) {
      max = value;
      maxBucket = bucket;
    }
  });

  if (maxBucket === -1) {
    return;
  }

  const frequency = maxBucket * bucketWidth;
  dom_frequency.innerText = `${Number.parseFloat(frequency).toFixed(2)} Hz`;

  const semitones = frequencyToSemitones(frequency);
  const margin = frequencyToSemitones(frequency + bucketWidth / 2) - semitones;

  dom_note.innerText = semitonesToNote(semitones);
  dom_tune.innerText = errorPercentage(semitones, margin);
  document.body.className = semitonesToClassname(semitones, margin);
};

const frequencyToSemitones = frequency => 12 * Math.log2(frequency / 440) + 69;

const semitonesToNote = semitones => {
  const rounded = Math.round(semitones - 69);

  const index = rounded >= 0 ? rounded % 12 : (12 + (rounded % 12)) % 12;

  return NOTE_NAMES[index];
};

const errorPercentage = (semitones, margin) => {
  const rounded = Math.round(semitones);

  const cents = Math.round((semitones - rounded) * 100);
  const accuracy = Number.parseFloat(margin * 100).toFixed(1);
  const sign = cents > 0 ? "+" : "";

  return `${sign}${cents} cents ± ${accuracy}`;
};

const semitonesToClassname = (semitones, margin) => {
  const rounded = Math.round(semitones);
  const error = Math.abs(semitones - rounded);

  const ok = margin > 0.05 ? margin : 0.05;

  if (error <= ok) {
    return "";
  }

  return Math.round(semitones) > semitones ? "flat" : "sharp";
};