freelizer/algorithm.js

Simple package on pure JavaScript with Audio Web API usage that was made for purpose of calculation sound frequency from microphone

// Implements modified ACF2+ algorithm
// Source: https://github.com/cwilso/PitchDetect
export const autoCorrelate = (buf, sampleRate) => {
  // Not enough signal check
  const RMS = Math.sqrt(buf.reduce((acc, el) => acc + el ** 2, 0) / buf.length)
  if (RMS < 0.001) return NaN

  const THRES = 0.2
  let r1 = 0
  let r2 = buf.length - 1
  for (let i = 0; i < buf.length / 2; ++i) {
    if (Math.abs(buf[i]) < THRES) {
      r1 = i
      break
    }
  }
  for (let i = 1; i < buf.length / 2; ++i) {
    if (Math.abs(buf[buf.length - i]) < THRES) {
      r2 = buf.length - i
      break
    }
  }

  const buf2 = buf.slice(r1, r2)
  const c = new Array(buf2.length).fill(0)
  for (let i = 0; i < buf2.length; ++i) {
    for (let j = 0; j < buf2.length - i; ++j) {
      c[i] = c[i] + buf2[j] * buf2[j + i]
    }
  }

  let d = 0
  for (; c[d] > c[d + 1]; ++d);

  let maxval = -1
  let maxpos = -1
  for (let i = d; i < buf2.length; ++i) {
    if (c[i] > maxval) {
      maxval = c[i]
      maxpos = i
    }
  }
  let T0 = maxpos

  let x1 = c[T0 - 1]
  let x2 = c[T0]
  let x3 = c[T0 + 1]
  let a = (x1 + x3 - 2 * x2) / 2
  let b = (x3 - x1) / 2

  return sampleRate / (a ? T0 - b / (2 * a) : T0)
}