dct/src/dct.js

Node.js implementation of the Discrete Cosine Transform (version 2 for now).

/*===========================================================================*\
 * Discrete Cosine Transform
 *
 * (c) Vail Systems. Joshua Jung and Ben Bryan. 2015
 *
 * This code is not designed to be highly optimized but as an educational
 * tool to understand the Mel-scale and its related coefficients used in
 * human speech analysis.
\*===========================================================================*/
var cosMap = null;

// Builds a cosine map for the given input size. This allows multiple input sizes to be memoized automagically
// if you want to run the DCT over and over.
var memoizeCosines = function(N) {
  cosMap = cosMap || {};
  cosMap[N] = new Array(N*N);

  var PI_N = Math.PI / N;

  for (var k = 0; k < N; k++) {
    for (var n = 0; n < N; n++) {
      cosMap[N][n + (k * N)] = Math.cos(PI_N * (n + 0.5) * k);
    }
  }
};

function dct(signal, scale) {
  var L = signal.length;
  scale = scale || 2;

  if (!cosMap || !cosMap[L]) memoizeCosines(L);

  var coefficients = signal.map(function () {return 0;});

  return coefficients.map(function (__, ix) {
    return scale * signal.reduce(function (prev, cur, ix_, arr) {
      return prev + (cur * cosMap[L][ix_ + (ix * L)]);
    }, 0);
  });
};

module.exports = dct;