File: src/filters/Convolution.js

/**
 * Based on Convolution.as originally written in as3.
 * Copyright (c) the Funnel development team
 * http://www.funnel.cc
 *
 * Ported to JavaScript by Jeff Hoefs
 * Copyright (c) 2011-2012 Jeff Hoefs <soundanalogous@gmail.com>
 *
 * Released under the MIT license. See LICENSE file for details.
 */

JSUTILS.namespace('BO.filters.Convolution');

/**
 * @namespace BO.filters
 */
BO.filters.Convolution = (function() {
  "use strict";

  var Convolution;

  // dependencies
  var FilterBase = BO.filters.FilterBase;

  /**
   * The Convolution object performs low-pass, high-pass and moving average
   * filtering on an analog input.
   * See [Breakout/examples/filters/convolution.html](https://github.com/soundanalogous/Breakout/blob/master/examples/filters/convolution.html) for an example application.
   *
   * @class Convolution
   * @constructor
   * @extends BO.filters.FilterBase
   * @param {Number[]} kernel An array of coefficients to be used with product-sum
   * operations for input buffers.
   */
  Convolution = function(kernel) {

    this.name = "Convolution";

    this._buffer = [];

    // use the coef setter
    this.coef = kernel;
  };


  Convolution.prototype = JSUTILS.inherit(FilterBase.prototype);
  Convolution.prototype.constructor = Convolution;

  /**
   * An array of coefficients to be used with product-sum operations for input buffers.
   * If assigned a new array, the input buffer will be cleared.
   * @property coef
   * @type Number[]
   */
  Object.defineProperty(Convolution.prototype, "coef", {
    get: function() {
      return this._coef;
    },
    set: function(kernel) {
      this._coef = kernel;
      this._buffer = new Array(this._coef.length);
      var len = this._buffer.length;
      for (var i = 0; i < len; i++) {
        this._buffer[i] = 0;
      }
    }
  });

  /**
   * Override FilterBase.processSample
   */
  Convolution.prototype.processSample = function(val) {
    this._buffer.unshift(val);
    this._buffer.pop();

    var result = 0;
    var len = this._buffer.length;

    for (var i = 0; i < len; i++) {
      result += this._coef[i] * this._buffer[i];
    }

    return result;
  };

  /**
   * Low-pass filter kernel. Use by passing this array to the constructor.
   * @property Convolution.LPF
   * @static
   */
  Convolution.LPF = [1 / 3, 1 / 3, 1 / 3];

  /**
   * High-pass filter kernel. Use by passing this array to the constructor.
   * @property Convolution.HPF
   * @static
   */
  Convolution.HPF = [1 / 3, -2.0 / 3, 1 / 3];

  /**
   * Moving average filter kernel. Use by passing this array to the constructor.
   * @property Convolution.MOVING_AVERAGE
   * @static
   */
  Convolution.MOVING_AVERAGE = [1 / 8, 1 / 8, 1 / 8, 1 / 8, 1 / 8, 1 / 8, 1 / 8, 1 / 8];

  return Convolution;

}());