rochars / wavefile

 * Copyright (c) 2019 Rafael da Silva Rocha.

 * Copyright (c) 2014 Florian Markert

 *

 * Permission is hereby granted, free of charge, to any person obtaining

 * a copy of this software and associated documentation files (the

 * "Software"), to deal in the Software without restriction, including

 * without limitation the rights to use, copy, modify, merge, publish,

 * distribute, sublicense, and/or sell copies of the Software, and to

 * permit persons to whom the Software is furnished to do so, subject to

 * the following conditions:

 *

 * The above copyright notice and this permission notice shall be

 * included in all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,

 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND

 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE

 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION

 * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION

 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

 *

 */

/**

 * @fileoverview Butterworth LPF. Based on the Butterworth LPF from Fili.js.

 * @see https://github.com/rochars/wavefile

 * @see https://github.com/markert/fili.js

 */

/**

 * Butterworth LPF.

 */

export class ButterworthLPF {

  /**

   * @param {number} order The order of the filter.

   * @param {number} sampleRate The sample rate.

   * @param {number} cutOff The cut off frequency.

   */

  constructor(order, sampleRate, cutOff) {

    let filters = [];

    for (let i = 0; i < order; i++) {

      filters.push(this.getCoeffs_({

        Fs: sampleRate,

        Fc: cutOff,

        Q: 0.5 / (Math.sin((Math.PI / (order * 2)) * (i + 0.5)))

      }));

    }

    this.stages = [];

    for (let i = 0; i < filters.length; i++) {

      this.stages[i] = {

        b0 : filters[i].b[0],

        b1 : filters[i].b[1],

        b2 : filters[i].b[2],

        a1 : filters[i].a[0],

        a2 : filters[i].a[1],

        k : filters[i].k,

        z : [0, 0]

      };

    }

  }

  /**

   * @param {number} sample A sample of a sequence.

   * @return {number}

   */

  filter(sample) {

    let out = sample;

    for (let i = 0, len = this.stages.length; i < len; i++) {

      out = this.runStage_(i, out);

    }

    return out;

  }

  getCoeffs_(params) {

    let coeffs = {};

    coeffs.z = [0, 0];

    coeffs.a = [];

    coeffs.b = [];

    let p = this.preCalc_(params, coeffs);

    coeffs.k = 1;

    coeffs.b.push((1 - p.cw) / (2 * p.a0));

    coeffs.b.push(2 * coeffs.b[0]);

    coeffs.b.push(coeffs.b[0]);

    return coeffs;

  }

  preCalc_(params, coeffs) {

    let pre = {};

    let w = 2 * Math.PI * params.Fc / params.Fs;

    pre.alpha = Math.sin(w) / (2 * params.Q);

    pre.cw = Math.cos(w);

    pre.a0 = 1 + pre.alpha;

    coeffs.a0 = pre.a0;

    coeffs.a.push((-2 * pre.cw) / pre.a0);

    coeffs.k = 1;

    coeffs.a.push((1 - pre.alpha) / pre.a0);

    return pre;

  }

  /**

   * Reset the filter.

   */

  reset() {

    for (let i = 0; i < this.stages.length; i++) {

      this.stages[i].z = [0, 0];

    }

  }

}