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/* |
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* Copyright (c) 2019 Rafael da Silva Rocha. |
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* Copyright (c) 2014 Florian Markert |
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* |
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* Permission is hereby granted, free of charge, to any person obtaining |
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* a copy of this software and associated documentation files (the |
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* "Software"), to deal in the Software without restriction, including |
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* without limitation the rights to use, copy, modify, merge, publish, |
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* distribute, sublicense, and/or sell copies of the Software, and to |
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* permit persons to whom the Software is furnished to do so, subject to |
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* the following conditions: |
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* |
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* The above copyright notice and this permission notice shall be |
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* included in all copies or substantial portions of the Software. |
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* |
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, |
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF |
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND |
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* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE |
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* LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION |
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* OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION |
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* WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. |
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* |
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*/ |
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/** |
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* @fileoverview Butterworth LPF. Based on the Butterworth LPF from Fili.js. |
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* @see https://github.com/rochars/wavefile |
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* @see https://github.com/markert/fili.js |
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*/ |
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/** |
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* Butterworth LPF. |
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*/ |
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export class ButterworthLPF { |
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/** |
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* @param {number} order The order of the filter. |
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* @param {number} sampleRate The sample rate. |
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* @param {number} cutOff The cut off frequency. |
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*/ |
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constructor(order, sampleRate, cutOff) { |
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let filters = []; |
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for (let i = 0; i < order; i++) { |
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filters.push(this.getCoeffs_({ |
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Fs: sampleRate, |
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Fc: cutOff, |
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Q: 0.5 / (Math.sin((Math.PI / (order * 2)) * (i + 0.5))) |
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})); |
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} |
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this.stages = []; |
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for (let i = 0; i < filters.length; i++) { |
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this.stages[i] = { |
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b0 : filters[i].b[0], |
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b1 : filters[i].b[1], |
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b2 : filters[i].b[2], |
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a1 : filters[i].a[0], |
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a2 : filters[i].a[1], |
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k : filters[i].k, |
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z : [0, 0] |
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}; |
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} |
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} |
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/** |
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* @param {number} sample A sample of a sequence. |
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* @return {number} |
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*/ |
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filter(sample) { |
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let out = sample; |
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for (let i = 0, len = this.stages.length; i < len; i++) { |
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out = this.runStage_(i, out); |
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} |
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return out; |
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} |
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getCoeffs_(params) { |
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let coeffs = {}; |
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coeffs.z = [0, 0]; |
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coeffs.a = []; |
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coeffs.b = []; |
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let p = this.preCalc_(params, coeffs); |
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coeffs.k = 1; |
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coeffs.b.push((1 - p.cw) / (2 * p.a0)); |
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coeffs.b.push(2 * coeffs.b[0]); |
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coeffs.b.push(coeffs.b[0]); |
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return coeffs; |
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} |
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preCalc_(params, coeffs) { |
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let pre = {}; |
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let w = 2 * Math.PI * params.Fc / params.Fs; |
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pre.alpha = Math.sin(w) / (2 * params.Q); |
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pre.cw = Math.cos(w); |
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pre.a0 = 1 + pre.alpha; |
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coeffs.a0 = pre.a0; |
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coeffs.a.push((-2 * pre.cw) / pre.a0); |
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coeffs.k = 1; |
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coeffs.a.push((1 - pre.alpha) / pre.a0); |
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return pre; |
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} |
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runStage_(i, input) { |
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let temp = |
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input * this.stages[i].k - this.stages[i].a1 * this.stages[i].z[0] - |
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this.stages[i].a2 * this.stages[i].z[1]; |
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let out = |
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this.stages[i].b0 * temp + this.stages[i].b1 * this.stages[i].z[0] + |
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this.stages[i].b2 * this.stages[i].z[1]; |
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this.stages[i].z[1] = this.stages[i].z[0]; |
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this.stages[i].z[0] = temp; |
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return out; |
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} |
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/** |
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* Reset the filter. |
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*/ |
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reset() { |
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for (let i = 0; i < this.stages.length; i++) { |
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this.stages[i].z = [0, 0]; |
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} |
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} |
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} |
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rochars /
wavefile
