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/* |
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* mulaw.js |
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* Copyright (c) 2018 Rafael da Silva Rocha. |
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* https://github.com/rochars/alawmulaw |
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* |
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* Reference: |
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* https://github.com/torvalds/linux/blob/master/sound/core/oss/mulaw.c |
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* |
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*/ |
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/** @private */ |
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const BIAS = 0x84; |
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/** @private */ |
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const SIGN_BIT = 0x80; |
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/** @private */ |
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const QUANT_MASK = 0xf; |
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/** @private */ |
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const SEG_MASK = 0x70; |
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/** @private */ |
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const SEG_SHIFT = 4; |
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/** @private */ |
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function valSeg_(val) { |
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let r = 0; |
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val >>= 7; |
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if (val & 0xf0) { |
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val >>= 4; |
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r += 4; |
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} |
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if (val & 0x0c) { |
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val >>= 2; |
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r += 2; |
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} |
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if (val & 0x02) { |
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r += 1; |
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} |
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return r; |
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} |
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/** |
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* Encode a 16-bit linear PCM sample as 8-bit mu-Law. |
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* @param {number} pcmSample A 16-bit sample |
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* @return {number} |
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*/ |
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function encodeSample(pcmSample) { |
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let mask; |
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let seg; |
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let uval; |
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if (pcmSample < 0) { |
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pcmSample = BIAS - pcmSample; |
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mask = 0x7F; |
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} else { |
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pcmSample += BIAS; |
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mask = 0xFF; |
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} |
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if (pcmSample > 0x7FFF) { |
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pcmSample = 0x7FFF; |
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} |
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seg = valSeg_(pcmSample); |
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uval = (seg << 4) | ((pcmSample >> (seg + 3)) & 0xF); |
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return uval ^ mask; |
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} |
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/** |
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* Decode a 8-bit mu-Law sample as 16-bit linear PCM. |
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* @param {number} muLawSample The 8-bit mu-Law sample |
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* @return {number} |
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*/ |
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function decodeSample(muLawSample) { |
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let t; |
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muLawSample = ~muLawSample; |
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t = ((muLawSample & QUANT_MASK) << 3) + BIAS; |
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t <<= (muLawSample & SEG_MASK) >> SEG_SHIFT; |
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return ((muLawSample & SIGN_BIT) ? (BIAS - t) : (t - BIAS)); |
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} |
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/** |
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* Encode 16-bit linear PCM samples into 8-bit mu-Law samples. |
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* @param {!Array<number>} samples A array of 16-bit linear PCM samples. |
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* @return {!Array<number>} |
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*/ |
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function encode(samples) { |
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let muLawSamples = []; |
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for (let i=0; i<samples.length; i++) { |
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muLawSamples.push(encodeSample(samples[i])); |
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} |
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return muLawSamples; |
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} |
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/** |
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* Decode 8-bit mu-Law samples into 16-bit linear PCM samples. |
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* @param {!Array<number>} samples A array of 8-bit mu-Law samples. |
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* @return {!Array<number>} |
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*/ |
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function decode(samples) { |
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let pcmSamples = []; |
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for (let i=0; i<samples.length; i++) { |
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pcmSamples.push(decodeSample(samples[i])); |
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} |
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return pcmSamples; |
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} |
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module.exports.encodeSample = encodeSample; |
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module.exports.decodeSample = decodeSample; |
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module.exports.encode = encode; |
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module.exports.decode = decode; |
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rochars /
alawmulaw
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