rochars / alawmulaw

 * alawmulaw: A-Law and mu-Law codecs in JavaScript.

 * https://github.com/rochars/alawmulaw

 *

 * Copyright (c) 2017-2018 Rafael da Silva Rocha.

 *

 * 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 A-Law codec.

 * References:

 * https://github.com/deftio/companders

 * http://dystopiancode.blogspot.com.br/2012/02/pcm-law-and-u-law-companding-algorithms.html

 */

/** @module alawmulaw/alaw */

/** @type {!Array<number>} */

  6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6, 

  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7, 

  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7 

];

/**

 * Encode a 16-bit linear PCM sample as 8-bit A-Law.

 * @param {number} sample A 16-bit linear PCM sample

 * @return {number}

 */

function encodeSample(sample) {

  /** @type {number} */

  let compandedValue; 

  sample = (sample ==-32768) ? -32767 : sample;

  /** @type {number} */

  let sign = ((~sample) >> 8) & 0x80; 

  if (!sign) {

    sample = sample * -1; 

  }

  if (sample > 32635) {

    sample = 32635; 

  }

  if (sample >= 256)  {

    /** @type {number} */

    let exponent = LOG_TABLE[(sample >> 8) & 0x7F];

    /** @type {number} */

    let mantissa = (sample >> (exponent + 3) ) & 0x0F; 

    compandedValue = ((exponent << 4) | mantissa); 

  } else {

    compandedValue = sample >> 4; 

  } 

  return compandedValue ^ (sign ^ 0x55);

}

/**

 * Decode a 8-bit A-Law sample as 16-bit linear PCM.

 * @param {number} aLawSample The 8-bit A-Law sample

 * @return {number}

 */

function decodeSample(aLawSample) {

  /** @type {number} */

  let sign = 0;

  aLawSample ^= 0x55;

    aLawSample &= ~(1 << 7);

    sign = -1;

  }

  /** @type {number} */

  let position = ((aLawSample & 0xF0) >> 4) + 4;

  /** @type {number} */

  let decoded = 0;

  if (position != 4) {

    decoded = ((1 << position) |

      ((aLawSample & 0x0F) << (position - 4)) |

      (1 << (position - 5)));

  } else {

    decoded = (aLawSample << 1)|1;

  }

  decoded = (sign === 0) ? (decoded) : (-decoded);

  return (decoded * 8) * -1;

}

/**

 * Encode 16-bit linear PCM samples into 8-bit A-Law samples.

 * @param {!Array<number>} samples A array of 16-bit PCM samples.

 * @return {!Array<number>}

 */

function encode(samples) {

  /** @type {!Array<number>} */

  let aLawSamples = [];

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

    aLawSamples.push(encodeSample(samples[i]));

  }

  return aLawSamples;

}

/**

 * Decode 8-bit A-Law samples into 16-bit linear PCM samples.

 * @param {!Array<number>} samples A array of 8-bit A-Law samples.

 * @return {!Array<number>}

 */

function decode(samples) {

  /** @type {!Array<number>} */

  let pcmSamples = [];

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

    pcmSamples.push(decodeSample(samples[i]));

  }

  return pcmSamples;

}

/** @export */

module.exports.encodeSample = encodeSample;

/** @export */

module.exports.decodeSample = decodeSample;

/** @export */

module.exports.encode = encode;

/** @export */

module.exports.decode = decode;