Source: lib/transmuxer/ac3.js

  1. /*! @license
  2.  * Shaka Player
  3.  * Copyright 2016 Google LLC
  4.  * SPDX-License-Identifier: Apache-2.0
  5.  */
  7. goog.provide('shaka.transmuxer.Ac3');
  10. /**
  11.  * AC3 utils
  12.  */
  13. shaka.transmuxer.Ac3 = class {
  14.   /**
  15.    * @param {!Uint8Array} data
  16.    * @param {!number} offset
  17.    * @return {?{sampleRate: number, channelCount: number,
  18.    *          audioConfig: !Uint8Array, frameLength: number}}
  19.    */
  20.   static parseFrame(data, offset) {
  21.     if (offset + 8 > data.length) {
  22.       // not enough bytes left
  23.       return null;
  24.     }
  26.     if (data[offset] !== 0x0b || data[offset + 1] !== 0x77) {
  27.       // invalid magic
  28.       return null;
  29.     }
  31.     // get sample rate
  32.     const samplingRateCode = data[offset + 4] >> 6;
  33.     if (samplingRateCode >= 3) {
  34.       // invalid sampling rate
  35.       return null;
  36.     }
  38.     const samplingRateMap = [48000, 44100, 32000];
  40.     // get frame size
  41.     const frameSizeCode = data[offset + 4] & 0x3f;
  42.     const frameSizeMap = [
  43.       64, 69, 96, 64, 70, 96, 80, 87, 120, 80, 88, 120, 96, 104, 144, 96, 105,
  44.       144, 112, 121, 168, 112, 122, 168, 128, 139, 192, 128, 140, 192, 160,
  45.       174, 240, 160, 175, 240, 192, 208, 288, 192, 209, 288, 224, 243, 336,
  46.       224, 244, 336, 256, 278, 384, 256, 279, 384, 320, 348, 480, 320, 349,
  47.       480, 384, 417, 576, 384, 418, 576, 448, 487, 672, 448, 488, 672, 512,
  48.       557, 768, 512, 558, 768, 640, 696, 960, 640, 697, 960, 768, 835, 1152,
  49.       768, 836, 1152, 896, 975, 1344, 896, 976, 1344, 1024, 1114, 1536, 1024,
  50.       1115, 1536, 1152, 1253, 1728, 1152, 1254, 1728, 1280, 1393, 1920, 1280,
  51.       1394, 1920,
  52.     ];
  54.     const frameLength = frameSizeMap[frameSizeCode * 3 + samplingRateCode] * 2;
  55.     if (offset + frameLength > data.length) {
  56.       return null;
  57.     }
  59.     // get channel count
  60.     const channelMode = data[offset + 6] >> 5;
  61.     let skipCount = 0;
  62.     if (channelMode === 2) {
  63.       skipCount += 2;
  64.     } else {
  65.       if ((channelMode & 1) && channelMode !== 1) {
  66.         skipCount += 2;
  67.       }
  68.       if (channelMode & 4) {
  69.         skipCount += 2;
  70.       }
  71.     }
  73.     const lowFrequencyEffectsChannelOn =
  74.       (((data[offset + 6] << 8) | data[offset + 7]) >> (12 - skipCount)) & 1;
  76.     const channelsMap = [2, 1, 2, 3, 3, 4, 4, 5];
  78.     // Audio config for DAC3 box
  79.     const bitStreamIdentification = data[offset + 5] >> 3;
  80.     const bitStreamMode = data[offset + 5] & 7;
  82.     const config = new Uint8Array([
  83.       (samplingRateCode << 6) |
  84.           (bitStreamIdentification << 1) |
  85.           (bitStreamMode >> 2),
  86.       ((bitStreamMode & 3) << 6) |
  87.           (channelMode << 3) |
  88.           (lowFrequencyEffectsChannelOn << 2) |
  89.           (frameSizeCode >> 4),
  90.       (frameSizeCode << 4) & 0xe0,
  91.     ]);
  93.     return {
  94.       sampleRate: samplingRateMap[samplingRateCode],
  95.       channelCount: channelsMap[channelMode] + lowFrequencyEffectsChannelOn,
  96.       audioConfig: config,
  97.       frameLength: frameLength,
  98.     };
  99.   }
  101.   /**
  102.    * @param {!Uint8Array} data
  103.    * @param {!number} offset
  104.    * @return {boolean}
  105.    */
  106.   static probe(data, offset) {
  107.     // look for the ac-3 sync bytes
  108.     if (data[offset] === 0x0b &&
  109.       data[offset + 1] === 0x77) {
  110.       // check the bsid (bitStreamIdentification) to confirm ac-3
  111.       let bsid = 0;
  112.       let numBits = 5;
  113.       offset += numBits;
  114.       /** @type {?number} */
  115.       let temp = null;
  116.       /** @type {?number} */
  117.       let mask = null;
  118.       /** @type {?number} */
  119.       let byte = null;
  120.       while (numBits > 0) {
  121.         byte = data[offset];
  122.         // read remaining bits, upto 8 bits at a time
  123.         const bits = Math.min(numBits, 8);
  124.         const shift = 8 - bits;
  125.         mask = (0xff000000 >>> (24 + shift)) << shift;
  126.         temp = (byte & mask) >> shift;
  127.         bsid = !bsid ? temp : (bsid << bits) | temp;
  128.         offset += 1;
  129.         numBits -= bits;
  130.       }
  131.       if (bsid < 16) {
  132.         return true;
  133.       }
  134.     }
  135.     return false;
  136.   }
  137. };
  139. /**
  140.  * @const {number}
  141.  */
  142. shaka.transmuxer.Ac3.AC3_SAMPLES_PER_FRAME = 1536;