Source: lib/transmuxer/ec3.js

  1. /*! @license
  2.  * Shaka Player
  3.  * Copyright 2016 Google LLC
  4.  * SPDX-License-Identifier: Apache-2.0
  5.  */
  7. goog.provide('shaka.transmuxer.Ec3');
  9. goog.require('shaka.util.ExpGolomb');
  12. /**
  13.  * EC3 utils
  14.  */
  15. shaka.transmuxer.Ec3 = class {
  16.   /**
  17.    * @param {!Uint8Array} data
  18.    * @param {!number} offset
  19.    * @return {?{sampleRate: number, channelCount: number,
  20.    *          audioConfig: !Uint8Array, frameLength: number}}
  21.    */
  22.   static parseFrame(data, offset) {
  23.     if (offset + 8 > data.length) {
  24.       // not enough bytes left
  25.       return null;
  26.     }
  28.     if (!shaka.transmuxer.Ec3.probe(data, offset)) {
  29.       return null;
  30.     }
  32.     const gb = new shaka.util.ExpGolomb(data.subarray(offset + 2));
  33.     // Skip stream_type
  34.     gb.skipBits(2);
  35.     // Skip sub_stream_id
  36.     gb.skipBits(3);
  37.     const frameLength = (gb.readBits(11) + 1) << 1;
  38.     let samplingRateCode = gb.readBits(2);
  39.     let sampleRate = null;
  40.     let numBlocksCode = null;
  41.     if (samplingRateCode == 0x03) {
  42.       samplingRateCode = gb.readBits(2);
  43.       sampleRate = [24000, 22060, 16000][samplingRateCode];
  44.       numBlocksCode = 3;
  45.     } else {
  46.       sampleRate = [48000, 44100, 32000][samplingRateCode];
  47.       numBlocksCode = gb.readBits(2);
  48.     }
  50.     const channelMode = gb.readBits(3);
  51.     const lowFrequencyEffectsChannelOn = gb.readBits(1);
  52.     const bitStreamIdentification = gb.readBits(5);
  54.     if (offset + frameLength > data.byteLength) {
  55.       return null;
  56.     }
  58.     const channelsMap = [2, 1, 2, 3, 3, 4, 4, 5];
  60.     const numBlocksMap = [1, 2, 3, 6];
  62.     const numBlocks = numBlocksMap[numBlocksCode];
  64.     const dataRateSub =
  65.         Math.floor((frameLength * sampleRate) / (numBlocks * 16));
  67.     const config = new Uint8Array([
  68.       ((dataRateSub & 0x1FE0) >> 5),
  69.       ((dataRateSub & 0x001F) << 3), // num_ind_sub = zero
  70.       (sampleRate << 6) | (bitStreamIdentification << 1) | (0 << 0),
  71.       (0 << 7) | (0 << 4) |
  72.       (channelMode << 1) | (lowFrequencyEffectsChannelOn << 0),
  73.       (0 << 5) | (0 << 1) | (0 << 0),
  74.     ]);
  76.     return {
  77.       sampleRate,
  78.       channelCount: channelsMap[channelMode] + lowFrequencyEffectsChannelOn,
  79.       audioConfig: config,
  80.       frameLength,
  81.     };
  82.   }
  84.   /**
  85.    * @param {!Uint8Array} data
  86.    * @param {!number} offset
  87.    * @return {boolean}
  88.    */
  89.   static probe(data, offset) {
  90.     // search 16-bit 0x0B77 syncword
  91.     const syncword = (data[offset] << 8) | (data[offset + 1] << 0);
  92.     if (syncword === 0x0B77) {
  93.       return true;
  94.     } else {
  95.       return false;
  96.     }
  97.   }
  98. };
  100. /**
  101.  * @const {number}
  102.  */
  103. shaka.transmuxer.Ec3.EC3_SAMPLES_PER_FRAME = 1536;