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ac3.js
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ac3.js
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/*! @license
* Shaka Player
* Copyright 2016 Google LLC
* SPDX-License-Identifier: Apache-2.0
*/
goog.provide('shaka.transmuxer.Ac3');
/**
* AC3 utils
*/
shaka.transmuxer.Ac3 = class {
/**
* @param {!Uint8Array} data
* @param {!number} offset
* @return {?{sampleRate: number, channelCount: number,
* audioConfig: !Uint8Array, frameLength: number}}
*/
static parseFrame(data, offset) {
if (offset + 8 > data.length) {
// not enough bytes left
return null;
}
if (data[offset] !== 0x0b || data[offset + 1] !== 0x77) {
// invalid magic
return null;
}
// get sample rate
const samplingRateCode = data[offset + 4] >> 6;
if (samplingRateCode >= 3) {
// invalid sampling rate
return null;
}
const samplingRateMap = [48000, 44100, 32000];
// get frame size
const frameSizeCode = data[offset + 4] & 0x3f;
const frameSizeMap = [
64, 69, 96, 64, 70, 96, 80, 87, 120, 80, 88, 120, 96, 104, 144, 96, 105,
144, 112, 121, 168, 112, 122, 168, 128, 139, 192, 128, 140, 192, 160,
174, 240, 160, 175, 240, 192, 208, 288, 192, 209, 288, 224, 243, 336,
224, 244, 336, 256, 278, 384, 256, 279, 384, 320, 348, 480, 320, 349,
480, 384, 417, 576, 384, 418, 576, 448, 487, 672, 448, 488, 672, 512,
557, 768, 512, 558, 768, 640, 696, 960, 640, 697, 960, 768, 835, 1152,
768, 836, 1152, 896, 975, 1344, 896, 976, 1344, 1024, 1114, 1536, 1024,
1115, 1536, 1152, 1253, 1728, 1152, 1254, 1728, 1280, 1393, 1920, 1280,
1394, 1920,
];
const frameLength = frameSizeMap[frameSizeCode * 3 + samplingRateCode] * 2;
if (offset + frameLength > data.length) {
return null;
}
// get channel count
const channelMode = data[offset + 6] >> 5;
let skipCount = 0;
if (channelMode === 2) {
skipCount += 2;
} else {
if ((channelMode & 1) && channelMode !== 1) {
skipCount += 2;
}
if (channelMode & 4) {
skipCount += 2;
}
}
const lowFrequencyEffectsChannelOn =
(((data[offset + 6] << 8) | data[offset + 7]) >> (12 - skipCount)) & 1;
const channelsMap = [2, 1, 2, 3, 3, 4, 4, 5];
// Audio config for DAC3 box
const bitStreamIdentification = data[offset + 5] >> 3;
const bitStreamMode = data[offset + 5] & 7;
const config = new Uint8Array([
(samplingRateCode << 6) |
(bitStreamIdentification << 1) |
(bitStreamMode >> 2),
((bitStreamMode & 3) << 6) |
(channelMode << 3) |
(lowFrequencyEffectsChannelOn << 2) |
(frameSizeCode >> 4),
(frameSizeCode << 4) & 0xe0,
]);
return {
sampleRate: samplingRateMap[samplingRateCode],
channelCount: channelsMap[channelMode] + lowFrequencyEffectsChannelOn,
audioConfig: config,
frameLength: frameLength,
};
}
/**
* @param {!Uint8Array} data
* @param {!number} offset
* @return {boolean}
*/
static canParse(data, offset) {
return offset + 64 < data.length;
}
/**
* @param {!Uint8Array} data
* @param {!number} offset
* @return {boolean}
*/
static probe(data, offset) {
// look for the ac-3 sync bytes
if (data[offset] === 0x0b &&
data[offset + 1] === 0x77) {
// check the bsid (bitStreamIdentification) to confirm ac-3
let bsid = 0;
let numBits = 5;
offset += numBits;
/** @type {?number} */
let temp = null;
/** @type {?number} */
let mask = null;
/** @type {?number} */
let byte = null;
while (numBits > 0) {
byte = data[offset];
// read remaining bits, upto 8 bits at a time
const bits = Math.min(numBits, 8);
const shift = 8 - bits;
mask = (0xff000000 >>> (24 + shift)) << shift;
temp = (byte & mask) >> shift;
bsid = !bsid ? temp : (bsid << bits) | temp;
offset += 1;
numBits -= bits;
}
if (bsid < 16) {
return true;
}
}
return false;
}
};
/**
* @const {number}
*/
shaka.transmuxer.Ac3.AC3_SAMPLES_PER_FRAME = 1536;