mp3player/src/mp3info/header.js

A mp3 player,get data by ajax and play by audiocontext or mediasource.it can download and reprocessing while playing

/**
 * MP3音频帧头信息模块
 * http://www.docin.com/p-1261649617.html
 * https://www.codeproject.com/Articles/8295/MPEG-Audio-Frame-Header
 */


import BitStream from '../common/bitstream';

//比特率对应表(bit/s) MEPG1.0 LayerIII
var bitRateMap = {
    '1': 32000,
    '10': 40000,
    '11': 48000,
    '100': 56000,
    '101': 64000,
    '110': 80000,
    '111': 96000,
    '1000': 112000,
    '1001': 128000,
    '1010': 160000,
    '1011': 192000,
    '1100': 224000,
    '1101': 256000,
    '1110': 320000
}
//采样率对应表(Hz) MEPG1.0 LayerIII
var sampleRateMap = {
    '0': 44100,
    '1': 48000,
    '10': 32000
}

var HEADER_MASK = 0xffe0 >> 5; //同步头
var MPEG1 = 3;
var LAYER3 = 1;
var MAX_TAG_OFF = 10 * 1024; //查找帧头，最多查找10K

var hasParseVbr = false;

function Header(_bitStream) {
    this.init(_bitStream);
}

var _proto_ = Header.prototype;

/**
 * 初始化
 * @param  {object} arrayBuffer 二进制数组
 */
_proto_.init = function(_bitStream) {
    this.bitStream = _bitStream;
}
/**
 * 解析帧头信息
 * @return object 比特流
 */
_proto_.parseHeader = function() {
    var mask = 0;
    var beginPos = this.bitStream.getBytePos();
    do {
        mask = this.bitStream.getBits(11); //获取11位同步头
        if (mask != HEADER_MASK) {
            if (this.bitStream.isEnd()) {
                break;
            }
            this.bitStream.rewindBits(3);
            continue;
        }
        this.verID = this.bitStream.getBits(2); //MPEG版本
        this.layer = this.bitStream.getBits(2); //MPEG层数
        this.protectionBit = this.bitStream.getBits(1); //保护位
        if (!this.frameSync) {
            this.bitStream.rewindBytes(2);
            this.frameSync = this.bitStream.getBits(8).toString(16) + ',' + this.bitStream.getBits(8).toString(16);
            this.frameSync = this.frameSync.toUpperCase(); //同步头
        }
        this.bitrateIndex = this.bitStream.getBitsStr(4);
        this.bitRate = bitRateMap[this.bitrateIndex]; //比特率
        this.sampleRateIndex = this.bitStream.getBitsStr(2);
        this.sampleRate = sampleRateMap[this.sampleRateIndex]; //采样路索引
        this.paddingBit = this.bitStream.getBits(1); //填充位
        this.privateBit = this.bitStream.getBits(1); //私有位
        this.channelMode = this.bitStream.getBits(2); //声道模式
        this.channelModeExtension = this.bitStream.getBits(2); //声道扩展模式
        this.copyright = this.bitStream.getBits(1); //版权
        this.original = this.bitStream.getBits(1); //填充位
        this.emphasis = this.bitStream.getBits(2); //强调

        if (this.verID != MPEG1 || this.layer != LAYER3) {
            return false;
        }

        this.frameSize = bitRateMap[this.bitrateIndex] * 1152 / 8;
        this.frameSize /= sampleRateMap[this.sampleRateIndex];
        this.frameSize += this.paddingBit; //帧长度
        this.sideInfoSize = (this.channelMode == 3) ? 17 : 32; //帧边信息长度
        this.duration = 1152 / sampleRateMap[this.sampleRateIndex]; //本帧时长

        //计算主数据长度
        this.mainDataSize = (this.frameSize - 4 - this.sideInfoSize) >> 0; //主数据长度
        if (this.protectionBit == 0)
            this.mainDataSize -= 2; //CRC
        break;

    } while (this.bitStream.getBytePos() - beginPos < MAX_TAG_OFF);

    if (this.bitStream.getBytePos() - beginPos >= MAX_TAG_OFF || this.bitStream.isEnd()) {
        return false;
    }

    this.endBytePos = this.bitStream.getBytePos();
    if (!hasParseVbr) {
        hasParseVbr = true;
        this.parseVbr();
        if (this.toc) {
            this.totalDuration = this.totalFrames * 1152 / sampleRateMap[this.sampleRateIndex]; //总时长
        }
    }
    return this.bitStream;
}
/**
 * 解析vbr信息
 */
_proto_.parseVbr = function() {
    var flags = 0;
    var tag = '';

    do {
        tag = String.fromCharCode(this.bitStream.getByte(), this.bitStream.getByte(), this.bitStream.getByte(), this.bitStream.getByte());
        if (tag == 'Xing' || tag == 'Info') { //Xing，Info头
            this.tocSize = 100; //vbr索引表长度
            flags = this.bitStream.getBits(32);
            if (flags & 1) {
                this.totalFrames = this.bitStream.getBits(32); //总帧数
            }
            if (flags & 2) {
                this.totalBytes = this.bitStream.getBits(32); //总长度
            }
            if (flags & 4) {
                this.toc = []; //vbr索引表
                for (var i = 0; i < this.tocSize; i++) {
                    this.toc[i] = this.bitStream.getByte();
                }
            }
            this.bitStream.setBytePos(this.endBytePos);
            return this.bitStream;
        } else if (tag == 'VBRI') { //VBRI头
            this.bitStream.skipBytes(6);
            this.totalBytes = this.bitStream.getBits(32);
            this.totalFrames = this.bitStream.getBits(32);
            this.tocSize = this.bitStream.getBits(16);
            this.bitStream.skipBytes(6);
            this.toc = [];
            for (var i = 0; i < tocSize; i++) {
                this.toc[i] = this.bitStream.getByte();
            }
            this.bitStream.setBytePos(this.endBytePos);
            return this.bitStream;
        } else {
            if (this.bitStream.isEnd()) {
                break;
            }
            this.bitStream.rewindBytes(3);
        }
    } while (tag != 'Xing' && tag != 'Info' && tag != 'VBRI' && this.bitStream.getBytePos() < MAX_TAG_OFF)

    this.bitStream.setBytePos(this.endBytePos);

    return false;
}

export default Header;