aac/src/adts_demuxer.js

An AAC decoder for Aurora.js

var AV = require('av');
var tables = require('./tables');

var ADTSDemuxer = AV.Demuxer.extend(function() {
    AV.Demuxer.register(this);
    
    this.probe = function(stream) {
        var offset = stream.offset;
        
        // attempt to find ADTS syncword
        while (stream.available(2)) {
            if ((stream.readUInt16() & 0xfff6) === 0xfff0) {
                stream.seek(offset);
                return true;
            }
        }
        
        stream.seek(offset);
        return false;
    };
        
    this.prototype.init = function() {
        this.bitstream = new AV.Bitstream(this.stream);
    };
    
    // Reads an ADTS header
    // See http://wiki.multimedia.cx/index.php?title=ADTS
    this.readHeader = function(stream) {
        if (stream.read(12) !== 0xfff)
            throw new Error('Invalid ADTS header.');
            
        var ret = {};
        stream.advance(3); // mpeg version and layer
        var protectionAbsent = !!stream.read(1);
        
        ret.profile = stream.read(2) + 1;
        ret.samplingIndex = stream.read(4);
        
        stream.advance(1); // private
        ret.chanConfig = stream.read(3);
        stream.advance(4); // original/copy, home, copywrite, and copywrite start
        
        ret.frameLength = stream.read(13);
        stream.advance(11); // fullness
        
        ret.numFrames = stream.read(2) + 1;
        
        if (!protectionAbsent)
            stream.advance(16);
        
        return ret;
    };
    
    this.prototype.readChunk = function() {
        if (!this.sentHeader) {
            var offset = this.stream.offset;
            var header = ADTSDemuxer.readHeader(this.bitstream);
            
            this.emit('format', {
                formatID: 'aac ',
                sampleRate: tables.SAMPLE_RATES[header.samplingIndex],
                channelsPerFrame: header.chanConfig,
                bitsPerChannel: 16
            });
            
            // generate a magic cookie from the ADTS header
            var cookie = new Uint8Array(2);
            cookie[0] = (header.profile << 3) | ((header.samplingIndex >> 1) & 7);
            cookie[1] = ((header.samplingIndex & 1) << 7) | (header.chanConfig << 3);
            this.emit('cookie', new AV.Buffer(cookie));
            
            this.stream.seek(offset);
            this.sentHeader = true;
        }
        
        while (this.stream.available(1)) {
            var buffer = this.stream.readSingleBuffer(this.stream.remainingBytes());
            this.emit('data', buffer);
        }
    };
});

module.exports = ADTSDemuxer;