adts.js
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'use strict';
var Stream = require('../utils/stream.js');
var AdtsStream;
var
ADTS_SAMPLING_FREQUENCIES = [
96000,
88200,
64000,
48000,
44100,
32000,
24000,
22050,
16000,
12000,
11025,
8000,
7350
];
/*
* Accepts a ElementaryStream and emits data events with parsed
* AAC Audio Frames of the individual packets. Input audio in ADTS
* format is unpacked and re-emitted as AAC frames.
*
* @see http://wiki.multimedia.cx/index.php?title=ADTS
* @see http://wiki.multimedia.cx/?title=Understanding_AAC
*/
AdtsStream = function() {
var buffer;
AdtsStream.prototype.init.call(this);
this.push = function(packet) {
var
i = 0,
frameNum = 0,
frameLength,
protectionSkipBytes,
frameEnd,
oldBuffer,
sampleCount,
adtsFrameDuration;
if (packet.type !== 'audio') {
// ignore non-audio data
return;
}
// Prepend any data in the buffer to the input data so that we can parse
// aac frames the cross a PES packet boundary
if (buffer) {
oldBuffer = buffer;
buffer = new Uint8Array(oldBuffer.byteLength + packet.data.byteLength);
buffer.set(oldBuffer);
buffer.set(packet.data, oldBuffer.byteLength);
} else {
buffer = packet.data;
}
// unpack any ADTS frames which have been fully received
// for details on the ADTS header, see http://wiki.multimedia.cx/index.php?title=ADTS
while (i + 5 < buffer.length) {
// Loook for the start of an ADTS header..
if (buffer[i] !== 0xFF || (buffer[i + 1] & 0xF6) !== 0xF0) {
// If a valid header was not found, jump one forward and attempt to
// find a valid ADTS header starting at the next byte
i++;
continue;
}
// The protection skip bit tells us if we have 2 bytes of CRC data at the
// end of the ADTS header
protectionSkipBytes = (~buffer[i + 1] & 0x01) * 2;
// Frame length is a 13 bit integer starting 16 bits from the
// end of the sync sequence
frameLength = ((buffer[i + 3] & 0x03) << 11) |
(buffer[i + 4] << 3) |
((buffer[i + 5] & 0xe0) >> 5);
sampleCount = ((buffer[i + 6] & 0x03) + 1) * 1024;
adtsFrameDuration = (sampleCount * 90000) /
ADTS_SAMPLING_FREQUENCIES[(buffer[i + 2] & 0x3c) >>> 2];
frameEnd = i + frameLength;
// If we don't have enough data to actually finish this ADTS frame, return
// and wait for more data
if (buffer.byteLength < frameEnd) {
return;
}
// Otherwise, deliver the complete AAC frame
this.trigger('data', {
pts: packet.pts + (frameNum * adtsFrameDuration),
dts: packet.dts + (frameNum * adtsFrameDuration),
sampleCount: sampleCount,
audioobjecttype: ((buffer[i + 2] >>> 6) & 0x03) + 1,
channelcount: ((buffer[i + 2] & 1) << 2) |
((buffer[i + 3] & 0xc0) >>> 6),
samplerate: ADTS_SAMPLING_FREQUENCIES[(buffer[i + 2] & 0x3c) >>> 2],
samplingfrequencyindex: (buffer[i + 2] & 0x3c) >>> 2,
// assume ISO/IEC 14496-12 AudioSampleEntry default of 16
samplesize: 16,
data: buffer.subarray(i + 7 + protectionSkipBytes, frameEnd)
});
// If the buffer is empty, clear it and return
if (buffer.byteLength === frameEnd) {
buffer = undefined;
return;
}
frameNum++;
// Remove the finished frame from the buffer and start the process again
buffer = buffer.subarray(frameEnd);
}
};
this.flush = function() {
this.trigger('done');
};
};
AdtsStream.prototype = new Stream();
module.exports = AdtsStream;