AAC是音频的一种编码格式，AAC通常压缩比为18：1，也有资料说为20：1，远胜mp3。
AAC音频格式有ADIF和ADTS：
ADIF：Audio Data Interchange Format 音频数据交换格式。这种格式的特征是可以确定的找到这个音频数据的开始，不需进行在音频数据流中间开始的解码，即它的解码必须在明确定义的开始处进行。故这种格式常用在磁盘文件中。
ADTS：Audio Data Transport Stream 音频数据传输流。这种格式的特征是它是一个有同步字的比特流，解码可以在这个流中任何位置开始。它的特征类似于mp3数据流格式。
简单说，ADTS可以在任意帧解码，也就是说它每一帧都有头信息。ADIF只有一个统一的头，所以必须得到所有的数据后解码。且这两种的header的格式也是不同的，目前一般编码后的和抽取出的都是ADTS格式的音频流。
ADTS是帧序列，本身具备流特征，在音频流的传输与处理方面更加合适。
下面我们对ADTS进行分析：
| ADTS AAC | ||||||
|---|---|---|---|---|---|---|
| ADTS_header | AAC ES | ADTS_header | AAC ES | ... | ADTS_header | AAC ES |
可以看到ADTS的每一帧都有头信息，即ADTS_header，ADTS头中相对有用的信息是采样率、声道数、帧长度。一般ADTS头信息都是7字节，如果有CRC则为9字节。
ADTS帧首部结构：
| 序号 | 域 | 长度（bits） | 说明 |
|---|---|---|---|
| 1 | Syncword | 12 | all bits must be 1 |
| 2 | MPEG version | 1 | 0 for MPEG-4, 1 for MPEG-2 |
| 3 | Layer | 2 | always 0 |
| 4 | Protection Absent | 1 | et to 1 if there is no CRC and 0 if there is CRC |
| 5 | Profile | 2 | the MPEG-4 Audio Object Type minus 1 |
| 6 | MPEG-4 Sampling Frequency Index | 4 | MPEG-4 Sampling Frequency Index (15 is forbidden) |
| 7 | Private Stream | 1 | set to 0 when encoding, ignore when decoding |
| 8 | MPEG-4 Channel Configuration | 3 | MPEG-4 Channel Configuration (in the case of 0, the channel configuration is sent via an inband PCE) |
| 9 | Originality | 1 | set to 0 when encoding, ignore when decoding |
| 10 | Home | 1 | set to 0 when encoding, ignore when decoding |
| 11 | Copyrighted Stream | 1 | set to 0 when encoding, ignore when decoding |
| 12 | Copyrighted Start | 1 | set to 0 when encoding, ignore when decoding |
| 13 | Frame Length | 13 | this value must include 7 or 9 bytes of header length: FrameLength = (ProtectionAbsent == 1 ? 7 : 9) + size(AACFrame) |
| 14 | Buffer Fullness | 11 | buffer fullness |
| 15 | Number of AAC Frames | 2 | number of AAC frames (RDBs) in ADTS frame minus 1, for maximum compatibility always use 1 AAC frame per ADTS frame |
| 16 | CRC | 16 | CRC if protection absent is 0 |
ADTS头部的生成：
/**
* 添加ADTS头部
*
* @param packet ADTS header 的 byte[]，长度为7
* @param packetLen 该帧的长度，包括header的长度
*/
private void addADTStoPacket(byte[] packet, int packetLen) {
int profile = 2; // AAC LC
int freqIdx = 3; // 48000Hz
int chanCfg = 2; // 2 Channel
packet[0] = (byte) 0xFF;
packet[1] = (byte) 0xF9;
packet[2] = (byte) (((profile - 1) << 6) + (freqIdx << 2) + (chanCfg >> 2));
packet[3] = (byte) (((chanCfg & 3) << 6) + (packetLen >> 11));
packet[4] = (byte) ((packetLen & 0x7FF) >> 3);
packet[5] = (byte) (((packetLen & 7) << 5) + 0x1F);
packet[6] = (byte) 0xFC;
}
其中profile表示使用哪个级别的AAC，在MPEG-2 AAC中定义了3种：
AAC三种级别
freqIdx表示使用的采样率下标，通过这个下标在 Sampling Frequencies[ ]数组中查找得知采样率的值：
- 0: 96000 Hz
- 1: 88200 Hz
- 2: 64000 Hz
- 3: 48000 Hz
- 4: 44100 Hz
- 5: 32000 Hz
- 6: 24000 Hz
- 7: 22050 Hz
- 8: 16000 Hz
- 9: 12000 Hz
- 10: 11025 Hz
- 11: 8000 Hz
- 12: 7350 Hz
- 13: Reserved
- 14: Reserved
- 15: frequency is written explictly
chanCfg表示声道数：
- 0: Defined in AOT Specifc Config
- 1: 1 channel: front-center
- 2: 2 channels: front-left, front-right
- 3: 3 channels: front-center, front-left, front-right
- 4: 4 channels: front-center, front-left, front-right, back-center
- 5: 5 channels: front-center, front-left, front-right, back-left, back-right
- 6: 6 channels: front-center, front-left, front-right, back-left, back-right, LFE-channel
- 7: 8 channels: front-center, front-left, front-right, side-left, side-right, back-left, back-right, LFE-channel
- 8-15: Reserved
AAC的解析：
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.HashMap;
import java.util.Map;
public class AACHelper {
// 采样频率对照表
private static Map samplingFrequencyIndexMap = new HashMap<>();
static {
samplingFrequencyIndexMap.put(96000, 0);
samplingFrequencyIndexMap.put(88200, 1);
samplingFrequencyIndexMap.put(64000, 2);
samplingFrequencyIndexMap.put(48000, 3);
samplingFrequencyIndexMap.put(44100, 4);
samplingFrequencyIndexMap.put(32000, 5);
samplingFrequencyIndexMap.put(24000, 6);
samplingFrequencyIndexMap.put(22050, 7);
samplingFrequencyIndexMap.put(16000, 8);
samplingFrequencyIndexMap.put(12000, 9);
samplingFrequencyIndexMap.put(11025, 10);
samplingFrequencyIndexMap.put(8000, 11);
samplingFrequencyIndexMap.put(0x0, 96000);
samplingFrequencyIndexMap.put(0x1, 88200);
samplingFrequencyIndexMap.put(0x2, 64000);
samplingFrequencyIndexMap.put(0x3, 48000);
samplingFrequencyIndexMap.put(0x4, 44100);
samplingFrequencyIndexMap.put(0x5, 32000);
samplingFrequencyIndexMap.put(0x6, 24000);
samplingFrequencyIndexMap.put(0x7, 22050);
samplingFrequencyIndexMap.put(0x8, 16000);
samplingFrequencyIndexMap.put(0x9, 12000);
samplingFrequencyIndexMap.put(0xa, 11025);
samplingFrequencyIndexMap.put(0xb, 8000);
}
private AdtsHeader mAdtsHeader = new AdtsHeader();
private BitReader mHeaderBitReader = new BitReader(new byte[7]);
private byte[] mSkipTwoBytes = new byte[2];
private FileInputStream mFileInputStream;
private byte[] mBytes = new byte[1024];
/**
* 构造函数，通过传递进来的文件路径创建输入流
*
* @param aacFilePath AAC文件路径
* @throws FileNotFoundException
*/
public AACHelper(String aacFilePath) throws FileNotFoundException {
mFileInputStream = new FileInputStream(aacFilePath);
}
/**
* 获取下一Sample数据
*
* @param byteBuffer 存放Sample数据的ByteBuffer
* @return 当前Sample的byte[]大小，如果为空返回-1
* @throws IOException
*/
public int getSample(ByteBuffer byteBuffer) throws IOException {
if (readADTSHeader(mAdtsHeader, mFileInputStream)) {
int length = mFileInputStream.read(mBytes, 0, mAdtsHeader.frameLength - mAdtsHeader.getSize());
byteBuffer.clear();
byteBuffer.put(mBytes, 0, length);
byteBuffer.position(0);
byteBuffer.limit(length);
return length;
}
return -1;
}
/**
* 从AAC文件流中读取ADTS头部
*
* @param adtsHeader ADTS头部
* @param fileInputStream AAC文件流
* @return 是否读取成功
* @throws IOException
*/
private boolean readADTSHeader(AdtsHeader adtsHeader, FileInputStream fileInputStream) throws IOException {
if (fileInputStream.read(mHeaderBitReader.buffer) < 7) {
return false;
}
mHeaderBitReader.position = 0;
int syncWord = mHeaderBitReader.readBits(12); // A
if (syncWord != 0xfff) {
throw new IOException("Expected Start Word 0xfff");
}
adtsHeader.mpegVersion = mHeaderBitReader.readBits(1); // B
adtsHeader.layer = mHeaderBitReader.readBits(2); // C
adtsHeader.protectionAbsent = mHeaderBitReader.readBits(1); // D
adtsHeader.profile = mHeaderBitReader.readBits(2) + 1; // E
adtsHeader.sampleFrequencyIndex = mHeaderBitReader.readBits(4);
adtsHeader.sampleRate = samplingFrequencyIndexMap.get(adtsHeader.sampleFrequencyIndex); // F
mHeaderBitReader.readBits(1); // G
adtsHeader.channelconfig = mHeaderBitReader.readBits(3); // H
adtsHeader.original = mHeaderBitReader.readBits(1); // I
adtsHeader.home = mHeaderBitReader.readBits(1); // J
adtsHeader.copyrightedStream = mHeaderBitReader.readBits(1); // K
adtsHeader.copyrightStart = mHeaderBitReader.readBits(1); // L
adtsHeader.frameLength = mHeaderBitReader.readBits(13); // M
adtsHeader.bufferFullness = mHeaderBitReader.readBits(11); // 54
adtsHeader.numAacFramesPerAdtsFrame = mHeaderBitReader.readBits(2) + 1; // 56
if (adtsHeader.numAacFramesPerAdtsFrame != 1) {
throw new IOException("This muxer can only work with 1 AAC frame per ADTS frame");
}
if (adtsHeader.protectionAbsent == 0) {
fileInputStream.read(mSkipTwoBytes);
}
return true;
}
/**
* 释放资源
*
* @throws IOException
*/
public void release() throws IOException {
mFileInputStream.close();
}
/**
* ADTS头部
*/
private class AdtsHeader {
int getSize() {
return 7 + (protectionAbsent == 0 ? 2 : 0);
}
int sampleFrequencyIndex;
int mpegVersion;
int layer;
int protectionAbsent;
int profile;
int sampleRate;
int channelconfig;
int original;
int home;
int copyrightedStream;
int copyrightStart;
int frameLength;
int bufferFullness;
int numAacFramesPerAdtsFrame;
}
}