Android音视频处理之AAC编码

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<Integer, Integer> 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;
    }
}

Android音视频处理之AAC编码

ADTS帧首部结构：

ADTS头部的生成：

AAC的解析：

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