AAC是音頻的一種編碼格式，AAC通常壓縮比為18：1，也有資料說為20：1，遠(yuǎn)勝mp3。

AAC音頻格式有ADIF和ADTS：

ADIF：Audio Data Interchange Format 音頻數(shù)據(jù)交換格式。這種格式的特征是可以確定的找到這個(gè)音頻數(shù)據(jù)的開始，不需進(jìn)行在音頻數(shù)據(jù)流中間開始的解碼，即它的解碼必須在明確定義的開始處進(jìn)行。故這種格式常用在磁盤文件中。

ADTS：Audio Data Transport Stream 音頻數(shù)據(jù)傳輸流。這種格式的特征是它是一個(gè)有同步字的比特流，解碼可以在這個(gè)流中任何位置開始。它的特征類似于mp3數(shù)據(jù)流格式。

簡單說，ADTS可以在任意幀解碼，也就是說它每一幀都有頭信息。ADIF只有一個(gè)統(tǒng)一的頭，所以必須得到所有的數(shù)據(jù)后解碼。且這兩種的header的格式也是不同的，目前一般編碼后的和抽取出的都是ADTS格式的音頻流。

ADTS是幀序列，本身具備流特征，在音頻流的傳輸與處理方面更加合適。

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下面我們對(duì)ADTS進(jìn)行分析：

可以看到ADTS的每一幀都有頭信息，即ADTS_header，ADTS頭中相對(duì)有用的信息是采樣率、聲道數(shù)、幀長度。一般ADTS頭信息都是7字節(jié)，如果有CRC則為9字節(jié)。

ADTS幀首部結(jié)構(gòu)：

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ADTS頭部的生成：

/**
 * 添加ADTS頭部
 *
 * @param packet    ADTS header 的 byte[]，長度為7
 * @param packetLen 該幀的長度，包括header的長度
 */
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表示使用哪個(gè)級(jí)別的AAC，在MPEG-2 AAC中定義了3種：

AAC三種級(jí)別

freqIdx表示使用的采樣率下標(biāo)，通過這個(gè)下標(biāo)在 Sampling Frequencies[ ]數(shù)組中查找得知采樣率的值：

• 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表示聲道數(shù)：

• 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

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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 {
    // 采樣頻率對(duì)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];

    /**
     * 構(gòu)造函數(shù)，通過傳遞進(jìn)來的文件路徑創(chuàng)建輸入流
     *
     * @param aacFilePath AAC文件路徑
     * @throws FileNotFoundException
     */
    public AACHelper(String aacFilePath) throws FileNotFoundException {
        mFileInputStream = new FileInputStream(aacFilePath);
    }

    /**
     * 獲取下一Sample數(shù)據(jù)
     *
     * @param byteBuffer 存放Sample數(shù)據(jù)的ByteBuffer
     * @return 當(dāng)前Sample的byte[]大小，如果為空返回-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;
    }

    /**
     * 從AAC文件流中讀取ADTS頭部
     *
     * @param adtsHeader      ADTS頭部
     * @param fileInputStream AAC文件流
     * @return 是否讀取成功
     * @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;
    }

    /**
     * 釋放資源
     *
     * @throws IOException
     */
    public void release() throws IOException {
        mFileInputStream.close();
    }

    /**
     * ADTS頭部
     */
    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;
    }
}