使用 Java 合并 2 个 AAC 文件

Question

我试图将 2 个 AAC 文件合并为一个，我发现在 AAC 文件中 - 标题元素一直存在到 FF8 字节，然后接下来的 4 个字节包含 AAC 数据的长度。我试着维护1个header数组，把2个AAC文件的大小相加，然后把两个文件的数据缓冲区一个接一个地加进去。

生成的文件仅播放第一个 AAC 文件。这是代码sn-p。

FileInputStream fs = new FileInputStream("./res/after.aac");

dis = new DataInputStream(fs);
headerData = new byte[0xFF8];
dis.read(headerData);


int lengthTotal = dis.readInt();
System.out.println("Length of After == "+lengthTotal);
dis.readInt();


data = new byte[dis.available()];

dis.readFully(data);
dis.close();
dis = null;
fs.close();
fs = null;


fs = new FileInputStream("./res/continue.aac");
dis = new DataInputStream(fs);

dis.skipBytes(0xFF8);

int length = dis.readInt();
System.out.println("Length of Ahead == "+length);
lengthTotal = lengthTotal + length -8;
System.out.println("Total Length== "+lengthTotal);
dis.readInt();
newData = new byte[dis.available()];
dis.read(newData);

FileOutputStream fos = new FileOutputStream("./res/combine.aac");
DataOutputStream dos = new DataOutputStream(fos);

dos.write(headerData);
dos.writeInt(lengthTotal);
dos.writeBytes("mdat");
dos.write(data);
dos.write(newData);

我知道在第 56 个字节中有关于 AAC 文件持续时间的信息，但我无法弄清楚。有人可以帮我吗？

Answer 1

我建议你看看jaad库中aac文件是如何解析的，特别有趣的是ADTS处理代码here和ADIF头解析here。

Answer 2

好吧，我不能告诉你你做错了什么。但我可以告诉你如何做你想做的事。

首先创建一个General Helper Functions：

    public static class General {
    public static void CopyBytes(byte[] dst, int dstOffset, byte[] src) {
        Buffer.BlockCopy(src, 0, dst, dstOffset, src.Length);
    }
}

public static class BitHelper {
    public static int Read(ref ulong x, int length) {
        int r = (int)(x >> (64 - length));
        x <<= length;
        return r;
    }

    public static int Read(byte[] bytes, ref int offset, int length) {
        int startByte = offset / 8;
        int endByte = (offset + length - 1) / 8;
        int skipBits = offset % 8;
        ulong bits = 0;
        for (int i = 0; i <= Math.Min(endByte - startByte, 7); i++) {
            bits |= (ulong)bytes[startByte + i] << (56 - (i * 8));
        }
        if (skipBits != 0) Read(ref bits, skipBits);
        offset += length;
        return Read(ref bits, length);
    }

    public static void Write(ref ulong x, int length, int value) {
        ulong mask = 0xFFFFFFFFFFFFFFFF >> (64 - length);
        x = (x << length) | ((ulong)value & mask);
    }

    public static byte[] CopyBlock(byte[] bytes, int offset, int length) {
        int startByte = offset / 8;
        int endByte = (offset + length - 1) / 8;
        int shiftA = offset % 8;
        int shiftB = 8 - shiftA;
        byte[] dst = new byte[(length + 7) / 8];
        if (shiftA == 0) {
            Buffer.BlockCopy(bytes, startByte, dst, 0, dst.Length);
        }
        else {
            int i;
            for (i = 0; i < endByte - startByte; i++) {
                dst[i] = (byte)((bytes[startByte + i] << shiftA) | (bytes[startByte + i + 1] >> shiftB));
            }
            if (i < dst.Length) {
                dst[i] = (byte)(bytes[startByte + i] << shiftA);
            }
        }
        dst[dst.Length - 1] &= (byte)(0xFF << ((dst.Length * 8) - length));
        return dst;
    }
}

public static class BitConverterBE {
    public static ulong ToUInt64(byte[] value, int startIndex) {
        return
            ((ulong)value[startIndex    ] << 56) |
            ((ulong)value[startIndex + 1] << 48) |
            ((ulong)value[startIndex + 2] << 40) |
            ((ulong)value[startIndex + 3] << 32) |
            ((ulong)value[startIndex + 4] << 24) |
            ((ulong)value[startIndex + 5] << 16) |
            ((ulong)value[startIndex + 6] <<  8) |
            ((ulong)value[startIndex + 7]      );
    }

    public static uint ToUInt32(byte[] value, int startIndex) {
        return
            ((uint)value[startIndex    ] << 24) |
            ((uint)value[startIndex + 1] << 16) |
            ((uint)value[startIndex + 2] <<  8) |
            ((uint)value[startIndex + 3]      );
    }

    public static ushort ToUInt16(byte[] value, int startIndex) {
        return (ushort)(
            (value[startIndex    ] <<  8) |
            (value[startIndex + 1]      ));
    }

    public static byte[] GetBytes(ulong value) {
        byte[] buff = new byte[8];
        buff[0] = (byte)(value >> 56);
        buff[1] = (byte)(value >> 48);
        buff[2] = (byte)(value >> 40);
        buff[3] = (byte)(value >> 32);
        buff[4] = (byte)(value >> 24);
        buff[5] = (byte)(value >> 16);
        buff[6] = (byte)(value >>  8);
        buff[7] = (byte)(value      );
        return buff;
    }

    public static byte[] GetBytes(uint value) {
        byte[] buff = new byte[4];
        buff[0] = (byte)(value >> 24);
        buff[1] = (byte)(value >> 16);
        buff[2] = (byte)(value >>  8);
        buff[3] = (byte)(value      );
        return buff;
    }

    public static byte[] GetBytes(ushort value) {
        byte[] buff = new byte[2];
        buff[0] = (byte)(value >>  8);
        buff[1] = (byte)(value      );
        return buff;
    }
}

public static class BitConverterLE {
    public static byte[] GetBytes(ulong value) {
        byte[] buff = new byte[8];
        buff[0] = (byte)(value      );
        buff[1] = (byte)(value >>  8);
        buff[2] = (byte)(value >> 16);
        buff[3] = (byte)(value >> 24);
        buff[4] = (byte)(value >> 32);
        buff[5] = (byte)(value >> 40);
        buff[6] = (byte)(value >> 48);
        buff[7] = (byte)(value >> 56);
        return buff;
    }

    public static byte[] GetBytes(uint value) {
        byte[] buff = new byte[4];
        buff[0] = (byte)(value      );
        buff[1] = (byte)(value >>  8);
        buff[2] = (byte)(value >> 16);
        buff[3] = (byte)(value >> 24);
        return buff;
    }

    public static byte[] GetBytes(ushort value) {
        byte[] buff = new byte[2];
        buff[0] = (byte)(value      );
        buff[1] = (byte)(value >>  8);
        return buff;
    }
}

现在实现 Audio Helper 类和接口：

    interface IAudioWriter
    {
    void WriteChunk(byte[] chunk, uint timeStamp);
    void Finish();
    string Path { get; }
}

    class AACWriter : IAudioWriter
    {
    string _path;
    FileStream _fs;
    int _aacProfile;
    int _sampleRateIndex;
    int _channelConfig;

    public AACWriter(string path) {
        _path = path;
        _fs = new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.Read, 65536);
    }

    public void WriteChunk(byte[] chunk, uint timeStamp)
            {
        if (chunk.Length < 1) return;

        if (chunk[0] == 0) { // Header
            if (chunk.Length < 3) return;

            ulong bits = (ulong)BitConverterBE.ToUInt16(chunk, 1) << 48;

            _aacProfile = BitHelper.Read(ref bits, 5) - 1;
            _sampleRateIndex = BitHelper.Read(ref bits, 4);
            _channelConfig = BitHelper.Read(ref bits, 4);

            if ((_aacProfile < 0) || (_aacProfile > 3))
                throw new Exception("Unsupported AAC profile.");
            if (_sampleRateIndex > 12)
                throw new Exception("Invalid AAC sample rate index.");
            if (_channelConfig > 6)
                throw new Exception("Invalid AAC channel configuration.");
        }
        else { // Audio data
            int dataSize = chunk.Length - 1;
            ulong bits = 0;

            // Reference: WriteADTSHeader from FAAC's bitstream.c

            BitHelper.Write(ref bits, 12, 0xFFF);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits,  2, 0);
            BitHelper.Write(ref bits,  1, 1);
            BitHelper.Write(ref bits,  2, _aacProfile);
            BitHelper.Write(ref bits,  4, _sampleRateIndex);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits,  3, _channelConfig);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits,  1, 0);
            BitHelper.Write(ref bits, 13, 7 + dataSize);
            BitHelper.Write(ref bits, 11, 0x7FF);
            BitHelper.Write(ref bits,  2, 0);

            _fs.Write(BitConverterBE.GetBytes(bits), 1, 7);
            _fs.Write(chunk, 1, dataSize);
        }
    }

    public void Finish() {
        _fs.Close();
    }

    public string Path {
        get {
            return _path;
        }
    }
}

现在你需要自己做的是，从第一个 AAC 文件中逐个读取块并写入它们，然后从第二个 AAC 文件中逐个读取块并将它们附加到中间文件。

注意，上面的代码是 C#，所以你必须使用 wrapping 来模拟 C# 的 ref 效果，只需替换：

ref Type variable_name

与：

_<Type> variable_name