Question

所以，我正在编写一个程序，以收集存储在ID3标签中的MP3信息。（ID3v1很简单，这很麻烦！）

我正在使用的一般过程是读取FrameID（4个字节），然后读取长度（4个字节），然后读取数据（长度字节）。

当读取长度时，它是一个Syncsafe整数，所以基本上每个字节的最高位应为0.这似乎适用于小数据帧，但是当它们变长时，偶尔我会遇到一个字节在最高位为1的长度字段中。根据ID3规范的惯例，我应该忽略这一位，但这样做会导致算法搞砸，只能获得部分标记。

特别是标签的frhed转储：

COMM<bh:00><bh:00><bh:00><bh:c6><bh:00><bh:00><bh:00>eng<bh:00>ABC123 ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT KYLE

此时，长度 $ 00 00 00 c6 ，作为BigEndian int 198 ，这在技术上是此处标记的长度。但是，二进制文件中的 c6 为 11000110 。同步安全算法核对第一位，然后计数变为 70 ，这是不正确的。我已经用Winamp 2.95和iTunes 10来完成帧长。我不明白。有没有一个条件我应该寻找同步保护整数，或者只是在读取这些帧时保持原样？

源代码：

FileStream fs = new FileStream(@"D:\commtag.dat",FileMode.Open,FileAccess.Read);
string frameID = ID3v2.ReadStream(fs, 4);
int i = ID3v2.ReadBEInt32(fs);
int j = ID3v2.UnSyncSafe(i);

[...]

public static int ReadBEInt32(Stream mp3File) {
    byte[] buffer = new byte[4];
    mp3File.Read(buffer, 0, 4);
    UInt32 rawInt = BitConverter.ToUInt32(buffer, 0);

    //if the endianness is not BigEndian, as provided in the ID3 tag, convert it
    if(BitConverter.IsLittleEndian) {
        rawInt = (rawInt >> 24) |
            ((rawInt << 8) & 0x00FF0000) |
            ((rawInt >> 8) & 0x0000FF00) |
            (rawInt << 24);
    }

    byte mask = 0x7F;

    return (int)rawInt;
}

public static int UnSyncSafe(int inData) {
    int outData = 0, mask = 0x7F000000;
    while(mask > 0) {
        outData >>= 1;
        outData |= inData & mask;
        mask >>= 8;
    }
    return outData;
}

运行此示例后，i和j的值分别为198和70。

这是完整的标题（frhed dump，它稍微进入了MP3标题的开头）：

ID3<bh:03><bh:00><bh:00><bh:00><bh:00><bh:10><bh:1d>TENC<bh:00><bh:00><bh:00><bh:01>@<bh:00><bh:00>WXXX<bh:00><bh:00><bh:00><bh:02><bh:00><bh:00><bh:00><bh:00>TCOP<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TOPE<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TCOM<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TCON<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TRCK<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TYER<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TALB<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TPE1<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>TIT2<bh:00><bh:00><bh:00><bh:01><bh:00><bh:00><bh:00>COMM<bh:00><bh:00><bh:00><bh:d3><bh:00><bh:00><bh:00>eng<bh:00>ABC123 ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT ID3v2COMMENT KYLE I AM ITUNES<bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh:00><bh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Answer 1

示例中的标记是2.3版。同步安全整数通常不用于2.3帧，只用于标记头。在2.4中，同步安全整数用于标题和帧中。

ID3 2.3 frame spec

Stackoverflow question

Answer 2

每个版本的ID3v2在存储帧头方面都有所不同。

ID3v2.2帧头长度为6个字节，最后3个字节为大端无符号整数。
ID3v2.3帧头长10个字节，后4个字节是大端无符号整数。当然，如果ID3v2标记未同步，则可能还有其他字节。
ID3v2.4帧头长10个字节，后4个字节是大端非同步无符号整数。当然，除非媒体播放器有缺陷。

以下是我在我的库中使用的代码：FrameHeader.cs Line 145

输入本质上是一个已经删除了任何unsynch字节的字节数组。

解码长ID3V2帧

2 个答案: