0

因此，我正在编写一个有趣的程序来收集存储在 ID3 标签中的 MP3 信息。（ID3v1 是微不足道的，这很麻烦！）

我使用的一般过程是读取 FrameID（4 个字节），然后是 Length（4 个字节），然后是 Data（Length Bytes）。

读取 Length 时，它是一个 Syncsafe 整数，所以本质上每个字节中的最高位应该是 0。这对于小数据帧来说似乎很好，但是当它们变长时，偶尔我会遇到长度中的一个字节最高位为 1 的字段。按照 ID3 规范的约定，我应该忽略此位，但这样做会导致算法搞砸，只能得到部分标签。

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，长度是$00 00 00 c6，作为 BigEndian int 是198，这在技术上是这里标签的长度。但是，二进制的c6是11000110。同步安全算法核对了第一位，然后计数变为70，这是不正确的。我已经让 Winamp 2.95 和 iTunes 10 用帧长度来做到这一点。我不明白。是否存在我应该寻找同步安全整数的条件，或者在阅读这些帧时保持原样？

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。

这是完整的标头（fhed 转储，它稍微进入 MP3 标头的开头）：

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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4

2 回答 2

3

您示例中的标签是 2.3 版。2.3 帧中通常不使用同步安全整数，仅在标签头中使用。在 2.4 中，同步安全整数用于标头和帧。

ID3 2.3 帧规格

堆栈溢出问题

于 2011-07-02T07:13:16.603 回答
1

ID3v2 的每个版本在存储帧头的方式上都不同。

  • ID3v2.2 帧头有 6 个字节长，最后 3 个字节是一个大端无符号整数。
  • ID3v2.3 帧头有 10 个字节长，第二个 4 个字节是一个大端无符号整数。当然，如果ID3v2标签不同步，可能会有额外的字节。
  • ID3v2.4 帧头长 10 个字节，第二个 4 个字节是一个大端非同步无符号整数。当然，除非媒体播放器有缺陷。

这是我在库中使用的代码：FrameHeader.cs 第 145 行

输入本质上是一个已经删除了任何不同步字节的字节数组。

于 2011-07-29T22:36:44.657 回答