Decoding Long ID3V2 Frames
So, I'm writing a program for fun to gather MP3 information stored in ID3 tags. (the ID3v1 was trivial, this is troublesome!)
The general process I'm using is to read a FrameID (4 bytes), then the Length (4 bytes), and then the Data (Length Bytes).
When reading the Length, it is a Syncsafe integer, so essentially the top bit in each byte should be 0. This seems to be fine for small data frames, but when they get long, occasionally I'll run into a byte in the length field which has a top bit of 1. By the convention of the ID3 spec, I'm supposed to ignore this bit, but doing so causes the algorithm to get screwed up and only get partial tags.
frhed dump of the tag in particular:
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
At this point, the length is $00 00 00 c6, which as a BigEndian int is 198, which is technically the length of the tag here. However, c6 in binary is 11000110. The sync safe algorithm nukes that first bit, and then the count becomes 70, which is not correct. I've gotten Winamp 2.95 and iTunes 10 to do this with the frame lengths. I don't get it. Is there a condition I'm supposed to look for to sync-safe the integer or just keep it as it is while reading through these frames?
Source Code:
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;
}
The values of i and j after running this example are 198 and 70, respectively.
Here is the complete header (frhed dump, it goes into the beginning of the MP3 header a little):
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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The tag in your example is version 2.3. Sync-safe integers are not typically used in 2.3 frames, only the tag header. In 2.4 sync-safe integers are used in both the header and frames.
ID3 2.3 frame spec
Stackoverflow question
Each version of ID3v2 is different in how it stores frame headers.
- ID3v2.2 frame headers are 6 bytes long with the last 3 bytes being a big endian unsigned integer.
- ID3v2.3 frame headers are 10 bytes long with the second 4 bytes being a big endian unsigned integer. Of course, if the ID3v2 tag is unsynchronized, there may be additional bytes.
- ID3v2.4 frame headers are 10 bytes long with the second 4 bytes being a big endian unsychronized unsigned integer. Unless, of course, the media player is defective.
Here is the code I used in my library: FrameHeader.cs Line 145
The input is essentially an array of bytes that have already had any unsynch bytes removed.
精彩评论