Книга: Practical Common Lisp

Structure of an ID3v2 Tag

Structure of an ID3v2 Tag

Before you can start cutting code, you'll need to be familiar with the overall structure of an ID3v2 tag. A tag starts with a header containing information about the tag as a whole. The first three bytes of the header encode the string "ID3" in ISO-8859-1 characters. In other words, they're the bytes 73, 68, and 51. Then comes two bytes that encode the major version and revision of the ID3 specification to which the tag purports to conform. They're followed by a single byte whose individual bits are treated as flags. The meanings of the individual flags depend on the version of the spec. Some of the flags can affect the way the rest of the tag is parsed. The "major version" is actually used to record the minor version of the spec, while the "revision" is the subminor version of the spec. Thus, the "major version" field for a tag conforming to the 2.3.0 spec is 3. The revision field is always zero since each new ID3v2 spec has bumped the minor version, leaving the subminor version at zero. The value stored in the major version field of the tag has, as you'll see, a dramatic effect on how you'll parse the rest of the tag.

The last field in the tag header is an integer, encoded in four bytes but using only seven bits from each byte, that gives the total size of the tag, not counting the header. In version 2.3 tags, the header may be followed by several extended header fields; otherwise, the remainder of the tag data is divided into frames. Different types of frames store different kinds of information, from simple textual information, such as the song name, to embedded images. Each frame starts with a header containing a string identifier and a size. In version 2.3, the frame header also contains two bytes worth of flags and, depending on the value of one the flags, an optional one-byte code indicating how the rest of the frame is encrypted.

Frames are a perfect example of a tagged data structure—to know how to parse the body of a frame, you need to read the header and use the identifier to determine what kind of frame you're reading.

The ID3 tag header contains no direct indication of how many frames are in a tag—the tag header tells you how big the tag is, but since many frames are variable length, the only way to find out how many frames the tag contains is to read the frame data. Also, the size given in the tag header may be larger than the actual number of bytes of frame data; the frames may be followed with enough null bytes to pad the tag out to the specified size. This makes it possible for tag editors to modify a tag without having to rewrite the whole MP3 file.[272]

So, the main issues you have to deal with are reading the ID3 header; determining whether you're reading a version 2.2 or 2.3 tag; and reading the frame data, stopping either when you've read the complete tag or when you've hit the padding bytes.

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