Songs of seven

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 ...»