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Inside the MP3 Codec
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Inside the MP3 Codec - Page 12
featured with permission from MP3: The Definitive Guide by Scot Hacker

ID3 Space

http://www.id3.org/ Tacked to the beginning or end of an MP3 file, " ID3" tag information may be stored (see Chapter 4), possibly including artist and title, copyright information, terms of use, proof of ownership, an encapsulated thumbnail image, and comments. There are actually two variants of the ID3 specification: ID3v1 and ID3v2, and while the potential differences between them are great, virtually all modern MP3 players can handle files with tags in either format (though a few older players will have problems with ID3v2 tags). Not only are ID3v2 tags capable of storing a lot more information than ID3v1 tags, but they appear at the beginning of the bitstream, rather than at the end. The reason for this is simple: When an MP3 file is being broadcast or streamed rather than simply downloaded, the player needs to be able to display all of this information throughout the duration of the track, not at the end when it's too late.

It's unfortunate that ID3 tags ever ended up being tagged onto the end of MP3 files to begin with; we'd be much better off if all MP3 files stored their ID3 data at the beginning rather than at the end of the file. As it stands, some MP3 players will simply give up if actual audio data is not encountered within the first few frames. While players developed to the actual ISO MPEG specification will know how to handle either type, the specification itself is unfortunately vague on this point. It simply states that a player should look for a " sync header," without specifying exactly where seeking should start and stop. This laxness in the spec has caused some controversy among developers of ID3-enabled applications, who naturally don't want their applications seeking blindly through 1GB image files, should the user happen to hand one to the application. Fortunately, the ID3v2 spec is more specific on the matter.

One of the more interesting portions of the ID3 specification is the numerical categorization of types of audio, as shown in the Appendix. The numerical identifiers are stored in the ID3 tag, and typically mapped to the actual names via a picklist or another widget in the MP3 player or ID3 tool.

 

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