ID3
From Wikipedia, the free encyclopedia
ID3 is a metadata container most often used in conjunction with the MP3 audio file format. It allows information such as the title, artist, album, track number, or other information about the file to be stored in the file itself.
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[edit] Versions of ID3
There are two unrelated versions of ID3: ID3v1 and ID3v2.
[edit] ID3v1
ID3v1 was created by Eric Kemp in 1996 and quickly became the de facto standard for storing metadata in MP3s. The ID3v1 tag occupies 128 bytes, beginning with the string TAG. To maintain compatibility with older media players the tag was placed at the end of the file. Some players played a small burst of static when they read the tag, but most ignored it, and almost all modern players will correctly skip it.
ID3v1 was extended by Michael Mutschler in 1997, by using the last byte of the little-used "comment" field to store the track number. Such tags are referred to as ID3v1.1.
ID3v1 had several shortcomings. The small tag size only allowed for 30 bytes for the title, artist, album, and a "comment", 4 bytes for the year, and a byte to identify the genre of the song from a list of 80 values (Winamp later extended this list to 148 values). Long song or album titles were simply truncated.
Many users criticized the predefined genre list, which did not contain common genres like minimalist or Baroque, but did contain entries like Christian Rap.
ID3v1 also lacked support for internationalization. While nominally the text was supposed to be encoded in ISO-8859-1, in practice the user's local encoding was usually used, and so mojibake are common in ID3v1 tags.
[edit] ID3v2
In response to these criticisms, a new standard called ID3v2 was created. Although it bears the name ID3, it has little to no relation to the ID3v1 standard.
ID3v2 tags are of variable size, and usually occur at the start of the file, to aid streaming media. They consist of a number of frames, each of which contains a piece of metadata. For example, the TIT2 frame contains the title, and the WOAR frame contains the URL of the artist's website. Frames can be 16MB in length. Textual frames are marked with an encoding bit, though mojibake are still common.
In the latest ID3v2 standard there are 84 types of frame, and applications can also define their own types. There are standard frames for containing cover art, BPM, copyright and license, lyrics, and arbitrary text and URL data, as well as other things.
There are three versions of ID3v2. ID3v2.2 was the first public version of ID3v2. It used three character frame identifiers rather than four (TT2 for the title instead of TIT2). Most of the common v2.3 and v2.4 frames have direct analogues in v2.2.
ID3v2.3 expanded the frame identifier to four characters, and added a number of frames. A frame could contain multiple values, separated with a / character.
ID3v2.4 is the latest version of the standard, dated November 1, 2000. Notably, it allows textual data to be encoded in UTF-8, which was a common practice in earlier tags despite the standard. It uses a null byte to separate multiple values, and so / can appear in text data again.
[edit] ID3v2 Chapters
The ID3v2 Chapter Addendum[1] was published in December 2005 but is not widely supported as yet. It allows users to jump easily to specific locations or chapters within an audio file and can provide a synchronized slide show of images and titles during playback. Typical applications include Enhanced podcasts and it can be used in ID3v2.3 or ID3v2.4 tags.
[edit] Criticisms and problems
Although the different versions of ID3v2 are conceptually similar, implementing an algorithm to read and write them is difficult. There are subtle but critical differences between all the versions. Even within a version, the structure of frames differs greatly. For example, the TIT2 frame which contains the title, and the USLT which contains a lyrics transcription, require different algorithms to extract the data. With 84 different frames, dozens of such sub-parsers are required. Other tagging formats such as APEv2 eschew this, and use a single key/value pair for the internal structure of every frame.
ID3v2 stores many things in the tag that are more commonly left to the audio format itself. Some examples are the TLEN frame which stores the audio length, or the AENC frame which contains the encryption scheme of the audio. (However, the information provided by TLEN is often not derived trivially. In general, the duration of a variable bitrate stream can be calculated only after examining each frame of the entire stream. Thus, TLEN can be useful for streaming audio and inadequate hardware.)
Despite being over six years old, ID3v2.4 has not seen much adoption. This is likely because the ID3v2 reference implementation still cannot read or write it. However, this is changing, as new audio players such as Amarok, Quod Libet, and foobar2000 will only write v2.4 tags by default.
Due to these problems, some tagging tools now use other metadata formats when tagging MP3s. Few new audio formats have adopted ID3v2 tags, opting instead for APEv2 tags or Vorbis comments.
[edit] Editing ID3 tags
ID3 tags may be edited in a variety of ways. On some platforms the file's properties may be edited by viewing extended information in the file manager. Additionally most audio players allow editing single or groups of files. Editing groups of files is often referred to as "batch tagging". There are also specialized applications, called taggers, which concentrate specifically on editing the tags of and related tasks. These often offer advanced features such as advanced batch tagging or editing based on regular expressions.
