The two biggest design goals were to be able to implement ID3v2 without disturbing old software too much and that ID3v2 should be as flexible and expandable as possible.
The first criterion is met by the simple fact that the MPEG decoding software uses a syncsignal, embedded in the audiostream, to 'lock on to' the audio. Since the ID3v2 tag doesn't contain a valid syncsignal, no software will attempt to play the tag. If, for any reason, coincidence make a syncsignal appear within the tag it will be taken care of by the 'unsynchronisation scheme' described in section 5.
The second criterion has made a more noticeable impact on the design of the ID3v2 tag. It is constructed as a container for several information blocks, called frames, whose format need not be known to the software that encounters them. At the start of every frame there is an identifier that explains the frames' format and content, and a size descriptor that allows software to skip unknown frames.
If a total revision of the ID3v2 tag should be needed, there is a version number and a size descriptor in the ID3v2 header.
The bitorder in ID3v2 is most significant bit first (MSB). The byteorder in multibyte numbers is most significant byte first (e.g. $12345678 would be encoded $12 34 56 78).
It is permitted to include padding after all the final frame (at the end of the ID3 tag), making the size of all the frames together smaller than the size given in the head of the tag. A possible purpose of this padding is to allow for adding a few additional frames or enlarge existing frames within the tag without having to rewrite the entire file. The value of the padding bytes must be $00.
1. ID3v2 header
The ID3v2 tag header, which should be the first information in the file, is 10 bytes as follows:
ID3v2/file identifier "ID3" ID3v2 version $03 00 ID3v2 flags %abc00000 ID3v2 size 4 * %0xxxxxxx
The first three bytes of the tag are always "ID3" to indicate that this is an ID3v2 tag, directly followed by the two version bytes. The first byte of ID3v2 version is it's major version, while the second byte is its revision number. In this case this is ID3v2.3.0. All revisions are backwards compatible while major versions are not. If software with ID3v2.2.0 and below support should encounter version three or higher it should simply ignore the whole tag. Version and revision will never be $FF.
The version is followed by one the ID3v2 flags field, of which currently only three flags are used.
- a - Unsynchronisation
Bit 7 in the 'ID3v2 flags' indicates whether or not unsynchronisation is used (see section 5 for details); a set bit indicates usage.
- b - Extended header
The second bit (bit 6) indicates whether or not the header is followed by an extended header. The extended header is described in section 3.2.
- c - Experimental indicator
- The third bit (bit 5) should be used as an 'experimental indicator'. This flag should always be set when the tag is in an experimental stage.
All the other flags should be cleared. If one of these undefined flags are set that might mean that the tag is not readable for a parser that does not know the flags function.
The ID3v2 tag size is encoded with four bytes where the most significant bit (bit 7) is set to zero in every byte, making a total of 28 bits. The zeroed bits are ignored, so a 257 bytes long tag is represented as $00 00 02 01.
The ID3v2 tag size is the size of the complete tag after unsychronisation, including padding, excluding the header but not excluding the extended header (total tag size - 10). Only 28 bits (representing up to 256MB) are used in the size description to avoid the introducuction of 'false syncsignals'.
An ID3v2 tag can be detected with the following pattern:
$49 44 33 yy yy xx zz zz zz zz
Where yy is less than $FF, xx is the 'flags' byte and zz is less than $80.
2. ID3v2 extended header
The extended header contains information that is not vital to the correct parsing of the tag information, hence the extended header is optional.
Extended header size $xx xx xx xx Extended Flags $xx xx Size of padding $xx xx xx xx
Where the 'Extended header size', currently 6 or 10 bytes, excludes itself. The 'Size of padding' is simply the total tag size excluding the frames and the headers, in other words the padding. The extended header is considered separate from the header proper, and as such is subject to unsynchronisation.
The extended flags are a secondary flag set which describes further attributes of the tag. These attributes are currently defined as follows
- x - CRC data present
- If this flag is set four bytes of CRC-32 data is appended to the extended header. The CRC should be calculated before unsynchronisation on the data between the extended header and the padding, i.e. the frames and only the frames.
Total frame CRC $xx xx xx xx
3. ID3v2 frame overview
As the tag consists of a tag header and a tag body with one or more frames, all the frames consists of a frame header followed by one or more fields containing the actual information. The layout of the frame header:
Frame ID $xx xx xx xx (four characters) Size $xx xx xx xx Flags $xx xx
The frame ID made out of the characters capital A-Z and 0-9. Identifiers beginning with "X", "Y" and "Z" are for experimental use and free for everyone to use, without the need to set the experimental bit in the tag header. Have in mind that someone else might have used the same identifier as you. All other identifiers are either used or reserved for future use.
The frame ID is followed by a size descriptor, making a total header size of ten bytes in every frame. The size is calculated as frame size excluding frame header (frame size - 10).
In the frame header the size descriptor is followed by two flags bytes. These flags are described in section 3.3.1.
There is no fixed order of the frames' appearance in the tag, although it is desired that the frames are arranged in order of significance concerning the recognition of the file. An example of such order: UFID, TIT2, MCDI, TRCK ...
A tag must contain at least one frame. A frame must be at least 1 byte big, excluding the header.
If nothing else is said a string is represented as ISO-8859-1 characters in the range $20 - $FF. Such strings are represented as <text string>, or <full text string> if newlines are allowed, in the frame descriptions. All Unicode strings use 16-bit unicode 2.0 (ISO/IEC 10646-1:1993, UCS-2). Unicode strings must begin with the Unicode BOM ($FF FE or $FE FF) to identify the byte order.
All numeric strings and URLs are always encoded as ISO-8859-1. Terminated strings are terminated with $00 if encoded with ISO-8859-1 and $00 00 if encoded as unicode. If nothing else is said newline character is forbidden. In ISO-8859-1 a new line is represented, when allowed, with $0A only. Frames that allow different types of text encoding have a text encoding description byte directly after the frame size. If ISO-8859-1 is used this byte should be $00, if Unicode is used it should be $01. Strings dependent on encoding is represented as <text string according to encoding>, or <full text string according to encoding> if newlines are allowed. Any empty Unicode strings which are NULL-terminated may have the Unicode BOM followed by a Unicode NULL ($FF FE 00 00 or $FE FF 00 00).
The three byte language field is used to describe the language of the frame's content, according to ISO-639-2.
All URLs may be relative, e.g. "picture.png", "../doc.txt".
If a frame is longer than it should be, e.g. having more fields than specified in this document, that indicates that additions to the frame have been made in a later version of the ID3v2 standard. This is reflected by the revision number in the header of the tag.
3.1. Frame header flags
In the frame header the size descriptor is followed by two flags bytes. All unused flags must be cleared. The first byte is for 'status messages' and the second byte is for encoding purposes. If an unknown flag is set in the first byte the frame may not be changed without the bit cleared. If an unknown flag is set in the second byte it is likely to not be readable. The flags field is defined as follows.
- a - Tag alter preservation
- This flag tells the software what to do with this frame if it is unknown and the tag is altered in any way. This applies to all kinds of alterations, including adding more padding and reordering the frames.
0 Frame should be preserved. 1 Frame should be discarded.
- b - File alter preservation
- This flag tells the software what to do with this frame if it is unknown and the file, excluding the tag, is altered. This does not apply when the audio is completely replaced with other audio data.
0 Frame should be preserved. 1 Frame should be discarded.
- c - Read only
- This flag, if set, tells the software that the contents of this frame is intended to be read only. Changing the contents might break something, e.g. a signature. If the contents are changed, without knowledge in why the frame was flagged read only and without taking the proper means to compensate, e.g. recalculating the signature, the bit should be cleared.
- i - Compression
- This flag indicates whether or not the frame is compressed.
0 Frame is not compressed. 1 Frame is compressed using [#ZLIB zlib] with 4 bytes for 'decompressed size' appended to the frame header.
- j - Encryption
This flag indicates wether or not the frame is enrypted. If set one byte indicating with which method it was encrypted will be appended to the frame header. See section 4.26. for more information about encryption method registration.
0 Frame is not encrypted. 1 Frame is encrypted.
- k - Grouping identity
- This flag indicates whether or not this frame belongs in a group with other frames. If set a group identifier byte is added to the frame header. Every frame with the same group identifier belongs to the same group.
0 Frame does not contain group information 1 Frame contains group information
Some flags indicates that the frame header is extended with additional information. This information will be added to the frame header in the same order as the flags indicating the additions. I.e. the four bytes of decompressed size will precede the encryption method byte. These additions to the frame header, while not included in the frame header size but are included in the 'frame size' field, are not subject to encryption or compression.
4. Default flags
The default settings for the frames described in this document can be divided into the following classes. The flags may be set differently if found more suitable by the software.
- Discarded if tag is altered, discarded if file is altered.
- Discarded if tag is altered, preserved if file is altered.
- Preserved if tag is altered, discarded if file is altered.
- Preserved if tag is altered, preserved if file is altered.
- The rest of the frames.