[ID3 Dev] Padding

Pyt py.thoulon at gmail.com
Mon Aug 22 22:16:45 PDT 2005

>Tag size included on tag header, dont include padding? 
 The tag size in the tag header includes the size of : (1) the extended 
header, (2) the actual frames, (3) the padding. See id3v2.4.0-structure.txt, 
section 3.1.
  >If padding is used to add aditional frames, where is the frame ID since 
ID is four null chars? or if padding is used to enlarge existing frames how 
to know what frame are enlarged?
 Let's say you have a tag with padding so that the total size of 
header+extended header+frames+padding = 4K (a typical case); the size in the 
tag header would be 4086 (4096 - size of header, which is ten bytes). Let's 
also say the actual frames use up only 2000 of those 4086 bytes. That leaves 
2086 bytes of padding.
Now, if you want to add a new frame to the tag, you can stick it right after 
the last of the existing frames, overwriting the padding bytes with the new 
frame contents. You would make no other change to your tag: the size remains 
the same, only the relative sizes of frames vs. padding have changed; if you 
new frame is say 50 bytes long, the tag size still indicates 4086, the total 
frame size is now 2050, and the padding size only 2036.
The advantage of this is that if you have enough padding, you can play with 
the frames without having to rewrite the whole MP3 file (which is generally 
a lengthy exercise, given the sizes of these beast).
 Enlarging a frame means a bit more extensive work because frames have to 
remain in one piece. Let's say you have a tag with TIT1, TALB, TIPL, TIT2 
frames in this order, followed by padding. If you want to enlarge TIPL for 
example, you will need to rewrite your whole tag (or at least the end of it, 
starting with TIPL) over the original one, eating up into the padding.
 >And if footer is writting to end of file, is'n possible to write an ID3v1 
Tag, right? since ID3v1 tag is located from 0 to 128 bytes at end of file.
Yes, you can. The ID3v2 footer should be located just before the ID3v1 tag. 
A good parsing algorithm looking for an ID3v2 footer should be able to skip 
the v1 tag.
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