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Topic: ID3v2 tag padding and resultant tag size< Next Oldest | Next Newest >
C Man
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Posted: Aug. 03 2005,00:15
Someone please explain this so I can understand it.  If I tag my files with the default 1500 byte tag pad size, shouldn't the tag sizes for all the files STAY at 1500 bytes as long as the 'used space' is less than 1500 bytes?  That's not what's happening though.

Example: I tag 2 files with the 1500 byte pad size.  One has a 'used space' of 85 bytes, and the other one 108 bytes.  The resultant tag sizes then become 1585 bytes & 1608 bytes respectively.  Why?

When using the 'browse & edit' tool to look at the advanced ID3v2 info of some files I downloaded, the tag sizes for one group of files is consistent at 1024 bytes despite the 'used space' varying from file to file (usually in the 70-90 byte range).  Most likely they didn't use MP3TS to tag the files, but still, why the difference?
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noddy
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Posted: Aug. 03 2005,00:18
Sorry C Man, this one's totally outside the realms of my expertise! Unless anyone else knows the answer, I think you're going to have to wait for Magnus to get back from his holiday around August 11. Sorry about that.

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Magnus Brading
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Posted: Aug. 11 2005,21:45
C Man, this is only different methodologies when adding padding to ID3v2 tags. I add 1500 extra bytes (as default value, fully configurable, just like you mention) in addition to the used space when creating the tags for the first time, while some other program that you mention might be using a constant value including the used space.

There are no rights or wrongs here, only a matter of taste, and I chose this strategy because is has a lower probability of the files needing tag enlargement (which is resource heavy) when something is added to them later.

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C Man
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Posted: Aug. 12 2005,18:20
Ok, thanks for the explanation.  So if someone wanted to be *really* anal about it and have all their tags be the same size, they could first mass tag all their files with the same info, like the 4 digit year, and then go back and tag in whatever their preferred way is?

I experimented a little last night after I read your reply.  When writing a tag for the first time, adding just one character adds 22 bytes to the tag size.  I don't understand why that is, but I'm not a computer expert so I just accepted it and moved on (any characters after that are 1 byte each though).  So adding the 4 digit year to a file will use 25 bytes (22+1+1+1).  I set the default padding to 1475 bytes and then added the 4 digit year to a group of files, and the resulting tag sizes were all 1500 bytes.  Then I re-tagged in my normal fashion and they stayed that way.  Makes sense that they would, of course, but it was nice to see it on screen when browsing the advanced ID3v2 info.
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Magnus Brading
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Posted: Aug. 12 2005,18:48
Yes, as you noticed, this technique will work. :)

And the reason for more than one byte being added to the tag size during your described conditions originates from some of the inner workings of ID3v2 tags, and is not that easy to explain here in simple words. For more info, please see the ID3v2.3 tag specification.

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