[ID3 Dev] ID3v2.4.0 Frames

Ion Todirel iontodirel at yahoo.co.uk
Thu Aug 25 12:43:09 PDT 2005

Hi Ben, i have some more questions about the frames, if you could help me...
In Frame Header:
<Frame ID (4Bytes)>
<Size     (4Bytes)>
<Flags    (2Bytes)>  
If in frame format flags is set: grouping identity, compression and encryption (with "1")
then, frame will look like that?:
<group identifier byte (1Byte)>
<Data Length Indicator byte (1Byte)> (compression)
<Encryption method byte (1Byte)      (encryption) 
Data lenght indicator value is stored like normal decimal number? if Data lenght Indicator flag is set (last bit in frame format flags) what this means?
Encription require a ENCR frame, right? but grouping identity require a GRID frame? or SIGN frame? if not for what is
SIGN and GRID frames, if not for working with grouping identity byte data.
<Data lenght indicator (4Bytes)
<frame contents>
she have same value like "Frame size"?, if all flags is zeroed (status and format flags)
is required to add a Data Lenght Indicator? why?
And what is the point of Unsyncronysation and what is that?, "The only purpose of unsynchronisation is to make the ID3v2 tag as
compatible as possible with existing software and hardware...", and Syncsafe. Its easy to make algorithms
to decode and encode synsafe integers, but way, what is the purpose of that, i dont find any explanation
on id3.org of that. On mp3-files, "unsyncronisation: is used?
About frames, in ID3v2.40 main structure its said that a frame must have at least one byte lenght
excluding header. Theoretically if i have these situation
<$00 00 00 01>
<$xx xx>
<Encoding $xx>
null             (here must be Information, but since size of the frame is one byte lenght, all its OK)
i must report a error that Information must be at least 1Byte lenght (minimum size of TIT1 must be 2Bytes)
right? what is the sense of encoding if text that must be encoded isnt present?
CRC is equal with Tag size - extended Header, if all is OK?
and finally least question, if a frame have  Tag alter preservation or File alter preservation flags set, leave this frame
and go to next?
Thanks, hope you help me.

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