[ID3 Dev] id3v2-3 extended header

[Clemens Hammerl]

Is there a test suite for id3v2 tags too? If not, can you send me a  
testfile with an extended header for v2.3 and 2.4, otherwise i cannot  
test my code becaus i have no mp3s with extended header.

How do i know how long the frame part of the tag is if there is an  
extended header?


Am 14.08.2007 um 21:31 schrieb Jim:

> I believe the 6 or 10 byte size only applies to v2.3 tags.  The  
> extended header size will be 10 bytes when the CRC flag is set (4  
> added bytes for the CRC value) and 6 byte when the CRC flag is unset.
>
> To find the beginning of the frames, you would start at position 10  
> + size of extended header if present.
>
> For v2.3 tags, you would read the first ten bytes (tag header) and  
> then, if there is an extended header, read 4 bytes to get its size  
> and then skip that many bytes.  (The padding that is given in the  
> extended header comes AFTER the frames.)
>
> For v2.4 tags, you would read the first ten bytes (tag header) and  
> then, if there is an extended header, read 4 bytes to get its size  
> and then skip (size - 4) bytes ahead.  (I believe in v2.4 tags, the  
> size given in  the extended header includes the 4 bytes you would  
> have already read.)  And don't forget that in v2.4 tags, the  
> extended header size is stored as a syncsafe integer.
>
> Check out this diagram.  It should help:
>
>    Overall tag structure:
>
>      +-----------------------------+
>      |      Header (10 bytes)      |
>      +-----------------------------+
>      |       Extended Header       |
>      | (variable length, OPTIONAL) |
>      +-----------------------------+
>      |   Frames (variable length)  |
>      +-----------------------------+
>      |           Padding           |
>      | (variable length, OPTIONAL) |
>      +-----------------------------+
>      | Footer (10 bytes, OPTIONAL) |
>      +-----------------------------+
>
>
>
> Clemens Hammerl wrote:
>> hi
>> more questions inline...
>> Am 14.08.2007 um 18:10 schrieb Jim:
>>> Hi Clemens.
>>>
>>> I'm not sure which version of ID3v2 you are asking about.   
>>> However, in both v2.3 and v2.4 tags, the extended header size is  
>>> the first four bytes.  For v2.3:
>>>
>>>      Extended header size   $xx xx xx xx
>>>      Extended Flags         $xx xx
>>>      Size of padding        $xx xx xx xx
>> So the extended header size is variable in 2.3/4 and not  
>> constantly 10 Bytesin size? Im just confused about the variable  
>> size of the header.
>> I will not implement extended headers im only interested in  
>> parsing the Text Information Frames, so i have to search for the  
>> beginning of the frames at position 10(Tag header) +  
>> extendedHeaderSize + SizeOfPadding. Am I right?
>>>
>>> And for v2.4:
>>>
>>>      Extended header size   4 * %0xxxxxxx
>>>      Number of flag bytes       $01
>>>      Extended Flags             $xx
>>>
>>>
>>> Is that what you were asking?  (Both of those descriptions were  
>>> taken
>>>> from the specs available on the ID3 website so you can look  
>>>> there for a
>>> more thorough description of each field.)
>>>
>>> I can only guess why the size is stored as 4 bytes.  My guesses  
>>> would be both for future compatability if the extended header  
>>> becomes larger in later versions and to make it standard with the  
>>> rest of the sizes (overall tag size & frame sizes).
>>>
>>> Jim
>>>
>>>
>>> Clemens Hammerl wrote:
>>>> Hi
>>>> Im new to id3 an this list and i'm currently working on an  
>>>> implementation of ID3 for Java.
>>>> I dont understand how the length of extended header is  
>>>> calculated. In the specification stands that:
>>>> "...Where the 'Extended header size', currently 6 or 10 bytes,  
>>>> excludes
>>>>    itself..."
>>>> But were in the header is the size of the header stored. Is it  
>>>> in the first 4 bytes. Why do you use 4 bytes when the maximum  
>>>> length is 10B?
>>>> best regards clemens
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