AW: [ID3 Dev] Purpose of Unsynchronization flag on ID3v24 tag header ?

[Mathias Kunter]

If the TAG header unsync bit is set this means that ALL frames within this tag have
been unsynced, and that ALL frames MUST contain the unsync bit set in the
FRAME header flags therefore (as defined by the specs). In my current ID3 implementation
I de-unsync a frame if either the TAG flag is set or if the FRAME flag is set for
the particular frame.

Note that if a frame has been unsynced this doesn't mean that this frame actually got
modified. (Well, the specs say that the flag SHOULD only be set if the frame became
actually modified because of a false sync, but you can't rely on that).

So if the frame header flag is set it is guaranteed that this frame (or all frames, if the
tag header flag is set) contain no false sync signal in it. It does NOT mean that the
frame content actually became modified, as mentioned before. From the decoder
point of view, this doesn't matter anyway. Just de-unsync the frame by replacing all
$FF $00 byte sequences with $FF bytes.


Cheers.
Mathias



----- Ursprüngliche Mail ----
Von: Paul Taylor <paul_t100 at fastmail.fm>
An: id3v2 at id3.org
Gesendet: Freitag, den 6. April 2007, 15:39:09 Uhr
Betreff: [ID3 Dev] Purpose of Unsynchronization flag on ID3v24 tag header ?

From IDv3v24 Spec ' Bit 7 in the 'ID3v2 flags' indicates whether or not unsynchronisation is applied on all frames (see section 6.1 for   details); a set bit indicates usage.'

What exactly does this mean, and what is its purpose? If I have a tag with 10 frames, and only one of those frames actually contains a FF FE pattern, and this frame is synchronized, 
and there is nothing to for the other nine, does that mean the tag unsynchronisation flag  is on/off. 

Or if one of the other frames does not contain a FF FE pattern but does contain a FF 00 pattern should that frame be unsynchronized (even though it 
creates more work at decoding and encoding ends) 
and only if all these cases were covered would the tag unsynchronisation flag be on. 

If the tag contains 10 frames and non of the frames contain an FF FE or FF 00 pattern, then unsynchronization 
would have no effect should the flag be on/off


thanks paul
 


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