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

[Mathias Kunter]

>Is a frame unsynchronized IF it  doesnt contain FF FE patterns

No. I don't know why you keep talking about the FF FE pattern, this has
nothing to do with the unsync scheme. As Ben said, unsync is only relevant
to a bit sequence of

%11111111  (which is  ==  FF)
%111xxxxx  (which is  >= E0)

So, if you encounter this bit sequence within the data of a frame AND you
decide to unsyc the frame, you HAVE to insert a 00 byte after the FF
byte. You HAVE to set the frame header flag now.

If you decide to don't unsync the frame content, do nothing. You MUST
NOT set the frame header flag. This also implies that the tag header flag
MUST NOT be set.

If you don't detect the mentioned bit sequence, you can still decide to
unsync the frame and set the flag in the frame header. However, this
would be silly as the unsync would have no effect for this frame. It is
just allowed from the specs point of view, but you shouldn't do that.

It is safe to insert a 00 byte after every FF byte WHEN applying the
unsync scheme. As Ben said, this may also inserts a 00 byte where
not really nescessary, but it is easier to implement and also works
fine because a decoder has to replace EVERY $FF $00 byte sequence.


>if you had a frame with a $FF $00 combination and you didnt unsynchronize
>the frame, [...], and then a decoder tried to de-unsync the frame it would
>remove the 00, and your data will be modified

The point is, if I didn't unsync the frame, the flag is NOT set. Therefore, a
decoder MUST NOT de-unsync the frame, of course. Therefore, it doesn't
matter from the decoder point of view.

Hope it is clear now.

Mathias








	
		
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