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

[Mathias Kunter]

>You say above the frame flag being set means 'it contains no false sync signal. It does NOT mean that the
>frame content actually became modified'

>whereas in your latest post you say 'If you decide to don't unsync the frame content, do nothing. You MUST
>NOT set the frame header flag.'

>These two statements seem to be the opposite of each other

They are not :)
Look at this:

***DECODER POINT OF VIEW***
IF the flag is set, the frame content MIGHT became modified by the unsync scheme and you MUST
de-unsync it to get the correct data. In the case that the content actually didn't became modified it's no
problem as the de-unsync process is going to simply have no effect then.

IF the flag is NOT set, you MUST NOT de-unsync the frame content as this COULD produce corrupted data.


***ENCODER POINT OF VIEW***
IF you want to unsync the content, replace all FF >=E0 byte sequences with FF 00 >=E0 bytes. In the case
that there is no FF >=E0 byte sequence, you CAN set the flag, although you SHOULDN'T. If there was a
FF >=E0 byte sequence in the data, you MUST set the flag. I explained earlier that it is sufficient to replace
FF bytes with FF 00 only, without checking for the entire FF >=E0 sequence.

IF you DON'T want to unsync the content, you MUST NOT set the flag and MUST NOT change the frame
data in any way, of course.


 
>>'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.'

>if  there were any FF 00 patterns there would be an effect. Replacing FF 00 with FF 00 00 is part of
>unsynchronization (it is not optional)

This is correct. However, when I wrote "you can still decide to unsync the frame" I meant that you also have
to care about the FF 00 combinations then (just because you're applying the unsync scheme). Therefore,
the decoder also reads the correct byte sequence again.

Mathias





		
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