I got this id3v24 tagged mp3 which I'm not sure is valid, though mp3tag and foobar reads it fine.

The problem is appears in an UTF-16 frame with Data length indicator (See http://www.id3.org/id3v2.4.0-structure -> 4.1. Frame header flags)

A good example is the TALB frame starting at position 39. At position 48 it is indicated that the frame has Data length indicator (i guess this is the next 4 bytes). After these 4 bytes another 4 bytes appears

01 FF 00 FE

I read the 01 as UTF-16 encoding, but what about FF 00 FE?

Is it a corrupted BOM?

http://stigc.dk/02%20-%20Baby%20Can%20I%20Hold%20You.mp3

Looks like valid ID3v2 unsynchronization to me.

OK I see it also got the Unsynchronization flag. Does the 4 bytes (01 FF 00 FE) belong to this flag? What does they mean?

FF FE means UTF-16 Little Endian. The reason why there is an 00 between them is unsynchronization. If a player is not capabile of identifying and skipping ID3v2 tags, it could interpret the FF FE BOM as synchronization word for an MPEG frame which also starts with FF F?. To avoid this, unsynchronization is used.

thanks i see it now. From the specification:

6.1. The unsynchronisation scheme

Whenever a false synchronisation is found within the tag, one zeroed
byte is inserted after the first false synchronisation byte. The
format of synchronisations that should be altered by ID3 encoders is
as follows:



Does this only alter the BOM or is it also possible that the actually UTF-16 data could be altered?

Everything in that specific frame - including the data / text, AFAIK.