[ID3 Dev] Finding the end of an ID3 Header

[Robert Sewell]

Hello,

 

I have a need to verify that mp3 files uploaded by our users comply with
certain minimum specs (bit and sample rates, etc.).  To get to the mp3
headers, I first have to check for an ID3 tag, and if found, use bytes
6-9 to calculate the location of the first mp3 frame.  My routine seems
to work in most cases.  However, I've got one mp3 files that doesn't
seem to fit the specs, and fails my verification process, although all
mp3 players play the file without a problem.  Below are the first 13
bytes of the file:

 

49 44 33 03 00 00 00 01 79 76 57 4F 41 46

 

If I am interpreting the ID3 specs correctly, the first mp3 frame should
be found at offset 7D00.  However, I only find 00 there; the first mp3
is really at 7EA1.

 

Am I missing something?  Or did the program that added the ID3 tag
miscalculate the ID3v2 size bytes?  If the latter, do all players allow
for mistakes and just start hunting for the 11-bit frame sync?

 

Thanks in advance,

 

Robert Sewell

 

 


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