[mad-dev] test if file is mp3

[Russell O'Connor]

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[To: sebastian@trueg.de, mad-dev@lists.mars.org]

On Thu, 7 Nov 2002, Sebastian Trueg wrote:

> Hy list,
> is there a good and fast way to test if a file actually is a mp3 file or not?
> My attempts with decoding some bytes at the beginning of the file do not
> work...

I was thinking about this problem recently.  I was planning on
implementing the following.

Read the first 4 bytes A B C D

if ABC = ID3, then it's MP3  (It might be better to skip the ID3 header
and then ensure the following holds)

or if ABCD form a valid MP3 header, then it's MP3.
Otherwise not MP3.

To check if it's a valid header, I'd ensure

A = FF
B & 0xE0 = E0
B & 0x18 != 0x08
B & 0x06 != 0
C & 0xF0 != 0xF0
C & 0x0C != 0x0C
D & 0x03 != 0x02

I'd be interested in other's opinions of this detection algorithm.

- -- 
Russell O'Connor            <http://www.math.berkeley.edu/~roconnor/>
``[Law enforcement officials] suggested that the activists were stopped
not because their names are on the list, but because their names resemble
those of suspected criminals or terrorists.'' -- SFGate.com
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