Re: Heuristic for determining valid MP3 file

Thanks, Glenn.

I'll have a look at your code.

Erik

----- Original Message ----- 
From: "Glenn Maynard" <[email protected]>
To: <[email protected]>
Sent: Thursday, April 01, 2004 9:58 PM
Subject: Re: [mad-dev] Heuristic for determining valid MP3 file


> On Thu, Apr 01, 2004 at 02:00:44PM +0100, Erik Jälevik wrote:
> > What's a good heuristic for determining whether a file is a valid MP3 or
not?
> >
> > My first thought was to grab a chunk at the beginning of the file and
repeatedly
> > call mad_header_decode until it comes back with a header and take that as an
> > indication that the file is valid.
> >
> > However, on trying this out on the first 10k of a plain text file,
> > mad_header_decode still returns a header found at 8k but with wrong data in
it.
> > Maybe this is due to it just looking for the sync bits and this file
happened to
> > have them within its first 10k?
> >
> > Is there a better way of doing this? Maybe start trying to decode and if
more
> > errors than a certain threshold occur, assume the file is not valid?
>
> I read up to 25000 (arbitrary) bytes of data.  Fatal errors are fatal;
> the only errors I handle are MAD_ERROR_LOSTSYNC, MAD_ERROR_BADCRC (which
> I don't have a test case for), and MAD_ERROR_BUFLEN/MAD_ERROR_BUFPTR.
>
> More specifically, I count the number of bytes read in a given pass; if
> more than the threshold is read without getting a good packet, I bail.
> (I also explicitly subtract things like Xing and ID3 headers from this
> count; ID3v2 headers can easily be larger than that.)
>
> The primary case where this matters is WAVs with MP3 data in them, and
> unknown headers.  This handles those fine (though why people are putting
> MP3 data in WAVs is well beyond my comprehension).
>
> My actual code is at
>
>
http://cvs.sourceforge.net/viewcvs.py/*checkout*/stepmania/stepmania/src/RageSoundReader_MP3.cpp?rev=HEAD
>
> See RageSoundReader_MP3::do_mad_frame_decode.
>
> (Any comments on the way I'm doing this are appreciated.  I havn't received
> any bug reports due to mis-detected files in quite a while.)
>
> -- 
> Glenn Maynard