An integrity-checking tool for MP3s
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What is it?

mp3sum is a utility for verifying the integrity of MP3 files. You can think of it like an SFV checker, except that it still works even after you've re-tagged the MP3.

mp3sum screen shot

How does it work?

When the LAME encoder creates an MP3 file, it embeds a CRC-16 check sum of the file's audio stream in the file's header. Since only the audio stream is used to compute the check sum, it is not affected by meta-data modifications (such as changing tags, adding lyrics, and applying cover art) that occur after the file has been created. This means that, no matter how much the file's tags are changed afterwards, the check sum still applies. Re-computing the check sum and comparing it to the one embedded in the file allows mp3sum to verify that the data in the file has not been corrupted.

How do i use it?

First, install the utility:

% git clone
% cd mp3sum
% sudo pip3 install -r requirements.txt
% sudo make install

(If you don't have Python 3, use pip instead of pip3 for the third step.)

Then, simply pass mp3sum any MP3s (or directories of MP3s) you wish to verify, as shown in the screen shot above.

What does the output mean?

mp3sum prints file results one per line in a format like the following:

P D4CB:D4CB C6FD:C6FD 01  -  Scissor Runner.mp3
  • P is the result code. This may be P for pass, U for unsupported, or F for fail. This code is provided primarily for easy batch scripting.

  • D4CB:D4CB is the computed and expected info tag CRC (respectively). The tag CRC must be valid before mp3sum will go on to check the audio stream. If the tag CRC is missing or uncomputable, it will be printed as all zeroes.

  • C6FD:C6FD is the computed and expected music CRC (respectively). The music CRC represents the actual audio data in the file. If the music CRC is missing or uncomputable, it will be printed as all zeroes.

  • 01 - Scissor Runner.mp3 is the path or name of the MP3 file.

What are pass, unsupported, and fail?

File checks may return one of three results:

  • Pass — mp3sum was able to compute both the tag CRC and the music CRC and confirm that they match what was embedded in the file. This result indicates a good (non-corrupt) MP3 file.

  • Unsupported — mp3sum can only verify the integrity of MP3s created by the LAME encoder, version 3.90 or later. If the file was created with another encoder (such as the one built into iTunes), or with an old version of LAME, or if there's any reason that the CRC could not be found or computed, an unsupported result will be returned. There's not necessarily anything wrong with an unsupported file — but, unless you have a valid SFV, you'll never really know.

  • Fail — Either the computed tag CRC or the computed music CRC didn't match the original value embedded in the file. This usually means that the file is corrupt, though there are non-corruption-related reasons that the info tag or audio stream might have been modified (see below).

What does a failed check mean?

Normally, a failed check means that the file is corrupt. This is especially likely if only one or two of the files in an album fails. Another reason only some of the files might fail is if the album was pieced together from multiple rips.

If all files in an album fail, it's more likely that there is some other, non-corruption-related, explanation. The three most common reasons are:

  • A scene group has modified the info tag, probably to change the encoder version string, rendering the info tag CRC invalid.

  • Someone has applied destructive volume normalisation (cf. MP3Gain) to the audio stream, rendering the music CRC invalid.

  • mp3sum simply doesn't know how to read the file, because its developer is stupid.

Why are the files out of order?

mp3sum is multi-threaded to improve check speed. Since checks are performed in parallel, it is likely that at least a few of the files will be out of order in the output. If this is bothersome for some reason, supplying --workers 1 will put the files back in order (at the cost of longer check times).

What's the licence?

As usual, mp3sum is provided under the MIT licence.

Where can i learn more?