mp3gain.js is the port of MP3Gain in javascript via Emscripten.

Online demo available here

mp3gain can analyze and adjust mp3 files so that they have the same volume.

mp3gain does not just do peak normalization, as many normalizers do. Instead, it does some statistical analysis to determine how loud the file actually sounds to the human ear. Also, the changes mp3gain makes are completely lossless. There is no quality lost in the change because the program adjusts the mp3 file directly, without decoding and re-encoding. Also, this works with all mp3 players, i.e. no support for a special tag or something similar is required.

mp3gain actually changes your file's gain only when you use one of the options -r, -a, -g, or -l. If none of these options is given, only a tag denoting the recommended gain change is written to the file. If you only want to print the recommended gain change (and not modify the file at all) you have to use the -s s (skip tag) option.

The method mp3gain uses to determine the desired volume is described at www.replaygain.org (link to URL http://www.replaygain.org/)

You can install the lib through NPM:

npm install --save mp3gain.js

Or download it on github release : https://github.com/ybootin/mp3gain.js/releases

The js files are available in the node_modules/mp3gain.js/dist folder :

• mp3gain.js, mp3gain.min.js - the all in one file, include the JS wrapper and the mp3gain worker
• mp3gain-wrapper.js, mp3gain-wrapper.min.js - The JS wrapper for the binary and the worker, for advanced use
• mp3gain-worker.js - the original mp3gain app compiled with emscriten, for use with the JS wrapper

This is project is only for browser use, Nodejs is not supported at this time. For a quick start, we recommand using the mp3gain.js file.

<script src="/path/to/mp3gain.js"></script>
<script>
  // instanciate the normalizer
  var normalizer = mp3gain.getInstance()

  // add files to be normalized from URL (must be accessible with CORS)
  // will return a promise
  normalizer.addFiles([
    'http://mydomain/myfile1.mp3',
    'http://mydomain/myfile2.mp3',
  ])
  .then(function(files) { // files is an array of {name: filename, data: UInt8Array}
    // at this point your files are loaded into the app and ready to be normalized
    // the run method take a list of arguments, available here, and return a promise with the normalized files once done
    // -a arg will normalize gain for the wholes files (album gain)
    return normalizer.run('-a')
  }).then(function(normalizedFiles) {
    // normalizedFiles is an array of {name: filename, data: UInt8Array} @see #common-interface-for-file
    console.log(normalizedFiles)
  }).catch(function(error) {
    // catch error from load or normalized here
    console.error(error)
  })
</script>

interface IFile {
  name: string,
  data: Uint8Array
}

instanciate mp3gain with mp3gain.getInstance()

You must attach callbacks before call the run function :

var normalizer = mp3gain.getInstance()

normalizer.on(mp3gain.ON_STDOUT, function(data) {
  console.log('Got stdout', data.stdout)
})

normalizer.addFiles([
  'http://mydomain/myfile1.mp3',
  'http://mydomain/myfile2.mp3',
])
.then(function(files) {
  return normalizer.run('-a -o')
}).then(function(files) {
  console.log('got normalized files', files)
})

Adapted from from debian documentation

If you do not specify -c, the program will stop and ask before applying gain change to a file that might clip

Here's the problem: The "global gain" field that mp3gain adjusts is an 8-bit unsigned integer, so the possible values are 0 to 255.

MOST mp3 files (in fact, ALL the mp3 files I've examined so far) don't go over 230. So there's plenty of headroom on top-- you can increase the gain by 37dB (multiplying the amplitude by 76) without a problem.

The problem is at the bottom of the range. Some encoders create frames with 0 as the global gain for silent frames. What happens when you lower the global gain by 1? Well, in the past, mp3gain always simply wrapped the result up to 255. That way, if you lowered the gain by any amount and then raised it by the same amount, the mp3 would always be exactly the same.

There are a few encoders out there, unfortunately, that create 0-gain frames with other audio data in the frame. As long as the global gain is 0, you'll never hear the data. But if you lower the gain on such a file, the global gain is suddenly huge. If you play this modified file, there might be a brief, very loud blip.

So now the default behavior of mp3gain is to not wrap gain changes. In other words,

1. If the gain change would make a frame's global gain drop below 0, then the global gain is set to 0.
2. If the gain change would make a frame's global gain grow above 255, then the global gain is set to 255.
3. If a frame's global gain field is already 0, it is not changed, even if the gain change is a positive number.

To use the original "wrapping" behavior, use the -w switch.

mp3gain.js is easy for a quick start, because it embed the binary and the worker, so you don't have to deal with others assets. But the file size is big (650k minified, 210k gzip), and probably you'll prefer use the mp3gain-wrapper.js which is less than 10k gzip.

mp3gain-wrapper.js use the exact same interface than mp3gain.js, only the instanciation change :

<script src="/path/to/mp3gain-wrapper.js"></script>
<script>
  // instanciate the normalizer, by specified the path of the worker
  var normalizer = new mp3gain.MP3GainWrapper('/path/to/mp3gain-worker.js')

  // ... then do the stuff as usual !
  normalizer.addFiles([...]).then(function() {
    return normalizer.run('-a')
  }).then(function(normalizedFiles) {
    // do what you want with your files
  }).catch(function(error) {
    // handle error
  })
</script>

Docker is required, as i use an emscripten docker image as a compiler to compile the main C code in javascript.

Use npm run build to build all the files into dist/.

Use npm run buildwrapper to compile only the dist/mp3gain-wrapper.js file

Use npm run buildbinary to compile the original mp3gain C code into dist/mp3gain-binary.js file

Use npm run buildworker to build the dist/mp3gain-worker.js file

Use npm run minify to minify all the assets

Use npm run clean to clean the dist folder

The homepage of mp3gain is located at http://mp3gain.sourceforge.net/