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Old 29th May 2012, 12:03   #3
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A 3rd Party Alternative for MP3s Only

"MP3Gain" can analyze and adjust mp3 files so that they have the same volume. It is an implementation of the ReplayGain spec. MP3Gain uses David Robinson's Replay Gain algorithm to calculate how loud the file actually sounds to a human's ears. With MP3Gain, the loudness adjustment is done on the data itself, in a lossless/reversible way. Another difference with MP3Gain is the fact that it can only adjust physical volume in 1.5 dB steps. There is no quality lost in the change because the app adjusts the mp3 file directly, without decoding and re-encoding.

Here's the technical reason on why it's lossless (despite operating on the data itself), and also why the smallest change possible is 1.5 dB:

The MP3 format stores the sound information in small chunks called "frames". Each frame represents a fraction of a second of sound. In each frame there is a "global gain" field. This field holds an 8-bit integer which can represent values from 0 to 255.

When an MP3 player decodes the sound in the frame, it uses the global gain field to multiply the decoded sound samples by 2^(gain/4). If you add 1 to this field in all the MP3 frames, you effectively multiply the amplitude of the whole file by 2^(1/4) = 119 % = +1.5 dB. Likewise, if you subtract 1 from this field, you multiply the amplitude by 2^(-1/4) = 84 % = -1.5 dB, reducing the amplitude of the whole file.

The way MP3Gain works actually has a very strong benefit. Since it is the data itself that is modified, MP3Gain does not require special support from music players.

MP3Gain can store 'analysis' and 'undo' information in special tags inside the mp3 file itself. These tags are in the APEv2 format. If you choose not to use these tags or delete them, then you will not be able to automatically undo changes made by MP3Gain. You will still be able to undo any changes, but you will have to manually keep track of what changes where made to your files and undo the changes manually. Logs of the analysis, changes, and errors can be saved to text files with names and locations of your choosing. MP3Gain must be used to undo any changes made with MP3Gain.

After MP3Gain completes an analysis, the results are displayed with an indication of whether clipping is present. Adjustments can then be chosen (up or down) before they are applied, to see their effect on clipping. This allows a minimum adjustment to be made when the main goal is to prevent clipping. Of course, the same adjustment (minimum or not) needs to be made to all files when the main goal is overall volume leveling.


There is another app with a similar name. "Mp3Gain PRO" does a form of volume normalization inside the mp3s. If you feel a song is too quiet or too loud at the beginning (or middle, or end), then it can boost or reduce the volume just for that part. Pretty cool, if that's what you need. In order to make its fine-tuned adjustments, it must re-encode the mp3 file. The changes Mp3Gain PRO makes are not undo-able. For this reason, I do not recommend it for general use.

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