by Alexander Thomas (aka Dr. Lex)
This command-line program calculates ReplayGain volume adjustments for MP3 files and stores the values in the file's ID3 tag, for use by players like Rockbox. Unlike MP3Gain, the MP3 stream is completely unaltered. Id3Gain uses ‘TXXX’ tags with labels like “replaygain_track_gain” to store the gain information. The drawback is of course that your MP3 player must support those tags, otherwise they won't have any effect.
Id3Gain can also update the Sound Check volume adjustment used by iTunes to the more accurate ReplayGain value, with the option to use album gain instead of track gain. Or, if you don't care about Sound Check, the program can also remove the ‘iTunNORM’ tag while processing MP3 files.
ReplayGain is an open source variant of the “Sound Check” feature built into iTunes. In other words, it calculates an optimal volume adjustment such that every song plays at the same perceived loudness. Because ReplayGain analyzes the entire file while Sound Check only seems to sample some parts of the song, ReplayGain generally gives much better results. I have seen cases where the Sound Check adjustment differred more than 7dB from the RG value, amounting to more than a doubling or halving of the volume. I have never encountered a case where Sound Check gave a more correct sounding result than RG. In some versions of iTunes the accuracy of Sound Check seems to have worsened to a point where it became useless, probably because the algorithm was further cannibalized to make it even faster. The result is that it doesn't really matter if SC is enabled or not: it will only change the distribution of too loud and too silent songs, without doing any useful volume equalisation. It just randomises the volumes. ReplayGain on the other hand really works.
If you have the impression that in the absence of volume adjustment music has steadily been becoming louder since 1990, you're right. I wrote an article about this pest that is plaguing the music industry.
This is not a polished, streamlined program with a glossy interface. It is a bare command-line tool. In fact, unless you're using OS X on a 64-bit Intel CPU, what you'll download is not even that: I only provide source code to avoid having to maintain binary releases for different platforms. To compile and run Id3Gain, libMAD and libid3tag are required. These can be installed through most package managers.
Version 2.0 works with iTunes 10.5 and makes it a lot easier to port the ReplayGain values to iTunes' “Sound Check”. You no longer need to perform digital voodoo rituals! Nevertheless, this is still only a console program which requires some familiarity with the Terminal. If you want an easy-to-use but non-free alternative for OS X that has a nice graphical interface, you may want to have a look at iVolume instead.
Read the included README file for instructions. The program includes a basic Makefile and a CMakeLists.txt file for easy compilation with CMake.
This binary will only work under 64-bit Intel Mac OS X. It was compiled under Lion but I believe it may also work under Snow Leopard and perhaps even Leopard, but I can't guarantee anything. For your convenience, the required libMAD and libid3tag libraries are built-in so you do not need to install them separately.
Id3Gain is released under the GPL. This software is provided "as is", without any implied warranty or claim of fitness for a particular purpose. Use of this software is completely at your own risk.