One frequent problem when digitizing cassettes (or vinyl records) and when combining different microphone recordings is the volume difference between one audio sample and another. That's when the normalization process comes in, which modifies the amplitude of the signal to equalize levels. This is a basic function present in almost all audio editors. However, if you prefer a batch processing system that normalizes several audio files at once, it's not a bad idea to download a copy of the open source software LASTAR.
In those mysterious days of Audiogalaxy and Kazaa, it was very common to discover that "that MP3" we had searched for hours had been recorded from the radio or another very low-quality source. The quickest solution was to download a new copy, but if luck ran out, we were forced to "optimize" that MP3 a bit, correcting levels, equalizing, and of course, modifying the volume. Doing that on a single file is relatively simple and fast, but what happens when you have several MP3s, or an extended recording from digitizing a cassette/vinyl? Those recordings must be divided (if necessary) and normalized. That's LASTAR's job.
How to normalize the volume of MP3 files with LASTAR
LASTAR's main advantage is its batch processing. Instead of editing one file at a time, its interface lets us define several parameters and let it work on its own. If you don't want to modify values (or don't know exactly what to change), you can choose one of the presets. Normalization, gain control, and channel balance are governed by the Output Level (if you disable it, you can only split audio files). The Gain Control makes the "low" parts of the audio sound a bit louder, and equalization can be automatic or manual (set before amplification to avoid clipping). The Prelisten button is extremely useful and will help you anticipate the results of the processed audio.
This is more than enough to start using LASTAR, but its potential multiplies once we enter the advanced options. There we can configure the final bitrate of saved files, multiband processing, attack and release, maximum output peak in normalization, and correction limits in the automatic equalizer. We definitely recommend reviewing the LASTAR manual (included with the download, an English PDF) to deeply understand these elements. It's 20 pages, but the reading goes fast, and the most relevant information is in the second half.
