Lossy Audio 101: MP3, iTunes, and others
File formats such as MP3, AAC, OGG Vorbis, and others, shrink the file size by throwing away information that is least likely to be missed, and maintaining as much sound quality as possible.
Lossy audio formats, while fine for consumers, are not fine for mixing and mastering. As a music producer, you want to get back exactly what you put into your music production system, not a version that’s close, but not quite the same.
Usually, audio producers output files in the WAV or AIF file formats, which are both lossless. If you take a lossless file, like a WAV file or an AIF file, convert it to a lossy file, and then convert it back, you do not get back the original signal.
Then, if you were to take that new WAV file, or AIF file, and convert it to MP3 again, you get a second round of distortion and loss.