When editing a file compressed in the MP3 or AAC formats, other audio editors decompress the file. After editing, the file must be re-encoded, resulting in an undesirable loss of quality. In simple terms, with most editors, you lose sound quality when you edit lossy formats such as AAC or MP3.
Fission, however, losslessly edits audio files in its native formats (AAC, MP3, Apple Lossless, FLAC, AIFF, and WAV). When editing a file in these formats with Fission, the resultant file will have no quality loss from the original file. Using Fission and saving losslessly means you'll never have to worry about losing quality when you edit your audio.