Can lossy audio files such as mp3 or AAC be mastered?
By Rob Stewart - JustMastering.com - September 8, 2012
You absolutely can master any file format, but, I strongly recommend using uncompressed (e.g. *.wav, or *.aiff) or "lossless" compressed (e.g. FLAC) versions of your mix when working with a mastering engineer. This ensures that you will receive the highest quality finished masters possible.
Most compressed formats such as *.mp3 are "lossy", meaning that in order to reduce the file size, something ends up being removed from your music.
On one hand, most lossy compression formats are very intelligent. They only remove portions of the music that are the least likely to be noticed by the listener. On the other hand, the portions removed are usually tranient details and stereo field information. Even though they aren't always obvious to a human listener, removing them decreases the resolution of your mix.
Taking it a step further, if those transient and stereo components are removed prior to mastering, and then the music is re-compressed to another lossy format after the music mastering process is completed, more information is removed again, which can greatly affect sound quality.
Here's a real-world example to illustrate.Below, are three audio samples that illustrate how converting to a lossy format can impact the sonic integrity of your music.
This first sample is an untreated, unmastered audio file. This is provided so that you can get a sense for the sound quality of the original file.