Interlude. Why Mastering Can’t Save a Bad Mix

The mastering engineer receives a song already cooked. Before him the voice, drums, bass, guitars and keyboards lived on separate tracks, and the mix engineer could do anything he liked with each of them. Then all of it was summed into two channels, left and right, and from then on the song can only be worked on as a whole, as a single wave in which everything is added to everything.

The craft has a ceiling. An equaliser at the mastering stage acts on the entire track at once. If the voice sounds dull because a guitar playing in the same frequency range is covering it, the mastering engineer can boost those frequencies, but the guitar will go up along with the voice, and the argument between them will not go anywhere, it will just get louder. This is the same masking MP3 is built on: a loud sound hides a quiet neighbour from the ear, and you can only move them into different places while they still sit on different tracks.