One model, two stems: how a vocal remover gets the instrumental for free

You feed a song in. You get two files back: one with only the singer, one with only the band. The obvious way to build that is to train two models, one per stem. The audio separator in my open-source app does something cheaper and, in practice, cleaner: it runs one model, then gets the second stem by subtraction. This post walks through the actual code that does it. It is short, it is specific, and once you see the subtraction trick you will use it in other places too. A mixing desk keeps every…