redlining

In broadcasting or sound engineering, redlining is when the levels are set too high and the lines on the screen/board are in the red instead of the more desired orange or yellow. Leaving levels at this high level may cause your audio to sound distorted and unpleasant.

Dude, your levels are up too high, you're redlining!

In the red

In music, when you have your amps turned up to 11 and the music is extremely loud and ear piercing. This is usually done deliberately.

Manowar want to be the loudest band so the deliberately play with the volume in the red.

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