Overdrive

Overdrive units distort the tone of an instrument by adding "overtones", creating a "warm" sound. To create a "dirty" or "gritty" sound, a unit further alters the tone by re-shaping or "clipping" its sound-waves so that they have flat, mesa-like peaks instead of curved ones. Distortion effects differ from overdrive effects in that that the former produces roughly the same amount of distortion at any volume. Overdrive units, on the other hand, produce “clean” sounds at quieter volumes and distorted sounds at louder volumes.