Key correction is a feature available on some turntables and most CD decks, whereby using the Pitch control does not affect the perceived pitch of the track. This is done by DSP on an onboard microchip, and is a very useful feature for mixing. It allows mixes to be made that are entirely in tune, whilst without key correction tracks vary in small percentages of pitch - resulting in microtones of variance in pitch, which sounds ugly. People with perfect pitch notice these differences especially.