Getting rid of background noise and white noise
One of the drawbacks of sound reproduction on tape is the background hiss that results from the magnetic action, which becomes most noticeable during quiet passages. To combat hiss, a noise reduction system known as Dolby (after R.M. Dolby, its American inventor) is widely used.
In quiet passages, electronic circuits automatically boost the signals before they reach the recording head, so helping to drown the hiss.
On playback, the signals are reduced to their correct level for feeding to the speakers. The hiss is reduced at the same time, becoming inaudible.