Time stretching is the process of changing the speed or duration of an audio signal without affecting its pitch.Pitch scaling or pitch shifting is the opposite: the process of changing the pitch without affecting the speed. Similar methods can change speed, pitch, or both at once, in a time-varying way.These processes are used, for instance, to match the pitches and tempos of two pre-recorded clips for mixing when the clips cannot be reperformed or resampled.