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 reverse: the process of changing the pitch without affecting the speed.

If you take a sample and assign it to a key in a sampler and then play the sample further up the key span you will invariably hear the sample get higher in pitch and shorter in length. It is the same as playing a record at a faster speed. The pitch changes and the time it takes for the record to play from beginning to end is far shorter than at the original speed.

Time stretching is simply a mathematical process and there are many software that provide time stretching as a standard function. For time stretching to work sensibly you need to know the original tempo of the sample so as to then advocate a new tempo for the sample to be stretched to or compressed to (time compression). Be aware that stretching a sample too much will cause all sorts of anomalies and sometimes these anomalies are used as an effect (robotic effect used in Electro genres etc).

Let’s take at a simple drum beat at 80 bpm and time stretch it to 125 bpm. Simply changing the pitch of the sample to speed it up will not work as the sounds within the beat will also be pitched up. We therefore must time stretch so as to maintain the pitch of the sounds within the drum beat..............

Excerpt taken from The Beat Production Bible.

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