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), however, most software will also have pitch/key detection algorithms built in. 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.
Time stretch beat 80 bpm
drum pattern with secondary kicks.mp3
The image shows that I have selected time stretch from Sound Forge’s Process menu.
Time stretch tempo select
I have selected ‘tempo’ as the defining parameter for the time stretching and, as with many software, I am using a preset template for the type of sound being processed, in this case drums.
Time stretched tempo
drum pattern with secondary kicks time stretched to 120 bpm.mp3
Just below ‘tempo’ I have input the original tempo of the drum beat (80 bpm) and above ‘final tempo’ I have selected 125 bpm as the new tempo. You can clearly hear the sped up new tempo but the important factor here is that the individual drum sounds have not been compromised in terms of pitch.
Let’s now listen to what happens if a sample is stretched/compressed too much. I mentioned earlier that there would be anomalies and that the robotic effect was the most common one. Well, here is an audio clip of a vocal sample at 96 bpm.
vocal stretch clean.mp3
And here is the same sample stretched/compressed down to 40 bpm.
vocal stretched to 40 bpm.mp3
Always try to match samples that are close in terms of bpm to your own compositions so as to avoid any of anomalies. This technique is used extensively in today’s genres and is a wonderful tool as it allows you to match tempos of differing samples to your compositions.
So, experiment and enjoy!