STRETCHING: Detailed Information

by Archer Endrich

Key Features

• Stretching the time of a sound (without changing the pitch) can be thought of as a horizontal process.
• The time stretch can either increase (> 1) or decrease (< 1) the length of the sound.
• Stretching the spectrum of a sound can be thought of as a vertical process. In this case the stretch factor will expand the frequency relationships inharmonically if > 1, and compress them if < 1. The timbre of the sound alters significantly.

Some possible musical results with STRETCH techniques:

• Time stretching can adjust the length of a sound without altering its pitch. This allows one to fit the sound to a required duration.
• The overall result can be made more 'plastic' by using a time-varying breakpoint file for the stretch factor.
• The time-stretch breakpoint file also makes it possible to play with the recognisability of the sound, such as retaining it by having the stretching begin only after the attack transient.
• Longer time stretches pull out the waveforms sideways, as it were, and slows down the oscillations. One starts to hear the frequency components slowly moving against each other.
• Long time stretches thus reveal the inner frequency activity of the sound, making its 'innerness' audible.
• Stretching the spectrum (values > 1) above the frequency divide tends to brighten the timbre of the sound, creating inharmonic (non-integer) relationships,
• Compressing the spectrum (values < 1) above the frequency divide will tend to deepen the tone.

Other forms of stretching in CDP

Stretching Time or Spectrum are special techniques. We can also think about extending (lengthening) a sound in more generic terms. CDP contains many different ways to do this. Here are some of them:

• BLUR SHUFFLE: create a pattern by which to shuffle analysis windows, with or without repetitions
• BLUR WEAVE: weaving a user-defined path through the windows of an analysis file; patterns can repeat windows and will lengthen the sound
• DISTORT REPEAT: N repetitions of (irregular) wavecycles
• DISTORT SHUFFLE: create a pattern by which to shuffle (rregular) wavecycles
• EXTEND DRUNK: segmentation with time-varying positioning and movement controls
• EXTEND LOOP: simple looping of segments end-to-end or overlapped
• GrainMill: timestretch of granularised forms of the original sound
• TEXTURE: building extended passages from part of or all of a sound, with many time-varying shaping controls

General observations

• Stretching is a basic spectral operation that is found in most sound design software.
• Stretching is usually done by a factor of up to about 2 in any one processing operation. The resultant output is then stretched again an indefinite number of times until the desired sonic result is reached.
• CUT can be applied where appropriate to keep the file being stretched to a reasonable length. In CDP the analysis file itself can be CUT.
• Long stretch operations can best be achieved by creating a command line batch file or, in the Sound Loom GUI, an 'Instrument' for sequential processing.
• CDP's STRETCH SPECTRUM can expand or compress frequency relationships either above or below a specified frequency divide. This means that there are four basic function options, and more aural possibilities result from the placement of the frequency divide.

Last updated: 4 December 2003