Katana VentraIP

Audio time stretching and pitch scaling

Time stretching is the process of changing the speed or duration of an audio signal without affecting its pitch. Pitch scaling is the opposite: the process of changing the pitch without affecting the speed. Pitch shift is pitch scaling implemented in an effects unit and intended for live performance. Pitch control is a simpler process which affects pitch and speed simultaneously by slowing down or speeding up a recording.

"Timestretch" redirects here. For the album, see Timestretch (album).

These processes are often used to match the pitches and tempos of two pre-recorded clips for mixing when the clips cannot be reperformed or resampled. Time stretching is often used to adjust radio commercials[1] and the audio of television advertisements[2] to fit exactly into the 30 or 60 seconds available. It can be used to conform longer material to a designated time slot, such as a 1-hour broadcast.

Resampling[edit]

The simplest way to change the duration or pitch of an audio recording is to change the playback speed. For a digital audio recording, this can be accomplished through sample rate conversion. When using this method, the frequencies in the recording are always scaled at the same ratio as the speed, transposing its perceived pitch up or down in the process. Slowing down the recording to increase duration also lowers the pitch, while speeding it up for a shorter duration respectively raises the pitch, creating the so-called Chipmunk effect. When resampling audio to a notably lower pitch, it may be preferred that the source audio is of a higher sample rate, as slowing down the playback rate will reproduce an audio signal of a lower resolution, and therefore reduce the perceived clarity of the sound. On the contrary, when resampling audio to a notably higher pitch, it may be preferred to incorporate an interpolation filter, as frequencies that surpass the Nyquist frequency (determined by the sampling rate of the audio reproduction software or device) will create usually undesired sound distortions, a phenomenon that is also known as aliasing.

Speed hearing and speed talking[edit]

For the specific case of speech, time stretching can be performed using PSOLA.


Time-compressed speech is the representation of verbal text in compressed time. While one might expect speeding up to reduce comprehension, Herb Friedman says that "Experiments have shown that the brain works most efficiently if the information rate through the ears—via speech—is the 'average' reading rate, which is about 200–300 wpm (words per minute), yet the average rate of speech is in the neighborhood of 100–150 wpm."[7]


Listening to time-compressed speech is seen as the equivalent of speed reading.[8][9]

In consumer software[edit]

Pitch-corrected audio timestretch is found in every modern web browser as part of the HTML standard for media playback.[10] Similar controls are ubiquitous in media applications and frameworks such as GStreamer and Unity.

Beatmatching

— real-time changes of tuning and timbre

Dynamic tonality

Pitch correction

Scrubbing (audio)

Nightcore

A comprehensive overview of current time and pitch modification techniques by Stephan Bernsee

Time Stretching and Pitch Shifting Overview

C source code for doing frequency domain pitch manipulation

Stephan Bernsee's smbPitchShift C source code

A Javascript pitchshifter based on smbPitchShift code, from the open source KievII library

pitchshift.js from KievII

- A good description of the phase vocoder

The Phase Vocoder: A Tutorial

New Phase-Vocoder Techniques for Pitch-Shifting, Harmonizing and Other Exotic Effects

A new Approach to Transient Processing in the Phase Vocoder

PICOLA and TDHS

Theory, equations, figures and performances of a real-time guitar pitch shifter running on a DSP chip

How to build a pitch shifter

Free and commercial versions of a popular 3rd party time stretching library for iOS, Linux, Windows and Mac OS X

ZTX Time Stretching Library

commercial cross-platform library, mainly used by DJ and DAW manufacturers

Elastique by zplane

from Qneo - specialized synthesizer for creative voice sculpting

Voice Synth

Free MATLAB implementations of various Time-Scale Modification procedures

TSM toolbox

at the Wayback Machine (archived 2023-02-02), a well-known algorithm for extreme (>10×) time stretching

PaulStretch

open source and commercial libraries for real time audio stretching

Bungee