Sound Definitions

Techniques : pitch shift : time domain

Rabiner and Schafer in 1978 put forth an alternate solution that works in the time domain: attempt to find the period (or equivalently the fundamental frequency) of a given section of the wave using some pitch detection algorithm (commonly the peak of the signal's auto correlation, or sometimes cepstral processing), and crossfade one period into another. This is called time domain harmonic scaling or the synchronized overlap-add method and performs somewhat faster than the phase vocoder on slower machines but fails when the auto correlation mis-estimates the period of a signal with complicated harmonics (such as orchestral pieces). Cool Edit Pro seems to solve this by looking for the period closest to a center period that the user specifies, which should be an integer multiple of the tempo, and between 30 Hz and the lowest bass frequency. For a 120 bpm tune, use 48 Hz because 48 Hz = 2,880 cycles/minute = 24 cycles/beat * 120 bpm.

This is much more limited in scope than the phase vocoder based processing, but can be made much less processor intensive, for real-time applications. It provides the most coherent results for single-pitched sounds like voice or musically monophonic instrument recordings.

High end systems

High-end commercial audio processing packages either combine the two techniques (for example by separating the signal into sinusoid and transient waveforms), or use other techniques based on the wavelet transform, or artificial neural network processing, producing the highest quality time stretching.

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