Phase vocoder

A phase vocoder is a type of vocoder which preserves both frequency and phase information.

A similar computer algorithm (referred to by the same name) allows frequency-domain modifications to a digital sound file (typically time expansion/compression and pitch shifting). At the heart of the phase vocoder is the STFT (short-time Fourier transform), typically coded using fast Fourier transforms. The STFT converts a time domain representation of sound into a time-frequency representation (the "analysis" phase), allowing modifications to the amplitudes or phases of specific frequency components of the sound, before resynthesis of the frequency domain representation into the time domain by the inverse STFT. The beauty of the phase vocoder is that the time scale of the resynthesis does not have to be the same as the time scale of the analysis, allowing for high-quality time-scale modification of the original sound file.

Since the boundaries between each component need to be in phase, this method results in an audible "phase smearing" effect, especially on transients. (Similar to phase dispersion.) Many other methods have been developed that mix aspects of the phase vocoder with other methods, to produce similar effects while maintaining phase coherence at transients.

See also

* Audio timescale-pitch modification

External links

*The Phase Vocoder: A Tutorial - A good description of the phase vocoder
*New Phase-Vocoder Techniques for Pitch-Shifting, Harmonizing and Other Exotic Effects
*A new Approach to Transient Processing in the Phase Vocoder

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