- Phase vocoder
A phase vocoder is a type of
vocoderwhich can scale both the frequencyand time domains of audio signals by using phase information. The computer algorithmallows frequency-domainmodifications 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 domainrepresentation 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 domainrepresentation into the time domain by the inverse STFT. 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 window need to be in phase, this method results in an audible "phase smearing" effect, especially on transients. (Similar to phase
dispersion.)Fact|date=April 2008 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.Fact|date=April 2008
Audio timescale-pitch modification
* [http://www.panix.com/~jens/pvoc-dolson.par The Phase Vocoder: A Tutorial] - A good description of the phase vocoder
* [http://www.ee.columbia.edu/~dpwe/papers/LaroD99-pvoc.pdf New Phase-Vocoder Techniques for Pitch-Shifting, Harmonizing and Other Exotic Effects]
* [http://www.ircam.fr/equipes/analyse-synthese/roebel/paper/dafx2003.pdf A new Approach to Transient Processing in the Phase Vocoder]
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