Phase vocoder

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A phase vocoder is a type of vocoder which can scale both the frequency and time domains of audio signals by using phase information. The computer algorithm 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 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.)^{[citation needed]} 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.^{[citation needed]}