Phase-vocoder: about this phasiness business

The phase-vocoder is a well-known tool for the frequency domain processing of speech or audio signals, with applications such as time compression or expansion, pitch-scale modification, noise reduction, etc. In the context of time-scale or pitch-scale modification, the phase-vocoder is usually considered to yield high quality results, especially when large modification factors are used on polyphonic or non-pitched signals. However, the phase-vocoder is also known for an artifact that plagues its output, and has been described in the literature as either "phasiness", "reverberation", or "loss of presence". Research has been devoted to understanding and reducing this artifact, and solutions have been proposed which either significantly improve the quality of the output at the cost of a very high additional computation time, or are inexpensive but only marginally effective. This paper examines the problem of phasiness in the context of time-scale modification of signals, and presents two new phase synchronization schemes which are shown to both significantly improve the sound quality, and reduce the computational cost of such modifications.