Phase Vocoder - History

History

The phase vocoder was introduced in 1966 by Flanagan as an algorithm that would preserve horizontal coherence between the phases of bins that represent sinusoidal components. This original phase vocoder did not take into account the vertical coherence between adjacent frequency bins, and therefore, time stretching with this system did produce sound signals that were missing clarity.

The optimal reconstruction of the sound signal from STFT after amplitude modifications has been proposed by Griffin and Lim in 1984. This algorithm does not consider the problem to produce a coherent STFT, but it allows to find the sound signal that has an STFT that is as close as possible to the modified STFT even if the modified STFT is not coherent (does not represent any signal).

The problem of the vertical coherence remained a major issue for the quality of time scaling operations until 1999 when the Laroche and Dolson proposed a rather simple means to preserve phase consistency across spectral bins. The proposition of Laroche and Dolson has to be seen as a turning point in phase vocoder history. It has been shown that by means of ensuring vertical phase consistency very high quality time scaling transformations can be obtained.

The algorithm proposed by Laroche did not allow to preserve horizontal phase coherence for sound onsets (note onsets). A solution for this problem has been proposed by Roebel.

A software implementation of the phase vocoder based signal transformation that is using means similar to what has been described here above to achieve high quality signal transformation is for example Ircam's SuperVP.