Written by Richard Dudas and Cort Lippe

Introduction

The phase vocoder is a tool used to perform time-stretching and pitch-shifting on recorded sounds. Its name derives from the early “vocoders” (contraction from “voice encoders”), which used a set of bandpass filters in parallel over many frequency bands, to crudely analyze and reconstruct speech. The infamous hardware vocoders of the 1960s and 1970s (as used, for example, by Wendy Carlos in the soundtrack of Kubrick’s film “A Clockwork Orange”) were based on this technology, and allowed the spectrum of one sound (in the Carlos example a synthesizer) to be controlled by that of another (a voice). In the Max/MSP examples folder, there is an example by Marcel Wierckx called “classic_vocoder.pat” (located in the “effects” sub-folder) which shows how this traditional vocoder works. Unlike the classic vocoder, which is based on bandpass filters, the phase vocoder is based on a Short-Term Fourier Transform (STFT) – a Fourier Transform performed sequentially on short segments of a longer sound – and in practical use has little to do with the hardware vocoders of the 1960s and 1970s. The phase vocoder can, however, be considered a type of vocoder because the Fourier Transform returns a set of amplitude values for a set of frequency bands spaced evenly across the sound’s spectrum, similar to the older vocoder’s set of bandpass filters. Of course the phase vocoder, as it’s name suggests, not only takes into account the amplitude of these frequency bands, but also the phase of each band.