24/10/03, pm-n.

Sound Manipulation in the Frequency Domain,

- a phase vocoder approach.

This page compiles material from the project "Sound Manipulation with a musical purpose" conducted by:

Peter Møller-Nielsen
Computer Science Dep.
Aarhus University
pmn@daimi.au.dk

and

Steffen Brandorff
Information and Media Studies
Aarhus University
sbrand@imv.au.dk

Time-Stretching with a Time Dependent Stretch Factor.

These notes describe how we implement time-stretching with a time dependent stretch factor using phase vocoder techniques. We have extended the classical time-stretching with a constant stretch factor to include a time dependent stretch factor. The stretch factor can be infinite, in which case the sound is "frozen" in time. Furthermore, the stretch factor can be negative which means that the input is played backwards. In other words, a sound is produced by sweeping forwards and/or backwards over the original sound with varying speed but keeping the pitches unchanged.

This is a rather detailed and mathematical oriented description of the techniques that underlie the MATLAB functions in pVoc. Emphasis is put on conversion between time- and frequency domain.

pVoc is a set of MATLAB-functions for manipulation of digitized sounds in the frequency domain. The functions form a testbed for new applications and algorithms. Most of the functions are designed to be building blocks for a variety of applications and experiments. However, a few applications are included to exemplify the use of the building blocks.
The section Getting started explains the most common use of the functions by means of a sample session. Sound examples are included. Some of the algorithms - especially those for conversion between time- and frequency domain - are described in Sound manipulation in the Frequency Domain.. The use of MATLAB puts a limit on the speed of the applications and the duration of the manipulated sounds. An application must be reprogrammed in C if speed or capacity is essential.

TimbreAnalysis is a set of MATLAB-functions for analysis of the temporal development of the spectral contents of a digitized sound.
The main purpose of timbreAnalysis is to plot the temporal development of the amplitudes in the spectra under various tunings and calibrations.

This is a Master's Thesis by Rune Orsval, Jan. 2001. ( In Danish )

The paper describes the development of a multi channel real time phase vocoder for musical applications. The main focus of the project is on achieving high quality sound. The paper discusses the prerequisits to achieving real time, it examines various algorithms for use in the phase vocoder, and it describes a test of the sound quality produced with the chosen algorithm. The structure of the phase vocoder program is decided upon, and the phase vocoder is implemented. Finally a series of measurements is presented in order to investigate if indeed the phase vocoder programme is capable of real time operation, and if so how many channels it can handle in real time. Both the tests of the quality of the sound produced and the real time capabilities show encouraging results.

P. Møller-Nielsen, Steffen Brandorff,
Phase Vocoder - et computerværktøj for musikere og komponister.

This booklet ( in danish ) describes the uses and underlying techniques of the phase vocoder for musicians and composers.