Audio effects based on AM/FM decomposition

Author: Antonio José Homsi Goulart

Advisor: Marcelo Queiroz

This research explores the design of audio effects based on decompositions that take a music instrument signal and unravel it to the AM/FM representation. The AM/FM decomposition produces a pair of signals, also in the time domain, that represent the analyzed signal’s envelope (AM portion) and instantaneous frequency (FM portion). This pair of signals act jointly, and when used to modulate an oscillator’s amplitude and frequency are able to reconstruct the original signal. However, by manipulating the AM and FM portions new possibilities arise for signal processing and implementation of musical effects.

In this work we discuss aspects of different techniques for the decomposition, either based on the Hilbert Transform, which results in an Analytic Signal, or on the Teager-Kaiser Energy Operator which is applied in the Energy Separation Algorithm. We present novel audio effects based on regular filtering (low-pass, band-pass, high-pass), dynamics processing (distortion, compressor, expander) and classic effects (tremolo, flanger, octaver, chorus) inspired manipulations of the AM and FM portions. Implementations for real-time operation are presented and discussed.

An experimental evaluation of the proposed effects was also realized, which compares audio descriptors related to amplitude (dynamics variation) and frequency (spectral variation) from the original and processed signals.

The publications related to this research work are:

GOULART, A.J.H..; QUEIROZ, M.; TIMONEY, J.; LAZZARINI, V. CPU consumption for AM/FM audio effects. Linux Audio Conference, Stanford, U.S.A., 2019.

GOULART, A.J.H..; QUEIROZ, M.; TIMONEY, J.; LAZZARINI, V. Interpretation and control in AM/FM-based audio effects. International Conference on Digital Audio Effects (DAFX), Aveiro, Portugal, 2018.

GOULART, A.J.H..; TIMONEY, J.; LAZZARINI, V. AM/FM DAFx. International Conference on Digital Audio Effects (DAFx), Trondheim, Norway, 2015.

GOULART, A.J.H..; TIMONEY, J.; LAZZARINI, V.; QUEIROZ, M. Psychoacoustic impact assessment of smoothed AM/FM ‘303’ resonance signals. In: Sound and Music Computing Conference, Maynooth, Ireland, pp. 193-200, 2015.

GOULART, A.J.H..; QUEIROZ, M.; TIMONEY, J.; LAZZARINI, V. Sound Synthesis with Periodic Linear Time-Varying Filters. In: Proceedings of the Linux Audio Conference 2015, Mainz, Germany, pp. 55-62, 2015.