DAFX: Digital Audio Effects, Second Edition

3.1 Introduction

The word modulate means to impress a modulator signal on a carrier signal. This can be accomplished by modifying the amplitude and/or the phase of the carrier signal by means of the modulator. The typical application domain of modulators are communication systems, where a low-frequency information-bearing signal modulates a radio-frequency carrier to obtain a signal with a frequency range suitable for transmission.

Numerous techniques have been designed to achieve this goal, some of which have found applications in digital audio effects. An interesting historical review of such audio effects can be found in [Bod84]. In the field of audio processing, these modulation techniques are mainly used with modulators having variations of very low frequency (up to 20 Hz), so that these variations are perceived as temporal fluctuations rather than a continuous sound, while the carrier is situated in the audible frequency region.

To gain a deeper understanding of the possibilities of modulation techniques, we will first introduce simple schemes for amplitude modulation, single-side-band modulation and phase modulation and point out their use for audio effects. We will then describe several demodulators, which extract parameters of the incoming signal for further effects processing. The combination of these techniques will lead to more advanced digital audio effects, which will be demonstrated by several examples.