Modulation and Delay Line Based Digital Audio Effects

In the field of musicians and recording engineers audio effects are mainly described and indicated by their acoustical effect. Audio effects can also be categorized from a technical point of view. The main criterion is found to be the type of modulation technique used to achieve the effect. After a short introduction to the different modulation types, three more sophisticated audio effect applications are presented, namely single sideband domain vibrato (mechanical vibrato bar simulation), a rotary speaker simulation, and an enhanced pitch transposing scheme. 1. EFFECT CLASSIFICATION Many audio effects are based on mixing the original signal with delayed and/or amplified copies of it. The delay is implemented in integer multiples of the unit delay. If the delay is greater than one unit delay, the chain of unit delays is referred to as delay line. If the delay time and the corresponding coefficients are constant, the well known FIR (finite impulse response) and IIR (infinite impulse response) filters are the result. In the case of time varying filters the coefficients are time variant, but the delays stay fixed. The resulting filter is a superposition of amplitude modulated (AM) and delayed copies of the original signal. If the coefficients are fixed and the delays are variable, audio effects like vibrato, flanging and chorus are built. The variation of the delay length is a phase modulation (PM) and its dynamic component is perceived as a frequency modulation. Some more elaborated effects use both types of modulation and will be presented in the following sections. 2. MODULATION 2.1. Amplitude Modulation Performing amplitude modulation (AM) in the digital domain requires just one multiplication, as shown in Fig. 1. The audio input signal x(n) is amplitude modulated by the modulation signal m(n). The classical amplitude modulation is expressed as m(n) = 1 + sin(!cnT ) 0 < < 1 (1) where is the modulation depth. The well known ring modulation is given by m(n) = sin(!cnT ): (2) m(n) x(n) y(n) Figure 1: Amplitude modulation.