DAFX: Digital Audio Effects, Second Edition

3.3 Demodulators

Each modulation has a suitable demodulation scheme and we will focus on the ring and amplitude modulations in this section. The demodulator for the ring modulator uses exactly the same scheme as the modulator, so no new effect is to be expected there. The demodulator for the amplitude modulator is called an amplitude follower in the realm of digital audio effects. Several implementation schemes are available, some are inspired from analog designs, some are much easier to realize using digital techniques. These demodulators comprise three parts: a detector, an averager and a scaler.

3.3.1 Detectors

The detector can be a half-wave rectifier dh(t), a full-wave rectifier df(t), a squarer dr(t) or an instantaneous envelope detector . The first two detectors are directly inspired by analog designs. They are still useful to achieve effects having typical analog behavior. The third and fourth types are much easier to realize in the digital domain (Figure 3.9). The four detectors are computed by

3.17

respectively, where x(n) denotes the input signal and its Hilbert transform.

Figure 3.9 Detectors: (a) half-wave, (b) full-wave, (c) squarer, (d) instantaneous envelope.

3.3.2 ...