DAFX: Digital Audio Effects, Second Edition

2.3 Equalizers

Introduction and Musical Applications

In contrast to low/highpass and bandpass/reject filters, which attenuate the audio spectrum above or below a cut-off frequency, equalizers shape the audio spectrum by enhancing certain frequency bands while others remain unaffected. They are typically built by a series connection of first- and second-order shelving and peak filters, which are controlled independently (see Figure 2.15). Shelving filters boost or cut the low- or high-frequency bands with the parameters cut-off frequency fc and gain G. Peak filters boost or cut mid-frequency bands with parameters center frequency fc, bandwidth fb and gain G. One often-used filter type is the constant Q peak filter. The Q factor is defined by the ratio of the bandwidth to center frequency . The center frequency of peak filters is then tuned while keeping the Q factor constant. This means that the bandwidth is increased when the center frequency is increased and vice versa. Several proposed digital filter structures for shelving and peak filters can be found in the literature [Whi86, RM87, Dut89a, HB93, Bri94, Orf96, Orf97, Zöl05].

Figure 2.15 Series connection of shelving and peak filters.

Applications of these parametric filters can be found in parametric equalizers, octave equalizers ...