Digital Audio Signal Processing, Second Edition

Chapter 5

Equalizers

Spectral sound equalization is one of the most important methods for processing audio signals. Equalizers are found in various forms in the transmission of audio signals from a sound studio to the listener. The more complex filter functions are used in sound studios. But in almost every consumer product like car radios, hifi amplifiers, simple filter functions are used for sound equalization. We first discuss basic filter types followed by the design and implementation of recursive audio filters. In Sections 5.3 and 5.4 linear phase nonrecursive filter structures and their implementation are introduced.

5.1 Basics

For filtering of audio signals the following filter types are used:

• Low-pass and high-pass filters with cutoff frequency f_c (3 dB cutoff frequency) are shown with their magnitude response in Fig. 5.1. They have a pass-band in the lower and higher frequency range, respectively.
• Band-pass and band-stop filters (magnitude responses in Fig. 5.1) have a center frequency f_c and a lower and upper f_l und f_u cutoff frequency. They have a passand stop-band in the middle of the frequency range. For the bandwidth of a band-pass or a band-stop filter we have

  

  Band-pass filters with a constant relative bandwidth f_b/f_c are very important for audio applications [Cre03]. The bandwidth is proportional to the center frequency, which is given by (see Fig. 5.2).

• Octave ...