ABSTRACT
This chapter covers how to implement and use a variety of common and useful filter types. It discusses how to build the sort of equalizer one might find in his/her digital audio workstation and how to build a crossover that one could use to build a multiband compressor. The biquad filter is simple yet versatile. A biquad is simply a few constants and the last two samples of input and output. In order for the biquad to actually do some useful filtering of the audio that is passed through it, one needs to initialize the coefficients so that the desired transfer function is applied. It is important to note how little state is actually required to represent a biquad. There is no cutoff frequency, sample rate, or quality factor stored here—everything is encoded into the coefficients for pair of quadratic equations, the scalars, and the stored input and output samples.
