This section just scratches the surface, there are many more commonly used filters. To go further, it would be helpful to look at the following.
- the state variable filter
- the biquad filter
- Robert Bristow-Johnson’s biquad filter cookbook
- IIR and FIR filters
- example filter code on musicdsp.org
If you want to go beyond using filters in your code, you will need to study the signal processing mathematics. It isn’t easy, but now that you are using digital filters in your code, you should have the motivation to dive in.
- One of many online courses in DSP
- Julius O. Smith’s excellent “INTRODUCTION TO DIGITAL FILTERS
WITH AUDIO APPLICATIONS” - UCSD’s MUS270A (occasionally undergrads are given permission to enroll)