ABSTRACT
The Infinite Impulse Response (IIR) Filter gets its name because if readers throw a single amplitude value at an IIR filter followed by digital silence, the output tails off forever, or at least until all the filter states fall into the noise floor. The impulse response length is governed by the amount of feedback. A designer may want to create a higher-order filter using the same method. They will just add more poles and zeros to suit—and lo and behold, the transfer function will be a ratio of higher-order polynomials. Robert Bristow-Johnson worked out a cookbook of formulas for approximating the classic Butterworth-shapes using bi-quadratic filter structures, and no doubt these formulas have since been implemented by expedient DSP coders many, many times over. Hal Chamberlin’s approach was significantly different because he was already very familiar with analog filter design and instead attempted to upgrade those circuits to digital ones using a component-based approach.
