CHAPTER 4 Crossover Types There are many types of crossover, classified in various ways. The categories listed here define what the crossover does, not how it does it. It is essentially a catalogue of target functions. It does not matter how the filter responses are obtained; that may be by filtering alone or a combination of filtering and drive unit responses. 4.1 All-Pole and Non-All-Pole Crossovers In discussions of crossover design, and in filter design generally, the terms "pole" and "zero" tend to be freely scattered through the text. These terms relate to the complex mathematical equation that describes the response of a filter or other frequency dependent system such as a servomechanism. A complex equation is one involving j, the square root of -1, and not just a complicated equation. This equation is often called the transfer function; it usually comes in the form of a fraction, and the poles derive from the bottom half (the denominator), while the zeros derive from the top half (the numerator). Both are