The desirable characteristics of a crossover system are easy to state, but not so easy to achieve in practice. There is a general consensus that there are five principal requirements that apply to all crossovers, be they active or passive, and that they should be ranked in order of importance thus:
1. Adequate flatness of summed amplitude/frequency response on-axis
2. Sufficiently steep rolloff slopes between the output bands
3. Acceptable polar response
4. Acceptable phase response
5. Acceptable group delay behaviour
To some extent, the amount of space devoted in this book to each requirement is dependent on their relative importance as set out in this list.