Unless a loudspeaker uses a single full-range driver, a crossover will be needed to split up the audio spectrum into discrete bands of frequencies. But should the crossover accomplish this task passively or actively? And if actively, with solid-state devices or with vacuum tubes?By applying two or more filters to the input signal, crossovers give each loudspeaker driver its own band of frequencies in which to operate. The filters used come in two fundamental types: digital and analogue, with the latter further subdividing into either passive or active.Each style of filter has its advantages and disadvantages. At extremely high frequencies, where active devices become bandwidth limited, passive filters work best.