ABSTRACT
This chapter deals with lowpass and highpass filters. Active filters are the building blocks of active crossovers. Allpass filters are so-called because they have a flat frequency response, and so pass all frequencies equally. Brickwall filters may be lowpass or highpass; their distinguishing characteristic is a very fast roll-off that looks almost vertical on the response plot. The cutoff frequency of a lowpass or highpass filter is a measure of where in the spectrum its roll-off starts. Strictly speaking, the order of a filter is the highest power of frequency that occurs in the complex algebraic equation that describes its behaviour. A filter made up only of cascaded 1st-order stages with the same cutoff frequency is called a synchronous filter. Second-order bandpass responses are basically all the same, being completely defined by centre frequency, Q, and gain. The chapter looks at the amplitude response, the phase-shift response, the response to a step input and the group delay curve.
