ABSTRACT
This chapter discusses most of the delay filter examples which will be designed for an 80 usec delay. It examines the use of Bessel lowpass filters to delay signals. In the analogue domain, delay equalisation is performed by using allpass filters, which have a flat amplitude-frequency response but a phase-shift that varies with frequency. Allpass filters are sometimes used to flatten the delay curve of a high-order lowpass or highpass filter. The delay required in a typical crossover is too long to be effectively provided by a single 1st-order allpass filter, because the longer the delay used, the lower the frequency at which that delay starts to roll off. Second-order allpass filters have the advantage that people can get a flatter delay response using fewer components than the cascaded-1st-order approach. The delay-line design may hopefully be of use to those wishing to explore delay-based crossovers.
