(b∂d’ ∂r w∂rth) A type of frequency filter characterized by a very flat passband, often used as an alias filter. Sometimes called a maximally flat filter or a trapezoidal filter. The slopes at the edges of the bandpass can be set arbitrarily. Filters can be specified by four numbers a,b,c,d, where b and c specify the frequency bandpass and a and d specify the frequencies that are down 3 dB in amplitude. See Sheriff and Geldart (1995, §15.7.4).