Minimax design of factorable Nyquist filters

The generation of zero-phase FIR (finite impulse response) low-pass Nyquist filters that can be split into minimum- and maximum-phase parts is considered. Two factorable minimax design methods are given. These methods use the McClellan-Parks algorithm as a first step to control the stopband behavior. The time domain constraints, imposed by solving a linear system of equations, determine the passband response. The final filter exhibits equiripple stopband behavior. The advantages of these methods are that the minimum- and maximum-phase parts are obtained without direct factorization and that arbitrary frequency weighting can be easily incorporated to allow for a nonequiripple behavior. The design approach is compared with other methods, and a practical design that conforms to a CCITT voiceband modem specification is shown.