Customizable Butterworth Digital Filter

An implementation is given for a low-pass or high-pass digital Butterworth filter of any order. Instead of specifying the cutoff frequency and order, the user specifies variables more relevant to applications. These are the frequency at the edge of the passband, the frequency at the edge of the stopband, the maximum passband attenuation, and the minimum stopband attenuation. Code is in FORTRAN with a complete C translation.