Specifications of ideal prototype filters for designing linear phase FIR filters

To design a linear phase FIR filter using a window technique, the infinite impulse response of an ideal filter to approximate a desired specifications must be truncated to a finite length. The parameters associated with this technique are the length of the impulse response and the type of the truncation window. We discuss another parameter, the specification of the frequency response of the ideal prototype filter. We show that a given specification may be met with a shorter length of the truncated impulse response, if the ideal filter is properly specified.