In practical implementation of sampling rate converters we must replace
the ideal lowpass filters of equations (9.20), (9.30), and (9.36) by a practical
finite-order filter. The lowpass filter can be designed to have linear
phase, a specified passband ripple, and stopband attenuation. Any of the
standard, well-known FIR filter design techniques (e.g., window method,
frequency sampling method) can be used to carry out this design. We
consider linear-phase FIR filters for this purpose because of their ease
of design and because they fit very nicely into a decimator stage where