To eliminate the images of Y(e) in the region outside 22 kHz, a sharp cutoff
(hence nonlinear phase) analog low-pass filter is required.
This problem is avoided by using an interpolation filter, as
in Fig. 7(b),which increases the sampling rate digitally.After
this, DIA conversion i s performed followed by analog filtering.
The interpolation filter H(z) is once again a linear phase
FIR low-pass filter and introduces no phase distortion.