10.8. Two-Stage Interpolation

Because we’re now familiar with the notion of polyphase filtering, we’re ready to consider the process of two-stage interpolation. When a desired interpolation factor L is large, say L > 20, significant interpolation filter computational savings may be had by implementing the interpolation in Figure 10–15(a) in two stages as shown in Figure 10–15(c). In the later figure we interpolate input sequence x_old(n) by integer factor L₁ followed by interpolation by integer factor L₂, where L = L₁L₂.

Figure 10–15. Interpolation: (a) single-stage; (b) lowpass filter (LPF) magnitude response and upsampled x_int(m) spectrum; (c) two-stage interpolation; (d) two-stage polyphase interpolation.