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Computational savings of polyphase interpolation/decimation

Assume that we design FIR LPF H z with N taps, requiring N multiplies per output. For standard decimation by factor M , we have N multiplies per intermediate sample and M intermediate samples per output, giving N M multiplies per output.

For polyphase decimation, we have N M multiplies per branch and M branches, giving a total of N multiplies per output. The assumption of N M multiplies per branch follows from the fact that h n is downsampled by M to create each polyphase filter. Thus, we conclude that thestandard implementation requires M times as many operations as its polyphase counterpart. (For decimation, we count multiplesper output, rather than per input, to avoid confusion, since only every M th input produces an output.)

From this result, it appears that the number of multiplications required by polyphase decimation isindependent of the decimation rate M . However, it should be remembered that the length N of the M -lowpass FIR filter H z will typically be proportional to M . This is suggested, e.g. , by the Kaiser FIR-length approximation formula N -10 10 logbase --> δ p δ s 13 2.324 Δ ω where Δ ω in the transition bandwidth in radians, and δ p and δ s are the passband and stopband ripple levels. Recall that, to preserve a fixed signal bandwidth, the transitionbandwidth Δ ω will be linearly proportional to the cutoff M , so that N will be linearly proportional to M . In summary, polyphase decimation by factor M requires N multiplies per output, where N is the filter length, and where N is linearly proportional to M .

Using similar arguments for polyphase interpolation, we could find essentially the same result. Polyphase interpolation byfactor L requires N multiplies per input, where N is the filter length, and where N is linearly proportional to the interpolation factor L . (For interpolation we count multiplies per input, rather thanper output, to avoid confusion, since M outputs are generated in parallel.)

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Source:  OpenStax, Digital signal processing (ohio state ee700). OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10144/1.8
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