FA-91545 / Digital signal filters / Member archive

Polyphase decimator splits the filter into contiguous blocks · case 05

Long filters produce outputs that do not match filter-then-downsample.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

Input [h, D, samples] (integers). Output y[m] = sum_k h[k] x[mD - k] for m = 0 .. ceil(len/D) - 1 with x = 0 outside the signal, computed through the polyphase split E_p[j] = h[jD + p].

Why this case matters

Polyphase decimators avoid computing discarded outputs; commutator and phase-split slips alias the output.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: random polyphase 10	`[9, 9]`	`[9, 3]`	Failed

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