FA-91652 / Digital signal filters / Member archive

Linear-phase classifier swaps types 1 and 2 · case 02

An odd-length symmetric filter is labelled type 2.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

Input integer taps. Strip leading and trailing zeros ("degenerate" if all zero). The trimmed support of length N is type 1/2 if symmetric (odd/even N), type 3/4 if antisymmetric (odd/even N), otherwise not linear phase. Return {"type", "group_delay": leading zeros + (N-1)/2 as a fraction string} or both None.

Why this case matters

Linear-phase type determines which responses an FIR can realize (e.g. no highpass for type 2); misclassification picks the wrong design.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: trailing zeros	`{"group_delay": "1", "type": 2}`	`{"group_delay": "1", "type": 1}`	Failed

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