Quantifying Nonlinear Behavior in Digital Moog Ladder Filters: Cross-Implementation Comparison and Common-Core Ablation
Digital Moog ladder filters are often compared by their linear frequency response, even though musically important differences emerge under nonlinear operation. This paper presents an open, reproducible SPICE-referenced evaluation framework that combines two components: a cross-implementation comparison of five digital ladder-filter implementations against the same SPICE reference, and a controlled common-core ablation. The cross-implementation comparison shows that close linear agreement can mask substantial nonlinear divergence, while the ablation shows that ladder nonlinear behavior depends not only on saturator choice and nonlinearity placement, but also on the non-uniform contribution of different ladder stages: in partial ablations of a SPICE-referenced TPT/ZDF core, retaining earlier-stage nonlinearities preserves harmonic behavior better than retaining later-stage nonlinearities alone. Beyond the specific comparisons reported here, the framework provides a reproducible basis for evaluating fidelity–cost trade-offs when nonlinear structure must be simplified.