Beyond SDR: How Music Source Separation Reshapes Rhythm-Relevant Signal Properties
Music source separation (MSS) is increasingly used not to remix music but to measure it: separated drum stems feed studies of microtiming, dynamics, and groove. The field evaluates separators almost exclusively by signal-to-distortion ratio (SDR), yet microrhythm research shows that a sound's perceived temporal location (its p-centre) is co-determined by its attack and envelope, precisely the properties SDR was not designed to protect. We quantify what four open separators spanning four architecture generations (Spleeter, HT-Demucs, BS-Roformer, SCNetXL) do to rhythm-critical signal properties, using the 50-track MUSDB18-HQ test set, where true stems make every claim falsifiable. Three findings emerge. (1) Onset timing is safe: onset F-measure tracks SI-SDR (Spearman rho = 0.62) and is invariant to input length. (2) Transient and dynamic shape are not: their distortion correlates only weakly with SI-SDR (|rho| <= 0.29), and the model ranking inverts - the SDR leader distorts drum attacks twice as much as its capability-matched CNN counterpart, while the SDR-worst model preserves dynamics better than a mid-pack one. Each model imposes a systematic, model-specific bias on the dynamic profile. (3) Input length reshapes the rendered attack of a fixed passage (marginally more for the transformer; paired p = 0.044) while leaving onset locations untouched. For rhythmic studies, separator choice and input conditions are methodological variables to be reported, and SDR alone cannot stand in for them.