Beyond Motion Artifacts: Optimizing PPG Preprocessing for Accurate Pulse Rate Variability Estimation

Wearable physiological monitors are ubiquitous, and photoplethysmography (PPG) is the standard low-cost sensor for measuring cardiac activity. Metrics such as inter-beat interval (IBI) and pulse-rate variability (PRV) -- core markers of stress, anxiety, and other mental-health outcomes -- are routinely extracted from PPG, yet preprocessing remains non-standardized. Prior work has focused on removing motion artifacts; however, our preliminary analysis reveals sizeable beat-detection errors even in low-motion data, implying artifact removal alone may not guarantee accurate IBI and PRV estimation. We therefore investigate how band-pass cutoff frequencies affect beat-detection accuracy and whether optimal settings depend on specific persons and tasks observed. We demonstrate that a fixed filter produces substantial errors, whereas the best cutoffs differ markedly across individuals and contexts. Further, tuning cutoffs per person and task raised beat-location accuracy by up to 7.15% and reduced IBI and PRV errors by as much as 35 ms and 145 ms, respectively, relative to the fixed filter. These findings expose a long-overlooked limitation of fixed band-pass filters and highlight the potential of adaptive, signal-specific preprocessing to improve the accuracy and validity of PPG-based mental-health measures.