Bandpass Filtering for PPG Preprocessing
Bandpass filtering is the fundamental preprocessing step for PPG, passing the cardiac frequency band (typically 0.5–8 Hz for HR, 0.1–0.5 Hz for respiratory rate) while rejecting DC offset, baseline wander, high-frequency noise, and powerline interference. Filter design must balance noise rejection against pulse waveform fidelity.
Standard PPG bandpass filter specifications depend on the target application. For heart rate extraction: 0.5–5 Hz passband captures the cardiac fundamental and 2–3 harmonics. For morphological analysis: 0.1–15 Hz preserves waveform shape including the dicrotic notch and high-frequency features. For respiratory rate extraction: 0.1–0.5 Hz isolates the respiratory modulation band. Cutoff frequencies below 0.5 Hz introduce baseline wander; cutoffs above 15 Hz pass high-frequency noise without adding useful cardiac information.
Butterworth IIR filters (order 2–4) are the most common choice for real-time PPG, offering maximally flat passband response and reasonable computational efficiency (2N multiply-accumulate operations per sample for order N). Chebyshev Type I filters provide sharper cutoff at the cost of passband ripple. FIR filters (order 50–200) provide linear phase response essential for morphological analysis but require higher computational resources and introduce group delay of N/2 samples.
Zero-phase filtering (applying the filter forward then backward) eliminates phase distortion for offline analysis, producing undistorted pulse waveforms. This approach is not applicable for real-time processing, where minimum-phase IIR filters are used with known, correctable group delay. For real-time clinical monitoring, filter settling time must be considered: a 4th-order Butterworth with 0.5 Hz cutoff requires ~4 seconds to settle after step inputs.
Frequently Asked Questions
What are the best cutoff frequencies for PPG heart rate?
0.5 Hz high-pass and 5 Hz low-pass (or 8 Hz for harmonic preservation) are standard for heart rate extraction. For neonatal PPG, the high-pass cutoff should be raised to 1 Hz to match higher heart rates (120–180 bpm).
Should IIR or FIR filters be used for PPG?
IIR (Butterworth, order 2–4) for real-time embedded applications due to computational efficiency. FIR (order 50–200) for offline analysis requiring linear phase. Zero-phase IIR for offline morphological analysis.
Does filtering affect HRV measurements?
High-pass filtering above 0.5 Hz removes respiratory modulation needed for LF and HF HRV analysis. For HRV applications, use high-pass at 0.05 Hz or lower. IBI extraction requires only peak detection, which is relatively robust to moderate filtering.