Table 4.

Comparison of the signal preprocessing methods of the proposed apnea system with the previous studies.

Reference	Signal Preprocessing Methods
Feature-Learning-Based Systems
Our Study	Bandpass Filtering + Z-score Normalization
Singh and Majumder [15]	Bandpass Filtering + Continuous Wavelet Transform + Zerocenter Normalization
Wang et al. [16]	FIR Filtering + R-peaks Detection + RR Interval Calculation + Median Filtering + Cubic Interpolation
Li et al. [17]	Bandpass Filtering + R-peaks Detection + RR Interval Calculation + Median Filtering + Interpolation
Feature-Engineering-Based Systems
Sharma and Sharma [12]	Bandpass Filtering + R-peaks Detection + RR Interval Calculation + QRS Complex Extraction + Zero Padding
Song et al. [13]	Filter-Bank-Based R-peaks Detection + RR Interval Calculation + Median Filtering + EDR Derivation
Varon et al. [14]	Notch Filtering + DC Component Remove + Up Sampling + R-peaks Detection + R-peaks Correction + RR Interval Calculation + EDR Derivation

FIR: Finite impulse response; DC: Direct current; and EDR: ECG-derived respiration.