Table 4.

Hardware-based methodologies for detecting stress from PPG signals.

Study	Signal Pre-Processing	Stress Detection Methodology
[12]	Butterworth low pass filter (2 Hz). Distribution filter, threshold detection	RR interval detection with time domain analysis
[30]	Finite Impulse Bandpass (0.8 Hz- 10 Hz)	Machine Learning
[15]	Low pass filter (5 Hz). Linear extrapolation based on rolling average	RR interval detection with time domain analysis
[18]	Butterband band pass (0.7 Hz–3.5 Hz); moving average filter	Machine Learning
[16]	Cubic spline interpolation on clipped signals, Savitsky-Golay filter	RR interval detection with time domain analysis
[26]	Savitsky-Golay filter	Support Vector Machine
[7]	Moving average filter	RR interval detection with time domain analysis
[27]	Normalization via subtracting baseline signal	T-test
[13]	3 stage band pass (0.5 Hz–11 Hz, 0.8 Hz–3 Hz, 0.9 Hz–1.6 Hz)	Sliding window RR detection with time domain analysis
[22]	Moving average filter	Sliding window PPG and PPG velocity signal analysis
[21]	None	RR interval detection
[17]	Noise removal via least mean squares	HR frequency analysis using IIR Bandpass filter and sinusoidal modeling.