## 4 Results
### Signal Processing Results
#### 4.2.3 Connection Between SNR and BPM Differences  
The scatter plots shown in Figure 9(a),b depict the BPM differences against the SNR values for two signal processing methods--the newly developed 'BPM DL' and the MATLAB's built-in function 'findpeaks', each applied using both BPF and Savitzky-Golay filters. The BPM differences are calculated according to Equation (20), while the calculation of SNR values is described in [23]. Ideally, a higher quality EBI signal would result in smaller differences between the BPM values obtained as the BPM difference from the EBI and PPG (ground truth) signals. In order to quantitatively compare BPF and Savitzky-Golay filtering methods, the centroid of the point cloud in the SNR-BPM difference plane was calculated for both filtering methods. These centroids provide an indication of the overall performance trend within each dataset, allowing for easier comparison between filtering techniques (shown as a diamond symbol in Figure 9(a)). Also, a linear regression line has been added to each dataset to illustrate their respective trends. The centroid of the band-pass filter with the 'BPM DL' method shows a BPM difference of 4.18 and an SNR of 2.61 dB, demonstrating its superior accuracy. In comparison, the 'findpeaks' method with the same filter has a BPM difference of 17.30, which is more than four times worse despite having a nearly identical SNR of 2.63 dB. When using the Savitzky-Golay filter, the 'BPM DL' method again performs better, with a BPM difference of 11.51 and an SNR of 1.8 dB. However, the 'findpeaks' method exhibits a much larger BPM difference of 29.37, making it more than two and a half times less accurate, with a similar SNR of 1.81 dB. These results clearly indicate that the 'BPM DL' method consistently outperforms 'findpeaks', especially in terms of BPM accuracy, regardless of the filtering technique used. We conducted a parametric study to assess the SNR value of the measured EBI signal at different electrode configurations in both bipolar and tetrapolar setups (the results as a colormap in Figure 11). Electrode placements, such as 1-2, 1-3, 1-4, 2-3, and 1-2-3-4, correspond to the specific electrode configurations depicted in Figure 3 and Table 1. The SNR values were calculated for signals that were processed using the BPF (letter B in Figure 11) and Savitzky-Golay (letter S in Figure 11) filters, prior to applying the 'findpeaks' and 'BPM DL' algorithms. Figure 11 provides a clear visualization of which electrode configurations yield the best and worst results across all test subjects and repeated measurements. The lowest SNR values manifest in bipolar configurations A-b10 and A-b17 for all test subjects and electrode sizes. The best results in the bipolar setup manifest using the configuration B-b10 (Table 1). Figure 9: Signal quality estimation results shown as a scatter plot in BPM difference—SNR plane for both signal filtering methods—band-pass signal and Savitzky–Golay. **(a)** The ‘findpeaks’ BPM estimation method; **(b)** the ‘BPM DL’ BPM estimation method. Diamond symbols show centroids of point clouds for both filtering methods and straight lines show the fitted linear regression model according to the color of each dataset. The best results are achieved in tetrapolar configuration with band-pass filtering, illustrated for BPM differences and MAPE values in Figures 12 and 13, respectively. It is evident in Figure 11 that the SNR values vary significantly between the configurations, particularly when BPF is applied. For example, configurations 1-3 and 1-4 show higher SNR values, suggesting a more accurate measured signal in Bipolar electrode setup. In the Savitzky-Golay filter case, the SNR decreases compared to the BPF. While both filters can improve signal clarity, the BPF is generally more effective in enhancing SNR. The variation in SNR across placements highlights the importance of electrode configuration for optimal EBI measurements. Similar to the SNR values shown earlier, Figure 12 illustrates the BPM differences.