Algorithm 1: Proposed Fetal Heart Rate Extraction Algorithm

Input: D, fetal heart sound data with the time of T seconds, H, peak margin matrix, F0 = 0, t = 0, w = 4. 1. the fetal heart sound data were obtained from t to t + w in D; 2. sampling down, from 8 k to 1 k; 3. find out the position of m peaks; 4. calculate the interval of m−1 peak and sort them by size (largest to smallest) to obtain matrix P; 5. calculate the time difference of m−2 peak interval and obtain matrix E; 6. find out the index i, correspond to the minimum in E; 7. set $E\left(i\right)=+\infty$, put P(i) and P(i + 1) into matrix H; 8. repeat step 5 and 6 until $min\left(E\left(i\right)\right)=\infty$; 9. calculate the mean of element in matrix H, At = mean(H); 10. calculate the fetal heart rate in window: ft = 60/At; 11. modify the fetal heart rate $F_t=\frac{\beta ·F_{t-1}+\left(1-\beta \right)·f_t}{1-{\beta }^t}$; 12. move window backward: t = t + 1; 13. repeat 1–11 until t + w > T; Output: F0, F1, L, Ft−1.