## 2 Theoretical Background
### ECG Data
#### 2.1.2 Preparation of the ECG Signals  
The ECG signals from the SCDH database were monitored for 24 h utilizing a sampling frequency of 250 Hz [13]; nevertheless, these signals have been resampled to 128 Hz (this procedure is accomplished by convolving the original ECG signal with a digital low-pass filter) in order to maintain consistency with the sampling frequency value of the NSR database. It is worth noticing that from the 24 h monitored for each subject, uniquely the first 30 min before SCD are extracted and evaluated in this work. The selected time window  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Patient**} & \multirow{2}{*}{**Gender**} & \multirow{2}{*}{**Age**} & \multicolumn{2}{c}{**Ventricular Fibrillation**} & **Subjacent** \\  & & & & **Onset Time** & **Cardiac** \\  & & & & **(Hours:Minutes:Seconds)** & **Rhythm** \\ \hline
1 & Male & 43 & 07:54:33 & Sinus \\
2 & Female & 72 & 13:42:24 & Sinus \\  & & & & & Sinus with \\  & & & & & sporadic \\
3 & Unnamed & 62 & 16:45:18 & demand \\  & & & & ventricular \\  & & & & pacing \\
4 & Female & 30 & 04:46:19 & Sinus \\
5 & Male & 34 & 06:35:44 & Sinus \\
6 & Female & 72 & 24:34:56 & Atrial fibrillation \\
7 & Male & 75 & 18:59:01 & Atrial fibrillation \\
8 & Female & 89 & 01:31:13 & Atrial fibrillation \\
9 & Unnamed & — & 08:01:54 & Sinus \\
10 & Male & 66 & 04:37:51 & Sinus \\
11 & Male & – & 02:59:24 & Sinus \\  & & & & Sporadic \\
12 & Male & 35 & 15:37:11 & ventricular \\  & & & & pacing \\
13 & Male & – & 19:38:45 & Sinus \\
14 & Male & 68 & 18:09:17 & Sinus \\
15 & Female & – & 03:41:47 & Sinus \\
16 & Male & 34 & 06:13:01 & Sinus \\
17 & Male & 80 & 02:29:40 & Sinus \\
18 & Female & 68 & 11:45:43 & Atrial fibrillation \\  & & & & Sinus with \\
19 & Female & 67 & 22:58:23 & sporadic pacing \\
20 & Female & 82 & 02:32:40 & Sinus \\ \hline \hline \end{tabular}
\end{table}
Table 1: General information patients provide by SCDH database.  
permits investigating and somehow comparing other works or investigations reported in the literature. The analyzed time window, i.e., 30 min, in this work is separated into 1 min lapses as follows: the first 1 min interval before the SCD onset, the second 1 min interval before the SCD onset, and so on. On the other hand, the control group (CG) is obtained by randomly extracting 1 min intervals from the NSR database. It is worth noticing that the length of the intervals, 1 min, is chosen because this time window enables the detection of reliable characteristics in the ECG signals in order to associate them with different phenomena [14; 16; 20; 21]. For this motive, the 1 min lapse is also investigated in this work in order to forecast until 30 min before an SCD episode. Figure 1 illustrates the monitored ECG signal for a subject with a normal cardiac rhythm (Figure 1b) and the first and second 1 min intervals prior to an SCD episode (Figure 1a), respectively. According to this figure, it is worth noticing that no one difference is visually obtained between both conditions; then, it is essential to provide a new methodology or method with the potential to identify suitable features in ECG signals in order to associate them with an SCD prediction.