Chapter 5. Process a Signal

After opening the ecg in the previous chapter, we want now to find the postions of the beats in the ECG. libRASCH supports this by providing a process plugin ('detect-simple') which performs a beat detection. Additionally we want to group the found heart beats in similar looking groups (templates) and we want to calculate some ECG specific values (e.g. the intervals between each heart beat, called RR intervals). The later two steps will be done in the process plugins 'template' and 'ecg' respectively.

In RASCHlab, all the above mentioned steps will be performed when selecting the menu entry 'Detect->Detect'. After selecting the menu entry, the channel selection dialog box will be opened (Figure 5-1) and the user has to select the channels in which the heart beats should be searched. (At the moment all available channels will be selectable, but this will be changed in the future, showing only the channels which makes sense.) To select the channels, click on the channel-name on the left side and then click on the '->' button. This will move the channel-name to the right side. Figure 5-2 shows the dialog box after selecting both channels.

After selecting the channels, press the 'OK' button. Now the beat detection, template grouping and the calculation of the ECG specific values will be done. For each processing step, a progress dialog will be shown. When all the processing is done, a window with ecg and the found beats and a window with the templates will be shown (Figure 5-3).

Note that the detection of heart beats are not limited to file formats which are used to store only ECG's. RASCHlab do not care which file format was used to store a signal. For example when you have an EDF file with ECG and EEG signals, you can perform the beat detection the same way as described above. As written above, at the moment the user has to be sure to select the correct channel containing an ECG recording.

RASCHlab allows you to correct the automatic evaluation of the ECG. A description how you can change an evaluation of an ECG is given in Section 9.1.

Evaluations created with RASCHlab are stored on disk at the same position where the signal data is stored. The data is stored in a file named using the name of the recording and for the file extension '.mte' is used. For example the evaluation created above is stored in the file '100s.mte'. The evaluation is stored using XML. The user can control with the menu entry 'General->Save ASCII eval-values' if array data (e.g. the positions of the heart beats) is stored as ASCII text (menu entry with check mark) or as binary (menu entry without check mark). When storing the data as ASCII text, it can be used easily with other programs. On the other side, when a lot of events are in an evaluation (e.g. heart beats in a 24 hour ECG), loading and saving took a lot of time. For this type of signals, the binary format is recommended.