6. Basic signal processing
This chapter explains how to transform signals with a set of basic processing. They all apply to the active signal viewer.
This chapter contains the following sections:
- 6.1. DC Removal
- 6.2. Frequency filters
- 6.3. Automatic Gain Control
- 6.4. Fast Fourier Transform
- 6.5. Tapering signals
- 6.6. Cutting signals
- 6.7. Merging signals
- 6.8. Subtracting signals
- 6.9. Rotate components
Some of these processing either modify existing signals or create new temporary signals. Each processing is performed in a specific domain (e.g. time of frequency). If the signals of the active viewer are not in the rigth domain, they are internally converted with the Fast Fourier Transform before processing. The initial domain is restored after processing.
Modified signals
On opening a database, each signal (either orignal or not) belong to one unique file. The amplitude vectors are copied into memory only upon request (e.g. calculating MAXAMPLITUDE in a table, viewing signals in a graphic). When a process modifies the signal (e.g. a filter), only the amplitude vector in memory is affected. In all cases, the original file remains unchanged. The signal is marked as modified.
For those signals, it is possible to erase the changes and to reload the amplitude vector stored in the disk file. Select "Revert to saved" from menu "Waveform". The signal is then marked as untouched.
On closing a database, if there are still modified signals, a warning will be issued to remind you to save the changes. When the database is saved, the modified signals are saved in the database directory with a binary format. Those signals become individual new signals in the database (not specifically linked to the original signals). They are just stored in a file name similar to the original file name (decorated by "_n", n being unique).
Temporary signals
Some processing create new temporary signals (e.g. cut). These signals have no storage on the disk to revert from. They are totally lost, without warning, on closing database, nor they are saved when you save a database. Temporary signals are easily recognized in a table which displays the FileName. The file name is set to "### Temporary signal ###".
If you want to check if there are temporary signals in your opened database, Drag "All signals" from "Files" list to a table. Look at the FileName field.
To export those files to a disk file, create a viewer with them gathered and select "Export ..." for menu "File". Select the appropriate format in the list. If you want to include them permanently in the database, "SAC" format is a good choice. Load the generated file(s) into the database and save the database. Note: the exported files must not be in a temporary location, create them in a permanent directory (e.g. the database directory).
