Usage: EDF files and ASCII files

Once you have installed edfAsc, you start the program in the usual way (calling the program from the text interface or clicking the icon). We are going to use the file Calib.rec. This file is also available at edf site.

. To get it follow the link located at the text ..here are two small EDF calibration recordings.. . You have to download and unzip the file edfcalib.zip

• Exporting a signal from en EDF file

  First of all we can glance the header of the file using edfEdit. This is the header

  The screen of edfEdit is shown in figure 3.1

  Figure 3.1: This is header of Calib.rec

  

  A lot of things can be learned from the header of an EDF file. Looking at the header we know that it includes one signal with two data records. Each data record lasts 10 seconds and includes 1000 samples. So it is sampled at 100 Hz. The signal is expressed in $\mu$V. We can also see a lot of information about how the file was obtained.

  Now we want to convert the signal to an ASCII file. These are the steps in edfAsc

  • We select the first tab called EDF > ASC
  • We select the file (in this case Calib.rec)
  • We select the signal that we want to export (In this case there is only one signal called Calibration)
  • We accept the default output name (in this case Calib.rec.txt)
  • We push the button called Export as ASCII

  That's all. A file called Calib.rec.txt was created. Let's see the first ten lines of the file

   
  je@unit1:~/tmp/tutorial> cat Calib.rec.txt | head -n 10
  0.000000e+00    -5.000000e+01
  1.000000e-02    -5.000000e+01
  2.000000e-02    -5.000000e+01
  3.000000e-02    -5.000000e+01
  4.000000e-02    -5.000000e+01
  5.000000e-02    -5.000000e+01
  6.000000e-02    -5.000000e+01
  7.000000e-02    -5.000000e+01
  8.000000e-02    -5.000000e+01
  9.000000e-02    -5.000000e+01
  

  The first column is the time in seconds. In this case the sampling period is 0.01 s (the signal was sampled at 100 Hz). The second column contains the signal in the units detailed in the header. We are going to plot the signal using Scilab. You can find more information on the use of Scilab in Clinical Neurophysiology in our site

  -->signal = fscanfMat("Calib.rec.txt");
   
  -->plot2d(signal(:,1),signal(:,2))
  

  The result is shown in figure 3.2

  Figure 3.2: This is the plot of the file such as it is plotted by Scilab

  

  An ASCII file can be a somewhat inefficient structure. Notice that the ASCII file is ten times bigger than the EDF file.

  je@unit1:~/tmp/tutorial> ls -l Calib.*
  -rw-r--r--  1 je users  4512 2002-10-04 14:47 Calib.rec
  -rw-r--r--  1 je users 52990 2004-10-05 19:58 Calib.rec.txt
  

  It is not only a matter of space. Reading an ASCII file is a very slow process. Because of this reason, edfAsc can also export float values. The resulting file can be read very fast by a lot of programs. On the next chapter we will face this issue some examples of how to import this kind of files in practice.

• Writing an EDF file from an ASCII file

  Now we are going to use the file Calib.rec.txt to create an EDF file called NewCalib.edf. Notice that since the original file contains two columns (time and values) the EDF file will have two signals. These are the steps needed to do it.

  • We select the second tab called ASC > EDF
  • We select the text file (in this case Calib.rec.txt)
  • We define the sampling rate as well as the duration of the data record (in this case we choose the default settings 100 Hz and 10 seconds to get a file similar to Calib.edf, although we could use other different parameters)
  • We define the name of the EDF file (in this case NewCalib.edf)
  • We push the button called Create EDF file

  We can see the result with RASCHlab The screen of RASCHlab is shown in figure 3.3

  Figure 3.3: NewCalib.edf as seen with RASCHlab after some arrangements

  

  Notice that units are not set by the program. The names of the signals as well as other parameters of the header should be modified using edfEdit.

  Also notice that time is included as a line. We should prepare the text file to include/eliminate the signals that we want to include in the EDF file. A simple way to do it is by using awk.

  je@unit1:~/tmp/tutorial> cat Calib.rec.txt | head -n 4
  0.000000e+00    -5.000000e+01
  1.000000e-02    -5.000000e+01
  2.000000e-02    -5.000000e+01
  3.000000e-02    -5.000000e+01
  je@unit1:~/tmp/tutorial> cat Calib.rec.txt | awk '{print $2}' | head -n 4
  -5.000000e+01
  -5.000000e+01
  -5.000000e+01
  -5.000000e+01
  je@unit1:~/tmp/tutorial> cat Calib.rec.txt | awk '{print $2}' > one_channel.txt
  je@unit1:~/tmp/tutorial>
  

  Using grep, awk, sed, head and tail you can easily modify a text file. These applications are also available on Windows OS