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Signal Processing in JETDSP

This on-line tutorial is aimed at people who have already used JetDSP, and now want to learn more about signal processing. It will cover two main areas,
  1. Looking at how to carry out existing jetdsp functions, including
    • all IDL functions
    • additional functions written by the development team.
  2. Adding new functions.

JETDSP allows the user to access the signal processing capabilities of IDL. The interface for this access is the signal processing dialogue. At the bottom are four buttons; 'Intepolation', 'Create Signal', 'Done' and 'Help'.The layout of the central part is determined by which button on the top row is selected. These three buttons also specify the type of signal processing expression you want to use. These are

  • Simple Expression. Shortcuts to IDL expressions to increase ease of use.

  • IDL Expression. Entering expressions for data, time and x using IDL expressions.

  • IBM Expression. Mainly for backwards compatability, the three boxes are identical to those on the IBM panel.

To start with load up three signals...

  • 46000/MAGN/IPLA
  • 46001/MAGN/IPLA
  • 46000/MAGN/ITOR

Plot these three signals. The 'Create Processed Signal' dialogue can be opened in two ways. This time it will be opened from the plot window, and later it will be opened from the main window.

Open it using the menu option 'File->Add Processed Signal', select 'IDL Expression', enter the following text and then click on 'Create Signal'.

This will have created a new signal with the same time axis as 'ITOR' but with the data scaled by a factor of 50.

This could also be done using the 'Simple Expression' dialogue. This time name the signal 'ITOR2' and the droplists should be set as follows.

This creates an identical signal to the previous signal.

To create a signal called DIFF using two existing signals, IPLA and ITOR1, then either set up the IDL dialogue or the simple dialogue as below.

All the new signals should have been plotted and also added to the main JETDSP window. The new signals can be used in the same way as any other signal in JETDSP. The 'Interpolation...' button is described later. This example uses the default interpolation settings. The 'Done' button closes the dialogue.

In general, JETDSP supports the following syntax for signal processing :

NEWSIGNAL = F(OLDSIGNALS) [&NEWSIGNAL_TIME=F(OLDSIGNALS)] [&NEWSIGNAL_X=F(OLDSIGNALS)]

where F(OLDSIGNALS) is a command that IDL recognises and newsignal is the name of your new signal.

The new signal can have any name that you like, provided that you don't use an IDL reserved keyword (in the same way that you are not allowed to have a FORTRAN variable called REAL). IDL reserved keywords are things like IF, THEN, END, BEGIN, LONG, FLOAT, NE, EQ. The only addition to this is that its not a good idea to use non-alphanumeric characters (e.g. symbols like $, #, +, =, -, *) in the name of signals since these are also used as special symbols by IDL. If you use a name that already exists, JETDSP will append a number to it, so you will get things like IPLA, IPLA1 and IPLA2.

Read the following signals.

  • 46000/LIDR/NE
  • 46000/CXSM/FLUX
  • 46000/CXSM/RCOR

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