Signal Processing Toolbox

Using SPTool: Filtering and Analysis of Noise

The following sections provide an example of using the GUI-based interactive tools to:

• Design and implement an FIR bandpass digital filter.
• Apply the filter to a noisy signal.
• Analyze signals and their spectra.

The steps include:

• Creating a noisy signal in the MATLAB workspace and importing it into SPTool
• Designing a bandpass filter using the Filter Designer
• Applying the filter to the original noise signal to create a bandlimited noise signal
• Comparing the time domain information of the original and filtered signals using the Signal Browser
• Comparing the spectra of both signals using the Spectrum Viewer
• Exporting the filter design to the MATLAB workspace as a structure
• Accessing the filter coefficients from the exported MATLAB structure

Importing a Signal into SPTool

To import a signal into SPTool from the workspace or disk, the signal must be either:

• A special MATLAB signal structure, such as that saved from a previous SPTool session
• A signal created as a variable (vector or matrix) in the MATLAB workspace

For this example, create a new signal at the command line and then import it as a structure into SPTool.

1. Create a random signal in the MATLAB workspace by typing

   randn('state',0);
   x = randn(5000,1);
   

2. If SPTool is not already open, open SPTool by typing

   sptool
   

The SPTool window is displayed.

3. Select Import from the File menu in SPTool. The Import to SPTool dialog opens.

   

Notice that the variable x is displayed in the Workspace Contents list. (If it is not, select the From Workspace radio button to display the contents of the workspace.)

4. Select the signal and import it into the Data field:
   1. Make sure that Signal is selected in the Import As pull-down menu.
   2. Select the signal variable x in the Workspace Contents list.
   3. Click on the arrow to the left of the Data field or type x in the Data field.
   4. Type 5000 in the Sampling Frequency field.
   5. Name the signal by typing noise in the Name field.
   6. Press OK.

At this point, the signal noise[vector] is selected in SPTool's Signals list.

Note    You can import filters and spectra into SPTool in much the same way as you import signals. See Importing Filters and Spectra into SPTool for specific details. You can also import signals from MAT-files on your disk, rather than from the workspace. See Loading Variables from the Disk for more information. Type help sptool for information about importing from the command line.

Getting Help

Designing a Filter