The Import/Boxcar Editor

The Import/Boxcar Editor is simply a handy interface that  allows  you  to  do two common, though different,  operations on an empty FIR filter design. It lets you:

  1. Import and analyze a FIR filter designed outside of ScopeFIR.
  2. Create and analyze a very simple type of FIR filter called a boxcar.

The Import/Boxcar Editor is shown below:

Importing

Sometimes you find yourself wanting to use ScopeFIR's powerful filter analysis and modification facilities to examine the characteristics of a FIR filter that  was not designed by ScopeFIR.   To import a filter:

  1. Hit the Sampling Frequency button on the upper left. You will be prompted to enter the numerical frequency and the units of frequency/time. (ScopeFIR needs to know the sampling frequency in order to determine the frequency response of the filter.)
  2. Hit the Import Coefficients button. You will be prompted to locate the file(s) of FIR filter coefficients to be imported. You will then see the characteristics of the filter depicted in ScopeFIR's Frequency/Impulse Response plots and its coefficient editor. You may modify the filter, if you like, using commands in ScopeFIR's FIR menu.

Creating a Boxcar Filter

A " boxcar" filter--sometimes also called a " comb" or " moving average" filter--is a FIR filter whose coefficients all have the same value.   This is desirable in certain applications.   For example, a boxcar with N taps can be implemented simply by adding the last N samples of a signal together--no multiplier is needed.

All boxcar filters have a lowpass response, and the cutoff frequency of a boxcar decreases as the number of taps increases.   Compared to a FIR whose taps can have any  value, a boxcar generally does not perform as well for a given number of taps.   But that's the price of simplicity.

The Import/Boxcar Editor's Create Boxcar button allows you to  design a boxcar filter by filling it with a constant. This is simply a shortcut to the Fill Filter With Constant command of  the FIR menu.

Analyzing the frequency response

The Import/Boxcar Editor provides a Frequency Response List which lets you see the exact attenuation of your filter,  in  dB, at specific frequencies.

If the Automatic option is enabled, ScopeFIR will show you the frequency magnitude response at zero and at half the Sampling Frequency. The Frequency Response List is refreshed whenever you Import Coefficients. (See Using the Frequency Response List for more information.)

When you save your project, the FIR coefficients, the Sampling Frequency, and the Frequency Response List are all saved in the project. Note, however, that if the underlying data file from which you imported changes, you must import it again.

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