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Filtering

To filter a .WAV file, you must first select a time segment.  This is done from either the Time Series, Spectrogram, or 3-D Surface plot.  Click the selection arrow located on the plot toolbar and then click and drag the mouse across the desired time segment.  This segment will then be displayed in inverse video.  Once the time segment has been selected, press the filter button on the toolbar or select the <Edit><Filter> menu command.  The filtering parameter dialog box will then allow you to select the frequencies to be filtered.

There are five filter types:
  • Low Pass - passes all frequency components below the specified cutoff frequency and attenuates all components above it.
  • High Pass - passes all frequency components above the specified cutoff frequency and attenuates all components below it.
  • Band Pass - passes the specified band of frequencies and attenuates all others.
  • Notch - rejects the specified band of frequencies and passes all others.
  • Custom - applies the specified filter response file to the data.

    Filter Sharpness - the number of filter taps (coefficients) directly affects the sharpness of the filter response and how closely it matches the ideal response.  More taps produce a sharper filter response but require more processing time.   

    Filter Gain - allows the overall gain to be adjusted.  Enter a value in dB; use a positive value to increase the gain, negative to decrease.

    Building a filter response file:
  • Run the NOTEPAD.EXE application, spreadsheet or word processor that can produce a standard ASCII text file.
  • Enter two columns of numbers representing the filter response curve.
  • The left column contains the frequency in Hertz. 
  • The right column contains the offset in decibels
  • Enter the information in ascending frequency order.
  • Separate the columns with a tab character.

    Example:
    ---------------------------
    100.0        -10.0
    500.0          -5.0
    1000.0         0.0
    2000.0         2.0
    10000.0      -5.0
    15000.0     -15.0

    Enter as many as32,768 pairs of numbers; at least 3 pair are required.  The analyzer will use a cubic spline algorithm to interpolate between these data points for each frequency in the spectrum.  Any line with a semicolon in the first column will be treated as a comment line and ignored.  By convention, filter response files have an extension of .FLT.

    Note:
    Filter Response files, Transdcuer Compensation files, and Overlay files have identical formats and can be interchanged.  This is useful if you want to verify that you have entered the filter response table correctly.  To do this, simply select load the filter response file into one of the overlays for display.

    The default folders used for storing the various files used by SpectraPLUS-DT can be customized by clicking on the <File><Set Paths> menu item.

    See also:  Opening and Saving Wave files, toolbar