Contents - Index

Overlays

The overlay feature allows you to display multiple spectral traces on the same plot for quick comparisons with previous measurements.  

Controls for creating and clearing overlays are located along the right edge of the spectrum view.  Up to 12 overlays can be displayed for each channel.  In addition, two "Composite" overlays can be set which is computed from other overlays.

Note: You can show/hide the entire set of overlay controls to save space on the plot by clicking the small box on the bottom right corner of the plot.  

Spectral Overlay Operations

Clicking one of the numbered "Set" overlay buttons will create an overlay using the contents of the current spectrum.  The overlays can be turned On and Off by clicking the checkbox labeled "On".  

Clicking the "Options" button brings up a dialog box with additional controls:

Legend: You may enter a descriptive legend up to 24 characters long.  This legend is displayed on the spectrum view and printed on any printouts.  The legend is saved as part of the overlay file format and restored whenever you load the file from disk.

Offset: You can apply an offset in dB to each overlay; this is useful for aligning overlays for comparison purposes.

Disk Storage:  Use these controls to save and load the overlay traces from disk. Temporary overlay files are used for storing the overlay data between sessions.  These files are named L1.OVL, L2.OVL, L3.OVL, L4.OVL, L5.OVL, L6.OVL, R1.OVL, R2.OVL, R3.OVL, R4.OVL, R5.OVL through R14.OVL.  Do not use these filenames for saving your own data as they will be overwritten.  Any offset is NOT included in the saved overlay file contents.

Trace Colors: these controls allow you to select the display colors for each of the overlay traces and  legends.  These colors are also used when printing the spectrum view on a color printer.  If you print to a monochrome printer, each of the overlays will have a unique line style.

Copy to Left/Right Channel: This allows you to quickly copy an overlay to the opposite channel.

Legend Display Size: this value will adjust the width of the rights border of the spectrum plot to show the specified number of legend characters.  Space may be reserved for up to 24 characters.  

Overlay File Format:  
Overlay files can be created manually.  Overlay files are simple ASCII text files containing two columns of numbers separated by a tab.  The left column contains the frequency; the right column contains the corresponding amplitude value.  If a semicolon is placed on the first column, the entire line will be treated as a comment and ignored.  Legends are the first line in the file and have a semicolon in the first column.  At least 3 data points are required and up to 32768 points can be used.  
A cubic spline algorithm is used to interpolate between each data point.  This file format is interchangeable with the microphone compensation file format.  

Composite Overlay
Pressing the overlay button labeled "C1" or "C2" will allow you to setup a "Composite" overlay.  The composite overlay can be either an average of up to 12 selected overlays or the difference between any 2 selected overlays.  Any offset selected for the dependent overlays is applied when the composite values are computed.   An offset can also be applied to the composite overlay itself.


Note:  If you regularly interchange overlay and microphone compensation files, you may want to set their file paths (See Set Paths command) to the same directory.  You can then easily select whether to use .MIC or .OVL file types from the file open/save dialog boxes.

Interpolation: When manually creating overlays the results can have unexpected excursions due to the cubic spline interpolation algorithm.  You can turn off this algorithm by adding a line to the beginning of the overlay file as follows

;SPLINE OFF

This can also be done with mic compensation files.