The overlay feature allows you to display multiple spectral traces on the same plot for quick comparisons with previous measurements. The overlay controls are shown on the right side of the spectrum plot. These can be quickly hidden using the small + or - button below the overlay controls.
Up to 6 overlays can be displayed for each channel. In addition, a "Composite" overlay can be set which is computed from other overlays.
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 plot 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. Any offset specified 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 plot on a color printer. If you print to a monochrome printer, each of the overlays will have a unique line style.
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 transducer compensation file format.
Pressing the overlay button labeled "C" will allow you to setup a "Composite" overlay. The composite overlay can be either an average of up to 6 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 transducer 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
This can also be done with transducer compensation files.