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SASLab Manual

Spectrogram Output

The spectrograms visible on the screen can be printed in different ways. One option was to print directly from the drop-down command File > Print Spectrogram. More flexibility can be achieved by copying the spectrograms into the clipboard or by saving them into graphics files. These images can then be inserted into different Windows applications (word processor, graphics editor) where they can be resized and textual descriptions can be added. Another advantage is that several spectrograms can be placed on a single page.

The quality of the printed spectrograms depends on the kind of printer used and the settings of the printer driver. Most standard printers cannot display true grayscales. One pixel can be either black or white, which means that shades of grey cannot be printed directly. In order to support grayscale output the printer driver software emulates different shades of gray (or other colors) by a method called dithering.

This means that the printer driver software produces a matrix of small points for every pixel of the spectrogram. The density or size of these points determines the shade of grey to be displayed. Because the resolution of most printers is limited, the dithering patterns can be recognized by the human eye, which is sometimes undesired. Most printer drivers allow adjusting the parameters of the dithering algorithm to optimize the obtained spectrogram prints.

To overcome the dithering problem a better printer with higher resolution (600 dpi or more) or a color sublimation printer could be used. The latter printers do not use dithering because they can display true colors or shades of gray directly. In binary black and white spectrogram prints the coarse raster of the single pixels is undesired. This effect can be reduced by selecting a higher frequency resolution (FFT length of 512 or 1024) and a higher temporal resolution (overlap of 87.5, 93.75%). Because high temporal resolution causes a longer spectrogram image with more spectra per time interval it migjht be necessary to use the commands File > Copy entire spectrogram or Save entire spectrogram. After inserting such long images into a word processor or a graphics editor it might be useful to adjust the aspect ratio in such a way that the image height will be increased.

The drop-down menu function Tools > Enlarge Image can be used to multiply the single pixels. The command Tools > Image Filter: Average can then be used to soften the edges of the enlarged pixels.

Avisoft Bioacoustics last modified on 27 April 2019