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SONAR Settings - What is Bias and Span?

Most SONAR controls including: range, gain, frequency, and color palette are generally intuitive, but what is this Bias and Span thing? The Bias and Span control consists of two sliders on an intensity scale. The scale maps the intensity of the SONAR return to a color value for display, with the lowest amplitude (0) on the left and the highest on the right. The yellow slider bars at each end are cut off filters for the amplitude of the return signal.

The left slider removes any signal with an amplitude less than its setting. Its purpose it to remove low level background noise. To use it, move the slider all the way to the left and then back to the right until you start to notice changes in the image (which means you are cutting out real data, not noise). Once you notice the changes, slide it back a little bit to the left to restore the data.

The right slider cuts off any signal with an amplitude higher than its setting. You might ask why you want to cut off actual data? Well, you don't. But, what you do want to do is map the range of actual data to the full color spectrum, not just part of it. In the image above, you can see the right slider is set very low. This will cause any Sonar return above this point to be completely white and wash out the image. Early versions of the software defaulted to these settings, which had to be modified to attain a usable image. You want to move this slider to match the intensity of the highest return. To do this, move the slider all the way to the right and then move it back to the left. When you see white areas of the image start to appear larger, stop and move the slider back a little bit to the right.

Now, you have removed any background noise and optimized the sound intesity range to the full color range. If the bars are too close together, you are cutting off low level data and replacing it with black, and cutting off high data and making the image more white, losing details on both ends. If the bars are too far apart, you are mapping the intensity to only part of the color scale, which will result in less apparent image resolution. In this case, rather than seeing colors from dark pink to white (say 0 - 255), you may only have data that maps from medium pink to light pink, so now instead of seeing the image with 256 colors, you are only seeing say 200 colors mapped and some of the finer detail will be lost.

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