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Humminbird 581i Manual

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Grayscale The use of varying shades of gray to represent the strength of the sonar signal
on the display. Traditionally, the strongest sonar signals are represented in black, and
progressively weaker signals are represented in progressively lighter shades of gray.
Noise The unintentional, external sound waves that interfere with the optimal operation
of sonar. Noise appears as random “dots“ on the display and is caused by a variety of
sources. Many Humminbird® products have a Noise Filter menu setting that allows the user
to clear the screen of noise that is difficult to eliminate (also, see Troubleshooting).
Pixels The “picture elements“, or small square blocks, that make up the image on the LCD.
Measured as a vertical by horizontal number (i.e. 640V x 320H), this key specification
typically indicates the quality of resolution. In fishfinders, the total resolution (vertical
multiplied by horizontal) is often less important than the “Vertical Pixel“ resolution because
a greater number of vertical pixels provide finer resolution of targets detected by sonar.
Sonar information on the horizontal axis can vary greatly, depending on boat speed and the
Chart Speed setting.
Power Output The amount of sound energy emitted into the water by the transducer’s
transmitter. Power output is measured using either RMS (Root Mean Square) or P-T-P (Peak-
to-Peak) measurement systems. Either method is acceptable, but it is important when
comparing power outputs, to make sure that the same measurement system is being used
for both outputs, because P-T-P numbers are 8 times higher than RMS numbers. Greater
power output allows the sonar signal to penetrate through weeds and thermoclines, reach
deeper depths and operate more effectively in noisy environments, such as when the boat
is running at high speed.
Pulse Width (Pulse Length) The length of time that a sonar sound burst is transmitted
into the water. Shorter pulse widths provide better target separation, but cannot travel to
great depths. Longer pulse widths provide better depth penetration, but result in poorer
target separation. Humminbird® varies pulse width based on depth to optimize both target
separation and depth performance. See
Target Separation.
Second Return Describes the appearance of a second sonar return below the primary
sonar return (bottom contour) at exactly twice the true depth. The second return is caused
by the same sonar energy bouncing off the bottom once, rebounding to the water surface
and then traveling back down to the bottom to be reflected again. Second returns are more
common in shallow water and over hard bottoms; it is possible to see a third sonar return
under some circumstances. The second return provides useful information to help
determine bottom hardness, as areas with harder bottoms will generally create a second
return. The second return can be used as a guide to set Sensitivity when in shallower water.
Glossary
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