Raymarine L750 Operating Handbook page 60

Chapter 3: Operating Instructions
To change the gain or sensitivity setting, press the Gain button. Four
new Soft Keys may appear at the bottom of the display, depending on
the frequency selected at the moment. See Fig. 3-15.
As we explained, the unit can use sound waves with two different
frequencies – 50 kHz and 200 kHz. Each frequency is better for a
particular purpose. The 50 kHz frequency scans a wide area. Because
it penetrates water well, 50 kHz is good for deep water. The 200 kHz
frequency scans a narrower area, but produces a more detailed view.
This frequency is good for seeing fish near the bottom or close together.
Notice that the settings you make here do not control which frequency is
used to generate the display. (You do that by pressing the Menu button,
then using the Soft Key for Frequency.) The Gain controls set up the
sensitivity for each transducer. You can decide later whether to use one
transducer or both to generate the display.
Gain Auto/Manual
When you choose the Auto setting, the Fishfinder will select the
gain or sensitivity by itself, automatically. When using the Auto
setting, the unit chooses the best sensitivity which will detect fish,
but will also show a minimum of background noise. If the amount of
background noise changes (due to changing depth or water
conditions), the Fishfinder will change the sensitivity to adjust.
You can also set the sensitivity manually. If you choose the Manual
setting and the amount of background noise changes, the
Fishfinder will not adjust for this. (To adjust the sensitivity manually,
use one of the next two Soft Keys.)
Even if you choose the Auto setting, you can fine-tune the gain
setting. Go to the Soft Key for the 50kHz or 200 kHz setting, and
adjust the gain as you would normally. (See the information on the
next two soft keys.) When the Fishfinder is turned off, then on
again, the Auto sensitivity setting will return to the normal or default
setting. Also, toggling between Auto and Manual will return the
Auto sensitivity setting to the default value.
50 kHz Setting
To make the gain or sensitivity setting when the unit is using the 50
kHz frequency, press this Soft Key. You will notice that the 50 kHz
frequency number is highlighted. Use the up- and down- controls
on the Trackpad to adjust the sensitivity. Making the dark bar
longer increases the sensitivity.
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