Furuno CSH-53 Operator's Manual page 30

Setting F-shift (data setting window: SONR•BEAM,
SONAR)
If interference cannot be suppressed by the interference rejector
(IR) or Tx cycle, change the setting of F-SHFT from 1 to 2.
Noise limiter (data setting window: SIGNAL, SIGNAL
PROCESS)
Weak, unwanted reflections, colored light-blue or green, appear
when the water is dirty, plankton layers exists, or due to ship's
noise. The noise limiter (NL) can reduce the effects of these
unwanted reflections–raising the setting causes them to become
bluish to background color. Normally a setting of 3 or 4 is suffi-
cient.
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