Furuno TS-331A - Manual (Page 31)

2. SONAR OPERATION
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2.9 Unit of Depth, Range Measurement
The unit of depth/range measurement may be chosen from meters, feet and
fathoms, on the Options menu. Check the unit desired.
Options menu, Units, Depth/Range selected
2.10 Noise Filter
You may encounter occasional or intermittent noise and interference on the
vertical sonar display or echo sounder display. Noise is mostly caused by
electrical equipment engine or propeller noise from own ship or noise from other
sonars being operated nearby. To suppress this noise, check Noise Filter on the
Options menu. Uncheck it if no noise is present.