Furuno NAVNET 1722 Operator's Manual page 150

4. VIDEO SOUNDER OPERATION
Dual-frequency display
The dual-frequency display provides both 50 kHz and 200 kHz pictures. This
display is useful for comparing the same picture with two different sounding
frequencies.
LF picture
The sounder uses ultrasonic pulses to detect bottom conditions. The lower the
frequency of the pulse, the wider the detection area. Therefore, the low
frequency is useful for general detection and judging bottom condition.
HF picture
The higher the frequency of the ultrasonic pulse the better the resolution.
Therefore, the high frequency is ideal for detailed observation of fish school.
4-4
LF
Dual-frequency display
Sounding area and transmission frequency
0.0
HF
low frequency
high frequency