Furuno GD-1700C - Manual (Page 148)

4. VIDEO SOUNDER OPERATION
4-4
Dual-frequency display
The dual-frequency display provides both HF and LF pictures. This display is
useful for comparing the same picture with two different sounding frequencies.
0.0
LF HF
Dual-frequency display
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.
low frequency
high frequency
Sounding area and transmission frequency