Furuno 1732 - Manual (Page 147)

4. VIDEO SOUNDER OPERATION
4-4
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.
0.0
50k 200k
Dual-frequency display
50 kHz picture
The sounder uses ultrasonic pulses to detect bottom conditions. The lower the
frequency of the pulse, the wider the detection area. Therefore, the 50 kHz
frequency is useful for general detection and judging bottom condition.
200 kHz picture
The higher the frequency of the ultrasonic pulse the better the resolution.
Therefore, the 200 kHz frequency is ideal for detailed observation of fish school.
50 kHz
200 kHz
Sounding area and transmission frequency