Furuno NAVNET 1823C - Manual (Page 150)

4. VIDEO SOUNDER OPERATION
4-4
Dual-frequency display
The dual-frequency display provides both LF and HF 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
Marker-zoom display
The marker-zoom display expands a selected area of the normal sounder picture
to full vertical size of the screen on the left-half window. You may specify the
portion to expand by operating the VRM (Variable Range Marker), which you can
shift with the [ENTER] knob. The area between the VRM and zoom marker is
expanded. The length of the segment is equal to one division of the depth scale.
Note: The VRM is set independently from other displays in case of multiple
displays.