Furuno COLOR VIDEO PLOTTER 1833C Operator's Manual page 148

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.
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.
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.
4-4
50k
Dual-frequency display
Sounding area and transmission frequency
0.0
200k
50 kHz
200 kHz