Furuno FSV-30 Operator's Manual page 181

Note: 1) To emphasis the difference between a weak and strong signal with
multi-color, lower the SIGNAL LEVEL setting or readjust the color
setting properly in the field.
2) The frequency is shifted to 22.4 kHz or 25.6 kHz to avoid the
interference with horizontal mode.
3) When H/V dual mode is used at hard and shallow bottom, the
bottom echo of horizontal mode may suppress the echo of vertical
mode. In this case, select the individual TX/ RX mode, or shift the
frequency of vertical mode further from the frequency of the
horizontal mode.
ORIGINAL SETTING
AP-7