Navigation and Seamanship (1+1)

3.1.8.4.2. Oscilloscope (CRT)

The depth range to be studied and the scale of display can easily be varied in an oscilloscope. The CRT display is therefore the best for the detailed study and identification of echoes, and also useful to distinguish the bottom dwelling fish echoes close from the echoes of bottom. This could be achieved by applying a special effect called White line Technique. The White line technique is a special signal treatment in the receiver whereby echoes above a certain strength either completely block the receiver ( displayed as white line in the screen) or are presented as a faint grey recording (grey line). This is useful in distinguishing the traces of fish close to the bottom from the bottom trace itself, even though fish echoes are considerably weaker than bottom echoes.

When the fish are close to the bottom the two traces will merge and the fire trace will be indistinguishable from that of the bottom. The white line or grey line effect will present the strong bottom echo as a white or greyish trace so that the black fish traces close to the bottom can better be identified.