8-6 Gain and threshold
Gain and threshold settings control the amount of detail displayed on a sonar display:
Gain: The gain of the sonar receiver. The gain should be high to display good detail, but if the gain is too high then information from the strong bottom signal is lost and false echoes might be displayed. There is a separate gain setting for each sonar frequency, 50 kHz and 200 kHz.
Threshold: Return echoes less than the threshold are ignored. The threshold should be as low as possible, but if the threshold is too low, unwanted noise will be displayed. Threshold is set as a percentage of gain. For example, if the threshold is 50 %, then return echoes less than 50 % of the maximum signal are ignored. There is a separate threshold setting for each sonar frequency, 50 kHz and 200 kHz.
The gain window
To display or change the current settings for gain,
select a sonar display and press 
. Select Threshold to display the thresholds.
This display is called the gain window.
Changing mode
The 657 has three operating modes, In Cruising and Fishing modes, the 657 automatically adjusts gain and threshold for good performance. In manual mode, you can hand adjust the settings.
To change the mode from the Gain window, Select Mode and then select Fishing, Cruising or Manual. When you select Manual, the 657 reverts to your last manual settings.
Changing gain and threshold
1In the Gain window, press 
or 
to select the setting to adjust.
2Press 
or 
adjust the setting. The 657 changes to Manual mode.
Tip: Use the
Northstar Explorer 657 Installation and Operation Manual | 39 |