Reducing noise and reverberation with the AGC (Automatic Gain Control) function

Identifying echoes from fish and schools can often be challenging when they are hidden by noise and reverberation. When sonar conditions are poor, effective tuning of the filters in the ST90 system may be necessary to achieve a satisfactory result. Automatic Gain Control (AGC) is one of the tools provided to remove noise and reverberation from the echo presentation.

The AGC (Automatic Gain Control) function runs an automatic analysis of the echo strength. Based on this analysis the ST90 system maintains a proper dynamic range based on all the current echo values. This is done by automatically reducing the gain if you have reverberation and noise, or increasing the gain if the acoustical conditions permit it.

Tip:

The most practical way to use the AGC (Automatic Gain Control) is to first switch it off. Adjust the echo presentation using the normal gain functionality until you have the best possible result, then activate the AGC.

The AGC works best with even background (homogenous noise conditions) and pronounced targets. With uniform environmental conditions you may benefit from strong compensation. However, if you are looking at scattered fish, reduce the automatic gain control, and try to add normal gain instead.

If you work close to other vessels, the noise from their propellers may be misinterpreted as echoes. This will also reduce the visibility of other echoes. The same applies if the bottom conditions place your vessel with decreasing depth on one side of the vessel and your targets on the other side. Reduce the automatic gain control.

If you have a large school located close to your vessel (typically closer than 150 meters) the school gives you a strong echo. The AGC may misinterpret the school echo for noise. Reduce or disable the automatic gain control.

The filters in the ST90 system are implemented in “series�. The gain compensations are made before the AGC. For this reason, your current gain setting will always have an effect on the automatic gain control.

Procedure

  1. Click in any view to make it active.

    The active view is identified with a thicker border.

  2. Open the Active menu.
  3. Select AGC (Automatic Gain Control).
  4. Select a value from the options provided.

    The choice you make is by default only applied to the currently selected (active) view.

    Select the middle of the button to open a small menu with the available options. Select the left side of the button to select a "lower"' menu choice. Select the right side of the button to select a "higher"' menu choice.