AGC (Automatic Gain Control) function

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.

How to open

You open this function from the Active menu.

Description

The AGC (Automatic Gain Control) function adjusts the gain using an image processing algorithm.

This processing depends on the strength of the echo signal. The image is analysed to identify reverberation and echoes. Based on the analysis the weak echoes are amplified, while the strong echoes are weakened. The result is a "normalizing" effect that reduces noise and reverberation.

The automatic gain control is functional up to the maximum range of the ST90 system.

You can choose from four different AGC settings. The settings define the difference between the maximum and minimum echo level for the automatic gain control to take effect.

The chosen AGC (Automatic Gain Control) setting is only be applied to the currently active view. Activate the relevant view before you select the AGC setting.

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.

  1. Bottom Filter
  2. Noise Filter (1)
  3. Gain & TVG (Time Variable Gain)
  4. Noise Filter (2)
  5. AGC (Automatic Gain Control)
  6. RCG (Reverberation Controlled Gain)
  7. Display Gain
  8. Ping‑Ping Filter

Note that Gain, TVG (Time Varied Gain) and Display Gain are not filters, but included only to illustrate the complete "signal path".

Details

Off

The automatic gain control (AGC) is disabled.

Weak

A mean value for all the echoes is established. The value is used as a threshold. Echoes below this threshold are suppressed.

Medium

The threshold value is increased by 3 dB compared to Weak.

Strong

The threshold value is increased by 3 dB compared to Medium.