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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 SN90 may be necessary to achieve a satisfactory result. Automatic
Gain Control (AGC) is one of the tools provided by the SN90 to remove noise and
reverberation from the echo presentation.
Context
The
(Automatic Gain Control) function runs an automatic analysis of the echo
AGC
strength. Based on this analysis the SN90 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 SN90 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
3
Select
(Automatic Gain Control).
AGC
381295/B
menu.
Operating procedures
85
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