Simrad ST90 - Reducing Noise and Reverberation with the AGC (Automatic Gain Control) Function

457420/A
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ReducingnoiseandreverberationwiththeAGC(Automatic
GainControl)function
Identifyingechoesfromshandschoolscanoftenbechallengingwhentheyarehidden
bynoiseandreverberation.Whensonarconditionsarepoor,effectivetuningofthelters
intheST90maybenecessarytoachieveasatisfactoryresult.AutomaticGainControl
(AGC)isoneofthetoolsprovidedbytheST90toremovenoiseandreverberationfromthe
echopresentation.
Context
TheAGC(AutomaticGainControl)functionrunsanautomaticanalysisoftheechostrength.
BasedonthisanalysistheST90maintainsaproperdynamicrangebasedonallthecurrent
echovalues.Thisisdonebyautomaticallyreducingthegainifyouhavereverberationand
noise,orincreasingthegainiftheacousticalconditionspermitit.
Tip
ThemostpracticalwaytousetheAGC(AutomaticGainControl)istorstswitchitoff.
Adjusttheechopresentationusingthenormalgainfunctionalityuntilyouhavethebest
possibleresult,thenactivatetheAGC.
TheAGCworksbestwithevenbackground(homogenousnoiseconditions)andpronounced
targets.Withuniformenvironmentalconditionsyoumaybenetfromstrongcompensation.
However,ifyouarelookingatscatteredsh,reducetheautomaticgaincontrol,andtryto
addnormalgaininstead.
Ifyouworkclosetoothervessels,thenoisefromtheirpropellersmaybemisinterpretedas
echoes.Thiswillalsoreducethevisibilityofotherechoes.Thesameappliesifthebottom
conditionsplaceyourvesselwithdecreasingdepthononesideofthevesselandyourtargets
ontheotherside.Reducetheautomaticgaincontrol.
Ifyouhavealargeschoollocatedclosetoyourvessel(typicallycloserthan150meters)
theschoolgivesyouastrongecho.TheAGCmaymisinterprettheschoolechofornoise.
Reduceordisabletheautomaticgaincontrol.
TheltersintheST90areimplementedin“series”.Thegaincompensationsaremade
beforetheAGC.Forthisreason,yourcurrentgainsettingwillalwayshaveaneffecton
theautomaticgaincontrol.
Procedure
1Clickinanyviewtomakeitactive.
Theactiveviewisidentiedwithathickerborder.
2OpentheActivemenu.
Operatingprocedures