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457420/A

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

Simrad ST90 User Manual

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