Simrad ES80 - Page 417

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Description
Bymeansofalgorithms,thetimevariablegaincompensationconvertstheecho
presentationasafunctionofrange.Thismakesthetargetswiththesamestrengthappear
withthesameintensityindependentoftheirphysicaldistancefromthetransducer.Inthe
ES80,theTVGcompensationismadeusingdigitalsignalprocessingsoftware.
Whenanacousticpulseissentthroughthewater,itwillgraduallyloseitsenergy.The
greaterthedistancebetweenthetransducerandthetarget(s),thegreaterthelossof
energy.Thelossofenergyisexplainedwithtwofactors.
•Geometricspread:Oncetransmitted,theacousticenergywillspreadouttoforma
circularbeam.Thewidthofthisbeamincreaseswiththephysicaldistancetothe
target(s).
•Absorptionloss:Dependingonthesalinityandtemperature,thewaterwillabsorb
someoftheenergyfromthetransmission.Theabsorptionlossincreasesasthe
physicaldistancetothetarget(s)increases.
Boththegeometricspreadandtheabsorptionwillalsohaveaneffectonthereturned
echosignal.Thatiswhywenormallyrefertothesefactorsasthetwo-waytransmission
loss.TVG(TimeV ariableGain)compensatesforthislossofacousticenergy.
TheTVGcompensationisexpressedasalogarithmiccurve.Youcanchoosefrom
aselectionofcurves.Eachcurvehasadifferentslopecreatingadifferentgain
compensation.EachcurveisidentiedwiththeequationXlogTVG.Thecoefcient"X"
isaninteger.Typicalvaluesfor"X"are10to40.
Tip
YoucanselectTVGusingthisfunction.YoucanalsoadjusttheTVGsettinginthe
Echogramdialogbox.TheTVGfunctionislocatedontheEchogrampage.
TheES80allowsyoutousedifferentTVGcompensationfortheechoesfromthewater
columnthanfromthebottom.Bymakingthebottomechoesindividuallyadjustable,
youcannetunethepresentationfromthebottomtoinvestigatethebottomtypeand
conditions.TheBottomTVGadjustmenthasnoeffectonshclosetothebottom.To
adjusttheBottomTVG,opentheEchogramdialogbox.
Details
TVG
SelecttheTVGsettingyouwanttouse.SeveralTVGcompensationsettingsare
available.
•NoTVG
TVGcompensationisnotimplemented.Thissettingishardlyeverused.
Functionsanddialogboxes