Noise Reduction.docx
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Noise Reduction.docx
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NoiseReduction
NoiseReduction
(http:
//en.wikipedia.org/wiki/Noise_reduction)
FromWikipedia,thefreeencyclopedia
Forsoundproofing,see soundproofing.Forscientificaspectsofnoisereductionofmachineryandproducts,see noisecontrol.
Thisarticle needsadditional citations for verification.Pleasehelp improvethisarticle byaddingcitationsto reliablesources.Unsourcedmaterialmaybe challenged and removed. (March2010)
Noisereduction istheprocessofremoving noise froma signal.
Allrecordingdevices,both analogue or digital,havetraitswhichmakethemsusceptibletonoise.Noisecanberandomor whitenoise withnocoherence,orcoherentnoiseintroducedbythedevice'smechanismorprocessing algorithms.
In electronic recordingdevices,amajorformofnoiseis hiss causedbyrandom electrons that,heavilyinfluencedbyheat,strayfromtheirdesignatedpath.Thesestrayelectronsinfluencethe voltage oftheoutputsignalandthuscreatedetectablenoise.
Inthecaseof photographicfilm and magnetictape,noise(bothvisibleandaudible)isintroducedduetothegrainstructureofthemedium.Inphotographicfilm,thesizeofthegrainsinthefilmdeterminesthefilm'ssensitivity,moresensitivefilmhavinglargersizedgrains.Inmagnetictape,thelargerthegrainsofthemagneticparticles(usually ferricoxide or magnetite),themorepronethemediumistonoise.
Tocompensateforthis,largerareasoffilmormagnetictapemaybeusedtolowerthenoisetoanacceptablelevel.
Contents
[hide]
∙1 Inaudio
o1.1 Dolbyanddbxnoisereductionsystem
o1.2 DynamicNoiseReduction
o1.3 Otherapproaches
∙2 Inimages
o2.1 Types
o2.2 Removal
▪2.2.1 Tradeoffs
▪2.2.2 Chromaandluminancenoiseseparation
▪2.2.3 Linearsmoothingfilters
▪2.2.4 Signalcombination
▪2.2.5 Anisotropicdiffusion
▪2.2.6 Non-localmeans
▪2.2.7 Nonlinearfilters
o2.3 Softwareprograms
∙3 Seealso
o3.1 Generalnoiseissues
o3.2 Audio
o3.3 Video
∙4 References
∙5 Externallinks
[edit]Inaudio
Noisereductionexample
More…
Exampleofnoisereductionusing Audacity with0 dB,5dB,12dB,and30dBreduction,150 Hz frequencysmoothing,and0.15secondsattack/decaytime.
Problemslisteningtothisfile?
See mediahelp.
Whenusing analog taperecordingtechnology,theymayexhibitatypeofnoiseknownas tapehiss.Thisisrelatedtotheparticlesizeandtextureusedinthemagneticemulsionthatissprayedontherecordingmedia,andalsototherelativetapevelocityacrossthe tapeheads.
Fourtypesofnoisereductionexist:
single-endedpre-recording,single-endedhissreduction,single-endedsurfacenoisereduction,andcodecordual-endedsystems.Single-endedpre-recordingsystems(suchasDolbyHXPro)worktoaffecttherecordingmediumatthetimeofrecording.Single-endedhissreductionsystems(suchas DNR)worktoreducenoiseasitoccurs,includingbothbeforeandaftertherecordingprocessaswellasforlivebroadcastapplications.Single-endedsurfacenoisereduction(suchas CEDAR andtheearlierSAE5000AandBurwenTNE7000)isappliedtotheplaybackof phonographrecords toattenuatethesoundofscratches,pops,andsurfacenon-linearities.Dual-endedsystems(suchas DolbyNR and dbxTypeIandII)haveapre-emphasisprocessappliedduringrecordingandthenade-emphasisprocessappliedatplayback.
[edit]Dolbyanddbxnoisereductionsystem
Whiletherearedozensofdifferentkindsofnoisereduction,thefirstwidelyusedaudionoisereductiontechniquewasdevelopedby RayDolby in1966.Intendedforprofessionaluse,DolbyTypeAwasanencode/decodesysteminwhichtheamplitudeoffrequenciesinfourbandswasincreasedduringrecording(encoding),thendecreasedproportionatelyduringplayback(decoding).TheDolbyBsystem(developedinconjunctionwith HenryKloss)wasasinglebandsystemdesignedforconsumerproducts.Inparticular,whenrecordingquietpartsofanaudiosignal,thefrequenciesabove1 kHzwouldbeboosted.Thishadtheeffectofincreasingthesignaltonoiseratioontapeupto10dBdependingontheinitialsignalvolume.Whenitwasplayedback,thedecoderreversedtheprocess,ineffectreducingthenoiselevelbyupto10dB.TheDolbyBsystem,whilenotaseffectiveasDolbyA,hadtheadvantageofremaininglistenableonplaybacksystemswithoutadecoder.
Dbx wasthecompetinganalognoisereductionsystemdevelopedby dbx laboratories.Itusedaroot-mean-squared(RMS)encode/decodealgorithmwiththenoise-pronehighfrequenciesboosted,andtheentiresignalfedthrougha2:
1 compander.DbxoperatedacrosstheentireaudiblebandwidthandunlikeDolbyBwasunusableasanopenendedsystem.Howeveritcouldachieveupto30dBofnoisereduction.SinceAnalog videorecordings usefrequencymodulationfortheluminancepart(compositevideosignalindirectcoloursystems),whichkeepsthetapeatsaturationlevel,audiostylenoisereductionisunnecessary.
[edit]DynamicNoiseReduction
DynamicNoiseReduction(DNR)isanaudionoisereductionsystem,introducedby NationalSemiconductor toreducenoiselevelsonlong-distance telephony.[1] Firstsoldin1981,DNRisfrequentlyconfusedwiththefarmorecommon Dolbynoisereductionsystem.[2] However,unlikeDolbyand dbx TypeI&TypeIInoisereductionsystems,DNRisaplayback-onlysignalprocessingsystemthatdoesnotrequirethesourcematerialtofirstbeencoded,anditcanbeusedtogetherwithotherformsofnoisereduction.[3] Itwasadevelopmentofthe unpatented Philips DynamicNoiseLimiter(DNL)system,introducedin1971,withthecircuitryonasingle chip.[4][5]
BecauseDNRisnon-complementary,meaningitdoesnotrequireencodedsourcematerial,itcanbeusedtoremovebackgroundnoisefromanyaudiosignal,including magnetictape recordingsand FMradiobroadcasts,reducingnoisebyasmuchas10dB.[6] Itcanbeusedinconjunctionwithothernoisereductionsystems,providedthattheyareusedpriortoapplyingDNRtopreventDNRfromcausingtheothernoisereductionsystemtomistrack.
OneofDNR'sfirstwidespreadapplicationswasinthe GM Delco carstereo systemsinU.S.GMcarsintroducedin1984.[7] Itwasalsousedinfactorycarstereosin Jeep vehiclesinthe1980s,suchastheCherokeeXJ.Today,DNR,DNL,andsimilarsystemsaremostcommonlyencounteredasanoisereductionsysteminmicrophonesystems.[8]
[edit]Otherapproaches
Asecondclassofalgorithmsworkinthetime-frequencydomainusingsomelinearornon-linearfiltersthathavelocalcharacteristicsandareoftencalledtime-frequencyfilters.[1] Noisecanthereforebealsoremovedbyuseofspectraleditingtools,whichworkinthistime-frequencydomain,allowinglocalmodificationswithoutaffectingnearbysignalenergy.Thiscanbedonemanuallybyusingthemousewithapenthathasadefinedtime-frequencyshape.Thisisdonemuchlikeinapaintprogramdrawingpictures.Anotherwayistodefineadynamicthresholdforfilteringnoise,thatisderivedfromthelocalsignal,againwithrespecttoalocaltime-frequencyregion.Everythingbelowthethresholdwillbefiltered,everythingabovethethreshold,likepartialsofavoiceor"wantednoise",willbeuntouched.TheregionistypicallydefinedbythelocationofthesignalInstantaneousFrequency,[2] asmostofthesignalenergytobepreservedisconcentratedaboutit.
Moderndigitalsound(andpicture)recordingsnolongerneedtoworryabouttapehisssoanalogstylenoisereductionsystemsarenotnecessary.However,aninterestingtwististhat dither systemsactuallyaddnoisetoasignaltoimproveitsquality.
[edit]Inimages
Imagestakenwithboth digitalcameras andconventional filmcameras willpickupnoisefromavarietyofsources.Manyfurtherusesoftheseimagesrequirethatthenoisewillbe(partially)removed-foraesthetic purposesasin artistic workor marketing,orforpracticalpurposessuchas computervision.
[edit]Types
In saltandpeppernoise (sparselightanddarkdisturbances), pixels intheimageareverydifferentincolororintensityfromtheirsurroundingpixels;thedefiningcharacteristicisthatthevalueofanoisypixelbearsnorelationtothecolorofsurroundingpixels.Generallythistypeofnoisewillonlyaffectasmallnumberofimagepixels.Whenviewed,theimagecontainsdarkandwhitedots,hencethetermsaltandpeppernoise.Typicalsourcesincludeflecksofdustinsidethecameraandoverheatedorfaulty CCD elements.
In Gaussiannoise,eachpixelintheimagewillbechangedfromitsoriginalvaluebya(usually)smallamount.Ahistogram,aplotoftheamountofdistortionofapixelvalueagainstthefrequencywithwhichitoccurs,showsa normaldistribution ofnoise.Whileotherdistributionsarepossible,theGaussian(normal)distributionisusuallyagoodmodel,duetothe centrallimittheorem thatsaysthatthesumofdifferentnoisestendstoapproachaGaussiandistribution.
Ineithercase,thenoiseatdifferentpixelscanbeeithercorrelatedoruncorrelated;inmanycases,noisevaluesatdifferentpixelsaremodeledasbeing independentandidenticallydistributed,andhenceuncorrelated.
[edit]Removal
[edit]Tradeoffs
Inselectinganoisereductionalgorithm,onemustweighseveralfactors:
▪theavailablecomputerpowerandtimeavailable:
adigitalcameramustapplynoisereductioninafractionofasecondusingatinyonboardCPU,whileadesktopcomputerhasmuchmorepowerandtime
▪whethersacrificingsomerealdetailisacceptableifitallowsmorenoisetoberemoved(howaggressivelytodecidewhethervariationsintheimagearenoiseornot)
▪thecharacteristicsofthenoiseandthedetailintheimage,tobettermakethosedecisions
[edit]Chromaandlu
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