Exfo EXpert IPTV Test Tools (FTB-200v2) Manual do Utilizador descarregar pdf (Página 48)

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causeobviousdistortionormaynotevenbenoticedbytheenduser,dependingonwhich

videoframetypesareimpaired.

Inaddition,impairmentscanbeintroducedduringtheencoding/decodingprocess,bythe

codecitselforaninappropriatelylowbitrates.Thevideocontent(e.g.,levelofdetailand

motiononscreen)canalsohaveasignificantimpacton

thevisibilityofproblems.

Furthermore,perceptualqualityisaffectedbysubjectivefactorsincludinghumanreaction

timeandthe‘recency effect’.Coupledwiththetypeofcontent,e.g.,fastmotion,highdetail,

orfrequentscenechanges,thequalityofexperiencefortheviewerwillvaryevenunderthe

sameimpairmentconditions.

Eachofth

eseobjectiveandsubjectivefactorsmustbetakenintoconsiderationinorderto

accuratelyestimateIPTVvideoperceptualquality.

A.1.1 Transmission‐RelatedImpairments

Packet‐basedvideocanbeverysensitivetonetworkimpairments.Packetlosscan

causesectionsofframesorcompleteframestobecorruptedordeleted.Forexample,the

MPEGcompressionalgorithmusesblock‐basedmotioncompensationforthereductionof

temporalredundancyandDiscreteCosineTransform(DCT)‐basedcompressionforthe

reductionofspatialredund

ancy.AnMPEGencodermaygeneratethreetypesofframe:Intra‐

coded(I),

Predictive(P),andBi‐directional(B)frames.

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Figure2‐1.Packet

vs.FrameLoss

RatesforMPEG

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Exfo EXpert IPTV Test Tools (FTB-200v2) Manual do Utilizador Página 48