Exfo EXpert IPTV Test Tools (FTB-200v2) Manuel d'utilisateur télécharger pdf (Page 56)

56



videoatagivenbitrateundernolosscondition,aswellasunderlossconditionsasdescribed

below.VQmon/HDautomaticallyadjusts thevideostreameffectivebitratebasedontheGOP

structureoftheencodedvideostream.

A.1.3 ImpactofSubjectiveFactorsonPerceptualQuality

Thedegreetowhichviewersfindvideoimpairme

ntsannoying—ornoticethemat

all—dependsinpartontheseverityanddurationoftheimpairmentevents,butalsoon

certaininherentcharacteristicsofhumanperception.Thesametypeofimpairmentmaybe

extremelyirritatingorbarelynoticed,dependingonfactorssuchasthescenecontentwhen

theerrortakesplace,

andwhetheritoccursaloneorsimultaneouslywithotherimpairments.

A.1.3.1 VideoContent

Thevisibilityofvideoproblemsdependspartiallyonthescenecontent;forexample,

framefreezestendtobemuchmorenoticeableinsequencescontaininghighlevelsofmotion

thaninrelativelystaticscenes,suchasfootageofatelevisionnewsanch

or.VQmon/HD

performscontentandsceneanalysis,detectinglevelsofdetail,motion,andpanning,andcan

detectandprovidealertsforcontentproblemssuchasnoise/snowandframefreezes.Scene

analysisdataisleveragedtoincreasetheaccuracyofVQmon/HD’sestimatedperceptual

qualityscores.

A.1.3.2 TemporalPhenomen

a:ReactionTime,MaskingandRecencyEffects

Perceptualqualityisaffectedsomewhatbyanaturaldelayinhumanreactiontime

whenimpairmentsoccur,orwhenqualityimprovesafteraperiodofdegradation;i.e.,the

viewer’sreactiontoeithereventisnotimmediate.Aswithaudio,whentwoormore

impairments

occursimultaneously(orinrapidsuccession),therecanbea“masking”effect

thataffectsthewayviewersperceivetheseverityofqualitydegradation.Inaddition,a

“recency”phenomenonexists,whereinviewerstendtoperceiveimpairmentsasmoresevere

whentheyoccurredrecently,butarewillingto“forgive”themtosomeextentastimepasses.

VQmon/

HD’squalityanalysisalgorithmconsidersthesetemporalphenomenainorderto

calculateperceptualqualityscoresthatcorrelateasaccuratelyaspossibletoscoresobtained

fromsubjectivetestsofliveviewers.

1 2 ... 51 52 53 54 55 56 57 58 59 60 61 ... 81 82

Pas de commentaire

Exfo EXpert IPTV Test Tools (FTB-200v2) Manuel d'utilisateur Page 56