Exfo EXpert IPTV Test Tools (FTB-200v2) Instrukcja Użytkownika Pobierz pdf (Strona 55)

55



A.1.2.2 PerformanceofVideoCoders



Therearemanystandardizedvideocodingalgorithms,suchasITU‐TH.261,H.263,

H.264,ISO/IECMPEG‐1,MPEG‐2andMPEG‐4,AVS,VC‐1,etc.Thesestandardsdonotexplicitly

definecodecs;theyonlydefinethesyntaxofanencodedvideobitstreamtogetherwiththe

methodsofdecodingthebitstream.Theconsequenceisthatth

eremightbesignificantquality

differencesbetweencodecsconformingtothesamestandard[16].TheWindowsMedia9

(VC‐1)ofMicrosoftandRealVideoofRealNetworksarewidelyusedproprietaryvideocoding

algorithmsforvideostreamingapplications.

Asaruleofthumb,ITU‐TH.263reducesthecodingbitratebyab

out50%againstH.261for

comparablevideoquality.ITU‐TH.264/AVCsignificantlyoutperformsallotherlisted

standards.Averagebit‐savingsofmorethan60%relativetoMPEG‐2arereported.H.264/AVC

MainProfileprovidesmorethan1/3bitratesavingrelativetoitscompetitors,MPEG‐4

AdvancedSimpleProfile(ASP)andH.263ConversationalHighCompr

ession(CHC)Profiles[25].

ReportsshowthatWMV9(VC‐1)achievedsimilarqualitytoMPEG‐2andMPEG‐4withonly

1/3

and1/2ofthebitrate,respectively[21].ApaperbyBennetandBock[13]concludesthat

comparingH.264/AVCandVC‐1,thereisverylittlepe

rformancedifferencebetweenthem.

A.1.2.3 EffectiveCodingBitrate

Thebitrateusedinestimatingthemeansquarederrorasreferencedinthebeginning

ofsection2.2.1canbeadjustedtoaneffectivebitratebasedonperformanceofavideocodec

describedabove.Theeffectivebitratealsoneedstobeadjustedbasedonth

eframe

resolutionandframerateofthevideosignal.Videoconferencingapplicationsgenerally

supportlowtomediumbitratesandframeresolution.Themostcommonlyusedresolutions

areQCIF(176x144)andCIF(352x288)at10to30framespersecond.Entertainment‐quality

IPTVapplicationsgenerallysupportvideoencodedwithSDresol

utionsof720x486(30Hz)or

720x576(25Hz),orHDresolutionsof1280x720(720p)or1920x1080(1080i)atanaverage

bitrateof3Mbits/secorhigher.

Typically,thenumberofbitsspentoncodingaP‐frameisabout20%ofthenumberrequired

foranI‐frame,whereasaB‐

frametakesabout5%ofthebitrateconsumptionoftheI‐frame.

Thisimpliesthatgroupofpictures(GOP)structurehasabigimpactonqualityofencoded

1 2 ... 50 51 52 53 54 55 56 57 58 59 60 ... 81 82

Brak uwag

Exfo EXpert IPTV Test Tools (FTB-200v2) Instrukcja Użytkownika Strona 55