Exfo EXpert IPTV Test Tools (FTB-200v2) Manuale Utente Scaricare il pdf (Pagina 50)

50

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Forinter‐frameormotion‐basedcoding(PandBframes),motionvectorsaredeterminedfor

eachblockandencoded.Asforintra‐framecoding,errorscanrenderawholesliceorframe

unusable.Insimpleinter‐framecodingsystems,thelossofoneIorPframecanmakeall

subsequentfr

amesunusableuntilthenextIframeisreceived—resultinginasignificant

periodofdegraded,frozen,orblankvideo.

Note:theH.264

(MPEG‐4AVC)codec

standardintroduces

twonewframetypes,



Figure2‐2.Error

propagationthrough

framesinatypical

GOP



“SwitchingI”(SI)and

“SwitchingP”(SP),whicharedesig nedtoenablethedecodertomoreeasilyswitchbetween

videostreamswithdifferentbitrates.VQmon/HDreportsSIandSPframemetricswhen

H.264isused.

Figure2‐2showstheimpactofencodingerrorsonvariousframetypes(I,B,andP)inatypical

GroupofPict

ures(GOP).

Inmostcases,thestandardsforvideocodingprovideconsiderableflexibilitytoboththe

encoderanddecoder,allowingarangeofcost/performancetradeoffstobemade.Thiscan

makeitdifficulttopreciselyassesstheimpactofnetworkimpairmentswithoutknowledgeof

theexactimplementation.

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Exfo EXpert IPTV Test Tools (FTB-200v2) Manuale Utente Pagina 50