User guide

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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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