Exfo EXpert IPTV Test Tools (FTB-200v2) Uživatelský manuál stáhnout pdf (Strana 51)

51

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A.1.1.2 ImpactofPacketLossonSpecificVideoCodecs

AsshowninFigure2‐3,asimplenon‐robustvideostreamcanbeseverelydegraded

withevenlowlevelsofpacketlossduetotheerrorpropagationeffectsdescribedabove.

PeakSignal‐to‐NoiseRatio(PSNR)isanobjectivemeasurementofvideoservice

quality

comparingthemaximumpowerofthevideosignaltothepowerofcorruptingnoiseaffecting

thesignal.Generally,aPSNRofunder20dBisregardedasunwatchable,andthislevelis

reachedforMPEGvideowithalossrateunder1percent.

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Figure2‐3.

EstimatedPSNR

forThree

CommonCodecs



Errormi

tigationalgorithmsarebeingincreasinglyappliedtohelptocompensateforpacket

loss[11][12].Methodsinclude:

ForwardErrorCorrection(FEC)‐redundancyisappliedtothedatastreamtoallowsome

proportionoflostorerroredpacketstobereplaced

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Exfo EXpert IPTV Test Tools (FTB-200v2) Uživatelský manuál Strana 51