PULSE CODE MODULATION.ppt
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PULSE CODE MODULATION.ppt

PULSECODEMODULATION.ppt

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PulseCodeModulationWhyaParticularEncodingTechniqueWhyaParticularEncodingTechniqueCriteriaForSignalEncodingFactorsUsedtoCompareEncodingSchemesReasonsforAnalogModulationBasicEncodingTechniquesSpectrumofAMsignalAmplitudeModulationSingleSideband(SSB)PulseModulationAnaloguemodulatedsystemsarequitewidelyused,becauseoftheirsimplicity.AnalternativetoanaloguemodulatedsystemsisPulsedsystems.Thissystemisbasedondigitalsignalsorpulses.Thebasisofsuchasystemistheuseofadigitalcarriersignal,whichismodulatedbyananaloguesignal.Therearevariouswaysinwhichthiscanbeachieved,givingrisedifferentsystems.Ananaloguesignalistransmittedcontinuouslyinitsentireform.Thisneednotbedoneprovidedcertainconditionsaresatisfied.Samplesoftheanaloguesignalmaybetransmittedatgivenintervalsoftime.Theoriginalsignalmaythenberecoveredatthereceivingendfromthetransmittedsamples.Thistechniqueisknownassamplinganditunderliespulsedsystems.Consideratrainofsignals,witharepetitionfrequencyfandperiodTwhereIfthepulsetrainwereamplitudemodulatedbytheanaloguesignal,theresultwillbepulseswhoseamplitudesaresamplesoftheanaloguesignalattimeintervalsT.Iftheamplitudemodulatedpulsetrainisthenanalysed,itsspectrumwillconsistofFouriercomponents,IfWandf-Wdonotoverlapthenitispossibletoseparatethegroupoffrequenciesatthereceivingend.Thiscanbeachievedbyuseofalow-passfilterwithacut-offfrequencyf=W.Theseparatedfrequenciesarethenthoseoftheanaloguesignaltransmitted.FromthisthefollowingconditionisderivedThismeanstherepetitionfrequencymustbeatleasttwicethehighestfrequencycomponentintheanaloguesignal.TheminimumsamplingfrequencyisthenTelephonesiganlsrangefrom300Hzto3.4kHz,theinternationallyagreedsamplingfrequencyis8kHz.Itmeansthereisaguardbandof1.2kHzbetweenthelowersidebandandthebaseband.Comment:ItmeansWhasanupperlimitThetechniquecanonlybeusedifthebandwidthcanberestrictedtoWwithoutdestroyingtheessentialinformation.Toachievethis,band-limitedsignalsareused.SamplingTheoremAnanaloguesamplercommonlyknownasthesample-and-holdi