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ID:40563538
大小:17.07 KB
页数:9页
时间:2019-08-04
《Pulse-code modulation》由会员上传分享,免费在线阅读,更多相关内容在教育资源-天天文库。
1、Pulse-codemodulationPulse-codemodulation(PCM)isadigitalrepresentationofananalogsignalwherethemagnitudeofthesignalissampledregularlyatuniformintervals,thenquantizedtoaseriesofsymbolsinanumeric(usuallybinary)code.PCMhasbeenusedindigitaltelephonesystemsand1980s-era
2、electronicmusicalkeyboards.Itisalsothestandardformfordigitalaudioincomputersandthecompactdisc"redbook"format.Itisalsostandardindigitalvideo,forexample,usingITU-RBT.601.UncompressedPCMisnottypicallyusedforvideoinstandarddefinitionconsumerapplicationssuchasDVDorDV
3、Rbecausethebitraterequiredisfartoohigh.ModulationInthediagram,asinewave(redcurve)issampledandquantizedforpulsecodemodulation.Thesinewaveissampledatregularintervals,shownasticksonthex-axis.Foreachsample,oneoftheavailablevalues(ticksonthey-axis)ischosenbysomealgor
4、ithm(inthiscase,thefloorfunctionisused).Thisproducesafullydiscreterepresentationoftheinputsignal(shadedarea)thatcanbeeasilyencodedasdigitaldataforstorageormanipulation.Forthesinewaveexampleatright,wecanverifythatthequantizedvaluesatthesamplingmomentsare7,9,11,12
5、,13,14,14,15,15,15,14,etc.Encodingthesevaluesasbinarynumberswouldresultinthefollowingsetofnibbles:0111,1001,1011,1100,1101,1110,1110,1111,1111,1111,1110,etc.Thesedigitalvaluescouldthenbefurtherprocessedoranalyzedbyapurpose-specificdigitalsignalprocessororgeneral
6、purposeCPU.SeveralPulseCodeModulationstreamscouldalsobemultiplexedintoalargeraggregatedatastream,generallyfortransmissionofmultiplestreamsoverasinglephysicallink.Onetechniqueiscalledtime-divisionmultiplexing,orTDM,andiswidelyused,notablyinthemodernpublictelephon
7、esystem.AnothertechniqueiscalledFrequency-divisionmultiplexing,wherethesignalisassignedafrequencyinaspectrum,andtransmittedalongwithothersignalsinsidethatspectrum.Currently,TDMismuchmorewidelyusedthanFDMbecauseofitsnaturalcompatibilitywithdigitalcommunication,an
8、dgenerallylowerbandwidthrequirements.Therearemanywaystoimplementarealdevicethatperformsthistask.Inrealsystems,suchadeviceiscommonlyimplementedonasingleintegratedcircu
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