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1、外文资料SignalsandSystemSignalsarescalar-valuedfunctionsofoneormoreindependentvariables.Oftenforconvenience,whenthesignalsareone-dimensional,theindependentvariableisreferredtoas“time”Theindependentvariablemaybecontinuesordiscrete.Signalsthatarecontinuousinbothampli
2、tudeandtime(oftenreferredtoascontinuous-timeoranalogsignals)arethemostcommonlyencounteredinsignalprocessingcontexts.Discrete-timesignalsaretypicallyassociatedwithsamplingofcontinuous-timesignals.Inadigitalimplementationofsignalprocessingsystem,quantizationofsig
3、nalamplitudeisalsorequired.AlthoughnotpreciselyCorrectineverycontext,discrete-timesignalprocessingisoftenreferredtoasdigitalsignalprocessing.Discrete-timesignals,alsoreferredtoassequences,aredenotedbyfunctionswhoseargumentsareintegers.Forexample,x(n)representsa
4、sequencethatisdefinedforintegervaluesofnandundefinedfornon-integervalueofn.Thenotationx(n)referstothediscretetimefunctionxortothevalueoffunctionxataspecificvalueofn.Thedistinctionbetweenthesetwowillbeobviousfromthecontest.Somesequencesandclassesofsequencesplaya
5、particularlyimportantroleindiscrete-timesignalprocessing.Thesearesummarizedbelow.Theunitsamplesequence,denotedbyδ(n)=1,n=0，δ(n)=0,otherwise(1)Thesequenceδ(n)playarolesimilartoanimpulsefunctioninanaloganalysis.Theunitstepsequence,denotedbyu(n),isdefinedasU(n)=1,
6、n≧0u(n)=0,otherwise(2)ExponentialsequencesoftheformX(n)=(3)Playaroleindiscretetimesignalprocessingsimilartotheroleplayedbyexponentialfunctionsincontinuoustimesignalprocessing.Specifically,theyareeigenfunctionsof18discretetimelinearsystemandforthatreasonformtheb
7、asisfortransformanalysistechniques.When︳α︳=1,x(n)takestheformx(n)=A(4)Becausethevariablenisaninteger,complexexponentialsequencesseparatedbyintegermultiplesof2πinω(frequency)areidenticalsequences,I.e:(5)Thisfactformsthecoreofmanyoftheimportantdifferencesbetweent
8、herepresentationofdiscretetimesignalsandsystems.Ageneralsinusoidalsequencecanbeexpressedasx(n)=Acos(n+Φ)(6)whereAistheamplitude,thefrequency,andΦthephase.Incontrastwithconti
