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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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684 Chapter 22 Difference equations and the z transform

22.2.4 Order

Theorderisthedifferencebetweenthehighestandlowestargumentsofthedependent

variable. The equation

3x[n +2] −x[n +1] −7x[n] =n

issecond order because the difference betweenn+2 andnis2.

x[n +1]x[n −1] = 7x[n −2]

isthirdorderbecausethedifferencebetweenn +1andn −2is3.Ingeneral,thehigher

the order of an equation, the more difficultitistosolve.

Insomedifferenceequationsthedependentvariableoccursonlyonce.Theseareclassified

as zero order. Engineers refer to them as non-recursive difference equations because

calculation of the value of the dependent variable does not require knowledge of

thepreviousvalues.Incontrast,differenceequationsoforder1orgreaterarereferredto

asrecursivedifferenceequationsbecausetheirsolutionrequiresknowledgeofprevious

values of the dependent variable. The difference equation

x[n]=n 2 +n+1

has zero order and so isanon-recursive difference equation.

22.2.5 Homogeneousandinhomogeneousequations

Themeaningsofhomogeneousandinhomogeneousasappliedtolineardifferenceequationsareanalogoustothosemeaningswhenappliedtodifferentialequations.Todecide

whether a linear equation is homogeneous or inhomogeneous it is written in standard

form, with all the dependent variable terms on the l.h.s. Any remaining independent

variable termsare writtenon the r.h.s.For example,

3nx[n +1] −2n 3 =x[n −1]

iswritten as

3nx[n +1] −x[n −1] = 2n 3 (22.1)

If the r.h.s. is 0, the equation is homogeneous; otherwise it is inhomogeneous. Equation

(22.1) isinhomogeneous but

3nx[n+1]−x[n−1]=0

ishomogeneous.

Engineeringapplication22.1

Signalprocessingusingamicroprocessor

In engineering, an increasing number of products contain a microprocessor or computer

which is solving one or more difference equations. The input to the microprocessorisasequenceofsignalvalues,inmanycasesformedasaresultofsamplinga

continuous inputsignal.Theoutput fromthemicroprocessorisasequence ofsignal

values whichmaybesubsequently convertedintoacontinuous signal.Forexample,

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