Proof of Convolution Theorem

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  • 7/27/2019 Proof of Convolution Theorem

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    TheConvolutionTheorem

    TheConvolutionTheoremstates:

    ! ! ! ! ! ! ! ! !"#! ! ! ! ! ! !(!)Proof:

    PartI:ProofoftheShiftTheoremorshift-invariance:

    Firstweprovetheshiftproperty,thatis,thattheFourierTransform(F),asa

    system,isshift-invariant:

    Thatis,ifthen

    Thatmeansthataddingashift(x-a)doesntchangethespectrum,F(u),itjust

    addsalinearphase,whichamountstomultiplicationby!!!!!"#.MultiplyingF(u)

    by!!!!!"#fordifferentatranslatesorshiftsf(x)bya.

    Letx=x-aanddx=dx

    Then! ! !

    !=

    ! ! !!= ! !! !!!!![! !

    !!! ]

    !"

    !

    !!

    Now!!!!!! !!!!

    = !!!!!!" !!!!!!"! !!"!!!!!!!" !"!!"#$%%:

    So! ! ! ! = !!!!!!"!(!)whichistheShiftTheorem

    Therefore,theFourierTransform,asaSystem,isshift-invariant.

    Significance:Shiftingortranslatingaspatialfunctionadistancex=aaddsalinear

    phase=uatotheoriginalphase.Conversely,alinearphasefilterproducesatranslationoftheimage.Themagnitudespectrumremainsunchanged.

    Ff(x) F(u) Ff(x-a) !

    !!!!"#

    F(u)

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