521 lines
		
	
	
		
			14 KiB
		
	
	
	
		
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			521 lines
		
	
	
		
			14 KiB
		
	
	
	
		
			HTML
		
	
	
	
	
	
| <!DOCTYPE html>
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| <!--Converted with LaTeX2HTML 2002-2-1 (1.71)
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| original version by:  Nikos Drakos, CBLU, University of Leeds
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| * revised and updated by:  Marcus Hennecke, Ross Moore, Herb Swan
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| * with significant contributions from:
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|   Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
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|   HREF="node186.html">Sawtooth waves and symmetry</A>
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| 
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| <H1><A ID="SECTION001410000000000000000">
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| Symmetries and Fourier series</A>
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| </H1>
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| 
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| <P>
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| Before making a quantitative analysis of the Fourier series of the classical
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| waveforms, we pause to make two useful observations about symmetries in
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| waveforms and the corresponding symmetries in the Fourier series.  First, a
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| Fourier series might consist only of even or odd-numbered harmonics; this is
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| reflected in symmetries comparing a waveform to its displacement by half
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| a cycle.  Second, the Fourier series may contain only real-valued or pure 
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| imaginary-valued coefficients (corresponding to the cosine or sine functions).
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| This is reflected in symmetries comparing the waveform to its reversal in time.
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| 
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| <P>
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| In this section we will assume that our waveform has an integer period <IMG
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|  WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img3.png"
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|  ALT="$N$">, and
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| furthermore, for simplicity, that <IMG
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|  WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img3.png"
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|  ALT="$N$"> is even (if it isn't we can just
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| up-sample by a factor of two).  We know from Chapter 9 that any (real or
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| complex valued) waveform <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> can be written as a Fourier series (whose
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| coefficients we'll denote by <IMG
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|  WIDTH="33" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1251.png"
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|  ALT="$A[k]$">):
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X[n] = A[0] + A[1]{U^n} + \cdots + A[N-1]{U^{(N-1)n}}
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="331" HEIGHT="28" BORDER="0"
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|  SRC="img1252.png"
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|  ALT="\begin{displaymath}
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| X[n] = A[0] + A[1]{U^n} + \cdots + A[N-1]{U^{(N-1)n}}
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| or, equivalently, 
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X[n] = 
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|         A[0] + A[1](\cos(\omega n) + i \sin(\omega n)) + \cdots
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="313" HEIGHT="28" BORDER="0"
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|  SRC="img1253.png"
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|  ALT="\begin{displaymath}
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| X[n] =
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| A[0] + A[1](\cos(\omega n) + i \sin(\omega n)) + \cdots
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| + A[N-1](\cos(\omega (N-1) n) + i \sin(\omega (N-1) n))
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="327" HEIGHT="28" BORDER="0"
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|  SRC="img1254.png"
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|  ALT="\begin{displaymath}
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| + A[N-1](\cos(\omega (N-1) n) + i \sin(\omega (N-1) n))
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| where <!-- MATH
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|  $\omega = 2\pi/N$
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|  -->
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| <IMG
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|  WIDTH="74" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1047.png"
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|  ALT="$\omega=2\pi/N$"> is the fundamental frequency of the waveform,
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| and
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| U = \cos(\omega) + i \sin(\omega)
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="146" HEIGHT="28" BORDER="0"
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|  SRC="img1049.png"
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|  ALT="\begin{displaymath}
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| U = \cos(\omega) + i \sin(\omega)
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| is the unit-magnitude complex number whose argument is <IMG
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|  WIDTH="14" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img27.png"
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|  ALT="$\omega $">.
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| 
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| <P>
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| To analyze the first symmetry we delay the signal <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> by a half-cycle.  Since
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| <!-- MATH
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|  ${U^{N/2}} = -1$
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|  -->
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| <IMG
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|  WIDTH="81" HEIGHT="37" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1255.png"
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|  ALT="${U^{N/2}} = -1$"> we get:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X[n+N/2] = A[0] - A[1]{U^n} + A[2]{U^{2n}} \pm \cdots
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="313" HEIGHT="28" BORDER="0"
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|  SRC="img1256.png"
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|  ALT="\begin{displaymath}
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| X[n+N/2] = A[0] - A[1]{U^n} + A[2]{U^{2n}} \pm \cdots
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| + A[N-2]{U^{(N-2)n}} - A[N-1]{U^{(N-1)n}}
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="274" HEIGHT="28" BORDER="0"
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|  SRC="img1257.png"
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|  ALT="\begin{displaymath}
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| + A[N-2]{U^{(N-2)n}} - A[N-1]{U^{(N-1)n}}
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| In effect, a half-cycle delay changes the sign of every other term in the
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| Fourier series.
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| We combine this with the original series in two different ways.  Letting
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| <IMG
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|  WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1258.png"
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|  ALT="$X'$"> denote half the sum of the two:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X'[n] = {{X[n] + X[n+N/2]}\over 2} =
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|     A[0] + A[2]{U^{2n}} + \cdots + A[N-2]{U^{(N-2)n}}
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="500" HEIGHT="40" BORDER="0"
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|  SRC="img1259.png"
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|  ALT="\begin{displaymath}
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| X'[n] = {{X[n] + X[n+N/2]}\over 2} =
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| A[0] + A[2]{U^{2n}} + \cdots + A[N-2]{U^{(N-2)n}}
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| and <IMG
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|  WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1260.png"
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|  ALT="$X''$"> half the difference:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X''[n] = {{X[n] - X[n+N/2]}\over 2} =
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|     A[1]{U^n} + A[3]{U^{3n}} + \cdots + A[N-1]{U^{(N-1)n}}
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="525" HEIGHT="40" BORDER="0"
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|  SRC="img1261.png"
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|  ALT="\begin{displaymath}
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| X''[n] = {{X[n] - X[n+N/2]}\over 2} =
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| A[1]{U^n} + A[3]{U^{3n}} + \cdots + A[N-1]{U^{(N-1)n}}
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| we see that <IMG
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|  WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1258.png"
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|  ALT="$X'$"> consists only of even-numbered harmonics (including DC) and
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| <IMG
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|  WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1260.png"
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|  ALT="$X''$"> only of odd ones.
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| 
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| <P>
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| Furthermore, if <IMG
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|  WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img670.png"
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|  ALT="$X$"> happens to be equal to itself shifted a half cycle, that
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| is, if <!-- MATH
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|  $X[n] = X[n+N/2]$
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|  -->
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| <IMG
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|  WIDTH="138" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1262.png"
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|  ALT="$X[n] = X[n+N/2]$">, then (looking at the definitions of <IMG
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|  WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1258.png"
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|  ALT="$X'$"> and <IMG
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|  WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img1260.png"
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|  ALT="$X''$">)
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| we get <IMG
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|  WIDTH="94" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1263.png"
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|  ALT="$X'[n] = X[n]$"> and <IMG
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|  WIDTH="73" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1264.png"
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|  ALT="$X''[n] = 0$">.  This implies that, in this case,
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| <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> has only even numbered harmonics.  Indeed, this should be no surprise,
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| since in this case <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> would have to repeat every <IMG
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|  WIDTH="32" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1265.png"
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|  ALT="$N/2$"> samples, so its
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| fundamental frequency is twice as high as normal for period <IMG
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|  WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img3.png"
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|  ALT="$N$">.
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| 
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| <P>
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| In the same way, if <!-- MATH
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|  $X[n] = -X[n+N/2$
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|  -->
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| <IMG
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|  WIDTH="146" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1266.png"
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|  ALT="$X[n] = -X[n+N/2$">], then <IMG
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|  WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img670.png"
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|  ALT="$X$"> can have only odd-numbered
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| harmonics.  This allows us easily to split any desired waveform into its
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| even- and odd-numbered harmonics.  (This is equivalent to using a comb filter
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| to extract even or odd harmonics; see Chapter <A HREF="node104.html#chapter-delay">7</A>.)
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| 
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| <P>
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| To derive the second symmetry relation we compare <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> with its time
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| reversal, <IMG
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|  WIDTH="48" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1267.png"
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|  ALT="$X[-n]$"> (or, equivalently, since <IMG
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|  WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img670.png"
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|  ALT="$X$"> repeats every <IMG
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|  WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img3.png"
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|  ALT="$N$"> samples, with
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| <IMG
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|  WIDTH="70" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1268.png"
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|  ALT="$X[N-n]$">).  The Fourier series becomes:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X[-n] = 
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|         A[0] + A[1](\cos(\omega n) - i \sin(\omega n)) + \cdots
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="326" HEIGHT="28" BORDER="0"
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|  SRC="img1269.png"
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|  ALT="\begin{displaymath}
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| X[-n] =
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| A[0] + A[1](\cos(\omega n) - i \sin(\omega n)) + \cdots
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| + A[N-1](\cos(\omega (N-1) n) - i \sin(\omega (N-1) n))
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="327" HEIGHT="28" BORDER="0"
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|  SRC="img1270.png"
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|  ALT="\begin{displaymath}
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| + A[N-1](\cos(\omega (N-1) n) - i \sin(\omega (N-1) n))
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| (since the cosine function is even and the sine function is odd).  In the same
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| way as before we can extract the cosines by forming <IMG
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|  WIDTH="40" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1271.png"
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|  ALT="$X'[n]$"> as half the sum:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X'[n] =  {{X[n] + X[-n]}\over 2} =
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|         A[0] + A[1]\cos(\omega n)+ \cdots + A[N-1]\cos(\omega (N-1) n)
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="544" HEIGHT="40" BORDER="0"
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|  SRC="img1272.png"
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|  ALT="\begin{displaymath}
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| X'[n] = {{X[n] + X[-n]}\over 2} =
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| A[0] + A[1]\cos(\omega n)+ \cdots + A[N-1]\cos(\omega (N-1) n)
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| and <IMG
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|  WIDTH="44" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1273.png"
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|  ALT="$X''[n]$"> as half the difference divided by <IMG
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|  WIDTH="9" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img646.png"
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|  ALT="$i$">:
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| <BR><P></P>
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| <DIV ALIGN="CENTER">
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| <!-- MATH
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|  \begin{displaymath}
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| X''[n] = {{X[n] - X[-n]}\over {2i}} =
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|         A[1]\sin(\omega n)+ \cdots + A[N-1]\sin(\omega (N-1) n)
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| \end{displaymath}
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|  -->
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| 
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| <IMG
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|  WIDTH="496" HEIGHT="40" BORDER="0"
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|  SRC="img1274.png"
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|  ALT="\begin{displaymath}
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| X''[n] = {{X[n] - X[-n]}\over {2i}} =
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| A[1]\sin(\omega n)+ \cdots + A[N-1]\sin(\omega (N-1) n)
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| \end{displaymath}">
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| </DIV>
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| <BR CLEAR="ALL">
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| <P></P>
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| 
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| <P>
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| So if <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> satisfies <IMG
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|  WIDTH="102" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1275.png"
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|  ALT="$X[-n] = X[n]$"> the Fourier series consists of cosine
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| terms only; if <IMG
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|  WIDTH="115" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img1276.png"
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|  ALT="$X[-n] = -X[n]$"> it consists of sine terms only; and as
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| before we can decompose any <IMG
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|  WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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|  SRC="img669.png"
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|  ALT="$X[n]$"> (that repeats every <IMG
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|  WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
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|  SRC="img3.png"
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|  ALT="$N$"> samples) as a sum
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| of the two.
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| 
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|   HREF="node186.html">Sawtooth waves and symmetry</A>
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| Miller Puckette
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| 2006-12-30
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