You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
 
 
 
miller-book/node185.html

520 lines
14 KiB

<!DOCTYPE html>
<!--Converted with LaTeX2HTML 2002-2-1 (1.71)
original version by: Nikos Drakos, CBLU, University of Leeds
* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan
* with significant contributions from:
Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
<HTML>
<HEAD>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<TITLE>Symmetries and Fourier series</TITLE>
<META NAME="description" CONTENT="Symmetries and Fourier series">
<META NAME="keywords" CONTENT="book">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">
<META NAME="Generator" CONTENT="LaTeX2HTML v2002-2-1">
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">
<LINK REL="STYLESHEET" HREF="book.css">
<LINK REL="next" HREF="node187.html">
<LINK REL="previous" HREF="node184.html">
<LINK REL="up" HREF="node184.html">
<LINK REL="next" HREF="node186.html">
</HEAD>
<BODY >
<!--Navigation Panel-->
<A ID="tex2html3341"
HREF="node186.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
SRC="next.png"></A>
<A ID="tex2html3335"
HREF="node184.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
SRC="up.png"></A>
<A ID="tex2html3329"
HREF="node184.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
SRC="prev.png"></A>
<A ID="tex2html3337"
HREF="node4.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents"
SRC="contents.png"></A>
<A ID="tex2html3339"
HREF="node201.html">
<IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index"
SRC="index.png"></A>
<BR>
<B> Next:</B> <A ID="tex2html3342"
HREF="node186.html">Sawtooth waves and symmetry</A>
<B> Up:</B> <A ID="tex2html3336"
HREF="node184.html">Classical waveforms</A>
<B> Previous:</B> <A ID="tex2html3330"
HREF="node184.html">Classical waveforms</A>
&nbsp; <B> <A ID="tex2html3338"
HREF="node4.html">Contents</A></B>
&nbsp; <B> <A ID="tex2html3340"
HREF="node201.html">Index</A></B>
<BR>
<BR>
<!--End of Navigation Panel-->
<H1><A ID="SECTION001410000000000000000">
Symmetries and Fourier series</A>
</H1>
<P>
Before making a quantitative analysis of the Fourier series of the classical
waveforms, we pause to make two useful observations about symmetries in
waveforms and the corresponding symmetries in the Fourier series. First, a
Fourier series might consist only of even or odd-numbered harmonics; this is
reflected in symmetries comparing a waveform to its displacement by half
a cycle. Second, the Fourier series may contain only real-valued or pure
imaginary-valued coefficients (corresponding to the cosine or sine functions).
This is reflected in symmetries comparing the waveform to its reversal in time.
<P>
In this section we will assume that our waveform has an integer period <IMG
WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img3.png"
ALT="$N$">, and
furthermore, for simplicity, that <IMG
WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img3.png"
ALT="$N$"> is even (if it isn't we can just
up-sample by a factor of two). We know from Chapter 9 that any (real or
complex valued) waveform <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> can be written as a Fourier series (whose
coefficients we'll denote by <IMG
WIDTH="33" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1251.png"
ALT="$A[k]$">):
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X[n] = A[0] + A[1]{U^n} + \cdots + A[N-1]{U^{(N-1)n}}
\end{displaymath}
-->
<IMG
WIDTH="331" HEIGHT="28" BORDER="0"
SRC="img1252.png"
ALT="\begin{displaymath}
X[n] = A[0] + A[1]{U^n} + \cdots + A[N-1]{U^{(N-1)n}}
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
or, equivalently,
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X[n] =
A[0] + A[1](\cos(\omega n) + i \sin(\omega n)) + \cdots
\end{displaymath}
-->
<IMG
WIDTH="313" HEIGHT="28" BORDER="0"
SRC="img1253.png"
ALT="\begin{displaymath}
X[n] =
A[0] + A[1](\cos(\omega n) + i \sin(\omega n)) + \cdots
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
+ A[N-1](\cos(\omega (N-1) n) + i \sin(\omega (N-1) n))
\end{displaymath}
-->
<IMG
WIDTH="327" HEIGHT="28" BORDER="0"
SRC="img1254.png"
ALT="\begin{displaymath}
+ A[N-1](\cos(\omega (N-1) n) + i \sin(\omega (N-1) n))
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
where <!-- MATH
$\omega = 2\pi/N$
-->
<IMG
WIDTH="74" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1047.png"
ALT="$\omega=2\pi/N$"> is the fundamental frequency of the waveform,
and
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
U = \cos(\omega) + i \sin(\omega)
\end{displaymath}
-->
<IMG
WIDTH="146" HEIGHT="28" BORDER="0"
SRC="img1049.png"
ALT="\begin{displaymath}
U = \cos(\omega) + i \sin(\omega)
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
is the unit-magnitude complex number whose argument is <IMG
WIDTH="14" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img27.png"
ALT="$\omega $">.
<P>
To analyze the first symmetry we delay the signal <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> by a half-cycle. Since
<!-- MATH
${U^{N/2}} = -1$
-->
<IMG
WIDTH="81" HEIGHT="37" ALIGN="MIDDLE" BORDER="0"
SRC="img1255.png"
ALT="${U^{N/2}} = -1$"> we get:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X[n+N/2] = A[0] - A[1]{U^n} + A[2]{U^{2n}} \pm \cdots
\end{displaymath}
-->
<IMG
WIDTH="313" HEIGHT="28" BORDER="0"
SRC="img1256.png"
ALT="\begin{displaymath}
X[n+N/2] = A[0] - A[1]{U^n} + A[2]{U^{2n}} \pm \cdots
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
+ A[N-2]{U^{(N-2)n}} - A[N-1]{U^{(N-1)n}}
\end{displaymath}
-->
<IMG
WIDTH="274" HEIGHT="28" BORDER="0"
SRC="img1257.png"
ALT="\begin{displaymath}
+ A[N-2]{U^{(N-2)n}} - A[N-1]{U^{(N-1)n}}
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
In effect, a half-cycle delay changes the sign of every other term in the
Fourier series.
We combine this with the original series in two different ways. Letting
<IMG
WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1258.png"
ALT="$X'$"> denote half the sum of the two:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X'[n] = {{X[n] + X[n+N/2]}\over 2} =
A[0] + A[2]{U^{2n}} + \cdots + A[N-2]{U^{(N-2)n}}
\end{displaymath}
-->
<IMG
WIDTH="500" HEIGHT="40" BORDER="0"
SRC="img1259.png"
ALT="\begin{displaymath}
X'[n] = {{X[n] + X[n+N/2]}\over 2} =
A[0] + A[2]{U^{2n}} + \cdots + A[N-2]{U^{(N-2)n}}
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
and <IMG
WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1260.png"
ALT="$X''$"> half the difference:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X''[n] = {{X[n] - X[n+N/2]}\over 2} =
A[1]{U^n} + A[3]{U^{3n}} + \cdots + A[N-1]{U^{(N-1)n}}
\end{displaymath}
-->
<IMG
WIDTH="525" HEIGHT="40" BORDER="0"
SRC="img1261.png"
ALT="\begin{displaymath}
X''[n] = {{X[n] - X[n+N/2]}\over 2} =
A[1]{U^n} + A[3]{U^{3n}} + \cdots + A[N-1]{U^{(N-1)n}}
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
we see that <IMG
WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1258.png"
ALT="$X'$"> consists only of even-numbered harmonics (including DC) and
<IMG
WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1260.png"
ALT="$X''$"> only of odd ones.
<P>
Furthermore, if <IMG
WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img670.png"
ALT="$X$"> happens to be equal to itself shifted a half cycle, that
is, if <!-- MATH
$X[n] = X[n+N/2]$
-->
<IMG
WIDTH="138" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1262.png"
ALT="$X[n] = X[n+N/2]$">, then (looking at the definitions of <IMG
WIDTH="22" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1258.png"
ALT="$X'$"> and <IMG
WIDTH="26" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
SRC="img1260.png"
ALT="$X''$">)
we get <IMG
WIDTH="94" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1263.png"
ALT="$X'[n] = X[n]$"> and <IMG
WIDTH="73" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1264.png"
ALT="$X''[n] = 0$">. This implies that, in this case,
<IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> has only even numbered harmonics. Indeed, this should be no surprise,
since in this case <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> would have to repeat every <IMG
WIDTH="32" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1265.png"
ALT="$N/2$"> samples, so its
fundamental frequency is twice as high as normal for period <IMG
WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img3.png"
ALT="$N$">.
<P>
In the same way, if <!-- MATH
$X[n] = -X[n+N/2$
-->
<IMG
WIDTH="146" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1266.png"
ALT="$X[n] = -X[n+N/2$">], then <IMG
WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img670.png"
ALT="$X$"> can have only odd-numbered
harmonics. This allows us easily to split any desired waveform into its
even- and odd-numbered harmonics. (This is equivalent to using a comb filter
to extract even or odd harmonics; see Chapter <A HREF="node104.html#chapter-delay">7</A>.)
<P>
To derive the second symmetry relation we compare <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> with its time
reversal, <IMG
WIDTH="48" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1267.png"
ALT="$X[-n]$"> (or, equivalently, since <IMG
WIDTH="17" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img670.png"
ALT="$X$"> repeats every <IMG
WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img3.png"
ALT="$N$"> samples, with
<IMG
WIDTH="70" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1268.png"
ALT="$X[N-n]$">). The Fourier series becomes:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X[-n] =
A[0] + A[1](\cos(\omega n) - i \sin(\omega n)) + \cdots
\end{displaymath}
-->
<IMG
WIDTH="326" HEIGHT="28" BORDER="0"
SRC="img1269.png"
ALT="\begin{displaymath}
X[-n] =
A[0] + A[1](\cos(\omega n) - i \sin(\omega n)) + \cdots
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
+ A[N-1](\cos(\omega (N-1) n) - i \sin(\omega (N-1) n))
\end{displaymath}
-->
<IMG
WIDTH="327" HEIGHT="28" BORDER="0"
SRC="img1270.png"
ALT="\begin{displaymath}
+ A[N-1](\cos(\omega (N-1) n) - i \sin(\omega (N-1) n))
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
(since the cosine function is even and the sine function is odd). In the same
way as before we can extract the cosines by forming <IMG
WIDTH="40" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1271.png"
ALT="$X'[n]$"> as half the sum:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X'[n] = {{X[n] + X[-n]}\over 2} =
A[0] + A[1]\cos(\omega n)+ \cdots + A[N-1]\cos(\omega (N-1) n)
\end{displaymath}
-->
<IMG
WIDTH="544" HEIGHT="40" BORDER="0"
SRC="img1272.png"
ALT="\begin{displaymath}
X'[n] = {{X[n] + X[-n]}\over 2} =
A[0] + A[1]\cos(\omega n)+ \cdots + A[N-1]\cos(\omega (N-1) n)
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
and <IMG
WIDTH="44" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1273.png"
ALT="$X''[n]$"> as half the difference divided by <IMG
WIDTH="9" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img646.png"
ALT="$i$">:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH
\begin{displaymath}
X''[n] = {{X[n] - X[-n]}\over {2i}} =
A[1]\sin(\omega n)+ \cdots + A[N-1]\sin(\omega (N-1) n)
\end{displaymath}
-->
<IMG
WIDTH="496" HEIGHT="40" BORDER="0"
SRC="img1274.png"
ALT="\begin{displaymath}
X''[n] = {{X[n] - X[-n]}\over {2i}} =
A[1]\sin(\omega n)+ \cdots + A[N-1]\sin(\omega (N-1) n)
\end{displaymath}">
</DIV>
<BR CLEAR="ALL">
<P></P>
<P>
So if <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> satisfies <IMG
WIDTH="102" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1275.png"
ALT="$X[-n] = X[n]$"> the Fourier series consists of cosine
terms only; if <IMG
WIDTH="115" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img1276.png"
ALT="$X[-n] = -X[n]$"> it consists of sine terms only; and as
before we can decompose any <IMG
WIDTH="36" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img669.png"
ALT="$X[n]$"> (that repeats every <IMG
WIDTH="18" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img3.png"
ALT="$N$"> samples) as a sum
of the two.
<P>
<BR><HR>
<!--Table of Child-Links-->
<A ID="CHILD_LINKS"><STRONG>Subsections</STRONG></A>
<UL>
<LI><A ID="tex2html3343"
HREF="node186.html">Sawtooth waves and symmetry</A>
</UL>
<!--End of Table of Child-Links-->
<HR>
<!--Navigation Panel-->
<A ID="tex2html3341"
HREF="node186.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
SRC="next.png"></A>
<A ID="tex2html3335"
HREF="node184.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
SRC="up.png"></A>
<A ID="tex2html3329"
HREF="node184.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
SRC="prev.png"></A>
<A ID="tex2html3337"
HREF="node4.html">
<IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents"
SRC="contents.png"></A>
<A ID="tex2html3339"
HREF="node201.html">
<IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index"
SRC="index.png"></A>
<BR>
<B> Next:</B> <A ID="tex2html3342"
HREF="node186.html">Sawtooth waves and symmetry</A>
<B> Up:</B> <A ID="tex2html3336"
HREF="node184.html">Classical waveforms</A>
<B> Previous:</B> <A ID="tex2html3330"
HREF="node184.html">Classical waveforms</A>
&nbsp; <B> <A ID="tex2html3338"
HREF="node4.html">Contents</A></B>
&nbsp; <B> <A ID="tex2html3340"
HREF="node201.html">Index</A></B>
<!--End of Navigation Panel-->
<ADDRESS>
Miller Puckette
2006-12-30
</ADDRESS>
</BODY>
</HTML>